Pulsejet Project

Fabrication of a valveless Pulsejet Engine

Project Report 2015

1. INTRODUCTION A pulsejet is one of the simplest of engines from a design and manufacturing aspect but this simplicity is offset by the complications involved in understanding its working. It should be borne in mind that there is no conclusively established comprehensive mathematical law governing the working of a pulsejet, hence all new and innovative modifications to pulsejets are done on a trial and error basis. This greatly hinders progress since the effect of a change in the design is 'unpredictable'. But this has not deterred academicians and scientist from attempting to develop a theoretical model of the working mechanism. A considerable number of analyses ranging from using an acoustic analogy to solving the flow-field internal to the pulsejet have been performed in the past and though each one sheds fresh insight into a specific process/processes occurring in the pulsejet, no single theoretical model has been able to sufficiently explain all the processes. The systemic nature of the processes involved in this jet engine leaves a fragmented analysis anal ysis of it wanting, hence requiring further understanding of 'how it works' and 'what makes it work'. The pulsejet operation cycle, as has been observed experimentally, can be summarized in four phases.

1. Combustion occurs in the combustion chamber and the ensuing heat release increases the pressure and drives out the hot gases through the exhaust and produces thrust. The hot gases expand down the exhaust and inlet tubes, but due to a difference in the crosssections of the inlet and exhaust pipes, a major portion of hot gases are expelled through the exhaust pipe.

2. Once the combustion gases have expanded to atmospheric pressure, over-expansion of the gases due to inertia (Kadenacy effect), inside the combustion chamber, causes the chamber pressure to decrease to sub-atmospheric sub-atmospheric levels.‟


Project Report 2015

3. The sub-atmospheric pressure causes fresh reactants to enter the combustion chamber through the inlet (the inlet air column has a lower inertia) and a small fraction of the exhaust gases from the exhaust tube.

4. The residual gases and heat transfer from the walls raise the reactants‟ temperature to the auto-ignition temperature, initiating combustion. The entire cycle repeats itself at a regular interval. 1.1 DESCRIPTION OF PULSE COMBUSTION

Pulsating combustion is a combustion process that occurs under oscillatory conditions. That means that the state variables, such as pressure, temperature, velocity of combustion gases, etc., that describe the condition in the combustion zone, vary periodically with time. Pulse combustion is a very old technology. The phenomenon of combustion-driven oscillations was first observed in the year 1777, subsequently explained by Lord Rayleigh in the year 1878, and used in a variety of applications around the turn of the Century. C entury.

Fig 1.1: General Pulse Combustion Process


Project Report 2015

3. The sub-atmospheric pressure causes fresh reactants to enter the combustion chamber through the inlet (the inlet air column has a lower inertia) and a small fraction of the exhaust gases from the exhaust tube.

4. The residual gases and heat transfer from the walls raise the reactants‟ temperature to the auto-ignition temperature, initiating combustion. The entire cycle repeats itself at a regular interval. 1.1 DESCRIPTION OF PULSE COMBUSTION

Pulsating combustion is a combustion process that occurs under oscillatory conditions. That means that the state variables, such as pressure, temperature, velocity of combustion gases, etc., that describe the condition in the combustion zone, vary periodically with time. Pulse combustion is a very old technology. The phenomenon of combustion-driven oscillations was first observed in the year 1777, subsequently explained by Lord Rayleigh in the year 1878, and used in a variety of applications around the turn of the Century. C entury.

Fig 1.1: General Pulse Combustion Process


Project Report 2015

One of the better known examples of a pulse combustor is the German V-1 "Buzz Bomb " of World

War

II; Although the technology of pulse combustion has been

known for many years, devices using pulse combustion have not been implemented widely despite their many attractive characteristics.

Fig1.2: Combustion chamber explosion

Compared to conventional combustion systems, their heat transfer rates are a factor of two to five higher than normal turbulent values, their combustion intensities are up to on order of magnitude magnitude higher, their emissions of oxides of of nitrogen are a factor of three lower, their thermal efficiencies are up to 40% higher, and they may be selfaspirating, obviating the need for a blower. This combination of attributes can result in favorable economic trade off with conventional combustors in many applications. Most of the research on pulse combustors has been directed toward applied examinations of the engineering engineering aspects of pulse combustors: heat transfer, efficiency, frequency of operation, pollutant formation, etc. There is also uncertainty over the behavior of frequency as a function of geometry, energy input, and mass input. Zinn states that the pulse combustor can be modeled as a Helmholtz resonator, while Dec and Keller found that the frequency of operation is a function of the magnitude of the energy input and of the magnitude of the mass flux.


Project Report 2015

These results indicate that a Helmholtz resonator model is insufficient to predict the frequency of operation. These fundamental questions must be answered before the prediction of an optimum resonant condition is possible.


Project Report 2015

2. LITERATURE SURVEY

2.1 STUDY OF EXISTING SYSTEM

A jet

engine is

a reaction

engine discharging

a

fast

moving jet that

generates thrust by jet propulsion in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, and pulse jets. In general, jet engines are combustion engines but non-combusting forms also exist. In common parlance, the term jet engine loosely refers to an internal combustion air breathing jet engine (a duct engine). These typically consist of an engine with a rotary (rotating) air compressor powered by a turbine ("Brayton cycle"), with the leftover power providing thrust via a propelling nozzle. Jet aircraft use these types of engines for long-distance travel. Early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines. These engines offer high speed and greater fuel efficiency than piston and propeller aeroengines over long distances. Jet engines power aircraft, cruise missiles and unmanned aerial

vehicles. In the form of rocket engines they

power fireworks,model rocketry, spaceflight, and military missiles. Jet engines have propelled high speed cars, particularly drag racers, with the all-time record held by a rocket car. A turbofan powered car, Thrust SSC, currently holds the land speed record. Jet engine designs are frequently modified for non-aircraft applications, as industrial gas turbines. These are used in electrical power generation, for powering water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. Industrial gas turbines can create up to 50,000 shaft horsepower. Many of these engines are derived from older military turbojets such as the Pratt & Whitney J57 and J75 models. There is also a derivative of the P&W JT8D low-bypass turbofan that creates up to 35,000 HP. Propeller engines are useful for comparison. They accelerate a large mass of air but by a relatively small maximum change in speed. This low speed limits the maximum thrust


Project Report 2015

of any propeller driven airplane. However, because they accelerate a large mass of air, propeller engines, such as turboprops, can be very efficient. On the other hand, turbojets accelerate a much smaller mass of intake air and burned fuel, but they emit it at the much higher speeds which are made possible by using a de Laval nozzle to accelerate the engine exhaust. This is why they are suitable for aircraft traveling at supersonic and higher speeds. Turbofans have a mixed exhaust consisting of the bypass air and the hot combustion product gas from the core engine. The amount of air that bypasses the core engine com pared to the amount flowing into the engine determines what is called a turbofan‟s bypass ratio (BPR). While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30 percent and 70 percent of the total thrust produced by a turbofan system. The Diffuser : The Diffuser is a low pressure circular vent that is responsible for

converting the kinetic energy of the atmospheric air into a static pressure rise. The pressure of the atmospheric air is high thus this air flows into the vent where it gets reduced in volume thus increasing its pressure. This intake air is then fed to the compressor. The Compressor : The compressor found in Turbo Jet engines are usually rotary

compressors. The rotary compressors are compressors that generate high volume of air at a low pressure thus having a lower pressure ratio compared to the reciprocating compressors. The air intake from the diffuser is fed to the inlet of the rotary axial or centrifugal compressor where the air gets compressed in various stages and reaches a high pressure. This high pressure is reached due to the various stages of the compressor adding to the pressure at each stage. The compressor outlet is the inlet to the air-fuel feed nozzle.


