JNTU III B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008
( Common to Mechanical Engineering and Automobile Engineering)
1. Identify the different modes of heat transfer in the following systems/ operations:
(a) Steam raising in a steam boiler.
(b) Air/ water cooling of an I.C. engine cylinder.
(c) Condensation of steam in a condenser
(d) Heat loss from a thermos flask.
(e) Heating of water in a bucket with an immersion heater
(f) heat transfer from a room heater.
(g) Heating of earth surface by sun.
2. (a) Define the overall heat transfer coefficient? Obtain the expression composite wall with three layer with convective conditions over the wall?
(b) A wall consists of three layers of 0.2 m concreter, 0.08 m of fibre glass insulation and 0.015 m gypsum board (0.04 W/mK). The convective heat transfer coefficients at inside and outside surfaces are 15 and 45 W/m2K respectively. The inside and outside surface temperatures are 25 0C and -10 0C respectively. Calculate the overall heat transfer coefficients for the wall and heat loss per unit area.
3. (a) The thermal conductivity of plane slab varies with temperature with the following relation: k = k0(1+bT+cT2). If the wall thickness is L and the surface temperatures are T1 and T2, Estimate the heat flux through the wall?
(b) Determine the heat loss per meter length through a thin walled tube of stainless steel (k=19 W/mK) with 20 mm ID and 40 mm OD which is covered with a 30 mm layer of asbestos insulation(0.2 W/mK). The inside and outside temperatures of this configuration are 600 0C and 100 0C respectively.
4. (a) A flat electrical heater of 0.4 m × 0.4 m size is placed vertically in still air at 20 0C. The heat generated is 1200 w/m2. Determine the value of convective heat transfer co-efficient and the average plate temperature.
(b) Explain Grashoff number significance in natural convective heat transfer.
5. (a) A thin 80 cm long and 8 cm wide horizontal plate is maintained at a temperature of 130 0C in a large tank full of water at 70 0C. Estimate the rate of heat input into the plate necessary to maintain the temperature of 130 0C.
(b) Differentiate velocity and thermal Boundary layers by a neat diagram.
6. (a) Distinguish between filmwise and dropwise condensation. Which of the two gives a higher heat transfer coefficient? Why?
(b) Dry saturated steam at a pressure of 2.5 bar condenses on the surface of a vertical tube of height 1.5m. The tube surface temperature is 120 0C. Estimate the thickness of the condensate film and the local heat transfer coefficient at a distance of 0.3m from the upper end of the tube.
7. Two large parallel planes having emissivities 0.3 and 0.5 are maintained at temperatures of 800K and 400K respectively. A radiation shield having an emissivity of 0.5 on both sides is placed between the two plates. Calculate:
(a) the heat transfer rate per unit area if the shield were not present
(b) the heat transfer rate per unit area with the shield present
(c) the temperature of the shield.
8. (a) Derive an expression for logarithmic mean temperature difference for the case of counter flow exchanger.
(b) A liquid chemical flows through a thin walled copper tube of 12 mm diameter at the rate of 0.5 kg/sec water flows in opposite direction at the rate 0.37kg/sec through the annular space formed by this tube and a tube diameter of 20 mm. The liquid chemical enters and leaves at 100 0C and 60 0C, while water enters at 10 0C. Find the length of tube required. Also find the length of tube required if the water flows in the same direction as liquid chemical. The properties of water and liquid chemical are:
PROPERTIES 27 0C LIQUID CHEMICALAT 80 0C WATERAT ?, Kg/m3 1078 995?, Kg/m-Sec2 3200*10?6 853 *10?6Cp, J/Kg-K 2050 4180K,W/mK 0.261 0.614