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JNTU B.Tech II THERMODYNAMICS Previous Question Paper (October - 2020 - R18)
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Main
A mass of 8 kg gas expands in a flexible container so that the p-v relationship is in the form of
Answer
Given data:
- Mass (m) = 8 kg
- Initial pressure (P1) = 1000 kPa
- Initial volume (V1) = 1 m3
- Final pressure (P2) = 5 kPa
- Polytropic index (n) = 1.2
- Change in specific internal energy (Δu) = -40 kJ/kg
First, calculate the final volume (V2) using the polytropic relation P1V1n = P2V2n:
Next, calculate the work done (W) during the polytropic expansion:
Calculate the total change in internal energy (ΔU):
Apply the First Law of Thermodynamics for a closed system (Q = ΔU + W):
The heat transfer is 2431 kJ. Since the value is positive, the heat is transferred to the system (heat addition).
A room for four persons has two fans, each consuming 0.18 kW power, and three 100 W lamps. Ventilation air at the rate of 80 kg/h enters with an enthalpy of 84 kJ/kg and leaves with an enthalpy of 59 kJ/kg. If each person puts out heat at the rate of 630 kJ/h, determine the rate at which heat is to be removed by a room cooler, so that a steady state is maintained in the room.
Answer
To maintain a steady state in the room, the total rate of heat generation must be balanced by the rate of heat removal by the room cooler and the heat carried away by the ventilation air. The energy balance equation for the room is:
First, calculate the total heat generation rate (Q̇gen) in kW:
- Fans: 2 × 0.18 kW = 0.36 kW
- Lamps: 3 × 100 W = 3 × 0.1 kW = 0.3 kW
- Persons: 4 × 630 kJ/h = 2520 kJ/h. Converting to kW: 2520 / 3600 = 0.7 kW
- Total Q̇gen = 0.36 + 0.3 + 0.7 = 1.36 kW
Next, calculate the rate of heat removed by the ventilation air (Q̇ventilation). Note that since enthalpy decreases as air leaves, the ventilation air is actually removing heat from the room:
Given ṁ = 80 kg/h = 80/3600 kg/s ≈ 0.02222 kg/s:
Finally, calculate the heat to be removed by the room cooler (Q̇cooler):
The room cooler must remove heat at a rate of approximately 1.916 kW to maintain steady state.
Prove that energy is a property.
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Apply steady flow energy equation for nozzle, compressor and condenser.
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Derive the Maxwell’s relations.
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Sketch the thermodynamic mnemonic diagram and explain its use to obtain Gibbs equations and Maxwell relations.
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A steam pressure vessel of holding capacity 4 m3 contains a mixture of saturated water and saturated steam at 250°C. The mass of the liquid present is 1 ton. Determine (a) Quality; (b) Specific Volume; (c) Specific Enthalpy; (d) Specific Entropy and (e) Specific Internal Energy of steam.
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Explain in detail about throttling and free expansion processes with a neat sketch and also mention their applications.
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State Avogadro’s Hypothesis.
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State and explain Dalton’s law of partial pressure and prove the statement mathematically.
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How does the vapour compression refrigeration system work? Explain with suitable diagrams.
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