Project Report 2015

Fig: 2.1 Typical Jet Engine interior

The Air-Fuel feed nozzle : The air-fuel feed nozzle mixes the compressed air with the

jet fuel. The fuel is mixed with a specific air-fuel ratio. This injector nozzle injects fuel at a constant rate in the combustion chamber where it is burned to form high pressure exhaust. The Combustion Chamber : This is a c

hamber where the air-fuel mixture is burned with the help of flame stabiizers. The flame stabilizers keep a constant flame ignited in the combustion chamber to continuously burn the fuel and also ensure that the flame does not go out. The chamber consists of two fuel injector nozzles and the flame stabilizer. The exhaust created in the combustion chamber is passed t o the Mechanical Turbine. The Mechanical Turbine : The mechanical turbine consists of a rotary element having

fan blades. The high pressure exhaust from the combustion chamber strikes the fans of the turbine causing it to rotate. This striking causes the exhaust to expand and lose its pressure.


Project Report 2015

The turbine is responsible for driving the axial compressor. the turbine rotation is connected to the rotor of the compressor with reduction gear. This causes the compressor to rotate. The exhaust nozzle from the turbine blades is further passed to the exhaust nozzle.

2.2 LIMITATION OF EXISTING SYSTEM 

They are heavier for the same power, because they need to be made from stronger material due to the higher compression ratio and because they need to have larger cylinder volume because of lower maximal rpm. On longer flights the reduction in fuel weight often makes up for the heavier engine.



Compared to a reciprocating engine of the same size, they are expensive.



Because of high speeds and high operating temperatures, designing and manufacturing gas turbines is a challenge from both the engineering and materials point of view.



Gas turbines also tend to use more fuel when they are idling.



Very heavy, so can‟t be implemented in light aircrafts and drones.





Need constant maintenance because of moving parts. Thrust production is often low at low subsonic speeds.


Project Report 2015

2.3 STUDY OF PROPOSED SYSTEM

Though a lot of research had been performed prior to Marconnet by researchers like Holtzwarth and Karavodine in the field of pulsed combustion, the use of pulsed combustion as a method of direct thrust generation was first carried out by Marconnet in his "reacture-pulsateur", which is in every way the precursor to the modern day valveless pulsejet. P. Schmidt applied the concepts of Marconnet's 'wave engine' to the development of an intermittent pulsejet engine, called the Schmidtrohr, directed towards use for vertical take-off and landing vehicles.

Once the potential of pulsejets as direct thrust producing engines had been demonstrated, the development of the same grew rapidly. The German Air Ministry, deciding to investigate all forms of jet engines, asked the Argus Motoren Gesellschaft of Berlin to develop the pulsejet. This project, under the development of Dr. Fritz Gosslau, led to the development of the famous Argus AS 109-014 powering the Vergeltungswaffe 1 (V-1) “Buzz Bomb” of World War -II [ref xiii]. It has been erroneously reported in numerous publications that the Argus work was in conjunction with P. Schmidt. Post World War-II, research in pulsejets was undertaken by the US Navy under Project Squid. French engineers at SNECMA did extensive research on pulsejets. Lockwood of Hiller Aircraft, with the support of the French engineers, investigated the working of pulsejets and this work is a landmark achievement as it is the only completely documented, systematic study in existence. He utilized analytical tools developed by J.V.Foa, which though not conclusive, were the most complete analytical approach available at that time. E.Tharratt's heuristic approach by *analytical evaluation of pulsejets. J.A.C. Kentfield and his associates from the University of Calgary pioneered work on developing computer simulations of the cyclic operations of the valveless pulsejet. Dudley Smith of the University of Texas, Arlington developed a numerical model of a valveless pulsejet to include combustion, while accessing the performance of a pulsejet with a synchronous injection ignition system.


Project Report 2015

Since 2004, a fair amount of research on pulsejets including experimental, analytical and numerical studies has been undertaken by North Carolina State University and these studies have demonstrated the feasibility of operating pulsejets of sizes as small as 8cm in length. A pulse jet engine is a type of jet engine in which combustion occurs in pulses. Pulsejet engines can be made with few or no moving parts, and are capable of running statically. Pulse jet engines are a lightweight form of jet propulsion, but usually have a poor compression ratio, and hence give a low specific impulse. Pulsejet is an unsteady propulsive device with its basic components being the inlet, combustion chamber, valve and valve head assembly and a tailpipe.

Fig2.2 Schematic of pulse combustion operation


Project Report 2015

2.4 TYPES OF PULSE JET ENGINES

There are two types of pulse jet engines: those with valves and those without. The ones with valves allow air to come in through the intake valve and exit through the exhaust valve after combustion takes place. Pulse jet engines without valves, however, use their own design as a valve system and often allow exhaust gases to exit from both the intake and exhaust pipes, although the engine is usually designed so that most of the exhaust gases exit through the exhaust pipe. A. Valved Pulsejet Engine

Valved engines use a mechanical valve to control the flow of expanding exhaust, forcing the hot gas to go out of the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake. The valved pulsejet comprises of a intake with a one-way valve arrangement. The valves prevent the explosive gas of the ignited fuel mixture in the combustion chamber from exiting and disrupting the intake airflow, although with all practical valved pulsejets there is some 'blowback' while running statically and at low speed as the valves cannot close fast enough to stop all the gas from exiting the intake.

Fig 2.3: Valved Pulsejet Engine

The hot exhaust gases exit through an acoustically resonant exhaust pipe. The valve arrangement is commonly a "daisy valve" also known as a reed valve. The daisy valve


Project Report 2015

is less effective than a rectangular valve grid, although it is easier to construct on a small scale. B. Valveless Pulsejet Engine

The valveless pulse jet engine operates on the same principle, but the 'valve' is the engine's geometry. Fuel as a gas or liquid vapor is either mixed with the air in the intake or directly injected into the combustion chamber. Starting the engine usually requires forced air and an ignition method such as a spark plug for the fuel-air mix. With modern manufactured engine designs, almost any design can be made to be 'self-starting' by providing the engine with fuel and an ignition spark, starting the engine with no compressed air. Once running, the engine only requires input of fuel to maintain a selfsustaining combustion cycle. Valveless pulsejets, have no moving parts and use only their geometry to control the flow of exhaust out of the engine. Valveless engines expel exhaust gases out of both the intake and the exhaust, most try to have the majority of exhaust go out the longer tail pipe, for more efficient propulsion.

Fig2.4:Valveless pulsejet engine

Fuel is drawn into the combustion chamber through the intake valve in either as an airgas mixture or in liquid form. The intake valve then closes and a spark plug is used to ignite the fuel in the combustion chamber. The fuel then expands rapidly and tries to fill the entire chamber in order to escape. The closed intake valve forces the fuel to the rear of the combustion chamber and allows the exhaust gases to exit through the exhaust valve.


