1. Consider an ideal air-standard Brayton cycle operating at steady state.
Operating data at
principal states in the cycle are given in the table below. The states are numbered as in the
figure given below. It produces 10 MW of power. Sketch the T–s diagram for the cycle
and determine
a. mass flow rate of air, in kg/s.
b. rate of heat transfer, in kW, to the working fluid passing
through the heat exchanger.
c. thermal efficiency.
2. Air enters the compressor of an air-standard Brayton cycle at 100 kPa, 300 K, with a
volumetric flow rate of 5 m3/s. The compressor pressure ratio is 10. The turbine inlet
temperature is 1400 K. The turbine and compressor each have an isentropic efficiency of
80%.
Determine
Mass flow rate of air
Power produced by turbine
Power consumed by compressor
Net power developed
Back work ratio
Work ratio
Rate of heat addition to the cycle
Thermal efficiency
3. Air is drawn in a gas turbine unit at 15°C and 1.01 bar and pressure ratio is 7:1. The
compressor is driven by the H.P. turbine and L.P. turbine drives a separate power shaft.
The isentropic efficiencies of compressor, and the H.P. and L.P. turbines are 0.82, 0.85
and 0.85 respectively. If the maximum cycle temperature is 610°C, calculate :
(i) The pressure and temperature of the gases entering the power turbine
(ii) The net power developed by the unit per kg/s mass flow
(iii) The work ratio
(iv) The thermal efficiency of the unit
Neglect the mass of fuel and assume the following :
For compression process Cpa = 1.005 kJ/kg K and γ = 1.4
For combustion and expansion processes : Cpg = 1.15 kJ/kg K and γ = 1.333.
4. Air enters the compressor at 100 kPa, 300 K and is compressed to 1000 kPa. The
temperature at the inlet to the first turbine stage is 1400 K. The expansion takes place
isentropically in two stages, with reheat to 1400 K between the stages at a constant
pressure of 300 kPa. A regenerator having an effectiveness of 100% is also incorporated
in the cycle. Determine:
(a) the thermal efficiency of the cycle
(b) the back work ratio
(c) the net power developed
(d) What percentage of the total heat addition occurs in the reheat process?
5. Air is compressed from 100 kPa, 300 K to 1000 kPa in a two-stage compressor with
intercooling between stages. The intercooler pressure is 300 kPa. The air is cooled back
to 300 K in the intercooler before entering the second compressor stage. Each compressor
stage is isentropic. Determine
(a) the temperature at the exit of the second compressor stage and
(b) the total compressor work input per unit of mass flow.
(c) Repeat (a) and (b) for a single stage of compression from the given inlet
state to the final pressure.
(d) the percentage reduction in compressor work with two-stage compression
and intercooling compared to a single stage of compression
6. What is athodyd and how is it different from a turbo jet?
7. Draw neat diagrams of ramjet and pulse jet engines and describe their operations. Also
state their advantages and disadvantages over each other.
8. A turbojet engine consumes air at the rate of 60.2 kg/s when flying at a speed of 1000
km/h. Calculate a) Exit velocity of jet when enthalpy change for nozzle is 230 kJ/kg and
velocity coefficient is 0.96, b) Fuel flow rate in kg/s when air fuel ratio is 70:1, c) Thrust
specific fuel consumption, d) Thermal efficiency of the plant when combustion efficiency
is 92% and calorific value of fuel used is 42000 kJ/kg, e) propulsive power, f)
propulsive efficiency, g) overall efficiency.
9. Why is thrust augmentation necessary in aircraft engines? Briefly describe the various
methods of thrust augmentation in jet engines.
10. The following data pertain to a jet engine flying at an altitude of 9000 m with a speed of
215 m/s.
Thrust power developed: 750 kW
Inlet pressure and temperature: 0.32 bar, – 42°C
Temperature of gases leaving the combustion chamber: 690°C
Pressure ratio 5.2
Calorific value of fuel: 42500 kJ/kg
Velocity in ducts (constant): 195 m/s
Internal efficiency of turbine: 86%
Efficiency of compressor: 86%
Efficiency of jet tube: 90%
For air : Cpa = 1.005, γ = 1.4, R = 0.287
For combustion gases, Cpg = 1.087
For gases during expansion, γ = 1.33.
Calculate the following :
(i) Overall thermal efficiency of the unit
(ii) Rate of air consumption
(iii) Power developed by the turbine
(iv) The outlet area of jet tube
(v) Specific fuel consumption in kg per kg of thrust
1. With the help of a neat sketch describe the working of a centrifugal compressor. Show the
variation of pressure and the velocity in the impeller and the diffuser.
2. Compare centrifugal and axial flow compressors.
3. Describe construction and working of an axial flow compressor with the help of a
diagram. Also draw its velocity diagrams.
4. Define the degree of reaction in axial flow compressors and explain its importance.
5. Explain the phenomenon of stalling, surging and choking in rotary compressors, and how
can they be controlled?
