JNTU
JNTU B.Tech III THERMAL ENGINEERING II Previous Question Paper (March - 2021 - R13/R15)
Answer any five questions. All questions carry equal marks.
Main
What are the different thermodynamic variables affecting efficiency and output of Rankine cycle. Explain their influence on Rankine cycle.
Answer
The efficiency and output of a Rankine cycle are primarily influenced by the following thermodynamic variables:
- Boiler Pressure: Increasing the boiler pressure raises the mean temperature of heat addition, which increases the thermal efficiency. However, it also increases the moisture content at the turbine exit, which can cause blade erosion.
- Condenser Pressure: Lowering the condenser pressure (vacuum) increases the enthalpy drop across the turbine, thereby increasing both the work output and thermal efficiency. The limit is set by the cooling water temperature.
- Superheat Temperature: Increasing the steam temperature at the turbine inlet increases the mean temperature of heat addition and reduces the moisture content at the turbine exhaust, improving both efficiency and turbine life.
- Reheat: Implementing reheat allows for higher boiler pressures while maintaining acceptable moisture levels at the turbine exit, which improves cycle efficiency and work output.
- Regeneration: Using feed water heating (regeneration) increases the temperature of the water entering the boiler, which improves the cycle efficiency by reducing the heat required from the external source.
What do you mean by stoichiometric air fuel ratio? Explain the method to estimate the air fuel ratio.
Answer
The stoichiometric air-fuel ratio is the chemically correct mass ratio of air to fuel required for complete combustion, where no excess air is present and no unburnt fuel remains. It is determined by the chemical composition of the fuel (carbon, hydrogen, and sulfur content).
The air-fuel ratio is estimated using the following method:
- Perform an elemental analysis of the fuel to determine the mass fractions of carbon (C), hydrogen (H₂), and sulfur (S).
- Write the balanced chemical combustion equations for each element: C + O₂ → CO₂; 2H₂ + O₂ → 2H₂O; S + O₂ → SO₂.
- Calculate the total mass of oxygen required for 1 kg of fuel: O₂ required = (8/3)C + 8H₂ + S (in kg).
- Subtract any oxygen already present in the fuel from the total oxygen required.
- Since air contains 23.3% oxygen by mass, the stoichiometric air required is: Air = (O₂ required / 0.233).
- The ratio is then expressed as the mass of air divided by the mass of fuel.
Mention any three boiler mountings and three Accessories with neat sketches.
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What is the condition for maximum discharge through the chimney in terms of flow gas temperature and boiler house temperature.
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In surface condenser the vacuum maintained is 700 mm of Hg. The barometer reads 754mm. If the temperature of condensate is 18°C, Determine (i) mass of air per kg of steam (ii) Vacuum Efficiency.
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Steam leaves the nozzle of a single-stage impulse turbine at 840 m/s. The nozzle angle is 18° and the blade angles are 29° at the inlet and outlet. The friction coefficient is 0.9. Calculate (a) blade velocity (b) steam mass flow rate in kg/h to develop 300 kW power.
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Explain the operating principle of Brayton cycle with a schematic diagram p-v and T-s diagrams.
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A simple gas turbine cycle works with a pressure ratio of 8. The compressor and turbine inlet temperatures are 300 K and 800 K respectively. If the volume flow rate of air is 250 m3/s, compute the power output and thermal efficiency.
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Dry saturated steam expands through a nozzle from a pressure of 13.7 bar down to 9.6 bar. Assuming the flow to be frictionless and adiabatic, estimate velocity of steam jet.
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What is degree of reaction for steam turbine? Derive its equation for reaction turbine.
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Explain working of turbo prop engine with a neat sketch.
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Explain the advantages and disadvantages of bipropellants used in rocket engines over monopropellants.
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Differentiate between jet condensers and surface condensers.
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Draw the schematic diagram of solid propellant rockets and discuss its working principle.
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