Project Report 2015

3. TYPES OF VALVELESS PULSEJETS The idea of pulsed combustion was conceived even before t he use of steady state combustion employed in gas turbine engines. Over the past hundred years various number of valveless pulsejet designs have been invented and tested. These are classified into three main systems 

Inline systems



U-shaped systems



Linear systems

3.1 INLINE SYSTEMS

The systems, which have an intake pipe, combustion chamber and exhaust pipe, all on the same axis with intake and exhaust held in opposite directions are called inline systems. The advantage of this system is that when the engine has positive forward air velocity the intake has air rushing into it creating a ram-air effect, similar to ram jet engines. Moreover the fabrication and fitting of inline systems is much easier than any other systems. The disadvantage is that these engines have lower thrust than other systems because the hot air exiting the intake after combustion does not to contribute to net thrust and actually creates negative trust that has to be overcome. To overcome this many complicated and mostly infeasible aerodynamic valves have been created to allow the ram air effect to work without allowing the air to move back through so as to increase thrust. However none have been proven effective. 3.1.1 Marconnet Design

In 1909 Georges Marconnet developed the first pulsating combustor without valves. It was the father of all valveless pulse jets. Marconnet found that a blast inside a chamber would prefer to go through a bigger exhaust opening rather than squeezing through a relatively narrow intake. In addition a long diffuser between the intake and the combustion chamber would direct the charge strongly towards exhaust, the way a trumpet directs sound.


Project Report 2015

Fig 3.1 Marconnet’s Valveless Pulsejet Engine

3.1.2 Schubert design

The principle of the valveless pulsating combustor was discovered by Lt.William Schubert of the US NAVY in the early 1940s. Schubert‟s design was called a “Resojet” on the account on its dependence on resonance. The taper less attachments of the inlet tube to the combustion chamber in Schubert‟s design creates strong turbulence for better mixing of fuel and air so that high intensity combustion takes place. Schubert carefully calculated the geometry of the intake so that the exhaust gas could not exit by the time the press ure inside fell below atmospheric. The resistance of a tube to the passage of gas depends steeply on the gas temperature. Thus, the same tube will offer a much greater resistance to outgoing hot gas than to the incoming cold air. The impedance is inversely proportional to the square root of the gas temperature. This degree of irreversibility seems to offer the possibility for the cool air necessary for combustion to get in during the intake part of the cycle, but for the hot gas to encounter too much resistance to get out of the intake during the expansion part.

Fig 3.2: Schubert’s valveless pulsejet engine


Project Report 2015

3.2 U-SHAPED SYSTEMS

The U-shape design overcomes the shortfall of the inline design by bending the exhaust pipe by 180 degrees, so that the exhaust and intake are aligned in the same direction. The advantage of this design is that the thrust generated by the inlet contributes to the net thrust of the engine as it flows in the same direction as the exhaust. The disadvantage is that the ram-air affect is lost. Moreover fabrication is quite complex. 3.2.1 Lockwood-Hiller

The U-shaped Lockwood-Hiller engine was invented by Raymond Lockwood. It is said that the Lockwood was the most effective pulse jet engine ever developed. The air fuel mixture is generated by mixing fuel which is injected through a jet built into the side of the combustion chamber or on a strut projecting into the chamber or on two crossed struts spanning the front part of the chamber. The chamber is the drum like broad part of the engine. The short straight tube attached to the combustion chamber is the inlet. And the long U tube attached to the combustion chamber is the tail pipe. The tailpipe is fitted with a flare at the end.

Fig3.3: U-shaped Lockwood Hiller engine

The Lockwood-Hiller design is the most successful example of U-shaped designs in both performance and efficiency. Conversely it is difficult to construct because of numerous cone sections are to be fabricated for i t.


Project Report 2015

3.3 LINEAR SYSTEMS

There are many designs of valveless pulsejet engines that cannot be categorized by either U-shape or inline designs. These engines a re generally variations of inline designs with the intake moved to the side of the combustion chamber. The typical feature of the linear engine is that the intake emanates from the side of the combustion chamber. The advantage of this type of engine is that the physical size is smaller than an equivalent Ushaped engine making integration into airframe more practical. These engines are also simpler to manufacture than U-shape design. The disadvantage of this design is the tuning difficulty for optimized performance as the intake length is directly proportional to exhaust length. Net thrust outputs are considerably greater than inline while performance is less than the equivalent U-shape design as the efficiency is limited by intake position.

3.3.1 Argus design

The capped tube design was first invented by the Argus Company (manufacturer of German V-1 bombs). It consisted of combustion chamber (plenum chamber), which formed a bottle shape design capped over with a hemispherical top. Fuel was injected through a nozzle located on the tip of the cap and protected from the chamber with metal grid. The grid functioned as a heat sink and prevented gas from burning at the nozzle. Pressurized air was forced into the plenum chamber continuously using a compressor, the combustion took place and the hot gases expanded. The continuous supply of the compressed air into the plenum chamber prevented hot gas from getting out of the plenum chamber and almost all of it were thrust into the exhaust. The engine did not self-sustain or resonate due to the reasons of smaller plenum chamber and exhaust length.


Fig 3.4: Capped tube-Argus

Project Report 2015


Project Report 2015

4. COMPONENTS AND SYSTEM DESCRIPTION The main components are used in this project are



SPARK PLUG



COMBUSTION CHAMBER



FUEL INLET



FRAME

4.1 SPARK PLUG

A spark plug (sometimes, in British in British English, a English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from anignition an ignition system to the combustion the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric an electric spark, while spark, while containing combustion pressure within the engine. A spark plug has

a

metal threaded metal threaded shell,

electrically

isolated

from

a

central electrode by central electrode by

aporcelain insulator. The central electrode, which may contain a resistor, a resistor, is connected by a heavily insulated heavily insulated wire to the output terminal of anignition anignition coil or magneto. magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded. electrically grounded. The central electrode protrudes through the porcelain insulator into the combustion the combustion chamber, forming chamber, forming one or more spark more spark gaps between gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the side, earth, or ground electrode(s).

Spark plugs may also be used for other purposes; in Saab in Saab Direct Ignition when they are not firing, spark plugs are used to measure ionization in the cylinders - this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function. Spark plugs may also be used in other applications such as furnaces as furnaces wherein a combustible fuel/air mixture must be ignited. In this case, they are sometimes referred to as flame igniters.


Project Report 2015

The plug is connected to the high voltage generated by an ignition coil or magneto. magneto. As the electrons flow from the coil, a voltage develops between the central and side electrodes. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. become ionized. The ionized gas becomes a conductor and allows electrons to flow across the gap. Spark plugs usually require voltage of 12,000 – 12,000 – 25,000 25,000 volts or more to "fire" properly, although it can go up to 45,000 volts. They supply higher current during the discharge process, resulting in a hotter and longer-durati on spark.

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" heard when observing a spark, similar to lightning to lightning and thunder. and thunder.

The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark the spark gap as the gases burn on their own. The size of this fireball, or kernel, depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition the ignition timing was retarded, and a large one as though the ti ming was advanced.


Project Report 2015

4.2 COMBUSTION CHAMBER

A combustion chamber is that part of an internal combustion engine (ICE) in which the fuel/air mix is burned. ICEs typically comprise reciprocating piston engines, rotary engines, gas turbine and jet turbines.The combustion process increases the internal energy of a gas, which translates into an increase in temperature, pressure, or volume depending on the configuration. In an enclosure, for example the cylinder of a reciprocating engine, the volume is controlled and the combustion creates an increase in pressure. In a continuous flow system, for example a jet engine combustor, the pressure is controlled and the combustion creates an increase in volume. This increase in pressure or volume can be used to do work, for example, to move a piston on a crankshaft or a turbine discin a gas turbine. If the gas velocity changes, thrust is produced, such as in the nozzle of a rocket engine.

