Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
UNIVERSITI TEKNOLOGI MARA FACULTY OF MECHANICAL MECHANICAL ENGINEERING ___________________ __________ __________________ __________________ ___________________ ___________________ __________________ ___________ __ Program : Bachelor Of Engineering (Hons) Mechanical Course : Thermodynamics Thermodynamics Lab Code : MEC 454 ___________________ __________ __________________ ___________________ ___________________ __________________ ___________________ ___________ _ LAB SHEET NO:
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THERMODYNAMICS THERMODYNAMICS LABORATORY SHEET TITLE : Determination Of Jet Velocity And Nozzle Efficiency
1. INTRODUCTION Nozzles are suitably shaped passages in which a fluid accelerates as its pressure decreases. If the fluid is “compressible”, (i.e. a gas or vapour ), ), very high velocities can be obtained with quite moderate pressure ratios, (e.g. the local speed of sound when the pressure is approximately approximately halved). Nozzles are vital components in a wide variety of engineering applications, such as turbines, Jet Propulsion, Rockets and Ejectors The high velocity jet of fluid leaving a nozzle may be used in several ways: In a turbine, turbine, the kinetic energy stored in the fluid forms the energy available to the blades or the rotor for conversion to shaft work. In rockets and jet propulsion, the change of momentum associated with the velocity changes in the nozzle provides most of the propulsion force. In ejectors and injectors, the injectors, the changes of momentum of the jet, with its entrained fluid, is used to bring about the desired pressure changes. The Ideal Nozzle And Nozzle Efficiency
Flow through a perfect nozzle would be reversible, (i.e. without heat transfer and without frictional effects, shocks, etc.), and will therefore be isentropic. If thermodynamic data related to the fluid is known, the theoretical velocities and other relationships for an isentropic nozzle may be calculated. Due to the effects of friction, uncontrolled expansion, shocks etc., the velocity of the jet of fluid leaving a nozzle will be lower than that from an ideal nozzle.
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Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
Theory Pressure ratio, P2
r p =
( ratio of outlet and inlet absolute pressures)
P1
Nozzle efficiency,
*
V2a2
Actual KE at nozzle exit N
=
= V2s2
Isentropic KE at nozzle exit
h P1 1
inlet ressure
Isentropic process Actual process P2 (exit pressure)
2a 2s
S
Finding the actual velocity : Air injet, V2a
Note that the air has no axial velocity when it leaves the impact head. From Newton’s Second Law, the force exerted ( in axial direction ) is equal to the rate of change of momentum ( in the same direction ).
F =
.
m
V2 .
V2a = F /
m
Finding isentropic velocity : 2
inlet
exit
2
Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
Energy balance equation between 1 and 2 :
h s h +
q – w =
2
V 2 s
1
2
V 1
2
2
2
q = 0 ( adiabatic )
+ g z z …………… 1 1
2
w = 0 ( no work transfer )
gz = negligible for gas and small difference in height. V12 = negligible compared to V22 V2s2
Eqn 1 become
=
h1 – h2s = C P (T1 – T2s) for perfect gas av
2
V2s
2
=
C Pav 2
=
1
2
=
1
T
1
1
C p
C p
C p
R 1
1
R T 1
C p
C v
1
R
1
R
C p
1
Finding theoretical air mass flow rate .
m
A V
P
For perfect gas
2
2
A2
V 2 s
R T
P 2 RT 2 s 3
1
1 r p
Cv + R
=
……… 2
R T T r p
Note that, Cp =
T 2 s
1
R ……….. 3
Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
2. APPARATUS THE HILTON NOZZLE PERFORMANCE TEST UNIT This unit has been specifically designed to allow students to investigate the performance of a range of nozzles ( i ) as kinetic energy producers, and ( ii ) as thrust producers. Since the unit works on air at ambient temperature it stabilizes immediately and its energy consumption is only the energy input needed to drive a relatively small compressor. MAIN COMPONENT Chamber long,
Stainless steel, 50mm dia. And approx. 300mm T shaped. End cover secured by stainless steel bolts and sealed by ‘O’ ring. The chamber is fitted with a drain valve.
Nozzles convergent.
Throat diameter 2.0mm (nominal). One Four convergent-divergent with Exit Are/Throat Area ratios of 1.2, 1.4, 1.6 and 2.0 respectively. Divergence 10 ° (included).
Pressure Gauges
Two, 0 to 1100 kN m-2, to measure inlet and chamber pressures.
Thermometers
Three mercury-in-glass, 150mm long – to measure inlet and chamber temperatures.
Flow Meter
Variable area type meter to measure air flow – range 1.0 to 9.0 gramme s -1. Calibrated for a standard atmosphere. Corrections are supplied for other conditions.
Valves
Diverter Valve – to direct air to a nozzle mounted in the wall of the chamber (for nozzle efficiency test), OR to the hollow cantilever (for jet reaction test). Needle Valve – to given fine control of nozzle inlet pressure. Back Pressure Valve – to control the pressure in the chamber.
100 kN m-2 = 1 bar = 14.5 1bf/in 2
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Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
3. EXPERIMENTAL : DETERMINATION OF JET VELOCITY AND NOZZLE EFFICIENCY 1)
Close the air inlet control valve and open the chamber pressure control valve. Before proceeding further, ensure that the contacts are clean, t hat the battery is in good condition and that the impact head is fitted to the end of the cantilever. Also, check that the micrometer dial has been correctly zeroed and that a cantilever load / deflection graph is available.
2)
Unscrew the knurled nut at the top right hand end of the chamber, withdraw the nozzle mounting sleeve and assemble Nozzle no. 1 into the unit.
3)
Turn the diverter valve handle to the upward position.
4)
With the chamber pressure control valve fully open, adjust the inlet control valve to give a constant air inlet pressure 600 kPa gage.
5)
Rotate the micrometer adjustment screw until the voltmeter and the lamp indicates that contact is just made. (Greatest sensitivity is obtained if the screw is adjusted so that the voltmeter indicates about 0.5V)
6)
Record the pressure, temperatures, air mass flow rate and dial reading.
7)
Increase the chamber pressure to about 100 kPa gage and repeat the above step.
8)
Making sure that the inlet pressure remains constant, repeat the test at other chamber pressures ( in increments of 100 kPa )
9)
Repeat the whole test with other nozzles.
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Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
4. RESULT AND DISCUSSION 1) Correct the observed air mass flow rate, if needed and tabulate the result against the pressure ratio (p 2 / p1) for each nozzle. 2) Calculate the nozzle efficiency for each test and tabulate the result against the pressure ratio (p 2/p1) for each nozzle. 3) Plot the corrected experimental air flow rates against the pressure ratio for all nozzles used on the graph. Similarly plot nozzle efficiency against pressure ratio. 4) From the graph and for each nozzle, estimate the pressure ratio (p2/p1) at which the air mass flow rate reaches its maximum value. Compare the theoretical and experimental air mass flow rates at this point. What is the effect on the air mass flow rate when this pressure ratio is reduced further? 5) From the graph and for each nozzle, estimate the pressure ratio (p2/p1) at which the nozzle efficiency is at its lowest.
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Thermodynamics - MEC 454/ LS 1/ WAN Rev. 01-2009
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