JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year PROCESS HEAT TRANSFER Previous Question Paper (May - 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
Write Fourier’s law for unsteady state heat conduction.
Answer
Fourier's law for unsteady state heat conduction is expressed as:
where q is the heat transfer rate, k is thermal conductivity, A is the area, and (∂T/∂x) is the temperature gradient at a specific time and location.
Write the physical significance of Fourier number.
Answer
The Fourier number (Fo = αt/L2) represents the ratio of the rate of heat conduction to the rate of thermal energy storage in a solid. It is a dimensionless parameter that characterizes unsteady state heat conduction and indicates the degree of temperature penetration into a body over a given time.
Define heat transfer coefficient.
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Why counter-current flow is preferred than co-current flow in a heat exchanger?
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What is the significance of Gratetz number?
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What is thermal boundary layer?
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What is film boiling?
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What is black body and gray body?
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What is the use of extended surface heat exchanger?
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Write the difference between drying and evaporation.
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Part B
Derive the equation for heat conduction through a sphere.
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A hollow sphere has an inside surface temperature 300°C and outside surface temperature of 30°C. Calculate the heat loss by conduction for an inside diameter of 5 cm and outside diameter of 15 cm. The thermal conductivity of material is 15 kcal/hr m °C.
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A flat furnace wall is constructed of a 115 mm layer of sil-o-cel brick with thermal conductivity of 0.4 kcal/hr m °C backed by a 230 mm layer of common brick of conductivity 4 kcal/hr m °C. The temperature of the inner face of the wall is 760°C and that of the outer face is 77°C.
a) What is the heat loss through the wall in kcal/m2 hr
b) What is the temperature of the interface between the refractory brick and the common brick?
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A copper wire, 5.2 mm in diameter, is insulated with a layer of PVC of thermal conductivity k = 0.43 W/ m°C. The wire carries current, and its temperature is 60°C. The film coefficient at the outer surface of the insulation is 11.35 W/m2°C. Calculate the critical insulation thickness.
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Methyl alcohol flowing in the inner pipe of a double-pipe heat exchanger is cooled with water flowing in the outer pipe. The inside and outside diameters of the inner pipe are 2.6 cm and 3.5 cm respectively. The thermal conductivity of the steel is 26 cal/cm hr °C. The individual film coefficients and fouling factors are: Alcohol coefficient=180 kcal/hr m2°C, Water coefficient= 300 kcal/hr m2°C, Inside fouling factor =1000 kcal/hr m2°C, Outside fouling factor =500 kcal/hr m2°C. Calculate the overall heat transfer coefficient based on the outside area of the inner pipe.
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What is Colburn j-factor and prove that relation.
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Write short notes on heat transfer to liquid metals.
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Estimate the average value of convective film coefficient on the inside surface of a tube of 50 mm i.d meant for heating water. The mass flow rate of water is 25000 kg/hr, which enters at a temperature of 20°C and leaves at 60°C. Calculate also the heat transferred per unit length of the tube if the wall temperature is 100°C above the bulk temperature of the fluid. Physical properties of water at the mean temperature of 40°C is given: ρ = 992 kg/m3, k = 54.5 × 10-2 kcal/m sec °C, ν = 0.659 × 10-6 m2/s, Pr = 4.31.
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Give the physical significance of Prandtl number and Grashof number.
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A laboratory water bath has an immersed horizontal heating element, 2.54 cm in diameter and 30 cm in length, with a power input of 500 W. If the bulk water temperature is 38°C and heat transfer occurs by free convection only, calculate the surface temperature of the heater?
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What is cosine law? Explain about diffuse radiation.
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A hot pipe (o.d = 0.06 m and wall temperature = 150°C) runs centrally through a bigger pipe, 0.31 m i.d., held at 30°C. If the emissivities of pipe surfaces are 0.6 and 0.75, respectively, calculate the net heat loss per metre length of the hot pipe.
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What is evaporation? Discuss its applications in the process industry.
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Discuss about the following methods of feeding in multiple effect evaporators: i) Forward feeding ii) Mixed feeding.
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Define ‘fin efficiency’? Give the equation to calculate it.
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Discuss about different types of extended surfaces with neat figures.
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