Head types

Various shapes of combustion chamber have been used, such as: L-head (or flathead) for side-valve engines; "bathtub", "hemispherical", and "wedge" for overhead valve engines; and "pent-roof" for engines having 3, 4 or 5 valves per cylinder. The shape of the chamber has a marked effect on power output, efficiency and emissions; the designer's objectives are to burn all of the mixture as completely as possible while avoiding excessive temperatures (which create NOx). This is best achieved with a compact rather than elongated chamber. Swirl & Squish

The intake valve/port is usually placed to give the mixture a pronounced "swirl" (the term is preferable to "turbulence", which implies movement without overall pattern) above the rising piston, improving mixing and combustion. The shape of the piston top also affects the amount of swirl. Another design feature to promote turbulence for good fuel/air mixing is "squish", where the fuel/air mix is "squished" at high pressure by the rising piston.


Project Report 2015

Flame front

Finally, the spark plug must be situated in a position from which the flame front can reach all parts of the chamber at the desired point, usually around 15 degrees after top dead centre. It is strongly desirable to avoid narrow crevices where stagnant "end gas" can become trapped, as this tends to detonate violently after the main charge, adding little useful work and potentially damaging the engine. Also, the residual gases displace room for fresh air/fuel mixture and will thus reduce the power potential of each firing stroke.

Why Valveless Pulsejets

A valveless pulse jet engine is a simple and ordinary engine. It is just a piece of metal tube cut to the required dimensions. In a valveless pulsejet engine there are no mechanical valves but they do have aerodynamic valves which for the most part resist the flow in a single direction. They have no mechanically moving parts and sothey are more reliable. All valveless engines have low thrust output, high fuel consumption and overall poor performance.

Fig4.1: A 4-Pound Valveless Pulse Jet


Project Report 2015

Pulsejets can be used on a large scale as industrial drying systems, and there has been a new surge to study and apply these engines to applications such as high output heating, biomass conversion, and alternative energy systems,as pulsejets can run on almost anything that burns including particulate fuels such as sawdust or coal powder.


Project Report 2015

5. PRINCIPLE OF OPERATION 5.1 RIJKE TUBE

Rijke's tube turns heat into sound, by creating a self-amplifying standing wave. It is an entertaining phenomenon in acoustics and is an excellent example of resonance.

Fig 5.1: Rijke Tube

The Rijke tube is simply a cylindrical tube with both ends open and a heat source placed inside it. The heat source may be a flame or an electrical heating element. It has a wire gauze inside about one quarter the way from the bottom. Traditionally, the tube is positioned vertically on a stand or even held in a hand and the heat source is introduced from below into the tube. For certain ranges of position of the heat source within the tube, the Rijke tube emits a loud sound. This phenomenon was discovered by Rijke around 1850, and is therefore called the Rijke phenomenon. Sound production in the Rijke tube is a classic example of a thermo-acoustic phenomenon. In the case of the Rijke tube air can move in and out of both ends. A heated metal mesh placed a quarter of the way up from the bottom heats the air flowing past it. This flow of air is a combination of the convection current caused by the transfer of heat from the metal mesh and the sound wave that is set up for the condition of two open ends.


Project Report 2015

For half of the oscillation cycle of the sound wave air moves in from both ends as it flows towards the center generating a pressure antinode (displacement node) there. Even though some of the air moving past the hot metal mesh has already been heated during the cycle prior to this, some additional cool air flows in, passing through it and acquiring thermal energy and further increasing the pressure, thus reinforcing the oscillation. For the remaining half cycle air passing by the metal mesh while flowing outward from the center of the tube is already heated and therefore energy transfer is minimal. The sound comes from a standing wave whose wavelength is about twice the length of the tube, giving the fundamental frequency. Lord Rayleigh, in his book, gave the correct explanation of how the sound is stimulated. The flow of air past the gauze is a combination of two motions. There is a uniform upwards motion of the air due to a convection current resulting from the gauze heating up the air. Superimposed on this is the motion due to the sound wave. For half the vibration cycle, the air flows into the tube from both ends until the pressure reaches a maximum. During the other half cycle, the flow of air is outwards until the minimum pressure is reached. All air flowing past the gauze is heated to the temperature of the gauze and any transfer of heat to the air will increase its pressure according to the gas law. As the air flows upwards past the gauze most of it will already be hot because it has just come downwards past the gauze during the previous half cycle. However, just before the pressure maximum, a small quantity of cool air comes into contact with the gauze and its pressure is suddenly increased. This increases the pressure maximum, so reinforcing the vibration. During the other half cycle, when the pressure is decreasing, the air above the gauze is forced downwards past the gauze again. Since it is already hot, no pressure change due to the gauze takes place, since there is no transfer of heat. The sound wave is therefore reinforced once every vibration cycle and it quickly builds up to very large amplitude.This explains why there is no sound when the flame is heating the gauze. All air flowing through the tube is heated by the flame, so when it reaches the gauze, it is already hot and no pressure increase takes place.


Project Report 2015

Fig 5.2: Working of a Rijke Tube

When the gauze is in the upper half of the tube, there is no sound. In this case, the cool air brought in from the bottom by the convection current reaches the gauze towards the end of the outward vibration movement. This is immediately before the pressure minimum, so a sudden increase in pressure due to the heat transfer tends to cancel out the sound wave instead of reinforcing it. The position of the gauze in the tube is not critical as long as it is in the lower half. To work out its best position, there are two things to consider. Most heat will be transferred to the air where the displacement of the wave is a maximum, i.e. at the end of the tube. However, the effect of increasing the pressure is greatest where there is the greatest pressure variation, i.e. in the middle of the tube. Placing the gauze midway between these two positions (one quarter of the way in from the bottom end) is a simple way to come close to the optimal placement. The Rijke tube is considered to be a standing wave form of thermo acoustic devices known as "heat engines" or "prime movers".


Project Report 2015

5.2 THE HELMHOLTZ RESONATOR

Helmholtz resonance is the phenomenon of air resonance in a cavity, such as when one blows across the top of an empty bottle. The name comes from a device created in the 1850s by Hermann von Helmholtz. The "Helmholtz resonator", which he, the author of the classic study of acoustic science, is used to identify the various frequencies or musical pitches present in music and other complex sounds. The Helmholtz resonator can best be demonstrated by taking a normal soft drink bottle and blowing over the mouth of the bottle. When air is forced into a cavity, the pressure inside it increases. When the external force pushing the air into the cavity is removed, the higher-pressure air inside will flow out. The cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreas ing each time. The air in the port (the neck of the chamber) has mass. Since it is in motion, it possesses some momentum. A longer port would make for a larger mass, and viceversa. The diameter of the port is related to the mass of air and the volume of the chamber. A port that is too small in area for the chamber volume will "choke" the flow while one that is too large in area for the chamber volume tends to reduce the momentum of the air in the port.


