JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III THERMAL ENGINEERING - II Previous Question Paper (January/February - 2023 - R18)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
PART – A
Explain different methods to improve the efficiency of Rankine cycle.
Answer
The efficiency of a Rankine cycle can be improved by the following methods:
- Increasing the average temperature of heat addition: This is achieved by increasing the boiler pressure or by incorporating superheating of the steam before it enters the turbine.
- Decreasing the temperature of heat rejection: This is accomplished by lowering the condenser pressure, which allows the steam to expand to a lower temperature and pressure, thereby increasing the net work output.
- Regenerative Feedwater Heating: Extracting a portion of steam from the turbine to preheat the feedwater before it enters the boiler reduces the amount of heat required from the external source.
- Reheating: Expanding steam in two stages with reheating between them increases the dryness fraction of the steam at the turbine exit, reducing moisture-related erosion and improving cycle efficiency.
Explain significance of boiler mountings and accessories.
Answer
Boiler mountings are essential safety devices required by law to ensure the secure and efficient operation of a boiler. They are directly fitted to the boiler shell or drum and are mandatory for preventing accidents; examples include safety valves, water level indicators, and pressure gauges, which monitor and control pressure and water levels to prevent explosions or damage.
Boiler accessories are auxiliary components installed to improve the overall efficiency and performance of the steam generation system. Unlike mountings, they are not strictly mandatory for safety but are vital for economic operation. Common examples include economizers, which recover waste heat from flue gases to preheat feedwater, and superheaters, which increase the temperature of saturated steam to improve turbine efficiency.
Draw different nozzle cross sections and explain them.
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What is the condition for maximum discharge through nozzle?
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Draw velocity triangle at exit for impulse turbine.
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Explain the effect of friction on the blade diagram efficiency in steam turbines.
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What is the purpose of regeneration in the gas turbine?
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Draw line diagram and explain the working of Evaporative Condenser.
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List out the factors which are to be considered for the comparison of different types of rockets.
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Explain thrust power and propulsion efficiency of jet propulsion.
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PART – B
Explain the effect of operating variables on Rankine cycle performance.
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A steam power plant operates on ideal Rankine cycle. The steam enters the turbine at 3 MPa, 350°C and is condensed in the condenser at 75 kPa, calculate thermal efficiency, back work ratio and work ratio of this cycle.
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What is meant by super critical boiler and explain one such boiler?
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Draw the line diagram and explain the working principle of economizer and discuss the precautions to be made in usage.
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Explain the effects of irreversibilities on the performance of nozzle.
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Calculate the throat and exit diameters of a convergent-divergent nozzle, which will discharge 820 kg of steam per hour at a pressure of 8 bar superheated to 220°C into a chamber having a pressure of 1.5 bar. The friction loss in the divergent portion of the nozzle may be taken as 0.15 of the isentropic enthalpy drop.
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Derive the equation for critical pressure ratio of nozzle for different conditions.
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In a convergent-divergent nozzle, the steam enters at 15 bar and 300°C and leaves at 2 bar. The inlet velocity to the nozzle is 150 m/s. Find the required throat and exit areas for a mass flow rate of 1 kg/s. Assume nozzle efficiency to be 90% and Cps = 2.4 kJ/kg K.
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Derive the equation optimum work out put in impulse turbine.
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The reaction turbine runs at 300 rpm and the steam consumption is 20000 kg/h. The pressure of steam at a certain pair is 2 bar, its dryness fraction is 0.93 and the power developed by the pair is 50 kW. The discharge blade angle is 20° for both the fixed and moving blades and the axial velocity of flow is 0.72 times the blade velocity. Find the drum diameter and the blade height. Take the tip leakage steam as 8%. Neglect the blade thickness.
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Derive the condition for maximum efficiency and blade height of reaction turbine.
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In a Parson reaction turbine, the angles of receiving tips are 35° and of discharging tips, 20°. The blade speed is 100 m/s. Calculate the tangential force, power developed, diagram efficiency and axial thrust of the turbine, if its steam consumption is 1 kg/min.
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Explain with neat line diagram the working of evaporative steam condenser.
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The following data refers to a test of the surface condenser of a steam turbine. Absolute pressure of the steam entering the condenser = 5.628 kPa, Temperature of condensate leaving the condenser = 32°C, Inlet temperature of cooling water = 15°C, Outlet temperature of cooling water = 30°C, Mass of cooling water per kg of steam = 32 kg. Assuming that all the heat lost by the exhaust steam is taken up by the circulating water; determine the dryness fraction of the steam as it enters the condenser.
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Explain the deviation of actual gas turbine cycle from ideal Brayton cycle.
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A gas turbine takes in air at 27°C and 1 bar. The pressure ratio is 4. The maximum temperature of the cycle is 560°C. The efficiency of the compressor and turbine is 0.83 and 0.85 respectively. Find the overall efficiency, if the regenerator effectiveness is 0.75.
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Explain the working of turboprop engine and its advantages.
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A jet propelled engine having two jets and working on turbojet has a velocity of 210 m/s, when flying at an altitude of 12000 m. The density of air at this altitude is 0.172 kg/m³. The resistance or drag of the plane is 6670.8 N and propulsive efficiency of the jet is 50%. The overall efficiency of the unit is 18%. Calorific value of the fuel is 4.895 × 10⁴ kJ/kg. Calculate i) Absolute velocity of jet, ii) Quantity of air compressed per minute, iii) Diameter of jet, iv) Net power output of the plant v) Thrust specific fuel consumption, vi) Air-fuel ratio.
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What are the desirable properties of a liquid propellant for a rocket engine?
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For a rocket engine, jet velocity is 1600 m/s, flight to jet speed ratio is 0.7. Oxidizer flow rate is 4 kg/s. Fuel flow rate is 1 kg/s. Heat of reaction per kg of exhaust gas is 2500 kJ/kg. Calculate the Thrust, specific impulse, propulsive efficiency, thermal and overall efficiency of rocket engine.
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