Semester Exam - Set-5

5HE - Heat Transfer - April 2000

Any missing data may be suitably assumed
Use of Steam tables and heat transfer data book are permitted

Part A (20 x 2 = 40 Marks)

1. Define thermal conductivity
2. What is thermal resistance and how is it calculated for conduction?
3. What is logarithmic mean radius for cylinder and write its significance
4. What is critical thickness of insulation for a cylinder?
5. What is Fourier number and explain its significance
6. What is nucleate boiling?
7. What is Lieden frost point?
8. What is Sieder Tate equation and why is it used?
9. What is LMTD correction factor?
10. What are compact heat exchangers?
11. When do you use double pipe and when do you use shell and tube heat exchanger?
12. What are recuperators and regenerators?
13. Define radiation shape factor
14. Differentiate black body and grey body
15. What are Fouling factors?
16. Define emissive power and intensity of radiation
17. What type of evaporator will you suggest for concentrating (a) heat sensitive materials? (b) scale forming liquids?
18. What are entrainment separators?
19. What are the various types of condensers used in evaporator system?
20. What are the advantages of thermal recompression?

    Part B (5 x 12 = 60 Marks)

21. A composite cylinder is made of 6 mm thick layers each of two materials of thermal conductivities of 30 W/m.K and 45 W/m.K. The inside is exposed to a fluid at 500^oC with a convection coefficient of 40 W/m².K and the outside is exposed to air at 35^oC with a convection coefficient of 25 W/m².K There is a contact resistance of 1 x 10^-3 m^{2 o}C/W between the layers. Determine the heat loss for a length of 2 m. The inside dia = 20 mm

    Or

22. It is desired to increase the heat dissipated over the surface of an electric device of spherical shape of 5 mm radius exposed to convection with h = 100 W/m².K by encasing it in a transparent spherical sheath of conductivity 0.04 W/m.K. Determine the diameter of the sheath for maximum heat flow. For a temperature drop of 120^oC from the device surface, determine the heat flow for the base and sheathed device

23. Steam at atmospheric pressure condenses on a 0.25 m² vertical plate. The plate temperature is 96^oC. Find the heat transfer coefficient and the mass of steam condensed per hour. The length of the plate is 50 cm. At 97^oC, r_c = 960 kg/m³; k = 0.68 W/m.K; m_c = 2.82 x 10^-4 kg/m.s; h_fg = 2255 kJ/kg

    Or

24. An air craft flies at an attitude where the temperature is -20^oC and the pressure is 0.08 atm. The air speed is 900 km/hr. The wind tank contains fuel at 20^oC. Keeping the surface at this temperature and assuming the effect of curvature to be small, determine the Reynolds number at the wing tip if the 3 m wide. Also determine the location from the leading edge where N_Re = 5 x 10⁵ if f_x = 0.0592 x N_Rex^-0.2, determine the value of local heat transfer coefficient

    Data: At -20^oC and 0.08 atm, the properties of air are, r = 0.116 kg/m³, m = 16.18 x 10^-6 N.s/m², k = 28.79 x 10^-3 W/m.K, N_Pr = 0.716

25. Explain the field of application, advantages and disadvantages of

    (a) double pipe heat exchanger

    (b) shell and tube heat exchanger

    (c) plate type heat exchanger

    (d) compact heat exchanger

    Or

26. Hot gases enter a finned tube heat exchanger at 300^oC and leave at 100^oC. It is used to heat water at a flow rate of 1 kg/s from 35^oC to 125^oC. The specific heat of exhaust hot gas is 1000 J/kg.K and the overall heat transfer coefficient based on the gas side is U_h = 100 W/m².K. Determine the required gas surface area using the NTU method

27. (a) Explain briefly the wave theory and the quantum theory. (6)

    (b) Derive the equation for heat transfer by radiation for

    (i) two large parallel plates

    (ii) one surface completely surrounded by another.

    Assume both surfaces are grey (6)

    Or

28. Calculate the radiation heat exchange for unit area between two parallel infinite walls, the temperature of which are 400^oC and 100^oC if

    (a) both are black surfaces

    (b) wall at 400^oC is black body and wall of 100^oC is grey with emissivity of 0.5

    (c) both walls are grey with emissivities of 0.8 and 0.5 respectively

    Assume Stefan-Boltzmann constant as 4.92 x 10^-8 kcal/hr.m².K⁴

29. A solution of organic colloids in water is to be concentrated from 8% to 45% in a single effect evaporator. Steam is available at a gauge pressure of 1.03 atm. A pressure of 102 mm Hg absolute is to be maintained in the vapor space. The feed rate to the evaporator is 12,000 kg/hr. The overall heat transfer coefficient can be taken as 2800 W/m².^oC. The solution has a negligible elevation in boiling point and a negligible heat of dilution. Calculate

    (a) steam consumption

    (b) the economy

    (c) the heating area required

    Data: The feed temperature is 21^oC. The specific heat of the feed solution is 3.77 kJ/kg.^oC and the latent heat of vaporization of the solution may be taken equal to that of water. Radiation losses may be neglected

    Or

30. Write short notes on:

    (a) Vapor compression evaporator

    (b) Methods of feeding of the evaporator.