JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech II PROCESS HEAT TRANSFER Previous Question Paper (August/September – 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Differentiate between natural convection and forced convection heat transfer.
Answer
In natural convection, fluid motion is induced by buoyancy forces resulting from density differences caused by temperature gradients within the fluid. In forced convection, fluid motion is externally induced by mechanical devices such as pumps, fans, or blowers.
Key differences include:
- Mechanism: Natural convection relies on density-driven circulation; forced convection relies on externally imposed flow.
- Heat Transfer Coefficient: Forced convection typically results in significantly higher heat transfer coefficients than natural convection.
- Governing Parameters: Natural convection is characterized by the Grashof number (Gr), while forced convection is characterized by the Reynolds number (Re).
Define thermal conductivity. Discuss the effects of temperature on thermal conductivity of materials.
Answer
Thermal conductivity (k) is a material property representing its ability to conduct heat, defined as the rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference.
The effect of temperature on thermal conductivity varies by material:
- Gases: Thermal conductivity increases with temperature because molecular kinetic energy and collision frequency increase.
- Liquids: Generally, thermal conductivity decreases as temperature increases (except for water).
- Solids: In metals, thermal conductivity often decreases with increasing temperature due to increased electron scattering. In non-metallic solids, it depends on the structure; for many insulating materials, it increases with temperature as radiation effects become significant at higher temperatures.
Define thermal diffusivity of a solid. Write the model equation for one dimensional unsteady state heat conduction and explain the variables in the equation.
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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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With a graphical representation, explain the variation of fluids temperature in counter current and parallel flow heat exchanger.
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Discuss the calculation of average temperature of fluid stream and overall heat transfer coefficient in a heat exchanger.
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Define overall heat transfer coefficients and individual coefficients.
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Methyl alcohol flowing in the inner pipe of a double-pipe exchanger is cooled with water flowing in the jacket. The inner tube is made of steel material with inside diameter 2.66 cm, outside diameter 3.34 cm and wall thickness of 0.34 cm. The thermal conductivity of steel is 45 W/m°C. The individual coefficients and fouling factors are given as: alcohol coefficient (hi) = 1020 W/m2°C, water coefficient (ho) = 1700 W/m2°C, inside fouling factor (hdi) = 5680 W/m2°C and outside fouling factor (hdo) = 2840 W/m2°C. What is the overall coefficient, based on the outside area of the inner pipe?
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Discuss the calculation of heat transfer coefficient using the correction factor for heating and cooling of fluids.
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Discuss the calculation of heat transfer coefficient for laminar flow heat transfer in tubes.
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Explain the heat transfer from condensing vapors.
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What is pool boiling? Explain different regimes of boiling.
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Discuss the calculation of radiation heat transfer to semi-transparent materials.
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Explain: Stefan-Boltzmann law, Planck’s law and Wien’s displacement law.
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Discuss the calculation of heat transfer coefficient and fin efficiency for extended surface exchangers.
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Explain the principle of forward feed and mixed feed method of feeding in evaporators.
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