) The wall of a refrigerator is constructed of fiberglass insulation (k =0.035 W/m °C) sandwiched between two layers of 1-mm-thick sheet metal (k =15.1 W/m °C). The refrigerated space is maintained at 3°C, and the average heat transfer coefficients at the inner and outer surfaces of the wall are 4 W/m2 °C and 9 W/m² °C, respectively. The kitchen temperature averages 25°C. It is observed that 2. condensation occurs on the outer surfaces of the refrigerator when the temperature of the outer surface drops to 20°C. (a). Sketch the thermal circuit (b) Determine the minimum thickness of fiberglass insulation that needs to be used in the wall in order to avoid condensation on the outer surfaces.

A. A family is preparing dinner and a pizza is being toasted in an oven whose wall is made of a 0.5-cm-thick layer of steel (ks = 15 W/m.K) and a layer of brick (kb = 0.72 W/m.K). At steady state, the oven inner surface temperature of the steel layer is 450°F, and a temperature drop of 0.5°C occurs across the steel layer. If the temperature of the outer surface of the brick must be no greater than 40°C for safety concerns, determine the thickness of brick in cm, that ensures this limit is met. What is the rate of conduction per unit wall surface area (also called heat flux), in kW/m²?

A solar collector and storage tank, shown in Figure (a) below, is to be optimized to achieve minimum first cost. During the day the temperature of water in the storage vessel is elevated from 25 °C (the minimum useful temperature) to tmax, as shown in Figure (b). The collector receives 260 W/m² of solar energy, but there is heat loss from the collector to ambient air by convection. The convection coefficient is 2 W/(m²*K), and the average temperature difference during the 10-hour day is (25 + tmax)/2 minus the ambient temperature of 10 °C. The energy above the minimum useful temperature of 25 °C that is to be stored in the vessel during the day is 200,000 kJ. The density of water is 1000 kg/m³, and its specific heat is 4.19 kJ/(kg*K). The cost of the solar collector in dollars is 20A, where A is the area in square meters, and the cost of the storage vessel in dollars is 101.5C, where V is the volume in cubic meters. Storage tank Collector (a) Temperature, C tmax 25°C, lowest useful…

Assume a steel ball has just been manufactured with a diameter of 10 cm and has a temperature of 877°C. The ball is cooling in air with an ambient temperature of 25°C and a convection coefficient of 20 W/m².°C. The thermal conductivity, density, and specific heat of steel are, respectively, 40 W/m.°C, 7800 kg/m³, and 450 J/kg. °C. Find the time it will take for the steel ball to reach a temperature of 35°C in the center? Neglect radiation. 13002 A