Exam 2: Energy, Energy Transfer, and General Energy Analysis

A 40-cm-long, 0.6-cm-diameter electric resistance wire is used to determine the convection heat transfer coefficient in air at 25°C experimentally. The surface temperature of the wire is measured to be 150°C when the electric power consumption is 90 W. If the radiation heat loss from the wire is calculated to be 30 W, the convection heat transfer coefficient is

A 1.5-m² black surface at 120°C is losing heat to the surrounding air at 30°C by convection with a convection heat transfer coefficient of 18 W/m².°C, and by radiation to the surrounding surfaces at 10°C. The total rate of heat loss from the surface is

A 200 W vacuum cleaner is powered by an electric motor whose efficiency is 70%. (Note that the electric motor delivers 200 W of net mechanical power to the fan of the cleaner). The rate at which this vacuum cleaner supplies energy to the room when running is

A 1.5-kW electric resistance heater in a room is turned on and kept on for 20 min. The amount of energy transferred to the room by the heater is

Heat is transferred steadily through a 0.15-m thick 3 m by 5 m wall whose thermal conductivity is 1.2 W/m.°C. The inner and outer surface temperatures of the wall are measured to be 18°C to 4°C. The rate of heat conduction through the wall is