ASEN 3113 Thermodynamics and Heat Transfer东盟3113个热力学与传热.doc

ASEN 3113 Thermodynamics and Heat TransferHomework #5Assigned: October 26, 2006Due: November 2, 2006 (class time)(Total points noted in each section; must clearly show equations with values and units, drawings, assumptions, etc.) 3. (20 points, 15 points for equations, 2/3 points for correct answer (a)/(b) Consider a 1.5 m high and 2 m wide glass window whose thickness is 6 mm and which has a thermal conductivity of k = 0.78 W/(m* C). Determine (a)the steady rate of heat transfer through this glass window and (b) the temperature of its inner surface for a day during which the room is maintained at 24 C while the temperature of the outdoors is -5 C. Take the convection heat transfer coefficients on the inner and outer surfaces of the window to be h1 = 10 W/(m2* C) and h2 = 25 W/(m2* C), and disregard any heat transfer by radiation.Solution:The steady rate of heat transfer through the window glass is The inner surface temperature of the window glass can be determined from4. (20 points, 15 points for equations, 5 points for correct answer) Problem 8-80 in Cengal, See Figure P8-80A 4-m high and 6-m wide wall consists of long 18 cm X 30 cm cross-section horizontal bricks k = 0.72 W/(m* C) separated by 3-cm-thick plaster layers k = 0.22 W/(m* C). There are also 2-cm-thick plaster layers on each side of the wall, and 2 cm thick rigid foam k = 0.026 W/(m* C) on the inner side of the wall. The indoor and the outdoor temperatures are 22 C and -4 C, and the convection heat transfer coefficients on the inner and the outer sides are h1 = 10 W/(m2* C) and h2 = 20 W/(m2* C), respectively. Assume one-dimensional heat transfer and disregarding radiation, determine the rate of heat transfer through the wall.SolutionThe steady rate of heat transfer through the wall per 0.33 m2 is The steady rate of heat transfer through the entire wall becomes5. (20 points, 15 points for equations, 2/3 points for correct answer (a)/(b) A 6-m internal diameter spherical tank made of 1.5 cm-thick stainless steel k = 15 W/(m* C) is used to store iced water at 0 C. The tank is located in a room whose temperature is 20 C. The walls of the room are also at 20 C. The outer surface of the tank is black (emissivity, =1), and heat transfer between the outer surface of the tank and the surroundings is by natural convection and radiation. The convection heat transfer coefficients at the inner and the outer surfaces of the tank are 80 W/(m2* C) and 10 W/(m2* C), respectively. Determine (a) the rate of heat transfer to the iced water in the tank and (b) the amount of ice at 0 C that melts during a 24-h period. The heat of fusion of water at atmospheric pressure is hif = 333.7 kJ/kg.SolutionThe inner and outer surface areas of the sphere are:We assume the outer surface temperature T2 to be 4 C after comparing convection heat transfer coefficients at the inner and outer surfaces of the tank. With this assumption, the radiation heat transfer coefficient can be determined from:The individual thermal resistances are(a) Then the steady rate of