TE_2008_May_2010.pdf - [3863 312*3863312...

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P.T.O. *3863312* [3863] – 312 T.E. (Mechanical) (Semester – I) Examination, 2010 HEAT TRANSFER (New) (2008 Course) Time : 3 Hours Max. Marks : 100 Instructions : 1) Answer 3 questions from Section I and 3 questions from Section II . 2) Answers to the two Sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Black figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed . 6) Assume suitable data, if necessary . SECTION – I Unit – I 1. a) Explain Fourier’s Law of heat conduction. 4 b) Explain variation of thermal conductivity in solids and liquids with temperature giving out reasons. 4 c) A steel tube with 5 cm ID and 7 cm OD (k = 28 W/mK), is covered with an insulation covering of thickness 15 mm (k = 0.2 W/m°C). A hot gas at temperature of 400° C with convective heat transfer coefficient of 300 W/m 2 K flows inside the tube. The outer surface of insulation is exposed to cold air at - 5°C with h = 20 W/m 2 °C. Calculate heat flow rate from the pipe and the interface temperature between steel and insulation. 8 OR 2. a) Derive general heat conduction equation in Cartesian coordinates for 3-dimensional unsteady state heat flow for an-isotropic material with uniform internal heat generation using standard notations. 8
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[3863] – 312 -2- *3863312* b) A spherical storage of steel of 20 cm ID and 30 cm OD (k = 50 W/mK) stores liquid oxygen, due to which its inside surface temperature is maintained at – 150°C. Outside surface is exposed to ambient air at 20°C with convective heat transfer coefficient of 10 W/m 2 K. Determine heat transfer rate. What shall be the percentage decrease in heat transfer rate, if the steel sphere is applied with layer of insulation (k = 0.5 W/m°C) of 25 mm thickness on its outside surface ? 8 Unit – II 3. a) Inner surface of radius r = r 1 and the outer surface of radius r = r 2 of a hollow
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