Download GTU BE/B.Tech 2018 Winter 7th Sem New 2170102 Theory Of Heat Transfer Question Paper

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Subject Code: 2170102

GUJARAT TECHNOLOGICAL UNIVERSITY

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Instructions:

1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.

Q.1 (a) What is meant by thermal resistance? Explain the electrical analogy for solving heat transfer problems. MARKS 03

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(b) Derive temperature variation profile (t-t₁/t-t₁) in a plane wall. 04

(c) Derive general heat conduction equation in cylindrical coordinates. 07

Q.2 (a) Define thermal conductivity. Explain physical significance of thermal conductivity. 03

(b) Explain the different modes of heat transfer. Describe conduction mode of heat transfer briefly. 04

(c) A double glazed window is made of 2 glass panes of 6 mm thick each with an air gap of 6 mm between them. Assuming that the air layer is stagnant and only conduction is involved, determine the thermal resistance and overall heat transfer coefficient. The inside is exposed to convection with h =15 W/m²K and the outside to 9 W/m²K. Compare the values with that of a single glass of 12 mm thickness. The conductivity of the glass = 1.4 W/mK and that for air is 0.025 W/mK. 07

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OR

Q.3 (a) A long rod 12 mm square section made of low carbon steel protrudes into air at 35°C from a furnace-wall at 200°C. The convective heat transfer coefficient is estimated at 22 W/m²K. The conductivity of the material is 51.9 W/mK. Determine the location from the wall at which the temperature will be 60°C. Also, calculate the temperature at 80 mm from base. 07

(b) Explain Stefan Boltzmann law. 03

(c) Explain the criteria of selection of fins. 04

Q.3 (a) A thermocouple is formed by soldering end-to-end wires of 0.5 mm dia. The thermal diffusivity of the material is 5.3 x 10^-6 m²/s. The conductivity of the material is 19.1 W/mK. The probe initially at 30°C is placed in a fluid at 600°C to measure the temperature of the fluid. If the convective heat transfer coefficient between the wire and the fluid is 85 W/m²K, determine the time constant for the probe and also the time taken for it to read 598°C. 07

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OR

(b) Explain thermal boundary layer. 03

(c) Some surfaces which do not appear black but have high values of absorptivity — Justify. 04

(c) Derive an expression for temperature distribution and heat dissipation for in fin of rectangular cross section insulated at tip. 07

Q.4 (a) Distinguish between mean film and bulk mean temperature. 03

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(b) Enumerate the factors on which the rate of emission of radiation by a body depends. 04

(c) Derive momentum equation for boundary layer. 07

OR

Q.4 (a) State physical significance of Reynolds Number and Prandtl Number. 03

(b) Assuming the sun to be a black body emitting radiation with maximum intensity at ? = 0.49 µm, calculate the following: 04

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1. The surface temperature of the sun, and
2. The heat flux at surface of the sun.

(c) By Buckingham’s p theorem method show that for free convection Nu = f (Gr, Pr) 07

Q.5 (a) State silent features of shape factor. 03

(b) Derive the expression for LMTD of counter flow heat exchanger, when ?₁=?₂=?. 04

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(c) A chemical having a specific heat of 3.3 kJ/kg K flowing at the rate 20,000 kg/h enters a parallel flow heat exchanger at 120 °C. The flow rate of cooling water is 50,000 kg/h with an inlet temperature of 20 °C. The heat transfer area is 10 m² and overall heat transfer coefficient is 1200 W/m² °C. Taking specific heat of water as 4.186 kJ/kg K. Find: 07

1. Effectiveness of the heat exchanger
2. Outlet temperature of water and chemical

OR

Q.5 (a) State the limitations of LMTD method in the analysis of heat exchanger. 03

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(b) Write a short note on heat pipe. 04

(c) Define condensation process. Explain drop wise and film wise condensation. 07