Thermodynamics

5–103 A long roll of 2-m-wide and 0.5-cm-thick 1-Mn manganesesteel plate (r 7854 kg/m 3 and c p 0.434 kJ/kg · °C)coming off a furnace at 820°C is to be quenched in an oil bathat 45°C to a temperature of 51.1°C. If the metal sheet is movingat a steady velocity of 10 m/min, determine the requiredrate of heat removal from the oil to keep its temperature constantat 45°C. Answer: 4368 kWFurnace10 m/min5–104 Reconsider Prob. 5–103. Using EES (or other)software, investigate the effect of the movingvelocity of the steel plate on the rate of heat transfer from theoil bath. Let the velocity vary from 5 to 50 m/min. Plot therate of heat transfer against the plate velocity, and discuss theresults.5–105 The components of an electronic system dissipating180 W are located in a 1.4-m-long horizontalduct whose cross section is 20 cm 20 cm. Thecomponents in the duct are cooled by forced air that entersthe duct at 30°C and 1 atm at a rate of 0.6 m 3 /min and leavesat 40°C. Determine the rate of heat transfer from the outersurfaces of the duct to the ambient. Answer: 63 W25°C30°C0.6 m 3 /minOil bath, 45°CFIGURE P5–103Naturalconvection180 W1.4 mFIGURE P5–105Steelplate40°C5–106 Repeat Prob. 5–105 for a circular horizontal duct ofdiameter 10 cm.5–107E The hot-water needs of a household are to be metby heating water at 55°F to 180°F by a parabolic solar collectorat a rate of 4 lbm/s. Water flows through a 1.25-in-diameterthin aluminum tube whose outer surface is black-anodized inorder to maximize its solar absorption ability. The centerlineof the tube coincides with the focal line of the collector, anda glass sleeve is placed outside the tube to minimize the heatlosses. If solar energy is transferred to water at a net rate ofChapter 5 | 265400 Btu/h per ft length of the tube, determine the requiredlength of the parabolic collector to meet the hot-waterrequirements of this house.5–108 Consider a hollow-core printed circuit board 12 cmhigh and 18 cm long, dissipating a total of 20 W. The widthof the air gap in the middle of the PCB is 0.25 cm. If thecooling air enters the 12-cm-wide core at 32°C and 1 atm at arate of 0.8 L/s, determine the average temperature at whichthe air leaves the hollow core. Answer: 53.4°C5–109 A computer cooled by a fan contains eight PCBs,each dissipating 10 W power. The height of the PCBs is 12cm and the length is 18 cm. The cooling air is supplied by a25-W fan mounted at the inlet. If the temperature rise of airas it flows through the case of the computer is not to exceed10°C, determine (a) the flow rate of the air that the fan needsto deliver and (b) the fraction of the temperature rise of airthat is due to the heat generated by the fan and its motor.Answers: (a) 0.0104 kg/s, (b) 24 percentAirinletFIGURE P5–109AiroutletPCB, 10 W5–110 Hot water at 90°C enters a 15-m section of a castiron pipe whose inner diameter is 4 cm at an average velocityof 0.8 m/s. The outer surface of the pipe is exposed to thecold air at 10°C in a basement. If water leaves the basementat 88°C, determine the rate of heat loss from the water.5–111 Reconsider Prob. 5–110. Using EES (or other)software, investigate the effect of the inner pipediameter on the rate of heat loss. Let the pipe diameter varyfrom 1.5 to 7.5 cm. Plot the rate of heat loss against thediameter, and discuss the results.5–112 A 5-m 6-m 8-m room is to be heated by anelectric resistance heater placed in a short duct in the room.Initially, the room is at 15°C, and the local atmospheric pressureis 98 kPa. The room is losing heat steadily to the outsideat a rate of 200 kJ/min. A 200-W fan circulates the airsteadily through the duct and the electric heater at an average