ME 2251 Heat and Mass Transfer, Fourth year, Department of Mechanical Sandwich Engineering, Model Exam Questions

Answer ALL Questions

Part-A (2 Marks)

1. State Fourier’s law of heat conduction.

2. State newton’s law of cooling

3. Define Radiation

4. What do you understand by conduction?

5. What is meant by veins displacement law?

6. Define pool boiling.

7. What is meant by convection?

8. What is meant by Fouling Factor?                                 

9. What is meant by laminar and turbulent flow?

10. Define LMTD.

PART B

11)       a. A furnace wall is made up of three layers of thickness 25 cm, 10 cm and 15 cm with thermal conductivities of 1.65 k and 9.2 W/mK respectively. The inside is exposed to gass at 1250^oC with a convective coefficient of 25 W/m²K and the inside surface is at 1100^oC, the outside surface is exposed to air at 25^oC with convective coefficient of 12 W/m2K. Determine i) the unknown thermal conductivity. ii) The overall heat transfer coefficient. iii) All the surface temperature.

Or

b. Derive the heat conduction equation for Cartesian coordinates

12)       a. Air at 20^oC at atmospheric pressure flows over a flat plate at a velocity of 3 m/s. If the plate is 1 m wide and 80^oC, Calculate the following at x=300mm.

1. Hydrodynamic boundary layer thickness

2. Thermal boundary layer thickness

3. Local friction coefficient

4. Average friction coefficient

5. Local heat transfer coefficient

6. Average heat transfer coefficient

7. Heat Transfer

Or

b. Air at 33k, 1.5 bar pressure, flows through 12 cm diameter tube. The surface temperature of the tube is maintained at 400K and mass flow rate is 75 kg/hr. Calculate the heat transfer rate for 1.5 m length of the tube.

13)       a. It is desired to use double pipe counter flow  heat  exchanger to cool 3 kg/s of oil     (Cp =2.1 kJ/kg)from 120ºC. Cooling water at 20ºC enters the heat exchanger at a rate of 10kg/s. The overall heat transfer co-efficient of the heat exchanger is 600 W/m²K and the heat transfer area is 6 m². Calculate the exit temperature of oil and water.                                    

Or

b. In a counter flow double pipe heat exchanger, water is heated from 25^oC to 65^oC by an oil with a specific heat of 1.45 KJ/Kg K and mass flow rate is 0.9 Kg/S. The oil is cooled from 230^oC to 160^oC. If the overall heat transfer coefficient is 420 w/m^{2 o}C, Calculate the following

1. The rate of heat transfer

2. The mass flow rate of water

3. The surface area of the heat exchanger                                                                              

14)       a. The temperature of a black surface 0.25 m² of area is 650 ^oC. Calculate

1. The total rate of energy emission (Total emissive power)

2. The intensity of normal Radiation

3. The wavelength of maximum mochromatic power.                       

Or

b. A furnace wall emits radiation at 2000K. Calculate

1. Monochromatic flux density at 1 wave length.

2. Wavelength at which emission is maximum and the corresponding emissive power.

3. Total emissive power.                                                                                

15)       a. What is meant by heat exchanger and explain the classification of heat exchangers.                                           

Or

b. Calculate the net radiant heat exchanger per m2 area for two large parallel plates at temperature of 427oC and 27oC respectively. Emissivity of hot plate is 0.9 and for cold plate is 0.6. If a polished aluminium shield is placed between them, find the percentage of reduction in the heat transfer. Emissivity of shield is 0.4.