UNIT-I AIRCRAFT GAS TURBINES •
Impulse and reaction blading of gas turbines
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Velocity triangles and power output
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Vortex theory
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Choice of blade profile, pitch and chord
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Estimation of stage performance
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Limiting factors in gas turbine design
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Methods of blade cooling
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Matching of turbine and compressor.
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Numerical problems
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University question paper solution
Gas Turbines •
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Work can be extracted from a gas at higher inlet pressure to the lower back pressure by allowing it to flow through the turbine. The work done by the gas is equivalent to the change of its enthalpy.
Turbine A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. A turbine is turbomachine with at least one moving part called a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. Early
Impulse and reaction turbine
Impulse turbines
An impulse stage is characterized by the expansion of the gas which occurs only in the stator nozzles. The rotor blades act as directional vanes to deflect the direction of the flow.
They convert the K.E. of the gas into work by changing the momentum of the gas more or less at constant pressure.
Reaction turbines •
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A reaction stage is one in which expansion of the gas takes place both in the stator & in the rotor. The function of the stator is the same as that of the impulse stage, but the function of the rotor is in two folds
Methods of blade cooling •
Convection cooling works by passing cooling air through passages internal to the blade. Heat is transferred by conduction through the blade, and then by convection into the air flowing inside of the blade. A large internal surface
area is desirable for this method, so the cooling paths tend to be serpentine and full of small fins.[
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A variation of convection cooling, impingement cooling , works by hitting the inner surface of the blade with high velocity air. This allows more heat to be transferred by convection than regular convection cooling does. Impingement cooling is often used on certain areas of a turbine blade, like the leading edge, with standard convection cooling used in the rest of the blade.
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The second major type of cooling is film cooling . This type of cooling works by pumping cool air out of the blade through small holes in the blade. This air creates a thin layer (the film) of cool air on the surface of the blade, protecting it from the high temperature air. The air holes can be in many different blade locations, but they are most often along the leading edge.
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Transpiration cooling , the third major type of cooling, is similar to film cooling in that it creates a thin film of cooling air on the blade, but it is different in that that air is "leaked" through a porous shell rather than injected through holes. This type of cooling is effective at high temperatures as it uniformly covers the entire blade with cool air.
Transpiration-cooled blades generally consist of a rigid strut with a porous shell. Air flows through internal channels of the strut and then passes through the porous shell to cool the blade.
UNIT-II RAMJET PROPULSION •
Operating principle of Ram jet engine
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Sub critical, critical and supercritical operation of Ramjet
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Combustion in Ramjet engine
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Ramjet performance
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Sample Ramjet design calculations
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Introduction to scramjet.
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Preliminary concepts in supersonic combustion –
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Integral ram- rocket
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Numerical Problems
WORKING PRINCIPLE OF RAMJET
RAMJET ENGINE
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A ramjet, sometimes referred to as a flying stovepipe or an athodyd (an abbreviation of aero thermodynamic duct), is a form of air breathing jet engine that uses the engine's forward motion to compress incoming air without an axial compressor. Ramjets cannot produce thrust at zero airspeed; cannot move anvehicle, aircraft therefore, from a standstill. they A ramjet-powered requires an assisted take-off like a rocket assist to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around Mach 3 (2,284 mph; 3,675 km/h). This type of engine can operate up to speeds of Mach 6 (4,567 mph; 7,350 km/h).
RAMJET OPERATION •
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jet engine without compressor and turbines very simple engine configuration
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there are no shafts, no rotating parts
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flow coming inside the intake M>1
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Intake should have a capability to handle shocks
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Shocks positioned in the intake system of the ramjets and the flow comes in through these shocks and they get supersonically decelerated or diffused or compressed
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At First, supersonic compression is happening through the intake system through a series of shocks. All of these are actually to begin with oblique shocks, ending finally, with a normal shock, at the end of which the flow actually does become subsonic and then the subsonic diffusion is happening through a normal diffusion process. supersonic compression followed by a subsonic diffusion or subsonic compression
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aerodynamic compression process (ram compression) high static pressure air is now fed into the combustion zone
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flame holders normal combustion phenomenon at low subsonic speeds. require low subsonic speeds to have a good efficient combustion Flow is fed on to the convergent divergent nozzle convergent divergent nozzle then exhausts the high temperature gas into the atmosphere back again, thereby creating an exit jet.
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overall momentum increase across the jet engine for the amount of mass that has come in, with a little bit of fuel addition and this change in momentum created by the jet engine creates the thrust. This thrust would then enable the aircraft to fly Joule Brayton cycle Now, in contrast to the air breathing engines the working cycle is performing without any compressor or turbine and it also does not need an enclosed combustion, which is often used in turbojet or turbofan engines where the combustion is isolated for very high efficiency.
