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Gas Turbines & Jet Propulsion (October 2010)

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Total No. of Questions : 12] P1036 [Total No. of Pages : 5 [3864]-134 B.E. (Mechanical) GAS TURBINES AND JET PROPULSION (2003 Course) (402044) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer 3 questions from Section - I and 3 questions from Section - II. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 6) Assume suitable data, if necessary. SECTION - I Q1) a) What do you understand by sonic velocity? Show that sonic velocity, a= b) [8] VRT . Air at 15 bar 1000 K enters convergent-divergent nozzle with negligible velocity under steady state condition. For a throat area of 30 cm2 find[8] i) Mass flow rate & throat conditions. ii) Pressure & temperature in divergent portion where Mach number is 2 & required exit area. OR Q2) a) b) Py 1 + Mx 2 = where Prove that for a normal shock Px 1 + My 2 is ratio of specific heats of air. [8] Air enters a pipe of 0.05m diameter at stagnation conditions of 10 bar & 400 K at Mach no. of 2.8. If Mach no. at exit is 1.2 & friction factor is 0.005. Find mass flow rate & length of pipe required. Assume Cp = 1.005 kJ/kg K. [8] P.T.O. Q3) a) b) c) Explain surging & choking in a compressor. [6] Explain : [4] i) Slip factor ii) Power input factor. A centrifugal compressor runs with tip rotor speed of 400 m/sec. & slip factor is 0.96. The compressor takes in air at 1 bar & 300K at ambient conditions. If mass flow rate of air is 20 kg/sec. & compressor works with an isentropic efficiency of 85%, calculate [6] i) Compressor power ii) Total pressure & tempr at exit of impeller. Assume Cp = 1.05 kJ/kg K & = 1.4. OR Q4) a) b) c) Discuss various performance characteristic curves on an axial flow compressors. [6] Define : [2] i) Coefficient of lift. ii) Coefficient of drag. An axial flow compressor comprises number of similar stages with equal work done per stage & velocity of flow is uniform throughout [8] compressor. Following is the data Overall stagnation pressure ratio = 3.5 Stagnation inlet temperature = 60 C Relative air angle at rotar inlet = 130 C Relative air angle at rotar outlet = 100 C Blade velocity = 185 m/sec. Degree of reaction = 0.5 Overall stagnation adiabatic efficiency = 0.87. Data refer to mean blade height & measurement of angle is done in same sense from blade velocity diagram. Calculate i) Stagnation outlet temperature ii) Number of stages [3864]-134 2 Q5) a) b) What is the effect of following operating variables on thermal efficiency of gas turbine? Discuss. [8] i) Pressure ratio ii) Turbine inlet temperature. iii) Compressor inlet temperature. iv) Turbine & compressor efficiency. In a gas turbine plant the pressure ratio through which air at 15 C is compressed is 6. The same air is then heated to maximum permissible temperature of 750 C first in a heat exchanger which is 75% efficient & then in combustion chamber. Air is then expanded in two stages such that expansion work is maximum. Air is reheated to 750 C after first stage. Determine cycle thermal efficiency & work ratio & net shaft work per kg of air. Assume compressor efficiency is 80% & turbine efficiency is 85% respectively. [10] OR Q6) a) Compare open cycle gas turbine & closed cycle gas turbine. [6] b) Explain open gas turbine cycle with intercooling. [6] c) A simple gas turbine operates at pressure ratio of 5:1. Turbine inlet tempr is 580 C. Air tempr at inlet to compressor is 15 C & its pressure is 1 bar. Compressor operates at an isentropic efficiency of 80%. Determine required isentropic efficiency of turbine so that overall efficiency of plant remains at 18%. [6] Take Cpa = 1.005 kJ/kg K. & Cpg = 1.1 kJ/kg K. SECTION - II Q7) a) b) Draw and explain the energy flow diagram for a stage of an impulse turbine. [8] Derive expression for maximum utilization factor of a 50% reaction stage. [10] [3864]-134 3 OR Q8) Write notes on : [18] a) Performance graphs of Gas Turbines. b) Reheat factor and stage efficiency. c) Multistage impulse turbines. Q9) a) Explain in detail the combustion theory based on three well known postulates as applied to gas turbines. [8] b) Discuss the factors affecting the combustion chamber design and performance. [8] OR Q10)Write notes on : [16] a) Fuels for Gas turbines. b) Annular combustion chamber. c) Recent developments and applications of gas turbines. d) Blade cooling of Gas Turbines. Q11)a) Discuss the working of a Turbojet engine with a neat sketch. Define [8] i) propulsive efficiency ii) overall efficiency and iii) thrust specific fuel consumption. b) A turbojet air craft with two jet engines is travelling with a velocity of 200 m/s. The other data is as given below : [8] i) Flying altitude = 12 km and air density at this altitude = 0.172 kg/m3. ii) The propulsive efficiency = 50% iii) Drag on the plane = 7000 N [3864]-134 4 Calculate : 1) Absolute velocity of the jet 2) 3) Net output of gas turbine 4) Assume Amount of air compressed per second Diameter of jet A) overall efficiency = 18%. B) calorific value of fuel = 48950 kJ . kg OR Q12)Write notes on : [16] a) Turbofan engine. b) Solar Rocket engine. c) Pulse jet engine d) Solid and liquid propellants. vvvv [3864]-134 5

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