JNTU
JNTU B.Tech 2018 HEAT AND MASS TRANSFER Previous Question Paper (April - 2018 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
PART- A
Comment on heat transfer in vacuum.
Answer
In a vacuum, heat transfer by conduction and convection is impossible because these modes require a physical medium. Heat transfer occurs exclusively through thermal radiation, which involves the emission and absorption of electromagnetic waves.
Differentiate between thermal conductivity and thermal conductance. State their respective units in SI system.
Answer
Thermal conductivity (k) is an intrinsic material property representing the ability of a material to conduct heat per unit thickness, measured in W/(m·K). Thermal conductance (C) is a property of a specific component or system geometry, representing the rate of heat flow per unit temperature difference, measured in W/K.
What is meant by critical thickness of insulation?
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State and explain the significance of geometric configuration factor.
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What are the various types of fins? Mention assumptions made during analysis of fins.
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What do you mean by dimensional analysis? List out a few applications.
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What is a heat exchanger? How heat exchangers are classified?
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What do you understand by the hydrodynamic and thermal boundary layers?
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State Fick’s law of diffusion.
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State and explain the steady state molecular diffusion in fluids at rest?
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PART- B
Derive general heat conduction equation in Cartesian coordinates.
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A furnace wall is made up of refractory brick, fire brick and an outside plaster. There is an air gap with thermal resistance of 0.15 m2 K/W between refractory brick and the fire brick. The refractory brick, 120 mm thick has k1 = 1.58 W/m K. The fire brick, 120 mm thick has k2 = 0.3 W/m K. The outside plaster, 15 mm thick, has k3 = 0.15 W/m K. The two extreme temperatures of this wall are 1000 °C and 100 °C. Determine the heat flow rate in kJ/h m2.
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Determine the interface temperatures.
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What do you understand by a black body and a gray body as applied to radiation problem? Explain how streaming out of a small hole in a large hollow body can be considered as black body radiation.
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Determine the radiation heat flux between two closely spaced, black parallel plates radiating only to each other if the temperatures are 850K and 425K respectively. Calculate the heat flux presuming that each of the parallel plates has an emissivity of 0.5. In each case the plates have an area of 4 m2.
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Explain the Rayleigh method and the Buckingham’s Π-theorem for dimensional analysis. What are repeating variables and how are they selected dimensional analysis?
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Differentiate between mechanisms of heat transfer by free and forced convection. Mention some of the areas where these mechanisms are predominant.
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A steam condenser is transferring 250kW of thermal energy at condensing temperature of 65°C. The cooling water enters the condenser at 20°C with a flow rate of 7500 kg/hr. Calculate the LMTD. If overall heat transfer coefficient for the condenser surface is 1250 W/m2-deg, what surface area is required to handle this load?
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Derive an expression for log mean temperature difference for counter flow heat exchanger.
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Distinguish between molecular diffusion and eddy diffusion and indicate which takes place predominantly in the following situations: a) Atomisation of oil droplet in a diesel engine cylinder, b) Vaporisation of petrol in carburettor, c) Diffusion of welding metals, d) Air pollution due to car exhaust.
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Estimate the diffusion coefficient for ammonia in air at 25°C temperature and one atmospheric pressure: For ammonia: molecular weight=17 and molecular volume=25.81 cm3/gm mole, For Air: molecular weight=29 and molecular volume=29.89 cm3/gm mole.
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