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Since the compression is dependent on the ram compression or aerodynamic compression, it is entirely dependent on the entry Mach number. Now, Mach number above 2 creates reasonable amount of ram compression, but as the entry Mach number starts going down, the amount of compression that would be available aerodynamically becomes less and less and less. As a result of which, a ramjet is actually very useful in supersonic speeds, but at very low speeds and especially during take off and landing, it actually cannot deliver much of compression and if it cannot deliver compression, it cannot really effectively work as a good jet engine.
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So, ramjet’s utility is restricted to supersonic speeds. It cannot really be used for take off and landing of an aircraft
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the graph has been plotted with reference to the Mach number and specific impulse, which is actually thrust per unit weight of flow Normally, in most of the rockets and other flying vehicles at high altitudes and space, the specific thrust is often designated as specific impulse; that is thrust per unit weight of flow of fuel and whatever other oxidizer are there. Now, in case ramjets and turbojets, the other oxidizer is only air; so, the I sp would be designated accordingly. BLUE - hydrocarbon fuels used
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reasonably good Isp configurations at very low Mach numbers. as the Mach number starts going up, typically the turbojets at Mach numbers about 2.5 or 3, turbojets become less and less competitive and ramjets become more and more competitive and then, upto a Mach number about 7 or 8, ramjets are very good and then from there onwards, you have the scramjets, which actually are the better fuel efficient engines.
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Rockets come in to use for even higher Mach number and they have a much lower I sp, but. at very high Mach numbers where the rockets fly to space, rockets actually are the more useful vehicles
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At very high Mach numbers above 10, the vehicles using ramjets, scramjets are still being designed; we still do not have them flying as yet. Red -hydrogen fuel high Isp – can able to fly at higher mach number problem of using hydrogen fuel- lighter-so need more space to carry much cleaner fuel in terms of environmental and pollution effects
Ramjet cycle •
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open cycle. So, the returnpath from 4 to 1 is open ideal cycle is the dotted cycle-1 to 02 prime and then from 02 prime to 03 prime and then down to 4 prime
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The real cycle of a ramjet is slightly different. The path from 1 to 2 is not isentropic; so, it does not go up straight. It goes up with a slight increase in entropy. So, there is a certain amount of efficiency that comes into the picture, which we shall normally be calling isentropic efficiency of the compression process. Then in the combustion process, there is likely to be certain amount of pressure loss most of which is a fluid mechanic pressure loss. That pressure loss needs to be accounted between 02 and 03. So, the 02 and 03 are on two different pressure lines. Ideally, as we know, from 02 prime to 03 prime, it is supposed to be constant pressure combustion.
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However, actual combustion is not exactly constant pressure, there is a small amount of pressure loss which is mainly fluid mechanic pressure loss and the combustion efficiency is typically of a very high order in constant pressure combustion. So, we will continue to treat it as more or less constant pressure combustion.
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Then you have the expansion process. There is no turbine. So, you have fully expansion in the nozzle and this expansion process is again not isentropic, unlike from 03 to 4. It is nonisentropic process or a polytrophic process and as a result, there is a slight increase in the entropy. So, there is a total increase in entropy from one end to the other and as a result of which, the whole cycle will have certain efficiency which again you are familiar with. Now, at the entry to the jet engine, you have certain amount of
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at the entry to the jet engine-certain amount of kinetic energy combustion at very high speeds is a big problem
Ramjet Inlet Operation : •
There are three distinct conditions under which a ramjet engine diffuser can operate, depending on the heat released in the combustor.
Critical When the heat released in the combustor is just enough that the back pressure at the exit section of the subsonic diffuser causes the normal shock to be positioned at the inlet throats, the operation is said to be critical; this is the design condition.
Subcritical If the heat release in the combustor is increased, the static pressure at the exit of the subsonic diffuser is greater than can be achieved under the design condition. The normal shock wave moves upstream, is expelled from the diffuser, and continues to move toward the vertex of the supersonic diffuser. Behind the normal shock wave, the flow is subsonic. Since the shock wave is detached from the inlet, the incoming air spills over the cowl of the diffuser. increasing vehicle drag and possibly leading to instability.
Supercritical When the heat released in the combustor is decreased, the back pressure at the outlet section of the diffusion system becomes too small to maintain the normal shock at the inlet. The excess pressure associated with the internal flow must therefore be dissipated inside the diffusion system by a strong shock wave forming in the diverging portion of the diffuser. In other words the normal shock moves into the inlet.
These three operating conditions can be related conveniently by means of pressure recovery and mass flow rate of air.
Ramjet combustion chamber
Scramjet •
A scramjet (supersonic combusting ramjet) is a variant of a ramjet air breathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, relies vehicleairspeed to forcefullya scramjet compress theon high incoming before combustion (hence ramjet), but a ramjet decelerates the air to subsonic velocities before combustion, while airflow in a scramjet is supersonic throughout the entire engine. This allows the scramjet to operate efficiently at extremely high speeds.
