(4.1)--HOMEWORK-Chapter1传热学传热学传热学.doc

Conduction 1.2 The thermal conductivity of a sheet of rigid, extruded insulation is reported to be k = 0.029 W/m ? K. The measured temperature difference across a 25-mm-thick sheet of the material is T1 ? T2 = 12°C. (a) What is the heat flux through a 3 m × 3 m sheet of the insulation? (b) What is the rate of heat transfer through the sheet of insulation? (c) What is the thermal resistance of the sheet due to conduction? Convection 1.12 You’ve experienced convection cooling if you’ve ever extended your hand out the window of a moving vehicle or into a flowing water stream. With the surface of your hand at a temperature of 30°C, determine the convection heat flux for (a) a vehicle speed of 40 km/h in air at ?8°C with a convection coefficient of 40 W/m2 ? K and (b) a velocity of 0.2 m/s in a water stream at 10°C with a convection coefficient of 900 W/m2 ? K. Which condition would feel colder? Contrast these results with a heat flux of approximately 30 W/m2 under normal room conditions. 1.14 A red brick wall has inner and outer surface temperatures of 16 and 6°C, respectively. The interior and exterior air temperatures are 20 and 5°C, respectively. The inner and outer convection heat transfer coefficients are 5 and 20 W/m2 ? K, respectively. Calculate the heat flux from the interior air to the wall, from the wall to the exterior air, and from the wall to the interior air. Is the wall under steady-state conditions? Radiation 1.22 An overhead 25-m-long, uninsulated industrial steam pipe of 100-mm diameter is routed through a building whose walls and air are at 25°C. Pressurized steam maintains a pipe surface temperature of 150°C, and the coefficient associated with natural convection is h = 10 W/m2 ? K. The surface emissivity is ε = 0.8. (a) What is the rate of heat loss from the steam line? (b) If the steam is generated in a gas-fired boiler operating at an efficiency of ηf = 0.90 and natural gas is priced at Cg = $0.02 per MJ, what is the annual cost of heat loss from