JNTU
JNTU B.Tech III THERMAL ENGINEERING II Previous Question Paper (January/February - 2021 - R16)
Answer any five questions
All questions carry equal marks
Main
A steam power plant working on Rankine cycle has the range of operation from 40 bar dry saturated to 0.05 bar. Determine: i) The cycle efficiency, ii) Work ratio, iii) Specific fuel consumption.
Answer
Given: P1 = 40 bar (dry saturated), P2 = 0.05 bar. From steam tables: h1 = hg at 40 bar = 2800.7 kJ/kg, s1 = sg at 40 bar = 6.070 kJ/kgK. At 0.05 bar: hf = 137.8 kJ/kg, hfg = 2423.8 kJ/kg, sf = 0.476 kJ/kgK, sfg = 8.396 kJ/kgK, vf = 0.001005 m3/kg.
1. Find dryness fraction (x2): s1 = sf2 + x2*sfg2 => 6.070 = 0.476 + x2(8.396) => x2 = 0.666.
2. Calculate h2: h2 = hf2 + x2*hfg2 = 137.8 + 0.666(2423.8) = 1752.6 kJ/kg.
3. Pump work (Wp): Wp = vf2(P1 - P2) = 0.001005(4000 - 5) = 4.01 kJ/kg.
4. Turbine work (Wt): Wt = h1 - h2 = 2800.7 - 1752.6 = 1048.1 kJ/kg.
5. Net work (Wnet): Wnet = Wt - Wp = 1044.09 kJ/kg.
6. Heat supplied (Qs): Qs = h1 - (hf2 + Wp) = 2800.7 - (137.8 + 4.01) = 2658.89 kJ/kg.
i) Cycle efficiency: η = Wnet / Qs = 1044.09 / 2658.89 = 39.27%.
ii) Work ratio: WR = Wnet / Wt = 1044.09 / 1048.1 = 0.996.
iii) Specific fuel consumption (SFC): Assuming calorific value of fuel (CV) = 42000 kJ/kg, SFC = 3600 / (η * CV) = 3600 / (0.3927 * 42000) = 0.218 kg/kWh.
What are the functions of boiler draught?
Answer
Boiler draught is the pressure difference required to maintain a constant flow of air and combustion gases through the boiler furnace and flues. Its primary functions are:
- To supply the necessary quantity of air to the furnace for the complete combustion of fuel.
- To remove the gaseous products of combustion (flue gases) from the furnace and discharge them into the atmosphere through the chimney.
- To control the rate of combustion by regulating the air supply based on the steam demand.
- To enable the use of lower-grade fuels by providing sufficient air flow to overcome the resistance of fuel beds.
Write the function of steam separator and steam trap.
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Explain working principle of Benson boiler with a diagram.
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Prove that the maximum discharge of fluid per unit area through a nozzle shall occur when the ratio of fluid pressure at throat to the inlet pressure is (2/(n+1))^(n/(n-1)), where n is the index of adiabatic expansion. Also obtain the expression for maximum mass flow through a convergent-divergent nozzle.
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A convergent divergent nozzle is required to discharge 2 kg of steam per second. The nozzle is supplied with steam at 6.9 bar and 180°C and discharge takes place against a back pressure of 0.98 bar. Expansion up to throat is isentropic and the frictional resistance between the throat and exit is equivalent to 62.76 kJ/kg of steam. Taking approach velocity of 75 m/s and throat pressure 3.9 bar, estimate suitable areas for the throat and exit.
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In a reaction turbine 6 kg/s steam is admitted at 15 bar dry saturated in the first stage. Turbine has eight pairs on mean diameter of 50 cm and run at 3000 rpm with mean blade speed to steam velocity ratio of 0.8. There occurs tip leakage of steam at all rows amounting to 10% of total and efficiency of working steam is 85%. Considering blade outlet angles for both fixed and moving blades to be 20°, determine the following analytically:
The output from turbine in hp
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The pressure of steam leaving turbine
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The mean blade height
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Derive the condition for optimum blade speed ratio for maximum blade efficiency of an impulse turbine. And also deduce the related blade efficiency.
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A gas turbine unit has a pressure ratio of 6:1 and maximum cycle temperature of 610°C. The isentropic efficiencies of the compressor and turbine are 0.80 and 0.82 respectively. Calculate the power output in kilowatts of an electric generator geared to the turbine when the air enters the compressor at 15°C at the rate of 16kg/s. Take Cp=1.005 kJ/kgK and γ=1.4 for the compression process, and take Cp=1.11 kJ/kgK and γ=1.333 for the expansion process.
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Explain with the help of entropy and enthalpy diagrams a turbojet gas turbine plant. How it is differs from Turbo Prop plant?
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What are the requirements of an ideal Rocket propellant and applications of Rockets.
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