Most of the energy we consume as food ¡s converted tothermal energy in the process of performing all our bodilyfunctions and is ultimately lost as heat from our bodies.Consider a person who consumes 2100 kcal per day (notethat what are commonly referred to as food calories areactually kilocalories), of which 20000 kcal is converted tothermal energy. (The remaining 100 kcal is used to dowork on the environment.) The person has a surface area of
(a) The person is in a room at 20°C, with a convectionheat transfer coefficient of
(b) If the temperature of the environment were 33°C,what rate of perspiration would be needed to maintain a comfortable skin temperature of 33°C?
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- Flue gases from an incinerator are released to atmosphere using a stack that is 0.6 m in diameter and 10.0 m high. The outer surface of the stack is at 40°C and the surrounding air is at 10°C. Determine the rate of heat transfer from the stack assuming (a) there is no wind and (b) the stack is exposed to 20 km/h winds.arrow_forwardConsider two concentric horizontal cylinders of diameters 55 cm and 65 cm, and length 125 cm. The surfaces of the inner and outer cylinders are maintained at 54C and 106C,respectively. If the annular cavity is filled with (a) water and (b) Air. With natural transport between rollersfind the speed of heat transfer.arrow_forwardThe top surface of the passenger car of a train moving at a velocity of 70 km/h is 2.8 m wide and 8 m long. The top surface is absorbing solar radiation at a rate of 200 W/m2, and the temperature of the ambient air is 30°C. Assuming the roof of the car to be perfectly insulated and the radiation heat exchange with the surroundings to be small relative to convection, determine the equilibrium temperature of the top surface of the car.arrow_forward
- Four power transistors, each dissipating 12 W, are mounted on a thin vertical aluminum plate 22 cm * 22 cm in size. The heat generated by the transistors is to be dissipated by both surfaces of the plate to the surrounding air at 25°C, which is blown over the plate by a fan. The entire plate can be assumed to be nearly isothermal, and the exposed surface area of the transistor can be taken to be equal to its base area. If the average convection heat transfer coefficient is 25 W/m2·K, determine the temperature of the aluminum plate. Disregard any radiation effects.arrow_forwardA 1.2-in-outer-diameter pipe is to span across a river at a 115-ft-wide section while being completely immersed in water. The average flow velocity of the water is 8 ft/s, and its temperature is 70°F. Determine the drag force exerted on the pipe by the river.arrow_forwardPlease help with this questionHot carbon dioxide exhaust gas at 1 atm is being cooled by flat plates. Thegas at 220°C flow is parallel over the upper and lower surfaces of a 3-m-longflat plate at a velocity of 3 m/s. If the flat plate surface temperature ismaintained at 80°C.Determine the local convection heat transfer coefficient at 1 m fromthe leading edge and calculate the average convection heat transfer coefficient over theentire plate.arrow_forward
- Consider a 50-cm-diameter and 95-cm-long hot water tank. The tank is placed on the roof ofa house. The water inside the tank is heated to 80℃ by a flat-plate solar collector during theday. The tank is then exposed to windy air at 18℃ with an average velocity of 40 km/h duringthe night. Estimate the temperature of the tank after a 45-min period. Assume the tank surfaceto be at the same temperature as the water inside, and the heat transfer coefficient on the topand bottom surfaces to be the same as that on the side surface. Evaluate the air properties at50℃. (Answer: 69.9 ℃)arrow_forwardAn automotive engine can be approximated asa 0.4-m-high, 0.60-m-wide, and 0.7-m-long rectangularblock. The bottom surface of the block is at a temperatureof 75°C and has an emissivity of 0.92. The ambient airis at 5°C, and the road surface is at 10°C. Determine therate of heat transfer from the bottom surface of the engineblock by convection and radiation as the car travels at avelocity of 60 km/h. Assume the flow to be turbulent over theentire surface because of the constant agitation of the engineblock. How will the heat transfer be affected when a 2-mmthickgunk (k = 3 W/m?K) has formed at the bottom surface asa result of the dirt and oil collected at that surface over time?Assume the metal temperature under the gunk still to be 75°C.arrow_forwardConsider laminar flow of air across a hot circular cylinder. At what point on the cylinder will the heat transfer be highest? What would your answer be if the flow were turbulent?arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning