Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year BASICS OF THERMODYNAMICS Previous Question Paper (September - 2020 - R16)
Answer any Five Questions. All Questions Carry Equal Marks
Main
Explain Joule's experiment and state the first law of thermodynamics applied to a closed system undergoing a cyclic process.
Answer
Joule's experiment, conducted by James Prescott Joule in the 1840s, was instrumental in establishing the First Law of Thermodynamics. The experiment involved a paddle wheel immersed in an insulated container of water. As a falling weight turned the paddle wheel, the mechanical work done on the system caused the temperature of the water to rise. Joule demonstrated that the work done on the system was directly proportional to the increase in internal energy (manifested as temperature rise), regardless of the method of energy transfer.
The First Law of Thermodynamics for a closed system undergoing a cyclic process states that the net heat transfer is equal to the net work transfer. Since a cycle returns the system to its initial state, the change in internal energy over the complete cycle is zero.
This implies that for any cyclic process, the net energy interaction with the surroundings must be zero, confirming that energy cannot be created or destroyed within the system.
A mass of gas is compressed in a quasi-static process from 80 kPa, 0.1 m3 to 0.4 MPa, 0.03 m3. Assuming that the pressure and volume are related by
Answer
Given data:
- Initial pressure P1 = 80 kPa = 80 × 103 Pa
- Initial volume V1 = 0.1 m3
- Final pressure P2 = 0.4 MPa = 400 × 103 Pa
- Final volume V2 = 0.03 m3
The process follows the law PVn = constant. First, we determine the polytropic index 'n':
(P1 / P2) = (V2 / V1)n
(80 / 400) = (0.03 / 0.1)n
0.2 = (0.3)n
ln(0.2) = n ln(0.3)
n = ln(0.2) / ln(0.3) ≈ 1.226
The work done in a polytropic process is given by the formula:
Substituting the values:
W = (8000 - 12000) / 0.226
W = -4000 / 0.226
W ≈ -17699.1 J
The net work done is approximately -17.7 kJ. The negative sign indicates that work is done on the gas system during the compression process.
Explain about thermodynamic system and control volume.
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A gas in a piston cylinder assembly undergoes an expansion process for which the relationship between pressure and volume is given by
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Why does free expansion have zero work transfer?
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A gas expands from an initial state where the pressure is 340 kPa and the volume is 0.0425 m3 to a final pressure of 136 kPa. The relationship between the pressure and volume of the gas is
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Distinguish between the terms 'change of state', path and process.
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A balloon is filled with air (200 kPa and 300K) such that it becomes a sphere of diameter 1m. It is then gradually heated till the pressure rises to 500 kPa. Determine the amount of work done during the process, assuming that the pressure inside the balloon is proportional to the diameter of the balloon.
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Show that the entropy change in a cyclic process is zero.
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Ten kg of water at 20°C is converted into ice at -10°C at constant atmospheric pressure. Assuming the specific heat of liquid water to remain constant at 4.2 kJ/kg K and that of ice to be half of this value and taking the latent heat of fusion of ice at 0°C to be 335 J/g, calculate the total entropy change of the system.
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In a gas turbine the gas enters at the rate of 5 kg/s with a velocity of 50 m/s and enthalpy of 900 kJ/kg and leaves the turbine with a velocity of 150 m/s and enthalpy of 400 kJ/kg. The loss of heat from the gases to the surroundings is 25 kJ/kg. Assume for gas R = 0.285 kJ/kg K and cp = 1.004 kJ/kgK and the inlet conditions to be at 100 kPa and 27°C. Determine the power output of the turbine and the diameter of the inlet pipe.
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Explain the construction of psychrometric chart at 1 bar pressure.
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Explain the carrier equation and its applications and the concept of adiabatic saturation.
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Mention merits and demerits of diesel cycle and otto cycles.
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In an air standard diesel cycle, the compression ratio is 15. Compression begins at 0.1 MPa, 40°C. The heat added is 1.675 MJ/kg. Find (i) the maximum temperature of the cycle, (ii) the work done per kg of air, (iii) the cycle efficiency, (iv) the temperature at the end of the isentropic expansion, (v) the cut-off ratio, (vi) the maximum pressure of the cycle and (vii) the m.e.p of the cycle.
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