Code No.: 4181 FACULTY OF ENGINEERING B.E. III/IV Year (Mech.) II Semester (Main) Examination, April/May 2008 HEAT TRANSFER Time: 3 Hours] [Max. Marks: 75 Answer all questions of Part A. Answer five questions from Part B. Part A - (Marks: 25) 1. Derive a differential equation for heat flow through a pipe neglecting axial conduction. 2. A plane slab is maintained at temperatures Tl and T2 on either side. The thermal conductivity of slab various according to K =Ko (1 + I3T). where Ko & 13 constants. are Derive an expression for temperature distribution within the slab. 3. Give three examples of heat generating systems. A plane slab generates heat uniformly. Sketch the temperature distribution within the slab if surface temperature on either side are same. 4. What is the physical significance of Biot number and what is the implication of Biot No = O? 5. Explain why heat transfer rates are higher when the flow is turbulent compared to laminar, in case of flow over a flat plate. 6. State Buckingham 1ttheorem, and its importance in convection heat transfer. 7. Differentiate between dropwise and filmwise condensation. What are the conditions under which dropwise condensation would occur? 8. Draw the boiling curve and mark various regimes. Why boilers are operated in the Nucleate boiling region? 9. A heat exchanger heats a fluid from 30°C to 50°C while hot fluid cools from 100°C to 60°C. Which of the following two arrangement gives smaller surface area required? (a) Parallel flow (b) Counter flow. 10. State planks law of radiation & draw the Plank's distribution between emissive power and wavelength of radiation at various temperature. How would you calculate total radiation at any temperature? [P.T.O. 2 4181 Part B - (Marks: 5 x 10 = 50) 11. A steam pipe (K = 45 W /m-K) has 70 mm ID and 85 mm OD. It is covered with two layers of insulation, the inside being asbestos (K = 0.15 W/m-K) & of 35 mm thick and over insulation is magnesia 25 mm thick (K = 0.075 W /m-K). The heat transfer coefficients for inside and outside being 220 W / m2 -oK and 6.5 W / m2 -oK respectively. If the steam temperature of steam is 350°C and the ambient temperature is 30° C; calculate the following: 10 (a) Heat loss from pipe for unit length (b) Interface temperature. 12. Calculate the amount of energy required to solder together two very long pieces of copper wires 1.625 mm diameter with solder that melts at 195°C. The . surrounding air temperature is 24°C and heat transfer coefficient between the wire and surrounding being 17 W /m2-K. The thermal conductivity of copper is 335W/m-K. 10 13. An iron plate of thickness 5 cm is initially at 250° C. Suddenly both surfaces are exposed to a fluid at 50° C and h = 500 W /m2-K. Determine 10 (a) Centre line temperature of plate after 120 seconds (b) Heat transferred to the fluid from one square meter surface area of plate during this time. Properties of Iron ore K = 60 W /m-K, Cp = 460 J /kgO K, P = 7850 kg/m3, a = 1.6 x 10- 5 m2/S 14. Air at 295° K flows over a flat plate at a velocity of 180 m/min. If the plate is 0.5 mx 0.25 m in size, fmd the average heat transfer coefficient and heat loss per hour if air flow is parallel to 0.5 m side of plate. 10 15. (a) Using electrical circuit analogy derive an expression for net radiation transfer between two infinitely long parallel plates. 5 (b) A 10 cm diameter pipe carrying steam at 400°C passes through a large room at 27°C. 5 The emissing of pipe E = 0.8 Estimate the heat loss from the pipe. 16. Using Nusselts theory of laminar film condensation derive an expression for Nusselt's number. 10 3 4181 17. A surface condenser condenses 2S000 kg/hr steam at 3SoC. The cooling water inlet and outlet temperatures are lSoC and 2SoC respectively. The tubes used for in condenser are 2 cm diameter and the velocity of water inside the tubes is 2 m/S. Assume heat transfer coefficient on steam side = SOOO 1m2 -K. Heat W transfer coefficient on waterside = 4000 W1m2 K. Neglect resistance due to tubewall. Determine 10 (a) Total surface area in m2 required (b) Number of tubes required (c) Length of each tube.