In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical hairdryer is idealized as a thin-walled cylindrical duct with a 6-cm diameter with a fan at the inlet that blows air over an electric heating coil as schematically shown in the figure. The design of this appliance requires two power settings, with which the air blown over the electric heating coil is heated from the ambient temperature of
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Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
- 1.60 Two electric resistance heaters with a 20 cm length and a 2 cm diameter are inserted into a well-insulated 40-L tank of water that is initially at 300 K. If each heater dissipates 500 W, what is the time required for bringing the water temperature in the tank to 340 K? State your assumption for your analysis.arrow_forwardThe blades of a wind turbine turn a large shaft at a relatively slow speed. The rotational speed is increased by a gearbox that has an efficiency of 0.93. In turn, the gearbox output shaft drives an electric generator with an efficiency of 0.95. The cylindrical nacelle, which houses the gearbox, generator, and associated equipment, is of length L = 6 m and diameter D = 3 m. If the turbine produces P = 2.5 MW of electrical power, and the air and surroundings temperatures are T = 25 oC and Tsur = 20 oC, respectively, determine the minimum possible operating temperature inside the nacelle. The emissivity of the nacelle is 0.83, and the convective heat transfer coefficient is h = 35 W/m2 .K. The surface of the nacelle that is adjacent to the blade hub can be considered to be adiabatic, and solar irradiation may be neglected. Use Fin or N number of fins to reduce the Ts of the nacelle less than 143 oCarrow_forwardA 10 hp fan is used in a room for circulation purposes. Assuming a sealed room. that is well insulated, determine the internal energy increase in Btu after 2 h of operation.arrow_forward
- Air with a temperature of 128 °C, a density of 4.89 kg/m3 and a velocity of 121 m/s enters a nozzle with an inlet to outlet area ratio of 2:1 and exits with a density of 2.36 kg/m3. Determine the temperature at the outlet (in K).arrow_forwardWater is to be heated from 15°C to 65°C as it flows through a 3-cm-internaldiameter 5-m-long tube . The tube is equipped with an electric resistance heater that provides uniform heating throughout the surface of the tube. The outer surface of the heater is well insulated, so that in steady operation all the heat generated in the heater is transferred to the water in the tube. If the system is to provide hot water at a rate of 10 L/min, determine the power rating of the resistance heater. Also, estimate the inner surface temperature of the tube at the exit.arrow_forwardConsider the process of baking potatoes in a conventional oven. Can the hot air in the oven be treated as a thermal energy reservoir? Explain.arrow_forward
- a. A fan is to accelerate quiescent air to a velocity of 10m/s at a rate of 4 m3/s. Determine the minimum power thatmust be supplied to the fan. Take the density of air to be 1.18kg/m3. b. A classroom that normally contains 40 people is tobe air-conditioned with window air-conditioning units of 5-kW cooling capacity. A person at rest may be assumed todissipate heat at a rate of about 360 kJ/h. There are 10 lightbulbs in the room, each with a rating of 100 W. The rate ofheat transfer to the classroom through the walls and the windows is estimated to be 15,000 kJ/h. If the room air is to bemaintained at a constant temperature of 21°C, determine thenumber of window air-conditioning units required c. A vertical piston–cylinder device contains water andis being heated on top of a range. During the process, 65 Btu of heat is transferred to the water, and heat losses from theside walls amount to 8 Btu. The piston rises as a result ofevaporation, and 5 Btu of work is done by the vapor.…arrow_forwardThe components of an electronic system dissipating 180 W are located in a 4-ft-long horizontal duct whose cross section is 6 in * 6 in. The components in the duct are cooled by forced air, which enters at 85°F at a rate of 22 cfm and leaves at 100°F. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 80°F, determine (a) the heat transfer from the outer surfaces of the duct to the ambient air by natural convection and (b) the average temperature of the duct. Evaluate air properties at a film temperature of 100°F and 1 atm pressure. Is this a good assumption?arrow_forwardAn array of power transistors, dissipating 6 W ofpower each, are to be cooled by mounting them on a 25-cm *25-cm-square aluminum plate and blowing air at 35°C overthe plate with a fan at a velocity of 4 m/s. The average temperatureof the plate is not to exceed 65°C. Assuming theheat transfer from the back side of the plate to be negligibleand disregarding radiation, determine the number of transistorsthat can be placed on this plate.arrow_forward
- The hot water needs of a household are to be met by heating water at 15°C to 95°C by a parabolic solar collector at a rate of 1.8 kg/s. Water flows through a 4-cm-diameter thin aluminum tube whose outer surface is blackanodized in order to maximize its solar absorption ability. The centerline of the tube coincides with the focal line of the collector, and a glass sleeve is placed outside the tube to minimize the heat losses. If solar energy is transferred to water at a net rate of 200 W per meter length of the tube, determine the required length of the parabolic collector to meet the hot water requirements of this house. Also, determine the surface temperature of the tube at the exit.arrow_forwardA fan is to accelerate quiescent air to 3m/s at a rate 81m^3/s. If density of air is 1.7kg/m^3, the minimum power in watts that must be supplied to the fan isarrow_forwardA 4-m × 5-m × 6-m room is to be heated by an electric resistance heater placed in a short duct in the room. Initially, the room is at 15°C, and the local atmospheric pressure is 98 kPa. The room is losing heat steadily to the outside at a rate of 150 kJ/min. A 200-W fan circulates the air steadily through the duct and the electric heater at an average mass flow rate of 40 kg/min. The duct can be assumed to be adiabatic, and there is no air leaking in or out of the room. If it takes 25 min for the room air to reach an average temperature of 25°C, find the temperature rise that the air experiences each time it passes through the heater.arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning