ME 6502 Heat and Mass Transfer, Third year, Department of Mechanical Sandwich Engineering, Model Exam Questions

Answer ALL questions

PART A – (10×2=20 marks)

1.      Define fin efficiency and fin effectiveness.

2.      List down the three types of boundary conditions.

3.      What do you understand by black body and grey body?

4.      Define radiosity and emissive power.

5.      Indicate the concept of boundary layer.

6.      Define Nusselt number and Stanton number.

7.      What do you understand by filmwise and dropwise condensation?

8.      How are heat exchangers classified based on flow arrangement?

9.      State Fick’s law of diffusion and mention its importance.

10.  Define mass concentration and molar concentration.

PART B – (5x16=80 marks)

11(a)  The inner dimensions of a freezer cabinates are 60*60cm. the cabinates wall consists of two 2mmthick steel wall k=40w/mk separated by a 4cm layer of  fiber glass insulation k=0.049w/mk.the inside  temperature is to be maintained at 15^0cthe outside on a hot summer day is 45º C.calculate the maximum amount of heat transfer, assuming  a heat transfer coewfficent of 10 w/mk.both  on side and outside  of the  cabinate also calculate the outer surface of the cabinate                                                                                                                                                                                     )                                                          (Or)

           (b.Derive general heat conduction equation in Cartesian co-ordinate?

12 (a) (i) Consider laminar hydrodynamically fully developed coquette flow (that is flow between parallel plates) fluid being viscous. The upper plate at temperature T₂ moves with a velocity U while the lower plate at T₁ less than T₂ is stationary. The distance between the plates is w. Write the appropriate governing flow and energy equations for the above and hence obtain expressions for the velocity and temperature profiles across the flow.

(ii) Air at 20ºC is flowing along a heated plate at 134ºC at a velocity of 3 m/s. The palte is 2 m long and 1.5 m wide. Calculate the thickness of the hydrodynamic boundary layer and the skin friction coefficient at 40 cm from the leading edge of the plate. The kinematic viscosity of air at 20ºC is 15.06x10^-6 m²/s.

(Or)

(b) (i) Distinguish between free and forced convection giving examples.

(ii) A steam pipe 10 cm OD runs horizontally in a room at 23ºC. Take outside temperature of pipe as 165ºC. Determine the heat loss per unit length of the pipe. Pipe surface temperature reduces to 80ºC with 1.5 cm insulation. What is the reduction in heat loss?

13(a) (i) Discuss the various regimes of pool boiling heat transfer.

(ii) An aluminium pan of 15 cm diameter is used to boil water and the water depth at the time of boiling is 2.5 cm. The pan is placed on an electric stove and the heating element raises the temperature of the pan to 110ºC. Calculate the power input for boiling and the rate of evaporation. Take C_sf = 0.0132.

(Or)

(b) (i) Describe the principle of parallel flow and counter flow heat exchangers showing the axial temperature distribution.

(ii) A parallel flow heat exchanger has hot and cold water stream running through it, the flow rates are 10 and 25 kg/min respectively. Inlet temperatures are 75ºC and 25ºC on hot and cold sides. The exit temperature on the hot side should not exceed 50ºC. Assume h_i = h_o = 600 W/m²K. Calculate the area of heat exchanger using Є–NTU approach.

14(a) (i) Explain absorptivity, reflectivity and transmissivity.

(ii) Two large parallel plates at 800 K and 600 K have emissivities of 0.5 and 0.8 respectively. A radiation shield having an emissivity of 0.1 on one side and an emissivity of 0.05 on the other side is placed between the plates. Calculate the heat transfer rate by radiation per square meter with and without the radiation shield. Comment on the results.

(Or)

(b) (i) Determine the view factor (F_1-4) for the figure shown below.

(ii) Discuss the radiation characteristics of carbon dioxide and water vapour.

15(a) A mixture of 0₂ and N₂ with their partial pressures in the ratio 1.21 to 0.79 is in a container at 25⁰C. Calculate the molar concentration the mass density and the mass fraction of each species for a total pressure of 1 bar. What would be the average molecular weight of the mixture?                                                                                                                                 

                                                                               Or

(b) Two large tanks, maintained at the same temperature and pressure are connected by a circular 0.15m diameter direct, which is 3m length. One tank contains a uniform mixture of 60 mole % ammonia and 40 mole % air and the other tank contains a uniform mixture of 20 mole % ammonia and 80 mole %air. The system is at 273 k and 1.013×10⁵ pa. Determine the rate of ammonia transfer between the two tanks. Assuming a steady state mass transfer.