Project Report 2015

Fig 5.3: Helmholtz Resonator

An important type of resonator with very different acoustic characteristics is the Helmholtz resonator. Essentially a hollow sphere with a short, small-diameter neck, a Helmholtz resonator has a single isolated resonant frequency and no other resonances below about 10 times that frequency. The resonant frequency (f) of a classical Helmholtz resonator, shown in Figure, is determined by its volume (V) and by the length (L) and area (A) of its neck:

Here,

   

f =

2


Project Report 2015

Figure 5.4: A Classic Helmholtz Resonator

where S is the speed of sound in air. As with the tubes discussed above, the value of the length of the neck should be given as the effective length, which depends on its radius. The isolated resonance of a Helmholtz resonator made it useful for the study of musical tones in the mid-19th century, before electronic analyzers had been invented. When a resonator is held near the source of a sound, the air in it will begin to resonate if the tone being analyzed has a spectral component at the frequency of the resonator. By listening carefully to the tone of a musical instrument with such a resonator, it is possible to identify the spectral components of a complex sound wave such as those generated by musical instruments. Helmholtz Resonator Analogy in Pulse Jet Engines

The simplest analytical model of the valveless pulsejet is that of a Helmholtz resonator in a combination with a quarter wave oscillator. While their analogy is one of the simplest forms, it allows for a wealth of understanding of the fundamental operation of a valveless pulsejet. The model assumes that the combustion chamber and inlet can be modeled as a Helmholtz resonator and the exhaust as a matched, or tuned, quarter wave oscillator (the familiar pipe organ) It is a classic element in the study of acoustics. The pressure of the gas within the cavity of the resonator changes as it is alternately compressed and expanded by the


Project Report 2015

influx and efflux of the gas through the opening and thus provide the stiffness element. At the opening, there is a radiation of sound into the surrounding medium, which leads to the dissipation of acoustic energy and thus provides a resistance element.


Project Report 2015

6. OPERATION OF A VALVELESS PULSE JET ENGINE

Fig 6.1: Valveless Pulse Jet during operation

The operation of valveless pulsejet requires a fundamental knowledge about mixing ignition, combustion and wave initiation, wave propagation and wave reflection. Any disturbance in the fluid medium creates a wave pattern. If the propagation of the wave is parallel to the motion of the fluid, then it is termed as longitudinal waves e.g. sound waves. This is the mode of wave propagation that occurs in a valveless pulsejet. When the deflagration begins, a zone of significantly elevated pressure travels outward through both air masses as a "compression wave". This wave moves at the speed of sound through both the intake and tailpipe air masses. (Because these air masses are significantly elevated in temperature as a result of earlier cycles, the speed of sound in them is much higher than it would be in normal outdoor air.) When a compression wave reaches the open end of either tube, a low pressure rarefaction wave starts back in the opposite direction, as if "reflected" by the open end. This low pressure region returning to the combustion zone is, in fact, the


Project Report 2015

internal mechanism of the Kadenacy effect. There will be no "breathing" of fresh air into the combustion zone until the arrival of the rarefaction wave. Mixing of air and fuel in a Valveless Pulsejet

In the combustion chamber fuel is injected into the flow of fresh air entering the engine. At the beginning of the charging cycle the mixture is very rich, then it gets leaned and at the end of the cycle it gets richer again but this mixing of fuel and air in a flow stream are affected by the parameters of molecular size, concentration, temperature, flow velocity in the vicinity of the injector and evaporation rate, vary within wide bounds, the mixture is very non-homogeneous. The combustion chamber consists of two distinct layers: a highly enriched layer with fuel and combustion products from the previous cycle and a cold la yer arising at the end of the suction cycle. This mixture in-homogeneously causes a noticeable drop in its combustible properties. The proper engine operation could be achieved with a mixture composition of air/fuel ratio 1.1 - 1.4. Ignition in a Valveless Pulsejet

Initially the fuel-air ignition is done manually with the help of blower and a spark plug. Since the pressure inside the combustion chamber is above atmospheric pressure, the combustion products along with the air flow towards the exhaust and continue so long as the pressure in the chamber falls below atmospheric pressure. Now the gases will retrace its path back into the combustion chamber since the atmospheric pressure is greater than the combustion pressure. Because of the momentum or the turbulence of the hot gas rushing back in, the pressure and temperature inside the combustion chamber will increase drastically. Once the chamber temperature is above the ignition temperature of the fuel the next ignition takes place and this cycle continues. Combustion process in a Valveless Pulsejet

The combustion process likely exists in two phases: an initial ignition which gradually takes over the entire combustion chamber and this increases pressure and temperature in the chamber and thereby facilitating the evaporation of the remaining unburned mixture, and a main combustion process occurring almost instantaneously in


Project Report 2015

the entire chamber and lasting about 25% of the entire cycle. The combustion chamber can reach up to a maximum approximate temperature of 2000K. Since the pressure difference between the combustion chamber and exhaust is oscillating, there will only be intermittent flow of air to the chamber to support combustion. A pulse jet engine is an ideal example for an unsteady combustion process. Here the combustion process is pulsating. The potential coupling between the unsteady components of pressure and heat release can lead to sustained, large amplitude acoustic oscillations which being driven by heat release is referred to as a thermo-acoustic instability. Rayleigh was the first to hypothesize the onset of the instability and define a criterion for positive coupling. According to Rayleighcriteria “if heat be periodically communicated to and abstracted from a mass of air vibrating in a cylinder, the effect produced will depend on the phase of vibration at which heat transfer takes place. If the heat be given to air at moment of greatest compression or taken at the moment of greatest rarefaction the vibration is encouraged. On the other hand heat is given moment of greatest rarefaction or abstracted at the moment of greatest condensation, the vibration is discouraged.” Expansion of gases

Due to pressure being setup only at a certain region of engine, the gases at high pressure migrate to low pressure regions in the engine and eventually out of the engine (atmosphere). This happens at a very high velocity since the potential difference in static pressure between atmosphere and the combustion chamber is very high. This phenomenon occurs at the cost of losing the achieved high static pressure in combustion chamber, a very high migration velocity implies a very high volume flow rate of the engine, hence a very quick and drastic drop in static pressure Suction of gases

Owing to the exit of the exhaust gases at very high velocities, the static pressure in the combustion chamber drops drastically, the drop is to such an extent that a negative gauge pressure (partial vacuum) is setup in the combustion chamber, which forces to cease any further exit to the combustion gases, instead the combusted products still dwelling in the engine is sucked back into the combustion chamber along with the


Project Report 2015

fresh atmospheric air. This leads to the fresh mixing of air and fuel inside the combustion chamber for subsequent combustions.


Project Report 2015

7. WORKING OF A VALVELESS PULSE JET ENGINE

Fig 7.1: Working of Valveless Pulsejet Engine

The figure below shows a layout of a valveless pulsejet engine. It has a chamber with two tubular ports of unequal length and diameter. The port on the right, curved backwards, is the intake pipe. The bigger, flared one on the left is the exhaust, or tailpipe. In some other engines, it is the exhaust pipe that is bent into the U-shape, but the important thing is that the ends of both ports point in the same direction. When the fuel-air mixture combusts in the chamber, the process generates a great amount of hot gas very quickly. This happens so fast that it resembles, an explosion.


