01. This problem has ten multiple questions. Please choose the best answer choice for each question. Each question is worth two points. 11 consider the following statements in relation to heat transfer. (A) conductance resistance of a plane wall is directly proportional to wall thickness (B) convection resistance of a cylinder s inversely proportional to its length TC) units of thermal resistance is wic Out of the above statements; Ceny Aland 8 are correct all are incorrect y only les correct (ii) only (A) is correct (v) all are correct 12 Acurrent carrying insulated long wire (kin=0.15 W/m.C) is exposed to an ambience that has a convective heat transfer coefficient (h) of 12 W/m2C. If the highest possible heat transfer rate per unit length of the wire to the ambience is desired, what should be the outside radius of of the insulation? 10 2 mm (11) 110 mm (iii) 12.5 mm (iv) 105mm (v) 95 mm 13 A5 cm internal diameter long pipe is made of 2 cm thick steel (k=15 W/mc) its thermal resistance per unit length is to 04c/w (m) 0.006C/W (iii)0.08c/w (iv) 0.00357C/W (v) not given 14 According to Fourier's law of heat conduction, A) steady state heat transfer rate is directly proportional to temperature gradient B) steady state heat transfer rate is directly proportional to thermal resistance (C) steady state heat transfer rate is inversely proportional to thermal resistance Out of the above statements (A) and B) are correct ( (A) and (C) are correct ( ail statements are wrong (wOnly (A) is correct iv) only (B) is correct 15 Consider the following statements in relation to heat transfer by radiation (A) a medium is necessary for radiation heat transfer (B) radiation energy is transferred through electromagnetic waves (C) the radiation heat energy emitted by all real surfaces is less than the radiation energy emitted by a blackbody at the same temperature Out of the above statements, (a) all are incorrect (ii) only (Al is correct (tv) only (B) and (C) are correct (iii) only (B) is correct iv) all are correct 16 In convection heat transfer, heat transfer rate is directly proportional to the temperature difference between a surface and a moving fluid (B) heat transfer mechanism involves both the conduction and fluid motion C) heat transfer coefficient is a property of the fluid. Out of above statements; (b) only Al is correct (A) and (C) are correct (A) and (8) are correct (B) and (C) are correct (V) (A), (B), and (c) correct 17 Heat is lost ata rate of 275 W per un t area of a 15-cm thick wall with a thermal conductivity (kel of wfm. C. The temperature drop across the wall is approximately 37 5C fuly 27.5C (i) 16.0C TV 8.00 v 4.0 18 Consider a wall that consists of two layers, A and B, with the following values. Ka = 12 W/( mK), LB cm, Ks = 0.2 W/(mK), L=5 cm. If the temperature drop across the wall is 18 C, the rate of heat transfer through the wall per unit area of the wall is approximately (i) 56.8 W/m2 ( 72.1 W/m2 (ii) 114 W/m2 (iv) 201 W/m2 (v) 270 W/m 19 A 10 m long, 8 cm outer radius cylindrical steam pipe is covered with 3 cm thick cylindrical insulation with a thermal conductivity of 0.05 W/(m. K). If the rate of heat loss from the pipe is 1000 W, the temperature drop across the insulation is approximately (i) 58c 1) 101 C dili) 143 C (iv) 282 (v) 600 C 110 Consider two wolis A and B with the same surface areas and the same temperature drops across their thicknesses. The ratio of thermal conductivities ka/ks is 4 and the ratio of the wall thicknesses is alles 2. The ratio of heat transfer rate heat transfer rate A/heat transfer rate B) is approximately (1) 05 ) 2 (iv) 4 (V) 8