JNTU
JNTU B.Tech II PROCESS HEAT TRANSFER Previous Question Paper (July/August - 2021 - R13)
Answer any five questions. All questions carry equal marks.
Main
Differentiate between natural convection and forced convection heat transfer.
Answer
Natural convection occurs when fluid motion is caused by buoyancy forces resulting from density differences due to temperature variations within the fluid. In contrast, forced convection occurs when fluid motion is induced by an external source such as a pump, fan, or blower. In natural convection, the heat transfer coefficient is generally lower, and the flow is driven by the temperature gradient itself. In forced convection, the flow velocity is independent of the temperature gradient, typically resulting in higher heat transfer coefficients.
Derive an equation for calculating the rate of heat flow through a flat wall constructed of series of different materials.
Answer
Consider a composite wall consisting of three layers of different materials with thicknesses L1, L2, and L3, and thermal conductivities k1, k2, and k3. Let the temperatures at the interfaces be T1, T2, T3, and T4. The heat flow rate Q through each layer in steady state is constant:
Rearranging these equations for the temperature differences:
Summing these equations eliminates the intermediate temperatures:
The total heat transfer rate is thus:
This can be expressed using the concept of thermal resistance R, where R = L / (kA), leading to Q = ΔT / ΣR.
With neat sketch, discuss the calculation of heat exchange parameters in a single – pass tubular condenser.
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Discuss the calculation of critical radius of insulation for a cylindrical pipe with outer radius r1 and outer surface temperature T1.
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Discuss the effects of fluid flow type, fluid properties and direction of flow on heat transfer rate.
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Explain the following: hydrodynamic boundary layer, thermal boundary layer and Prandtl number.
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Define Grashof number and Nusselt number with their physical significance.
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Describe the mechanism of heat transfer to boiling liquids.
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A double pipe parallel flow heat exchanger use oil (Cp = 1.88 kJ/kg.K) at an initial temperature of 205C to heat water (Cp = 4.18 kJ/kg.K), flowing at 225 kg/hr from 16C to 44C. The oil flow rate is 270 kg/hr. Calculate the following: (a) what is the heat transfer area required for an overall heat transfer coefficient of 340 W/m2.K. (b) Determine the number of transfer unit (NTU) and (c) Calculate the effectiveness of the heat exchanger.
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Derive expression for heat transfer rate through hollow cylinder. State the assumptions made.
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What are the causes of fouling? How fouling factor affects heat flux?
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Explain the various correlations to calculate the heat transfer coefficient in liquid metal heat transfer.
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Discuss the mechanism of radiation heat transfer to semi-transparent materials.
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With neat sketch discuss the constructional features, working principle and uses of long tube vertical evaporator with upward flow of the liquid.
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Define capacity and economy of tubular evaporators.
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