Project Report 2015

Fig 7.2: Layout of a Valveless Pulse Jet Engine

The immediate, explosive rise in internal pressure first compresses the gas inside and then pushes it forcefully out of the chamber, two powerful spurts of hot expanding gas are created – a big one that blows through the tailpipe and a smaller one blowing through the intake. Leaving the engine, the two jets exert a pulse of thrust – they push the engine in the opposite direction. As the gas expands and the combustion chamber empties, the pressure inside the engine drops. Due to inertia of the moving gas, this drop continues for some time even after the pressure falls back to atmospheric. The expansion stops only when the momentum of the gas pulse is completely spent. At that point, there is a partial vacuum inside the engine. The process now reverses itself. The outside (atmospheric) pressure is now higher than the pressure inside the engine and fresh air starts rushing into the ends of the two ports. At the intake side, it quickly passes through the short tube, enters the chamber and mixes with the fuel. The tailpipe, however, is rather longer, so that the incoming air does not even get as far as the chamber before the engine is refilled and the pressure peaks. One of the prime reasons for the extra length of the tailpipe is to retain enough of the hot exhaust gas within the engine at the moment the suction starts. This gas is greatly rarified by the expansion, but the outside pressure will push it back and increase its density again. Back in the chamber, the gases of the previous combustion mix vigorously with the fresh fuel/air mixture that enters from the other side. The heat of the chamber and the free radicals in the retained gas will cause ignition and the process repeats.


Project Report 2015

The spark plug shown on the picture is needed only at start-up. The retained hot gas provides self-ignition and the spark plug becomes unnecessary. Indeed, if spark ignition is left on, it can interfere with the normal functioning of the engine.In the Jshaped and U-shaped valveless engines, gas spews out of two ports. Some valveless pulsejet designers have developed engines that are not bent backwards, but employ various tricks that work in a similar fashion to valves -- i.e. they allow fresh air to come in but prevent the hot gas from getting out through the intake. A gentler, more gradual entry would not generate the necessary swirling of gases. In addition, turbulence increases the intensity of combustion and the rate of the heat release.

7.1 THERMODYNAMIC CYCLE The thermodynamic working principle of a pulsejet engine does not have an exact explanation; hence a popular and commonly accepted thermodynamic model is a Lenoir cycle. The Lenoir Cycle is an idealized thermodynamic cycle, where the ide al gas undergoes basically 3 processes to produce work. The most interesting part of this cycle is that the output work is obtained with no energy spent on compressing the working fluid. The cyclic process are as follows, (1) Constant volume (isochoric) heat addition and then (2) Adiabatic expansion and (3) Constant pressure (isobaric) heat rejection. As Pulsejets typically have a very small compression ratio that reaches a maximum at around (1.7). The Lenoir three cycle process can be se en below in.


Project Report 2015

Fig 7.3: Lenoir Cycle

As the expansion process is isentropic and hence involves no heat interaction. Energy is absorbed as heat during the constant volume process and rejected as heat during the constant pressure process. Hence the (P-V) diagram from fig (3.5-1) represents the thermodynamic process of the Lenoir cycle. Due to the finite time of combustion and incomplete filling of the chamber with the fresh charge, the pressure at the end of the heat supply process depends on both the fuel-air composition and on the relative volume of the fresh mixture entering through the inlet valve. In this case the heat supply process is not isochoric. This deviation from the ideal process demands for implementation of modifications to the existing ideal process.

Designing of a valveless pulse jet engine

Valveless Pulse jets are much simpler in design than the valved engines, but with simplicity you have to sacrifice kgs of thrust and loose the ram air effect. The following section breaks a valveless pulsejet engine into major components and investigates design approaches used in other designs for each component. The most important components are the combustion chamber, the exhaust and intake pipes, the fuel injection system, the spark ignition system and the air assist starting system. For


Project Report 2015

each of the components, various solutions are considered to guide in designing a suitable pulsejet engine. Combustion Chamber

The combustion chamber is arguably the most important component of a valveless pulsejet design. For a valveless pulsejet engine, the combustion chamber geometry is critical as any flow inconsistency can disrupt the pulsating combustion cycle, as pressure waves may be reflected at sudden area changes. The most suitable solution depends heavily on the selected configuration but there are several design parameters that apply to all cases. The most significant attribute of a combustion chamber is the circular cross section. This is because the pressure inside the combustion chamber, positive or negative depending on the cycle, causes stress within the wall. This stress is more evenly distributed by a circular cross-section design.

Fig 7.4: Comparison of conical sections

Combustion chambers also have conical sections leading into the intake and exhaust pipes. These sections maintain smooth gas flow throughout the engine.The above figure depicts the gas flow after combustion in both a conical section and a stepped transition. The example on the left has a higher pressure increase because the post ignition confinement is improved, but produces lower thrust because the gas suffers choking due to entrance effects upon entering the exhaust, limiting the exiting velocity. Conversely, a tapered cone that is too shallow has poor levels of post ignition confinement, meaning thrust is also low. A good compromise is required in order to have a practical engine.


Project Report 2015

Fig 7.5: Lockwood Hiller Combustion Chamber

Fig 7.6: Logan Combustion Chamber Section

The Logan combustion chamber section shows the implementation of the cone sections on two different design solutions. Notably, the Lockwood-Hiller design has steep cones while the Logan design features shallower tapers. This is because the Lockwood- Hiller design has much larger intake and exhaust openings that allow the flow to move relatively smoothly so post ignition confinement is the most critical component of that design. Conversely, the Logan design has smaller openings and requires unimpeded air flow exiting and entering the engine thus the conical section is much shallower. From Simpson (2005), the optimum cone angle for an inline or linear valveless configuration is approximately 30 degrees, depending on the size of the engine. The cone section is a critical compromise between the flow of the gases and


Project Report 2015

post ignition confinement and as such, is a relatively critical consideration for our design. Exhaust and Intake

The exhaust and intake pipes of a valveless pulsejet engine are generally straight, circular cross-section tubes with a critical length. The length is critical as it must promote the acoustic resonance necessary to sustain engine operation. The diameter of the pipe is also an important consideration as it needs to allow sufficient flow to produce the required thrust; however, some degree of pressure must be retained to aid in combustion chamber pressure increase.

Fig 7.7: Standard Exhaust Runner Design

The fig shows an arbitrary exhaust pipe section. The length to diameter ratio is not as critical as the length is the critical dimension. Generally, however, the length to diameter ratio is 7 to 10 percent of the length to give sufficient volume for gas flow. This is similar for intake pipes to allow a sufficient fresh air charge into the combustion chamber. Standard exhaust runner design also depicts the diffuser on the end of the pipe. This is the same for both intake and exhaust and is necessary to control the flow of gas exiting and entering the engine.


Project Report 2015

Fig 7.8: Sudden Expansion Exit Conditions

For the exit condition, this fig shows that when a sharp expansion occurs, the flow creates turbulent eddies as it separates from these edges. This separation causes the flow to lose energy, thus reducing the overall thrust developed by the engine. By making this transition conical or bell-shaped these effects are negated keeping the flow smooth and directing more of the energy of the flow into generating thrust from the engine. Conversely, for the intake condition, the fig shows that the flow separates from the surface at the sharp corner creating a vena contractor that effectively limits thecrosssectional area through which the air can flow. This limits the effectiveness of the intake to draw in the fresh air charge and the exhaust to ingest the cool dense air required to confine the combustion event. Conical or bell-shaped diffusers limit flow separation allowing smooth tran sition of the air into the engine.

