Semester Exam - Set-4

5HE - Heat Transfer - October 1999

Use of Steam Tables is permitted

Part A (20 x 2 = 40 Marks)

1. What are the three modes of heat transfer? Mention about the mechanism in each mode
2. Write briefly about the effect of temperature on thermal conductivity of materials/substances
3. Write the one-dimensional steady state heat conduction for a hollow cylinder
4. Determine the heat flux across 15.2 cm thick slab when one face is kept at 500 K and the other face at 280 K. The thermal conductivity of the slab material varies linearly with temperature in accordance with the equation k = k_o(1 + BT) where k_o = 0.0346 W/m.K and B = 3.6 x 10^-3 oK^-1.
5. Write the two dimensional steady state heat transfer conduction equation
6. What is transient heat conduction?
7. Write a short note on heat transfer in packed beds
8. Write the equation giving the individual heat transfer coefficients as a function of relevant variables in the case of natural convection
9. Is a counter flow heat exchanger more efficient than a parallel heat exchanger? If so, why?
10. Draw a neat sketch of 1-2 counterflow heat exchanger
11. List out the various types of heat exchangers
12. Explain the significance of NTU in heat exchangers
13. How does a scale deposit affect the heat transfer rate?
14. Explain the concept of log mean temperature difference
15. Explain the mechanism of thermal radiation
16. Write a note on radiation between surfaces
17. What are the applications of tubular furnaces?
18. Define 'evaporation'
19. Give the statement of Duhring's rule
20. Is multiple effect operation more economical than single effect operation, in evaporation? Why?

Part B (5 x 12 = 60 Marks)

21. (a) Discuss briefly about heat conduction through a series of resistances (6)

(b) The two faces of a slab at x = 0 and x = L are kept at t₁ and t₂ ^oC respectively. The 'k' of the material is given by as a temperature dependent value by k = k_o(t² - t_o²) where t_o and k_o are constants. Deduce the expression for (i) heat flow/unit area and (ii) k_av for this material (6)

Or

22. (a) A long hollow cylinder has its inner and outer surfaces maintained at temperatures T_b and T_a respectively. The inner and outer radii are b and a respectively. Calculate the temperature profile in the solid section of the cylinder and determine the flux at both surfaces. Assume steady state condition (8)

(b) Write a short note on conduction through liquids (4)

23. (a) Heat is generated within a sphere at 2.07 x 10⁸ W/m³. The sphere is 8 cm in diameter. The surface temperature is 370 K. (i) Calculate the temperature at the center of the sphere (ii) Calculate the temperature at a radial distance of 2 cm. (8)

(b) Briefly explain the concept of heat transfer by convection (4)

Or

24. (a) Determine the rate of heat transfer between two fluids separated by a copper tube 2 mm thick, 2 cm OD and 1.5 m long, if the inner fluid (water) temperature is 30^oC and the outer fluid (steam) temperature is 100^oC. The water side film coefficient is 2000 W/m².K and the steam side heat transfer coefficient is 15000 W/m².K. The thermal conductivity of copper is 400 W/m.K (8)

(b) Write a note on drop-wise condensation and film-type condensation (4)

25. (a) Derive the expression for log mean temperature difference in heat exchangers (8)

(b) Explain the terms capacity ratio and effectiveness (4)

Or

26. Discuss briefly about the design of furnaces and design of tubular reactors
27. (a) Explain the concept of thermal radiation (6)

(b) Explain the significance of Stefan-Boltzmann's law (6)

Or

28. Write a short note on: (3 x 4 = 12)

(a) Radiation error in temperature measurement

(b) Clouds of particles and luminous flames

(c) Tubular furnaces

29. (a) Discuss about the capacity and economy of multiple effect evaporators (8)

(b) State Duhring's rule and explain its significance in evaporation (4)

Or

30. Write short notes on: (3 x 4 = 12)

(a) Single and multiple effect evaporation

(b) Boiling point elevation

(c) Types of evaporators