JNTU
JNTU B.Tech III THERMAL ENGINEERING - II Previous Question Paper (March - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Explain the concept of mean temperature of heat addition.
Answer
The mean temperature of heat addition (Tm) is a theoretical temperature at which the total heat supplied to a thermodynamic cycle could be added isothermally to produce the same thermal efficiency as the actual cycle. It is defined by the ratio of the total heat added to the change in entropy during the heat addition process.
In a Rankine cycle, this represents the average temperature of the working fluid during the phase change (evaporation) and superheating processes. A higher mean temperature of heat addition directly correlates to a higher thermal efficiency of the cycle, as it approximates the efficiency of a Carnot cycle operating between the mean temperature of heat addition and the sink temperature.
In a regenerative cycle the inlet conditions are 40 bar and 400oC for the Rankine cycle steam is bled at 10 bar for regenerative heating. The exit pressure of the turbine is 0.8 bar. Neglecting pump work, determine the efficiency of the cycle.
Answer
To determine the efficiency of the regenerative cycle, we analyze the states using steam tables for the given pressures: P1 = 40 bar, T1 = 400°C; P2 = 10 bar (bleed); P3 = 0.8 bar (condenser).
- At state 1 (40 bar, 400°C): h1 = 3213.6 kJ/kg, s1 = 6.769 kJ/kg·K.
- At state 2 (10 bar, isentropic expansion from 1): s2 = s1 = 6.769 kJ/kg·K. At 10 bar, sg = 6.585 kJ/kg·K, so steam is slightly superheated. Interpolating, h2 ≈ 2860 kJ/kg.
- At state 3 (0.8 bar, isentropic expansion from 1): s3 = s1 = 6.769 kJ/kg·K. At 0.8 bar, sf = 1.138, sfg = 6.485. Quality x3 = (6.769 - 1.138) / 6.485 = 0.868. h3 = hf + x3hfg = 356.5 + 0.868(2308.8) ≈ 2360 kJ/kg.
- Let m be the mass of steam bled. Energy balance at the feed water heater: m(h2 - hf2) = (1 - m)(hf2 - hf3), where hf2 = 762.8 kJ/kg and hf3 = 356.5 kJ/kg. Solving for m: m(2860 - 762.8) = (1 - m)(762.8 - 356.5) yields m ≈ 0.162.
- Work output WT = (h1 - h2) + (1 - m)(h2 - h3) = (3213.6 - 2860) + (1 - 0.162)(2860 - 2360) = 353.6 + 419 = 772.6 kJ/kg.
- Heat input Qin = (h1 - hf2) = 3213.6 - 762.8 = 2450.8 kJ/kg.
- Efficiency η = WT / Qin = 772.6 / 2450.8 ≈ 0.315 or 31.5%.
A convergent-divergent nozzle is required to discharge 2 kg of steam per second. The nozzle is supplied with steam at 10 bar and 200oC and the discharge takes place against a back pressure of 0.34 bar. Estimate the throat and exit areas. Assume isentropic flow and take the index n=1.3. Nozzle efficiency is assumed to be 85%.
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An impulse turbine is provided with single row wheel whose mean diameter is 100 cm and it is rotating at 50 rps. The nozzle in inclined at an angle of 20o and the velocity of steam coming out of the turbine is 350 m/sec. Determine the power developed if the axial thrust on the end bearings is limited to 118 N. Take blade friction factor=0.8. Assume the blades are equi-angular.
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Explain the working of a single-stage impulse turbine. Sketch pressure and velocity variations along the axis of the turbine.
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Describe the factors which influence of condenser and vacuum efficiencies of a condensing plant.
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Discuss briefly the methods employed for improvement of thermal efficiency of an open cycle gas turbine plant.
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Why mountings are essential in boilers? Name different mountings and give functions of each.
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A gas turbine is operated with a temperature limits of 300 K and 1200 K. Calculate the optimum pressure ratio for maximum network output and also calculate the thermal efficiency.
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What are the different methods of compounding of steam turbine stages? List the advantages and limitations of velocity compounding.
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Differentiate between jet condensers over surface condensers.
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What are different savers of air leakage from the condenser plant? Explain the method of removing the air.
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Explain different types of propellants used in solid propellant rockets.
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A turbojet engine indicates 45 kg of air per second and propels an aircraft with a uniform flight speed of 880 km/h. The isentropic enthalpy change for nozzle is 188.37 kJ/kg and its velocity coefficient is 0.96. The fuel-air ratio is 0.012, the combustion efficiency is 0.95 and the lower heating value of the fuel is 44,000 kJ/kg. Calculate: i) the thermal efficiency of the engine, ii) the fuel flow rate in kg/h, iii) the propulsion power in kW, iv) the thrust power, v) the propulsive efficiency.
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Describe with a sketch a solid propellant rocket. What is gain? What are the applications of solid propellant rockets?
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