Scramjet
Scramjet
Scramjet •
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The scramjet is composed of three basic components: a converging inlet, where incoming air is compressed; a combustor, where gaseous fuel is burned with atmospheric oxygen to produce heat; a diverging nozzle, where the heated air is accelerated to produce thrust.
Scramjet •
Scramjet engines operate on the same principles as ramjets, but do not decelerate the flow to subsonic velocities. Rather, a scramjet combustor is supersonic: the inlet decelerates the flow to a lower Mach number for combustion, after which it is accelerated to an even higher Mach number through the nozzle. By limiting the amount of deceleration, temperatures within the engine are kept at a tolerable level, from both a material and combustive standpoint. Even so, current scramjet technology requires the use of high-energy fuels and active cooling schemes to maintain sustained operation, often using hydrogen and regenerative cooling techniques
Bow shock •
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A bow shock, also called a detached shock or normal shock, is a curved, stationary shock wave that is found in a supersonic flow past a finite body. The name comes from the example of a bow wave that forms at the bow of a ship when it moves through the water. Unlike an oblique shock, the bow shock is not necessarily attached to the tip of the body. Oblique shock angles are limited in formation and are based on the flow deflection angle, upstream Mach number. When these limitations are exceeded (greater deflection angle or lower Mach number), a detached bow shock forms instead of an oblique shock. As bow shocks form for high flow deflection angles, they are often seen forming around blunt objects
Performance of ramjet FACTORS AFECTING RAMJET PERFORMANCE •
Velocity
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Specific Impulse
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Thrust
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Altitude
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Weight
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Cost:
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Development Time
RAMJET PERFORMANCE
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SCRAMJET
UNIT-III FUNDAMENTALS OF ROCKET PROPULSION •
Operating principle
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Specific impulse of a rocket
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Internal ballistics of rocket engines Rocket nozzle classification - Explanation
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Rocket performance considerations
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Numerical problems
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University question paper solution
SOLID PROPELLANT ROCKET MOTOR
PRINCIPLE •
All the conventional propulsion systems work by causing a change of momentum in a working fluid in a direction opposite to the intended motion. Rockets fall under the category of direct acting engines – since the energy liberated by the chemical process is directly used to obtain thrust. Being non-air breathing devices the basic component of a rocket are
(i) Combustion chamber where exothermic processes produces gases at high temperature and pressure, and (ii) Nozzle, which accelerate the fluid to high velocities and discharge them into surrounding atmosphere thereby deriving the desired force or thrust.
Classification of Chemical Rockets •
Depending on the context, the chemical rockets are classified in many ways as follows:
(a) Type of propellant: Solid, Liquid (mono propellant / bipropellant and hybrid rockets) (b) Application: Launch vehicle, JATO’s, ICBM, IRBM, SAM etc.
ABM’s,
(c) Size of Unit (and thrust level sometimes ): 10 ton, 100 kg etc. (d)
Type
clusterin
of
subsystem: rain t
e etc.
Turbopump
fed,
Specific Impulse •
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The specific impulse of a rocket, Isp, is the ratio of the thrust to the flow rate of the weight ejected, that is where F is thrust, is the rate of mass flow, and g is the acceleration of gravity at ground level.
• Specific impulse is expressed in seconds. When the thrust and the flow rate remain constant
UNIT-IV CHEMICAL ROCKETS •
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Solid propellant rockets – Selection criteria of solid propellants
Hardware components of solid rockets– Propellant grain design considerations
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Liquid propellant rockets – Selection of liquid propellants
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Cooling in liquid rockets
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Hybrid rockets
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Numerical problems
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University question paper solution
LIQUID ROCKET MOTOR
UNIT-V ADVANCED PROPULSION TECHNIQUES •
Electric rocket propulsion
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Ion propulsion techniques
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Nuclear rocket
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Solar sail Concepts in nozzleless propulsion
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Numerical problems
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University question paper solution
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Revision
Electric rocket propulsion •
ELECTRO THERMAL
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ELECTRO MAGNETIC(PLASMA THRUSTERS)
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ELECTRO STATIC(ION PROPULSION
ELECTRO THERMAL PROPULSION Electro-thermal propulsion systems are those systems in which electrical energy is used to heat propellants, thus producing thrust. Principle Electro-thermal systems heat propellants , which produce gases. The gases are then sent through a supersonic nozzle to produce thrust.
Ion propulsion Technique •
This technique of propulsion utilizes electrostatic energy, i.e. energy due to electric charges on materials is used to propel rockets. Since ions are used for this, the technique is also called as ion propulsion technique.
Nuclear Rocket •
Nuclear energy is used as propellant.
Solar sail
NOZZLELESS PROPULSION
Nozzleless solid propellant rocket motor
THE END
