Download me 010 602 heat and mass transfer (m)

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Reg. No ......................................
Name ..........................................
B.TECH. DEGREE EXAMINATION, NOVEMBER 2005
Sixth Semester
Branch—Mechanical Engineering
ME 010 602 HEAT AND MASS TRANSFER (M)
(New Scheme—2002 Admission onwards)
Time : Three Hours
Maximum : 100 Marks
Use of approved data book is permitted.
Assume any missing data if required.
Part A
Answer all questions. Each
question carries 3 marks.
1. Discuss the mechanism of heat conduction in gases and solids.
2. Why is the insulated-tip solution important for the fin problems?
3. What is the basic procedure in setting up a numerical solution to a two-dimensional conduction
problem?
4. Describe the relation between fluid friction and heat transfer.
What is the approximate criterion for transition to turbulence in a free -convection
boundary layer?
(3 x 5 - 15 marks)
5.
Part B
Answer all questions. Each
question carries 5 marks.
6. Differentiate between thermally black and visually black bodies with examples.
7. Explain the laws of radiation.
8. Why does a "mixed" or "unmixed" fluid arrangement influence heat exchanger performance ?
9. What advantage does the effectiveness—NTU method have over the LMTD method?
10. Why cannot atmospheric evaporation rates be calculated with ordinary molecular-diffusion
equations?
(5 x 5 - 25 marks)
Part C
11. Derive an expression for the temperature distribution in a sphere of radius rQ with uniform heat
generation q and constant surface temperature Tw.
Or
12. Aim. long steel plate is well insulated on its sides, while the top surface is at 100 °C and the
bottom surface is convectively cooled by a fluid at 20°C. Under steady state conditi on without
heat generation, a thermocouple at the mid point of the plate reveals a temperature of 85°C.
What is the value of the heat transfer coefficient at the bottom surface ?
Turn over
2
13. Consider the velocity boundary layer profile for flow over a flat plate to be of the form
u = Cj + C2y + Cgj2 + C^3 . Applying appropriate boundary conditions, obtain an expression for
the velocity profile in terms of the boundary layer thickness 8 and free stream velocity U.Q. using
the integral form of the boundary layer momentum equation, obtain an expression for the boundary
layer thickness in terms of the local Reynolds number.
Or
14. Engine oil at a rate of 0.02 kg./s flows through a 3 mm. diameter tube 30 m. long. The oil has an
inlet temperature of 60°C, while the tube wall temperature is maintained at 100°C by steam
condensing on its outer surface.
(a)
Estimate the average heat transfer coefficient for internal flow of the oil.
(b)
Determine the outlet temperature of the oil.
15. Consider a concentric tube heat exchanger characterized by a uniform overall heat
coefficient and operating under the following conditions.
m
Cp
TV
T0
kg./s
J/kg. K
°C
°C
Cold fluid ...
0.125
4200
40
95
Hot fl ui d ...
0.125
2100
210
What is the maximum possible heat transfer rate ? What is the heat exchanger effectiveness ?
Should the heat exchanger be operated in parallel flow or in counter flow ? What is the ratio of the
required areas for these two flow conditions ?
Or
16. In an automobile radiator, water, which has a flow rate of 0.05 kg./s enters at 400 K. and is to
leave at 330 K. The water is cooled by air which enters at 0.75 kg./s and 300 K. If the overall heat
transfer coefficient is 200 W/m. 2 K, what is the required heat transfer surface area. What is its
effectiveness ?
17. Two Parallel disks, each 1 m. in diameter are spaced 25 cm. apart. One disk is maintained at
300°C while the other disk is insulated on the back side. Both disks have an emissivity of 0.5 and
are placed in a large room which is maintained at 30°C. Calculate the radiation energy lost by the
300°C disk.
Or
18. Two rectangles 50 by 50 cm. are placed perpendicularly with a common edge. One surface has
Tj = 1000 K. , EJ = 0.6, while the other surface is insulated and in radiant balance with a large
surrounding room at 300 K. Determine the temperature of the insulated surface and heat lost by
the surface at 1000 K.
3
19. Water at a rate of 1.0 liter/hr in the form of 0.4 mm. droplets at 25°C is sprayed on a hot surface
at 280°C with an impact velocity of 3 m./s. Estimate the maximum heat transfer which can be
achieved with this arrangement.
Or
20. An open pan 15 cm. in diameter and 7.5 cm. deep contains water at 25 °C and is exposed to
atmospheric air at 25°C and 50 percent relative humidity. Calculate the evaporation rate of water.
(5 x 12 - 60 marks)