Fig 7.9: Entrance Flow Conditions


Project Report 2015

8. OBJECT AND APPROACH Although not all waves have a speed that is independent of the shape of the wave, and this property therefore is an evidence that sound is a wave phenomenon, sound does nevertheless have this property .For instance, the music in a large concert hall or stadium may take on the order of a second to reach someone seated in the nosebleed section, but we do not notice or care, because the delay is the same for every sound. Bass, drums, and vocals all head outward from the stage at 340 m/s, regardless of their differing wave shapes. The speed of sound in a gas is related to the gas's physical properties. It is a series of compressions and expansions of the air.

Fig 8.1: Propagation of Sound during the Operation of Pulse Jet

Even for a very loud sound, the increase or decrease compared to normal atmospheric pressure is no more than a part per million, so our ears are apparently very sensitive instruments. In a vacuum, there is no medium for the sound waves, and so the y cannot exist. The roars and whooshes of space ships in Hollywood movies are fun, but scientifically wrong.


Project Report 2015

8.1 KADENACY EFFECT

Fig 8.2: Kadenacy Effect

In the explanation of the working cycle, inertia keeps driving the expanding gas out of the engine all the way until the pressure in the chamber falls below atmospheric. The opposite thing happens in the next part of the cycle, when the outside air pushes its way in to fill the vacuum. The combined momentum of the gases rushing in through the two opposed ports causes the chamber briefly to be pressurized above atmospheric before ignition. There is thus an oscillation of pressure in the engine caused by inertia. The gases involved in the process (air and gaseous products of combustion) are stretched and compressed between the inside and outside pressures. In effect, those fluids behave like an elastic medium, like a piece of rubber. This is called the “Kadenacy Effect”. The elastic character of gas is used to store some of the energy created in one combustion cycle and use it in the next. The energy stored in the pressure differential (partial vacuum) makes the aspiration (replacement of the burned gas with fresh fuel-air mixture) possible. Without it, pulsejets would not work.


Project Report 2015

8.2 PROPAGATION OF SOUND IN PULSE JETS

The phenomenon of sound is easily found to have all the characteristics we expect from a wave phenomenon Sound waves obey superposition. Sounds do not knock other sounds out of the way when they collide, and we can hear more than one sound at once if they both reach our ear simultaneously. The medium does not move with the sound. Even standing in front of a titanic speaker pla ying earsplitting music, we do not feel the slightest breeze. The velocity of sound depends on the medium. Sound travels faster in helium than in air, andfaster in water than in helium. Putting more energy into the wave makes it more intense, not faster. Acoustic Theory

The pressure wave travels up and down the tube. When the wave front reaches an end of the tube, part of it reflects back. Reflections from opposed ends meet and form the so-called „standing wave‟.

Fig 8.3: Standing Wave A standing wave in a transmission line is a wave in which the distribution of current, voltage, or field strength is formed by the superposition of two waves of the


Project Report 2015

same frequency propagating in opposite directions. The effect is a series of nodes (zero displacement) and anti-nodes (maximum displacement) at fixed points along the transmission line. Such a standing wave may be formed when a wave is transmitted into one end of a transmission line and is reflected from the other end by an impedancemismatch, i.e., discontinuity, such as an open circuit or a short. The failure of the line to transfer power at the standing wave frequency will usually result in attenuation distortion. In practice, losses in the transmission line and other components mean that a perfect reflection and a pure standing wave are never achieved. The result is a partial standing wave, which is a superposition of a standing wave and a traveling wave. The degree to which the wave resembles either a pure standing wave or a pure traveling wave is measured by the standing wave ratio.

Fig 8.4: Wave Formation at the Exhaust Another example is standing waves in the open ocean formed by waves with the same wave period moving in opposite directions. These may form near storm centers, or from reflection of a swell at the shore, and are the source of microbaroms and microseisms. Graphically, the standing wave is best represented by a double sine curve. The same is true for the pulsejet cycle. The undulations of a single sine curve depict the changes of gas pressure and gas speed inside a pulsejet engine very well. The


Project Report 2015

doubling of the curve – the addition of a mirror image, so to say – shows that the places where the pressure and speed are the highest in one part of the cycle will be the places where they are the lowest in the opposite part. The changes of pressure and the changes of gas speed do not coincide. They follow the same curve but are offset from each other. One trails (or leads) the other by a quarter of the cycle. If the whole cycle is depicted as a circle – 360 degrees – the speed curve will be offset from the pressure curve by 90 degrees.The resonance establishes a pattern of gas pressures and speeds in the engine duct that is peculiar to the pulsejet and not found in the other jet engines. In some ways it resembles a 2-stroke piston engine resonant exhaust system more than in does a conventional jet engine. Understanding this pattern is very important, for it helps determine the way the events in the engine unfold. When considering a pulsejet design, it is always good to remember that those machines are governed by a complex interaction of fluid thermodynamics and acoustics. Elements of Resonance

In acoustic terms, the combustion chamber is the place of the greatest impedance, meaning that the movement of gas is the most restricted. However, the pressure swings are the greatest. The chamber is thus a speed node but a pressure antinode. The outer ends of the intake and exhaust ports are the places of the lowest impedance. They are the places where the gas movement is at the maximum and the speed changes are the greatest – in other words, they are speed antinodes. The pressure swings are minimal, so that the port ends are pressure nodes. The pressure outside the engine is constant (atmospheric). The pressure in the combustion chamber seesaws regularly above and below atmospheric. The pressure changes make the gases accelerate through the ports in one direction or another, depending on whether the pressure in the chamber is above or below atmospheric. The distance between a node and an antinode is a quarter of the wavelength. This is the smallest section of a standing wave that a resonating vessel can accommodate. In a valveless pulsejet, this is the distance between the combustion chamber (pressure antinode) and the end of the tailpipe(pressure node). This length will determine the fundamental wavelength of the standing wave that will govern the engine


Project Report 2015

operation.The distance between the chamber and the end of the intake is rather shorter. It will accommodate a quarter of a wave of a shorter wavelength. This secondary wavelength must be an odd harmonic of the fundamental.


Project Report 2015

9. DESIGNING AND MODELLING 9.1 CAD/CAE

Computer aided design or CAD has very broad meaning and can be defined as the use of computers in creation, modification, anal ysis and optimization of a design. CAE (Computer Aided Engineering) is referred to computers in Engineering analysis like stress/strain, heat transfer, flow analysis. CAD/CAE is said to have more potential to radically increase productivity than any development since electricity. CAD/CAE builds quality form concept to final product. Instead of bringing in quality control during the final inspection it helps to develop a process in which quality is there through the life cycle of the product. CAD/CAE can eliminate the need for prototypes. But it required prototypes can be used to confirm rather predict performance and other characteristics. CAD/CAE is employed in numerous industries like manufacturing, automotive, aerospace, casting, mould making, plastic, electronics and other general purpose industries. CAD/CAE systems can be broadly divided into low end, mid end and high-end systems.

Low-end systems are those systems which do only 2D modeling and with only little 3D modeling capabilities. According to industry static‟s 70-80% of all mechanical designers still uses 2D CAD applications. This may be mainly due to the high cost of high-end systems and a lack of expertise.Mid-end systems are actually similar high-end systems with all their design capabilities with the difference that they are offered at much lower prices. 3D sold modeling on the PC is burgeoning because of many reasons like affordable and powerful hardware, strong sound software that offers windows case of use shortened design and production cycles and smooth integration with downstream application. More and more designers and engineers are shifting to mid end system.

