Download GTU BE/B.Tech 2019 Summer 6th Sem Old 161906 Heat And Mass Transfer Question Paper

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Subject Code: 161906

BE - SEMESTER-VI(OLD) - EXAMINATION - SUMMER 2019

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Subject Name: Heat And Mass Transfer

Time: 10:30 AM TO 01:00 PM

Instructions:

1. Attempt all questions.
2. Make suitable assumptions wherever necessary.

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3. Figures to the right indicate full marks.

1. Q1 (a) Derive general heat conduction equation in Cylindrical coordinates with general notations 07

   (b) A metal plate of 4 mm thickness with thermal conductivity 95.5W/m °C is exposed to vapour at 100°C on one side and cooling water at 25°C on the opposite side. The heat transfer coefficients on vapour side and water side are 14500 W/m²°C and 2250 W/m² °C respectively. Determine 07

   1. The rate of heat transfer
   2. The overall heat transfer coefficient and

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   3. Temperature drop at each side of heat transfer

2. Q2 (a) Write a short note on critical thickness of insulation 07

   (b) Derive an expression for heat dissipation in rectangular fin of uniform cross-section which is insulated at tip. 07

   OR

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   (b) An egg with mean diameter of 4 cm and initially at 25 °C is placed in a boiling water pan for 4 minutes and found to be boiled. For how long should a similar egg be boiled when taken from a refrigerator at 5 °C? 07

3. Q3 (a) Define Reynolds number, Nusselt number and Prandtl number. Explain their importance in convection heat transfer. 07

   (b) Air at atmospheric pressure and 200°C flows over a plate with a velocity of 5 m/s. The plate is 15 mm wide and is maintained at a temperature of 120 °C. Calculate the thicknesses of hydrodynamic and thermal boundary layers and the local heat transfer coefficient at a distance of 0.5 m from the leading edge. Assume that flow is on one side of the plate. Density of air is 0.815 kg/m³, dynamic viscosity is 24.5x10?6 Ns /m² °C, Prandtl number is 0.7 and thermal conductivity is 0.0364 W/m °C 07

   OR

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   Q3 (a) Obtain a general form of equation for forced Convective heat transfer using dimensional analysis 07

   (b) A horizontal fluorescent tube which is 3.8 cm in diameter and 120 cm long stands in still air at 1 bar and 20 °C. if the surface temperature is 40 °C and radiation is neglected, what is heat transfer rate by convection. Use equation Nu = 0.53 (Gr.Pr)^0.25. Kinematic viscosity is 15.6x10?6 m²/s, Prandtl number is 0.701 and thermal conductivity is 2.673 x10?² W/mK. 07

4. Q4 (a) Describe briefly the factors considered for the selection criteria for heat exchanger. 07

   (b) 55000 kg/h of oil is cooled from 95 °C to 65 °C. At the same time 50000kg/h of cooling water at 30 °C enters the heat exchanger. The flow conditions are such that overall heat transfer coefficient remains constant at 2270 W/m² K. Determine the heat transfer area required and the effectiveness, assuming two streams are in parallel flow. For the both the streams, assume specific heat at constant pressure to be 4.2 kJ/kg K. 07

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   OR

   Q4 (a) Explain the typical boiling curve for water at 1 atm pressure with a neat sketch. 07

5. Q5 (a) Write short notes: 07

   1. Film condensation

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   2. Dropwise condensation

   (b) State the following laws for radiation heat transfer 07

   1. Plank’s law
   2. Wien’s law
   3. Stefan Boltzmann law

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   4. Kirchhoff’s law
   5. Lambert cosine law

   OR

   Q5 (a) A furnace emits radiation at 2000 K treating it as a black body radiation, calculate the following: 07

   1. Monochromatic radiant flux density at 1 µm wavelength.

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   2. Wavelength at which emission is maximum and corresponding radiant flux density.
   3. Total emissive power.

   (b) State and explain Fick’s law of diffusion. Express Fick’s law in terms of partial pressures of gases 07

   Q5 (a) A 30 mm deep pan is filled with water to a level of 15 mm and is exposed to dry air at 40 °C. Assuming the mass diffusivity as 0.25x10?4 m²/s, calculate the time required for all the water to evaporate. 07

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Date: 31/05/2019

Total Marks: 70

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GUJARAT TECHNOLOGICAL UNIVERSITY

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