Question

5 A hollow sphere of inside radius 30 mm and outside radius 50 mm is (5 electrically heated at its inner surface at a constant rate of 10 KW/m2. The outer surface is exposed to a fluid at 30°C, with heat transfer coefficient of 170 W/m2.K. The thermal conductivity of the material is 20 W/m.K. Calculate *.inner and outer surface temperatures 301.5°C Not all the above 30.15°C. 57.14'c. O 60.14'c. O تقع The handle of a saucepan, 30 cm long and 2 cm in diameter is partially (6 immersed in boiling water at 100°C. The average unit conductance over the handle surface is 7.35 W/m2.K in the kitchen air at 24°C. The cook is likely to grasp the last 10 cm of the handle and hence, the temperature of this portion should not exceed 32°C. What should be the material conductivity of * handle? The handle may be treated as a fin of insulated tip 10.0 W/m.K. 10.72 W/m.K. 10.5 W/m.K. 10.4 W/m.K. Not all the above

Answer 1

Take # Ti-To q. Ai= 1 (rori 47 km, da - 10 x 0.0 1130g 2 T-30 0.053054 0.18724 so T= 301.7°C Take : 0 - 201T-T2 RW q .A;= more 47kr, 52 301.7 -T2 7 T2W 10 x8.0 1130g = 0.05305 -) T2 = 241.7°C Quessin Grium, L=30 cm = 0.3m D = 2 cm = 0.02m To = 100°c h = 7.35 W/m2K "To = 24°c Tatx=0.2m a=02m = 320c

rolun For insulated tip Cosh m (2-x) The Top To-Top IT ဆီ 0. Cosh mL 32-24 cash in (0.3-0.2) coshm (0.3) P100-24 0.105- Cosh (alm) Cosh (0.3m) Cosh (0.3m) x 0.105= cosh (0.1m) ott from het & trial method we get m= 11.71 hp KA 11.71 7.35XFXD K & A 2² = 11.71 7.35 x 4 KX 0.02 11.71 ving 80 K= 10.72 W/mk