High-end CAD/CAE software‟s are for the complete modeling, analysis and manufacturing of products. High-end systems can be visualized as the brain of


Project Report 2015

concurrent engineering. The design and development of products, which took years in the passed to complete, is now made in days with the help of high-end CAD/CAE systems and concurrent engineering.

9.2 MODELING

Model is a Representation of an object, a system, or an idea in some form other than that of the entity itself. Modeling is the process of producing a model; a model is a representation of the construction and working of some system of interest. A model is similar to but simpler than the system it represents. One purpose of a model is to enable the analyst to predict the effect of changes to the system. On the one hand, a model should be a close approximation to the real system and incorporate most of its salient features. On the other hand, it should not be so complex that it is impossible to understand and experiment with it. A good model is a judicious trade off between realism and simplicity. Simulation practitioners recommend increasing the complexity of a model iteratively. An important issue in modeling is model validity. Model validation techniques include simulating the model under known input conditions and comparing model output with system output. Generally, a model intended for a simulation study is a mathematical model developed with the help of simulation software. 9.3 SOFTWARE FOR MODELING 

Solid works



Creo



CATIA



Unigraphics, etc


Project Report 2015

9.3.1 Creo

Creo Elements/Pro (formerly Pro/ENGINEER), PTC's parametric, integrated 3D CAD/CAM/CAE solution, is used by discrete manufacturers for mechanical engineering, design and manufacturing. Created by Dr. Samuel P. Geisberg in the mid1980s, Pro/ENGINEER was the industry's first successful rule-based constraint (sometimes called "parametric" or "variational") 3D CAD modeling system. The parametric modelling approach uses parameters, dimensions, features, and relationships to capture intended product behaviour and create a recipe which enables design automation and the optimization of design and product development processes. This design approach is used by companies whose product strategy is family-bas ed or platform-driven, where a prescriptive design strategy is fundamental to the success of the design process by embedding engineering constraints and relationships to quickly optimize the design, or where the resulting geometry may be complex or based upon equations. Creo Elements/Pro provides a complete set of design, analysis and manufacturing capabilities on one, integral, scalable platform. These required capabilities include Solid Modeling, Surfacing, Rendering, Data Interoperability, Routed Systems Design, Simulation, Tolerance Anal ysis, and NC and Tooling Design. Like any software it is continually being developed to include new functionality. The details below aim to outline the scope of capabili ties to give an overview rather than giving specific details on the individual functionality of the product.

Creo Elements/Pro is a software application within the CAD/CAM/CAE category, along with other similar products currently on the market.Creo Elements/Pro is a parametric, feature-based modeling architecture incorporated into a single database philosophy with advanced rule-based design capabilities. It provides in-depth control of complex geometry, as exemplified by the trajpar parameter. The capabilities of the product can be split into the three main headings of Engineering Design, Analysis and Manufacturing.


Project Report 2015

9.3.2 Engineering Design

Creo Elements/Pro offers a range of tools to enable the generation of a complete digital representation of the product being designed. In addition to the general geometry tools there is also the ability to generate geometry of other integrated design disciplines such as industrial and standard pipe work and complete wiring definitions. Tools are also available to support collaborative development.

A number of concept design tools that provide up-front Industrial Design concepts can then be used in the downstream process of engineering the product. These range from conceptual Industrial design sketches, reverse engineering with point cloud data and comprehensive free-form surface tools.

We created 3D model of this project by using CREO software. The models are shown below…


Fig 9.1: Model representation in creo

Project Report 2015


Project Report 2015

9.4 MATERIAL EMPLOYED

The material used in the manufacturing process is Mild Steel.Mild steel is a type of steel that only contains a small amount of carbon and other elements. It is softer and more easily shaped than higher carbon steels. It also bends a long way instead of breaking because it is ductile. It is used in nails and some types of wire, it can be used to make bottle openers, chairs, staplers, staples, railings and most common metal products. Its name comes from the fact it only has less carbon than steel. Some mild steel properties and uses: 

Mild steel has a maximum limit of 0.2% carbon. The proportions of manganese (1.65%), copper (0.6%) and silicon (0.6%) are approximately fixed, while the proportions of cobalt, chromium, niobium, molybdenum, titanium, nickel, tungsten, vanadium and zirconium are not.



A higher amount of carbon makes steels different from low carbon mild-type steels. A greater amount of carbon makes steel stronger, harder and very slightly stiffer than a low carbon steel. However, the strength and hardness comes at the price of a decrease in the ductility of this alloy. Carbon atoms get trapped in the interstitial sites of the iron lattice and make it stronger.



What is known as mildest grade of carbon steel or 'mild steel' is typically low carbon steel with a comparatively low amount of carbon (0.16% to 0.2%). It has ferromagnetic properties, which make it ideal for manufacture of many products.



The calculated average industry grade mild steel density is 7.85 gm/cm3. Its Young's modulus, which is a measure of its stiffnes s is around 210,000 MPa.



Mild steel is the cheapest and most versatile form of steel and serves every application which requires a bulk amount of steel.

The thickness of 2mm for both the pulsejets is maintained.


Project Report 2015

9.5 MANUFACTURING PROCESS

Manufacturing processes are the steps through which raw mate rials are transformed into a final product. The manufacturing process begins with the creation of the materials from which the design is made. These materials are then modified through manufacturing processes to become the required part. Manufacturing processes can include treating (such as heat treating or coating), machining, or reshaping the material. The manufacturing process also includes tests and checks for quality assurance during or after the manufacturing, and planning the production process prior to manufacturing.

Fig 9.2: Manufacturing process


Fig 9.3 Detailed Design

Project Report 2015


Project Report 2015

10.ADVANTAGES

Pulse jet engines are easy to build on a small scale and can be constructed using few or no moving parts. This means that the total cost of each pulses jet engine is much cheaper than traditional turbine engine. Pulse jet engines do not produce torque like turbine engine do, and have a higher thrust-to-weight ratio. Pulse jet engines can also run on virtually any substance that can burn, possibly making them a milestone in alternative fuel innovations.

Department of Aeronautical Engineering

56

Mount Zion college of engineering,Kadammanitta


Project Report 2015

11. DISADVANTAGES While pulse jet engines can be beneficial to many industries, they do have several disadvantages. For example, pulse jet engines are very loud which only makes them practical for military and industrial purposes. Also, pulse jet engines do not ha ve very good thrust specific fuel consumption levels. Likewise, pulse jet engines use acoustic resonance rather than external compression devices to compress fuels before combustion


57



Project Report 2015

12.APPLICATIONS

Pulsejets are used today in target drone aircraft, flying control line model aircraft (as well as radio-controlled aircraft), fog generators, and industrial drying and home he ating equipment. Because pulsejets are an efficient and simple wa y to convert fuel into heat, experiments are using them for new industrial applications such as biomass fuel conversion, boiler and heater systems, and other application.


58



Project Report 2015

13.CONCLUSION

This project work has provided us an excellent opportunity and experience, to use our limited knowledge. We gained a lot of practical knowledge regarding, planning, purchasing, assembling and machining while doing this project work. We feel that the project work is a good solution to bridge the gates between institution and industries.

We are proud that we have completed the work with the limited time successfully. The “PULSE JET ENGINE” is working with satisfactory conditions. We are able to understand the difficulties in maintaining the tolerances and also quality. We have done to our ability and skill making maximum use of available facilities.


59


Pulsejet Project

Recommend Documents