Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B. Tech II PROCESS HEAT TRANSFER Previous Question Paper (April/May - 2023 - R16)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
Define forced convection heat transfer. List the factors affecting heat transfer rate in forced convection.
Answer
Forced convection is a mechanism of heat transfer where fluid motion is generated by an external source such as a pump, fan, or blower. The heat transfer rate is affected by fluid velocity, fluid properties (density, viscosity, thermal conductivity, specific heat), surface geometry, and the nature of the flow (laminar or turbulent).
Mention the analogy between “thermal resistances in series” and “electrical resistance in series”.
Answer
In thermal systems, the heat flow rate (Q) is analogous to electrical current (I), the temperature difference (ΔT) is analogous to voltage difference (V), and the thermal resistance (R_th) is analogous to electrical resistance (R_e). For resistances in series, the total thermal resistance is the sum of individual resistances, similar to Kirchhoff's voltage law for electrical circuits.
Define “heat transfer coefficient”.
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Define critical radius of insulation. Why it is important?
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What is the relationship between thermal conductivity and thickness of the thermal boundary layer?
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What is the effect of heating and cooling of highly viscous liquids on rate of heat transfer?
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What is natural convection? Mention the importance of Grashof number.
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State Kirchhoff's law in radiation. Mention the applications of Kirchhoff's law.
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Define baffle pitch. What are the design conditions followed for baffle spacing?
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What are the applications, advantages and disadvantages of agitated-film evaporator?
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Part B
Consider a hollow sphere of thermal conductivity (k), inner radius (ri) and outer radius (ro) with corresponding surface temperature Ti and To respectively. Derive expression for rate of heat transfer through this hollow sphere.
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An exterior wall of a house may be approximated by a 4 inch layer of common brick [k = 0.7 W/(m °C)] followed by a 1.5 inch layer of gypsum plaster [k =0.48 W/(m °C)]. What thickness of loosely packed rock-wool insulation [k =0.065 W/(m °C)] should be added to reduce the heat loss (or gain) through the wall by 80 percent?
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Discuss the variation of temperatures of the condensing vapor and of the liquid against the tube length in a condenser.
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Explain the classification of individual heat transfer coefficients based on various heat transfer methods in fluids.
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Consider a layer of insulation which might be installed around a circular pipe. The inner temperature of the insulation is fixed at Ti and the outer surface is exposed to a convection environment at T∞. Derive expression for critical-radius-of-insulation.
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What is the advantage of using an effective coefficient for internal heat transfer?
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Discuss the calculation of heat transfer coefficient and temperature distribution for heat transfer in fully developed flow.
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Define Peclet number and Graetz number. Mention the physical significance.
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Discuss the correlations to calculate Nusselt number for heat transfer in liquid metal.
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State and explain Colburn analogy between heat transfer and fluid friction. Mention the limitations of this analogy.
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Discuss the mechanism of drop wise condensation and film type condensation.
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Explain the applications of Nusselt equation in film-type condensation. Summarize the assumptions to derive Nusselt equation.
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Draw the typical heat transfer coefficients vs. ΔT plot for boiling of water at 1 atm and explain the mechanism of boiling based on four segments of the graph.
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What is “square of the distance effect” in radiation heat transfer?
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With a diagram, explain the mechanism of heat transfer in Calandria reboiler.
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Derive expression for heat-transfer coefficient in a scraped surface exchanger.
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