JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III THERMAL ENGINEERING – II Previous Question Paper (November/December - 2020 - R16)
Answer any five questions. All questions carry equal marks.
Main
Describe the regenerative cycle with the help of neat sketch. Also represent the cycle on T-S diagram.
Answer
The regenerative cycle improves the thermal efficiency of a steam power plant by extracting a portion of steam from the turbine at one or more stages (bleed steam) and using it to preheat the feedwater before it enters the boiler. This process reduces the amount of heat required from an external source to raise the feedwater to the saturation temperature, thereby increasing the mean temperature of heat addition.
In a typical regenerative cycle, the feedwater is passed through a feed water heater (regenerator). The bled steam mixes with the cold feedwater, raising its enthalpy. This minimizes the temperature difference between the heating medium and the fluid, reducing irreversibility. On a T-S diagram, the cycle deviates from the standard Rankine cycle as the heat addition process begins at a higher temperature, effectively shifting the heat addition curve upward.
A steam power plant is to operate with a boiler pressure of 50 bar and a condenser pressure of 0.07 bar. The steam leaving the boiler is at a temperature of 350°C. Determine the efficiency, specific steam consumption and work ratio of the Rankine cycle.
Answer
Given: P1 = 50 bar, T1 = 350°C, P2 = 0.07 bar.
From steam tables:
At 50 bar, 350°C: h1 = 3068.4 kJ/kg, s1 = 6.448 kJ/kgK.
At 0.07 bar: hf = 163.4 kJ/kg, hfg = 2409.1 kJ/kg, sf = 0.559 kJ/kgK, sfg = 7.717 kJ/kgK, vf = 0.001007 m³/kg.
1. Determine dryness fraction (x2) at turbine exit: s1 = sf + x2*sfg => 6.448 = 0.559 + x2(7.717) => x2 = 0.763.
2. Enthalpy at turbine exit (h2): h2 = hf + x2*hfg = 163.4 + 0.763(2409.1) = 2002.5 kJ/kg.
3. Pump work (Wp): Wp = vf(P1 - P2) = 0.001007(5000 - 7) = 5.03 kJ/kg.
4. Enthalpy at boiler inlet (h3): h3 = hf + Wp = 163.4 + 5.03 = 168.43 kJ/kg.
5. Net work (Wnet): Wnet = (h1 - h2) - Wp = (3068.4 - 2002.5) - 5.03 = 1060.87 kJ/kg.
6. Heat supplied (Q1): Q1 = h1 - h3 = 3068.4 - 168.43 = 2899.97 kJ/kg.
7. Efficiency (η): η = Wnet / Q1 = 1060.87 / 2899.97 = 0.3658 or 36.58%.
8. Specific Steam Consumption (SSC): SSC = 3600 / Wnet = 3600 / 1060.87 = 3.39 kg/kWh.
9. Work Ratio: Work Ratio = Wnet / Turbine Work = 1060.87 / (3068.4 - 2002.5) = 1060.87 / 1065.9 = 0.995.
Establish a condition for maximum discharge of flue gases through a chimney of given height.
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Sketch and describe the working of Babcock and Wilcox water tube boiler.
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Derive an expression for maximum mass flow through a convergent-divergent nozzle when the steam is expanded isentropically from rest.
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A group of convergent-divergent nozzles are supplied with steam at a pressure of 2 MN/m² and a temperature of 325°C. Supersaturated expansion according to the law PV^1.3 = constant, occurs in the nozzle down to an exit pressure of 0.36 MN/m². Steam is supplied at the rate of 7.5 kg/s. Determine the required throat and exit areas.
Answer
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Derive the expression relating the critical pressure ratio to index of expansion n, for expansion in a nozzle.
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Steam at 10 bar and 250°C is expanded in a nozzle to a pressure of 2 bar. The area at exit is 2 cm² and inlet velocity is negligible. Calculate the mass flow rate, if phase equilibrium is assumed throughout the expansion and if steam is assumed to be supersaturated.
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A simple impulse turbine has one ring of moving blades running at 150 m/s, absolute velocity of steam at exit is 85 m/s at an angle 80° with the tangent of wheel, friction coefficient is 0.82, rate of steam flowing 2 Kg/s. Assuming the moving blades to be symmetrical, find the a) Blade angles b) Nozzle angle c) absolute velocity of steam at entrance and d) power developed.
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At a particular stage of Parson's reaction turbine the mean blade speed is 60 m/s and the steam pressure is 3.5 bar with a temperature of 175°C. The identical fixed and moving blades have inlet angles of 30° and outlet angles of 20°. Determine a) power developed by the stage, b) the blade height if it is 1/10th of the blade ring diameter for a flow rate of 810 kg/min, and c) the specific enthalpy drop if the stage efficiency is 85%.
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Explain the operation of central flow surface condenser with the help of neat sketch.
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The following readings were taken during a test on a surface condenser: Mean condenser temperature = 35°C, Hot well temperature = 30°C, condenser vacuum = 69 cm of Hg, barometer reading 76 cm of Hg. Condensate collected 16 kg/min, cooling water enters at 20°C and leaves at 32.5°C, flow rate being 37,500 kg/h. Calculate i) mass of air present per cubic metre of condenser, ii) quality of steam at condenser inlet.
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Describe the operation of turbojet engine with a neat sketch and explain its thermodynamic cycle.
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Derive expression for the thrust and propulsion efficiency of rocket and compare with those of turbojet.
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