JNTU
JNTU B.Tech II PROCESS HEAT TRANSFER Previous Question Paper (April - 2018 - R15)
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.
PART - A
Write the equation for heat flow through a sphere.
Answer
The rate of heat transfer through a hollow sphere under steady-state conduction with constant thermal conductivity is given by the equation:
where Q is the heat flow rate, k is the thermal conductivity of the material, T1 and T2 are the temperatures at the inner and outer surfaces, and r1 and r2 are the inner and outer radii of the sphere, respectively.
What is forced convection? Explain.
Answer
Forced convection is a mechanism of heat transfer in which fluid motion is generated by an external source, such as a pump, fan, blower, or compressor, rather than by buoyancy forces resulting from density differences.
In this process, the fluid is forced to flow over a solid surface or through a conduit. Because the flow velocity is externally controlled, the heat transfer coefficient is significantly higher compared to natural convection. The rate of heat transfer is governed by Newton's law of cooling:
where h is the convective heat transfer coefficient, which depends on the fluid properties, flow geometry, and the Reynolds number of the flow.
What is heat transfer coefficient?
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What is LMTD? Give the limitations of it?
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Define boundary layer.
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What is the physical significance of Prandtl number?
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Define pool boiling.
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What is Wien’s displacement law?
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What is economy of an evaporator?
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What are the uses of extended surfaces used in heat exchangers?
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PART - B
Estimate the rate of heat loss through the wall of a spherical boiling pan with an inner diameter of 1.0 m and total boiler wall and insulation thickness of 100mm. The temperature of the inner surface is 1400C and that of the outer surface is 500C. The equivalent thermal conductivity of material of the spherical wall is 0.12 W/m K.
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An ice box has walls constructed of a 10cm layer of cork board contained between two wooden walls, each two 2 cm thick. Find the rate of heat removal in kcal/m2 hr if the inner wall surface is kept at -100C, while the outer surface temperature is 300C. Find out the zone in the wall where the temperature is 200C. Thermal conductivities of cork board and wood respectively are 0.035 and 0.09 kcal/hr m 0C.
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Explain the concept of critical insulation thickness.
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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/ m0 C. The wire carries current, and its temperature is 600 C. The film coefficient at the outer surface of the insulation is 11.35 W/m2 0C. Calculate the critical insulation thickness.
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Discuss the advantages of counter-current flow over parallel flow in heat exchangers, Explain these using graphs.
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Water enters a two-fluid heat exchanger at 550 C and leaves at 850 C. Hot gases enter at 3050 C and leaves at 1600 C. If the total heat transfer area is 500 m2 and the overall heat transfer coefficient is 600 kcal/hr m2 0C, determine the total heat transferred per hour for parallel flow of the two fluids.
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Describe about Reynolds analogy and prove that NSt=f/2.
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Derive the equations for heat transfer by forced convection in turbulent flow.
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Determine the heat transfer coefficient for water flowing in a tube of 16 mm diameter at a velocity of 3 m/s. The temperature of the tube is 240 C, and the water enters at 800 C and leaves at 360 C. Use the Dittus-Boelter equation Nu = 0.023 Re0.8 Pra, where a=0.4 when the fluid is heated and a=0.3 when the fluid is cooled. Properties: ρ=984 kg/m3, k=0.657 W/m K, μ=485×10-6 Pa s, CP= 4178 J/kg K.
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A room has a nearly horizontal flat roof of thin GI (galvanized iron) sheet. On a sunny day the roof receives 700 W/m2 radiation. Assuming that 70% of this energy is transferred to the ambient medium at 320C by free convection from both upper and lower surfaces of the roof, calculate the steady state temperature of the roof.
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Describe the mechanism of boiling with the help of q/A Vs ΔT curve. Indicate different regimes of boiling.
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250 kg/hr of dry carbon dioxide at normal pressure is to be heated from 260C to 1000C in a heat exchanger using steam at 1.7 atmosphere pressure. The gas is passed through copper tubes (2.54 cm OD and 0.124 cm wall thickness) and the inlet velocity is to be 6.1 m/s. The mean specific heat of carbon dioxide is 0.204 cal/gm 0C. Use the gas film coefficient equation h = 2.5 G0.8, where G is mass velocity in kg/m2s. How many tubes are required and what is their length? Data: Steam film coefficient = 1.13×104 W/m2 0C, k_copper = 391 W/m 0C.
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Compare the merits and demerits of different feeding methods in multiple effect Evaporators.
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