Thermodynamics

104 | Thermodynamics2–95 The inner and outer surfaces of a 5-m 6-m brick wallof thickness 30 cm and thermal conductivity 0.69 W/m · °C aremaintained at temperatures of 20°C and 5°C, respectively.Determine the rate of heat transfer through the wall, in W.Brickwall20°C 5°C30 cmconvection heat transfer coefficient and surface emissivity onthe heat transfer rate from the ball. Let the heat transfer coefficientvary from 5 to 30 W/m 2 · °C. Plot the rate of heattransfer against the convection heat transfer coefficient forthe surface emissivities of 0.1, 0.5, 0.8, and 1, and discuss theresults.2–102 Hot air at 80°C is blown over a 2-m 4-m flat surfaceat 30°C. If the convection heat transfer coefficient is 55W/m 2 · °C, determine the rate of heat transfer from the air tothe plate, in kW.2–103 A 1000-W iron is left on the ironing board with itsbase exposed to the air at 20°C. The convection heat transfercoefficient between the base surface and the surrounding airis 35 W/m 2 · °C. If the base has an emissivity of 0.6 and asurface area of 0.02 m 2 , determine the temperature of thebase of the iron.FIGURE P2–95 1000-Wiron2–96 The inner and outer surfaces of a 0.5-cm-thick 2-m 2-m window glass in winter are 10°C and 3°C, respectively.If the thermal conductivity of the glass is 0.78 W/m · °C,determine the amount of heat loss, in kJ, through the glassover a period of 5 h. What would your answer be if the glasswere 1-cm thick?2–97 Reconsider Problem 2–96. Using EES (or other)software, investigate the effect of glass thicknesson heat loss for the specified glass surface temperatures. Letthe glass thickness vary from 0.2 to 2 cm. Plot the heat lossversus the glass thickness, and discuss the results.2–98 An aluminum pan whose thermal conductivity is237 W/m · °C has a flat bottom whose diameter is 20 cmand thickness 0.4 cm. Heat is transferred steadily to boilingwater in the pan through its bottom at a rate of 500 W. Ifthe inner surface of the bottom of the pan is 105°C, determinethe temperature of the outer surface of the bottom ofthe pan.2–99 For heat transfer purposes, a standing man can bemodeled as a 30-cm diameter, 170-cm long vertical cylinderwith both the top and bottom surfaces insulated and with theside surface at an average temperature of 34°C. For a convectionheat transfer coefficient of 15 W/m 2 · °C, determine therate of heat loss from this man by convection in an environmentat 20°C. Answer: 336 W2–100 A 5-cm-diameter spherical ball whose surface ismaintained at a temperature of 70°C is suspended in the middleof a room at 20°C. If the convection heat transfer coefficientis 15 W/m 2 · C and the emissivity of the surface is 0.8,determine the total rate of heat transfer from the ball.2–101 Reconsider Problem 2–100. Using EES (orother) software, investigate the effect of theAir20°CFIGURE P2–1032–104 A thin metal plate is insulated on the back andexposed to solar radiation on the front surface. The exposedsurface of the plate has an absorptivity of 0.6 for solar radiation.If solar radiation is incident on the plate at a rate ofα = 0.625°C700 W/m 2FIGURE P2–104