Anna University Examination May/June 2012

Important Questions

Common to

**ME41:Heat and Mass Transfer**

**ME1251 Heat And Mass Transfer**

**080120015:Heat And Mass Transfer**

**113401:Heat and Mass Transfer**

Unit I

1. Derive the general heat conduction equation in cylindrical coordinates.

2. Explain briefly the concept of critical thickness of insulation and state any two applications of the same

3. A composite wall consists. of 10 cm thick layer of building brick, k = 0.7 W/mK and 3 cm thick plaster, k = 0.5 W/mK. An insulating material of k = 0.08 W/mK is to be added to reduce the heat transfer through the wall by 40%. Find its thickness. [same model may be asked]

4 .Circumferential aluminium fins of rectangular profile (1.5cmwide and 1mm thick) are fitted on to a 90 mm engine cylinder with a pitch of 10 mm. The height of the cylinder is 120 mm. The cylinder base temperature before and after fitting the fins are 200°C and 150°C respectively. Take ambient at 30°C and h(average) =100 W/m K2 Estimate the heat dissipated from the finned and the unfinned surface areas of cylinder body. [same model may be asked]

5. Derive the general heat conduction equation for a hollow cylinder.

Unit II

1. Air at 400 K and 1 atm pressure flows at a speed of 1.5 m/s over a flat plate of 2 m long. The plate is maintained at a uniform temperature of 300 K. If the plate has a width of 0.5 m, estimate the heat transfer coefficient and the rate of heat transfer from the air stream to the plate. Also estimate the drag force acting on the plate[same model may be asked]

2. A steam pipe 10 cm OD runs horizontally in a room at 23° C. Take outside temperature of pipe as 165 ° C. Determine the heat loss per unit length of the pipe. Pipe surface temperature reduces to 80° C with 1.5 cm insulation. What is the reduction in heat loss? [same model may be asked]

3. Define Reynold’s, Nusselt and Prandtl numbers

4. A 6 – m long section of an 8 cm diameter horizontal hot water pipe passes through a large room in which the air and walls are at 20°C. The pipe surface is at 70°C and the emissivity of the pipe surface is 0.7. Find the rate of heat loss from the pipe by natural convection and radiation.

Unit III

1. Define effectiveness of a heat exchanger. Derive an expression for the effectiveness of a double pipe parallel flow heat exchanger. State the assumptions made.

2. A tube of 2 m length a nd 25 mm outer diame te r is to be used to condense saturated steam at 100°C while the tube surface is maintained at 92°C. Estimate the average heat transfer coefficie nt and the ra te of condensation of steam if the tube is kept horizonta l. The steam condenses on the outside of the tube. [same model may be asked]

3. Derive the LMTD for a parallel flow heat exchanger stating the assumptions

4. Consider laminar film condensation of a stationary vapour on a vertical flat plate of length L and width b. Derive an expression for the average heat transfer coefficient. State the assumptions made.

5. Discuss the various regimes of pool boiling heat transfer.

Unit IV

1. State and prove the following laws:

(1) Kirchoffs law of radiation

(2) Stefan - Boltzmann law

2. Liquid Helium at 4.2 K is stored in a dewar flask of inner diameter = 0.48 m and outer diameter = 0.5 m. The dewar flask can be treated as a spherical vessel. The outer surface of the inner vessel and the inner surface of the outer vessel are well polished and the emissivity of these surfaces is 0.05. The space between the two vessels is thoroughly evacuated. The inner surface of the dewar flask is at 4.2 K while the outer surface is at 300 K. Estimate the rate of heat transfer between the surfaces[same model may be asked]

3. Two large parallel plates of 1m×1m spaced 0.5m apart in a very large room whose walls are at 27°C. The plates are at 900°C and 400°C with emissivities 0.2 and 0.5 respectively. Find the net heat transfer to each plate and to the room. [same model may be asked]

4. Explain briefly the following:

(i) Specular and diffuse reflection

(ii) reflectivity and transmissivity

Unit V

1. Discuss briefly the following

(i) Fick’s law of diffusion

(ii) Equimolar counter diffusion

(iii) Evaporation process in the atmosphere

2. Air at 1.01 bar and 30°C flows past a tray full of water with a velocity of 2 m/s. The partial pressure of water vapour is 0.7 kPa and the saturation pressure is 3.17 kPa. The tray measures 40 cm along the flow direction and has a width of 20 cm. Calculate the evaporation rate of water if the temperature on the water surface is 25°C. Assume the following properties for air: density, ? 1.2 kg/m3, kinematic viscosity, ? = 15 × 10-6 m2/s and diffusivity, D = 0.145 m2/h[same model may be asked]

3. The temperature recorded by a thermometer whose bulb covered by a wet wick in dry air at atmospheric pressure is 22°C. Estimate the true air temperature. [same model may be asked]

4. Write short notes on the following:

(i) Analogy between heat and mass transfer

(ii) Evaporation process in the atmosphere.

(i) Analogy between heat and mass transfer

(ii) Evaporation process in the atmosphere.