**SIXTH SEMESTER B.TECH DEGREE EXAMINATION,**

**Branch: Mechanical Engineering**

**HEAT AND MASS TRANSFER**

**Time: 3 Hours Max. Marks: 100**

**Part A**

**(Answer all questions; each carries 2 marks)**

2. What is conduction shape factor? How it is related to thermal resistance?

3. Define Biot number and give its significance in Newtonian heating.

4. State Buckingham's Pi theorem.

5. Distinguish between thermal and hydrodynamic boundary layer.

6. Define effectiveness of the fin.

7. What is LMTD and discuss its importance in heat exchanger problem?

8. Draw boiling curve for water and show different regimes.

9. Define Wien's distribution law.

10. Define the terms (a) Mass concentration (b) Molar concentration

(10 X 2 = 20 Marks)

**Part B**

**(Answer any ONE question from each Module; each carries 20 marks)**

**Module - I**

(b) A furnace wall is made up of three layers of thicknesses 250 mm, 100 mm and 150 mm

with thermal conductivities of 1.65, K and 9.2 W/m.K respectively. The inside is exposed

to gases at 1250 oC with a convection coefficient of 25 W/m2

.K. and the inside surface is

at 1100 oC, the outside surface is exposed to air at 25 oC with convection coefficient of 12

W/m2

.K. Determine (a) the unknown thermal conductivity K (b) the overall heat transfer

coefficient (c) all the intermediate temperatures.

OR

12. (a) Derive an expression for steady state temperature distribution in a slab with internal heat

generation.

(b) A steel ball (specific heat = 0.46 kJ/kgK, and thermal conductivity 35W/mK) having 5

cm diameter and initially at a uniform temperature of 450 oC is suddenly placed in a

control environment in which the temperature is maintained at 100 oC. Calculate the time

required for the ball to attain a temperature of 150 oC.

**Module - II**

13. (a) Air at 20 oC at atmosphere pressure flows over a flat plate at a velocity of 3.5m/s. If the

plate is 5m long and 2m wide. Calculate the following (i) Length of the plate over which the boundary layer is laminar

(ii) Thickness of the boundary layer

(iii) Shear stress on the location where boundary layer is laminar

(iv) Total drag force on the both sides of the plate where boundary layer is laminar

(Take, Density = 1.205 kg/m3

; kinematic viscosity= 15.06x10-6 m

2

/s)

(b) When 0.6kg of water/min is passed through a tube of 2cm diameter. It is found to be

heated from 20 oC to 60 oC. 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 oC.

Determine the length of the tube required for fully developed flow.

OR

14. (a) Air at 40 oC flows over a tube with a velocity of 30 m/s. The tube surface temperature is

120 oC. Calculate the heat transfer coefficient for the following cases:

(i) Tube is square with a side of 6cm

(ii) Tube is circular cylinder with a diameter of 6cm.

(b) Show by dimensional analysis tha in free convection heat transfer the Nusselt number is a

function of Grashoff number and Prandtl number.

**Module - III**

oil with a specific heat of 1.45 kJ/kg.K and mass flow rate of 0.9 kg/sec. The oil is cooled

from 230 oC to 160 oC. If the overall heat transfer coefficient is 420 W/m2

.K. Calculate

the following. (a) Rate of heat transfer, (b) Mass flow rate of water, (c) Surface area of

the heat exchanger.

(b) Derive an expression for temperature distribution in long fin.

OR

16. (a) Derive the expression for LMTD in counter flow heat exchanger.

(b) Write a note on Heat pipe.

**Module - IV**

17. (a) Two parallel, infinite grey surfaces are maintained at temperature of 127 oC and 227 oC

respectively. If the temperature of the hot surface is increased to 327 °C, by what factoris the net radiation exchange per unit area increased? Assume the emissivities of cold

and hot surface to be 0.9 and 0.7 respectively.

(b) Explain the analogy between heat and mass transfer

OR

18. (a) Determine the view factor (F1-4) for the figure shown below

(b)Explain the phenomenon of equimolar counter diffusion. Derive an expression for

equimolar counter diffusion between two gases or liquids.

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