ME6502 Heat and Mass transfer Important questions

Anna University, Chennai

Department of Mechanical Engineering

Subject Name : Heat and Mass Transfer

Subject Code  : ME6502

Regulation      : 2013

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Click: Unit I Conduction 2 Marks with Answers      Click: Unit II Convection 2 Marks with Answers Click:Unit III Phase change Heat Transfer and Heat exchangers 2 Marks with Answers    Click: Unit IV Radiation 2 Marks with Answers Click: Unit V Mass Transfer 2 Marks with Answers

                           PART-B (16 Marks )

1.A furnace wall consists of three layers. The inner layer of 10 cm thickness is made of firebrick (k =1.04 W/mK). The intermediate layer of 25 cm 

 thickness is made of masonry brick (k = 0.69 W/mK) followed by a 5 cm thick concrete wall (k = 1.37 W/mK). When the furnace is in continuous 

operation the inner surface of the furnace is at 800°C while the outer concrete surface is at 50°C. Calculate the rate of heat loss per unit area of the 

 wall, the temperature at the interface of the firebrick and masonry brick and the temperature at the interface of the masonry brick and concrete. 

2.A plane wall 20 cm thickness generates heat at the rate of 5 x 104 W/m3 when an electric current is passed through it. The convective heat transfer 

coefficient between each face of the wall and the ambient air is 60 W/m2K. Determine

(a) The surface temperature 

(b) The maximum temperature in the wall. Assume ambient air temperature to be 25°C and the thermal conductivity of the wall material to be 16 W/mK.

 3. A 6 cm long copper rod (k = 300 W/mK) 6mm in diameter is exposed to an environment at 20°C. The base temperature of the rod is maintained at 160°C. The heat transfer co-efficient is 20 W/m2K. Calculate the heat given by the rod and efficiency and effectiveness of the rod. 

4.A steel ball 100 mm diameter was initially at 50ºC and is placed in air which is at 35°C. Calculate time required to attain 400°C .Take  k(steel) = 35 W/mK, Cp = 0.46 kJ/kgK, ρ = 7800 kg/m3, h = 10W/m2K.  

5.A large vertical plate 5 m high is maintained at 100°C and exposed to air at 30°C Calculate the convection heat transfer coefficient. 

6.    0.6 kg of water per minute is passed through a tube of 2 cm diameter, it is found to be heated from 20 C to 60 C. the heating is achieved by condensing steam on the surface of the tube and subsequently the surface temperature of the tube is maintained at 90 C. Determine the length of the tube required for fully developed flow.                                                                                                      

7. 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. 

8.Define the term convection and distinguish between free and forced convection                

                                                                                                                                                                9.A counter flow concentric tube heat exchanger is used to cool engine oil (C=2130 J/kgK ) from 160ºC to 60ºC with water available at 25ºC as the cooling medium. The flow rate of cooling water through the inner tube of 0.5m is 2kg/s while the 

 flow rate of oil through the outer annulus, outer diameter is 0.7 m is also 2kg/s. If overall heat transfer coefficient is 250 W/m2 K, how long must the heat exchanger to be meet it’s cooling requirement. 

10.  A black body at 3000 K emits radiation. Calculate the following:

i) Monochromatic emissive power at 7m.m wave length.

ii) Wave length at which emission is maximum.

iii) Total emissive power,

iv) Calculate the total emissive of the furnace if it is assumed as a real 

surface having emissivity equal to 0.85.           

11. A vertical flat plate in the form of fin is 500m in height and is exposed to steam at 40 C. calculate the atmospheric pressure. If surface of the plate is maintained at 60

following.

1. The film thickness at the trailing edge

2. Overall heat transfer coefficient 

3. Heat transfer rate

4. The condensate mass flow rate. 

12.Water is boiled at the rate of 24 kg/h in a polished copper pan, 300 mm in diameter, at atmospheric pressure. Assuming nucleate boiling conditions calculate the temperature of the bottom surface of the pan.

13. In a double pipe counter flow heat exchanger, 10,000 kg/hr of an oil having a specific heat of 2095 J/kgK is cooled from 80ºC to 50ºC by 8000 kg/hr of water entering at 25ºC. Determine the heat exchanger area for an overall heat transfer coefficient of 300 W/m2K. 

14. 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 to 

50ºC. Assume hi=ho=600 W/m2K. Calculate the area of heat exchanger using €-NTU approach.

15.Emissivities of two large parallel plates maintained at T1  K and T2  K are 0.6 and

0.6 respectively. Heat transfer is reduced 75 times when a polished aluminium radiation shields of emissivity 0.04 are placed in between them. Calculate the number of shields required. 

                                                           

16.Two black square plates of size 2 by 2 m are placed parallel to each other at a distance of 0.5 m. One plate is maintained at a temperature of 1000C. Find the heat exchange between the plates .

17.Hydrogen gases at 3 bar and 1 bar are separated by a plastic membrane having thickness 0.25 mm. the binary diffusion coefficient of hydrogen in the plastic is 9.1*10-3  m2/s. The solubility of hydrogen in the membrane is 2.1 ´ 10-3 is assumed. Calculate the following uniform temperature condition of 20 C

1. Molar concentration of hydrogen on both sides

2. Molar flux of hydrogen

3. Mass flux of hydrogen 

 18.Define convective mass transfer and it’s types. Also mention the dimensionless parameters used on it.                                                                                                                                 

                                                                 

19. An open pan of 150 mm diameter and 75 mm deep contains water at 25 C and 50% R.H. Calculate the evaporation rate of water in grams per hour exposed to atmospheric air at 25C .

20. An open pan 210 mm in diameter and 75 mm deep contains water at 25 exposed to dry atmospheric air. Calculate the diffusion coefficient of water in air. Take the rate of diffusion of water vapour is 8.52 10-4 kg/h

21. Write short notes on   isothermal evaporation water into air.                 

22.Emissivities of two large parallel plates maintained at T1  K and T2  K are 0.6 and 0.6 respectively. Heat transfer is reduced 75 times when a polished aluminium radiation shields of emissivity 0.04 are placed in between them. Calculate the number of shields required. 

                                                       

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