Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech II THERMODYNAMICS Previous Question Paper (September - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
During one cycle the working fluid in an engine engages in two work interactions: 15 kJ to the fluid and 44 kJ from the fluid, and three heat interactions, two of which are known: 75 kJ to the fluid and 40 kJ from the fluid. Evaluate the magnitude and direction of the third heat transfer.
Answer
According to the first law of thermodynamics for a closed system undergoing a cycle, the net heat transfer is equal to the net work transfer, expressed as ∑Q = ∑W.
Given interactions:
Work interactions: W1 = -15 kJ (to the fluid), W2 = +44 kJ (from the fluid). Net work ∑W = 44 - 15 = 29 kJ.
Heat interactions: Q1 = +75 kJ (to the fluid), Q2 = -40 kJ (from the fluid), Q3 = ?
Applying the cycle energy balance:
Solving for Q3:
The magnitude of the third heat transfer is 6 kJ, and the negative sign indicates that the heat is transferred from the fluid.
Explain the significance of absolute scale of temperature.
Answer
The absolute scale of temperature, or thermodynamic temperature scale, is significant because it is independent of the physical properties of any specific substance used to measure it. Unlike empirical scales (such as Celsius or Fahrenheit) which rely on the expansion of mercury or the pressure of a gas, the absolute scale is based on the second law of thermodynamics.
Key points of its significance include:
- It provides a universal reference point, with the absolute zero (0 K) representing the state where all molecular motion ceases according to classical kinetic theory.
- It is essential for defining the efficiency of heat engines, as the Carnot efficiency is expressed solely in terms of the absolute temperatures of the heat source and sink.
- It allows for the calculation of entropy changes in thermodynamic processes, which is fundamental for evaluating the reversibility and feasibility of cycles.
- It ensures consistency in thermodynamic relations, such as the ideal gas law (PV = mRT), where temperature must be expressed in Kelvin to maintain the validity of the gas constant R.
Using an engine of 30% thermal efficiency to drive a refrigerator having a COP of 5, what is the heat input into the engine for each MJ removed from the cold body by the refrigerator? If this system is used as a heat pump, how many MJ of heat would be available for heating for each MJ of heat input to the engine?
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Describe the corollaries of first law of thermodynamics.
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Water at 150 kPa and 100°C enters a mixing chamber at a rate of 136 kg/min where it is mixed steadily with steam entering at 140 kPa and 150°C. The mixture leaves the chamber at 140 kPa and 50°C and heat loss to the surrounding air at 20°C at a rate of 190 kJ/min. Neglecting KE and PE, determine the entropy generation, available energy and irreversibility.
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0.45 kg of air undergoes two processes that are in series. The first process consists of an isothermal expansion during which the air is supplied with 23.2 kJ at a temperature of 187°C; this process is then followed by a reversible adiabatic process that takes the gas to a temperature of 57°C, determine the change in entropy for these combined processes.
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Differentiate between unavailable energy and irreversibility.
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Steam having a quality of 0.80 is contained in a rigid vessel at a pressure of 20.7 bar. Heat is added until the temperature reaches 260°C. Determine the final pressure.
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Explain in detail about a typical P–V–T diagram for a substance that expands on freezing.
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Two Carnot engines are operated in a series with the exhaust of the first engine being the input of the second engine. The upper temperature of this combination is 700°C, and the lower temperature is 200°C; if each engine has the same thermal efficiency, determine the exhaust temperature of the first engine and the inlet temperature to the second engine.
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How to measure the dry bulb temperature and wet bulb temperature? Explain.
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Derive the Vander Waals equation for real gases.
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Explain the steps involved in the construction of Psychrometric chart at 2 bar pressure and also explain the process of adiabatic saturation.
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Derive an expression for the thermal efficiency of a dual cycle.
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Determine the mean effective pressure of a four-cycle, six-cylinder engine that has a 75 kW output at 4000 rpm. The cylinder bore is 75 mm and the stroke is 102 mm.
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