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One experimental method of measuring an insulating material’s thermal
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- The surface area of an unclothed person is 1.50 m2, and his skin temperature is 33.0C. The person is located in a dark room with a temperature of 20.0C, and the emissivity of the skin is e = 0.95. (a) At what rate is energy radiated by the body? (b) What is the significance of the sign of your answer?arrow_forwardThe exhaust duct from a heater has an inside diameter of 114.3 mm with ceramic walls 6.4 mm thick. The average k = 1.52 W/mK. Outside this wall, an insulation of rock wool 102 mm thick is installed. The thermal conductivity of the rock wool is k = 0.046+1.56*10-4T (°C) (W/mK). The inside surface temperature of the ceramic is T1= 588.7 K, and the outside surface temperature of the insulation is T3= 311 K. Calculate the heat loss for 1.5 m of duct and the interface temperature T2between the ceramic and the insulation.Assumesteady heat transfer.Hint: The correct value of km for insulation is that evaluated at the mean temperature of T2+T3/2. Hence, for the first trial assume a mean temperature of, say, 448 K. Then, calculate the heat loss and T2. Using this new T2, calculate a new mean temperature and proceed as before.arrow_forwardA 880 cm X 1680 cm house is built on a 15.6 cm thick concrete slab of thermal conductivity 0.62 W/m.K.. If the ground temperature of the slab is 7.6ºC while the interior of the house is 21ºC. Calculate the following: a) The temperature difference in kelvin Answer for part 1 . b) The temperature gradient (ΔT/Δx) in kelvin/metre Answer for part 2 . c) The heat loss rate through the concrete slab in kilowatt Answer for part 3 .arrow_forward
- In an experiment to determine thermal conductivity of a new metal alloy, a bar of the metal is completely surrounded by insulation and exposed to 750 W of constant heat energy at one end. The bar has a radius 0.063 m. The length is 0.454 m. The entire set-up is in a cold room and once the system has reached steady state conditions, the temperature at the hot end is measured to be 70°C and the temperature at the cold end is measured to be 20°C. What is the thermal conductivity in W/m•K of the metal alloy.arrow_forwardConsider an opaque plate that is well insulated on the edges and it is heated at the bottom with an electric heater. The plate has an emissivity of 0.67, and is situated in an ambient surrounding temperature of 7°C where the natural convection heat transfer coefficient is 7 W/m2?K. To maintain a surface temperature of 80°C, the electric heater supplies 1000 W/m2 of uniform heat flux to the plate. Determine the radiosity of the plate under these conditions.arrow_forwardTwo 5-cm-diameter, 15-cm-long aluminum bars (k = 176 W/m·K) with ground surfaces are pressed against each other with a pressure of 20 atm. The bars are enclosed in an insulation sleeve and, thus, heat transfer from the lateral surfaces is negligible. If the top and bottom surfaces of the two-bar system are maintained at temperatures of 150°C and 20°C, respectively, determine (a) the rate of heat transfer along the cylinders under steady conditions and (b) the temperature drop at the interface.arrow_forward
- A rectangular window in a home has a length of 1.5 m and a height of 0.80 m. If the window allows heat to escape from the home at a rate of 2,000 watts, how thick must the window be if the inside temperature of the home is 220 C and the outside temperature is 3.00C? (Assume that the coefficient of thermal conduction of glass is 0.80 W/mK.) a. 7.1 mm b. 124 mm c. 9.1 mm d. 8.1 mm e. 11 mmarrow_forwardThe north wall of an electrically heated home is 20 ft long, 10 ft high, and 1 ft thick, and is made of brick whose thermal conductivity is k = 0.42 Btu/h·ft·°F. On a certain winter night, the temperatures of the inner and the outer surfaces of the wall are measured to be at about 62°F and 25°F, respectively, for a period of 8 h. Determine (a) the rate of heat loss through the wall that night and (b) the cost of that heat loss to the home owner if the cost of electricity is $0.07/kWh.arrow_forwardThe walls of a house consists of L=0.02 m thick plywood backed byinsulation with the same thickness. The temperature of the inside surface of the wall (the insulation side) is T2=25 oC, while the temperature at the outside surface (the plywood side) is T1= -15 oC, both being constant. The thermal conductivities of the plywood and insulation are, respectively k1=0.09 J/(s m oC) and k2=0.03 J/(s m oC).a) Find the temperature T at the plywood – insulation interface. b) If the total surface area of the walls is 70 m2, find the amount of heat the house looses in 24 hours. (Assuming all the heat losses are due to the thermal conductivity of the walls).c) Does the entropy of the house change? If it does, by how much?d) Does the entropy of the Universe change? If it does, by how much?e) If the cost of electric power is $0.15 per kilowatt hour, how much one has to pay to keep the house warm for 24 hours?arrow_forward
- One of the windows in a house has the shape of a square 1.0 m on a side. The glass in the window is 0.50 cm thick, andhas a coefficient of thermal conductivity of k = 0.84 W>1m # K2. (a) How much heat is lost through this window in oneday if the temperature difference between the inside and outside of the window is 21 C°? (b) Suppose all the dimensionsof the window—height, width, and thickness—are doubled. If everything else remains the same, by what factor doesthe heat flow change?arrow_forwardA short brass cylinder (r = 8530 kg/m3, cp =0.389 kJ/kg·K, k = 110 W/m·K, and a 5 *.39 3 1025 m2/s) of diameter 8 cm and height 15 cm is initially at a uniform temperature of 150°C. The cylinder is now placed in atmospheric air at 20°C, where heat transfer takes place by convection with a heat transfer coefficient of 40 W/m2·K. Calculate (a) the center temperature of the cylinder; (b) the center temperature of the top surface of the cylinder; and (c) the total heat transfer from the cylinder 15 min after the start of the cooling.arrow_forward
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