JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III THERMAL ENGINEERING II Previous Question Paper (August/September - 2024 - R16)
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. iv) Approved Steam Tables and Gas Tables are permitted.
Part A
Draw the T-s diagram for a Rankine cycle and denote the processes.
Answer
The Rankine cycle T-s diagram consists of four processes: 1-2 is isentropic compression in the pump, 2-3 is constant-pressure heat addition in the boiler, 3-4 is isentropic expansion in the turbine, and 4-1 is constant-pressure heat rejection in the condenser. On the T-s plot, temperature (T) is on the vertical axis and entropy (s) is on the horizontal axis, with the cycle bounded by the saturated liquid and saturated vapor lines forming a dome.
What are the benefits of boiler accessories? Give examples.
Answer
Boiler accessories are auxiliary components installed to improve the overall efficiency, safety, and performance of a boiler plant. Unlike mountings, which are essential for safe operation, accessories are primarily used to enhance the heat transfer rate and reduce fuel consumption.
The primary benefits include:
- Increased Efficiency: By preheating feed water or air, they recover waste heat that would otherwise be lost in flue gases.
- Improved Capacity: They allow the boiler to generate more steam for the same amount of fuel consumed.
- Operational Control: They help in maintaining stable steam pressure and temperature levels.
Common examples of boiler accessories include:
- Economizer: Used to heat the feed water using the heat from exhaust flue gases.
- Air Preheater: Used to increase the temperature of the combustion air before it enters the furnace.
- Superheater: Used to increase the temperature of saturated steam above its saturation point, increasing the thermal efficiency of the power cycle.
- Feed Pump: Used to supply the required amount of water to the boiler against the internal pressure.
Define stagnation properties in the context of steam nozzles. Why are they important in nozzle design?
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What is the critical pressure ratio in steam nozzle operation and its implications for flow behavior?
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State the effects of friction on the performance of steam turbines.
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What conditions are necessary for achieving maximum efficiency in a reaction turbine?
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State the common sources of air leakage in steam condensers. How do they affect performance?
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Compare closed and open gas turbine cycles.
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Differentiate between pressure thrust and momentum thrust.
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Distinguish solid propellant rockets and liquid propellant rockets.
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Part B
Explain the methods and merits of regeneration systems in a steam-based thermal power plant.
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Steam generated at a power plant at 8.0 MPa and 500°C is fed to a turbine. Exhaust from the turbine enters a condenser at 15 kPa where it is condensed to a saturated liquid, which is then pumped to the boiler. What is the thermal efficiency of a Rankine cycle with an isentropic turbine operating condition?
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Explain the working of Benson boiler with neat diagram.
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In a coal-fired power plant boiler, 10,000 kg/hr of coal with a calorific value of 30,000 kJ/kg is burned. The excess air factor is 1.2, and the flue gas temperature leaving the boiler is 200°C. If the steam output from the boiler is 8,000 kg/hr at 10 bar and 250°C, calculate the overall efficiency of the boiler. Assume 12% excess air and 25% moisture in the coal.
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Derive the maximum mass flow rate relation for a steam nozzle.
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Dry saturated steam at 6.5 bar with negligible velocity expands isentropically in a convergent divergent nozzle to 1.4 bar and a dryness fraction of 0.956. Determine the final velocity of steam from the nozzle if 13% heat is lost in friction. Find the percentage reduction in the final velocity.
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Distinguish between ideal and actual expansion in a steam nozzle, highlighting the factors contributing to deviations from ideal behavior.
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Explain the off-design pressure ratio for convergent and C-D nozzles.
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A velocity-compounded impulse turbine operates in three stages, each having a pressure drop of 5 bar. If the initial steam velocity is 200 m/s, calculate the final velocity of steam at the exit of the last stage, considering an efficiency of 90% for each stage.
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A steam turbine operates with an inlet steam velocity of 300 m/s and an outlet velocity of 150 m/s. If the blade speed ratio is 0.5, calculate the blade efficiency of the turbine.
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Explain the concept of degree of reaction in reaction turbines and derive the expression for degree of reaction in terms of inlet and outlet velocities and blade angles.
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A Parsons reaction turbine operates with an inlet steam velocity of 200 m/s and an outlet velocity of 50 m/s. If the degree of reaction is 0.4, determine the change in enthalpy of steam across the turbine stage.
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Discuss the requirements of a modern surface condenser.
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A condenser removes 12 MW of heat from the steam at a cooling water flow rate of 2000 m3/hr. If the inlet and outlet temperatures of the cooling water are 20 °C and 30 °C respectively, calculate the heat transfer coefficient of the cooling water.
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Explain the working of a simple gas turbine plant with a suitable schematic layout.
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A gas turbine plant operates with a pressure ratio of 15. If the compressor inlet temperature is 300 K and the turbine inlet temperature is 1400 K, calculate the Brayton cycle efficiency of the plant.
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Differentiate among turbojet, turbofan and turbo-propeller aircraft engines.
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A turbojet engine, flying at an altitude, receives air at 0.6 bar and 255 K and it is compressed through a compression ratio of 8, with an isentropic efficiency of 80%. Fuel with a heating value of 40 MJ/kg is used to raise the temperature to 1200 K before entering the turbine with an isentropic efficiency of 95%. The mechanical transmission efficiency is 97%. A convergent nozzle with an exit area of 0.5 m2 is used to produce a gas jet. Determine the jet velocity and thrust.
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Explain with a neat sketch the working of a gas pressure feed system used in liquid propellant rocket engines.
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The effective jet velocity from a rocket is 2700 m/s. The forward flight velocity is 1350 m/s, and the propellant consumption is 78.6 kg/s. Calculate thrust power and propulsive efficiency.
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