Consider an aluminum pan used to cook food on top of an electric range. The bottom section of the pan has thickness of L = 0.3 cm and diameter of D = 20 cm. The electric heating unit transfers 800 W of power to the pan. During steady operation, the temperature of the inner surface of the pan is measured to be 110°C. Which of the following boundary conditions is correct during this cooking process; Water

Consider an aluminum pan used to cook food on top of an electric range. The bottom section of the pan has thickness of L = 0.3 cm and diameter of D = 20 cm. The electric heating unit transfers 800 W of power to the pan. During steady operation, the temperature of the inner surface of the pan is measured to be 110°C. Which of the following boundary conditions is correct during this cooking process; Water

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.67P: 1.67 In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical...

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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.
1.67 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 to an outlet temperature of and with exit air velocities of 1.0 m/s and 1.5 m/s. Estimate the electric power required for the heating coil to meet these conditions, assuming that heat loss from the outside of the dryer duct is neglected.
8.15 A mercury bath at is to be heated by immersing cylindrical electric heating rods, each 20 cm tall and 2 cm in diameter. Calculate the maximum electric power rating of a typical rod if its maximum surface temperature is .
A heater plate measures 40 cm x 40 cm in the tank and is kept at a constant temperature of 70 oC.Show the calculations in which case it can heat the oil at a temperature of 30 oC in the tank. For oil: ν=0.00026 m2 / s, k=0.15 W / mK, Pr=2900
Question 2 Consider a 1.5-m-high electric hot-water heater that has a diameter of 40 cm and maintains the hot water at 60 °C. The tank is located in a small room whose average temperature is 27 °C, and the heat transfer coefficients on the inner and outer surfaces of the heater are 50 and 12 W/m2⋅K, respectively. The tank is placed in another 46-cm-diameter sheet metal tank of negligible thickness, and the space between the two tanks is filled with foam insulation (k = 0.03 W/m⋅K). The thermal resistances of the water tank and the outer thin sheet metal shell are very small and can be neglected. The price of electricity is $0.08/kWh, and the homeowner pays $280 a year for water heating. Determine the fraction of the hot-water energy cost (in %) of this household that is due to the heat loss from the tank. Heat transfers through the top and bottom of the tank are negligible. In continuation of Question 2, hot-water tank insulation kits consisting of 3-cm-thick fiberglass insulation…
A cylindrical tube has an inner diameter of 2 cm and a wall thickness of 1 cm. The tube is evacuated. In the center, there is a radiation source along its axis that sends 1e^5 W/m^2 to the inner surface of the tube. K for the tube wall is 2 W/m K. If the outer surface of the tube is cooled with water at 298 K with a convective heat transfer coefficient of 100 W/m^2 K, determine the minimum temperature rating for the tube material for safe operation.
A 2-m x 1.8-m section of wall of an industrial furnace burning natural gas is not insulated, and the temperature at the outer surface of this section is measured to be 80°C. The temperature of the furnace room is 30°C, and the combined convection and radiation heat transfer coefficient at the surface of the outer furnace is 10 W/m² • °C. It is proposed to insulate this section of the furnace wall with expanded perlite insulation (k = 0.052 W/m • °C) in order to reduce the heat loss by 90%. Assuming the outer surface temperature of the metal section still remains at about 80°C, determine the thickness of the insulation that needs to be used. ANSWER:_______cm
A refrigerator door of area 0.6 sq.m consists of 25 mm of insulation ontop of a thin metal sheet. The film heat transfer coefficients inside andoutside the refrigerator are 10.0 and 15.0 W m-2 K-1, respectively, and the thermal conductivity of the insulation is 0.25 W m-1 K-1. If the working temperature of the refrigerator is 0oC and ambient temperatureis 20oC, determine the heat flow through the refrigerator door and thetemperature of the inside surface of the door.
The 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 oC
The roof of a house is electrically heated. This roof is 6 m long, 8 m highwide and 0.25 m thick, and is made with a flat layer of concrete whose conductivityis ? = 0.8 ?⁄? ?. The temperatures of the inner and outer surfaces are15 and 4 °C, respectively, for a period of 10 hrs. Determine a) the reason for the lossof heat through the roof that night and b) the cost of that heat loss to the homeownerof the house, if the cost of electricity is $0.08/kWh.
A 2-m × 1.8-m section of wall of an industrial furnace burning natural gas is not insulated, and the temperature at the outer surface of this section is measured to be 80°C. The temperature of the furnace room is 30°C, and the combined convection and radiation heat transfer coefficient at the surface of the outer furnace is 10 W/m2·C. It is proposed to insulate this section of the furnace wall with perlite insulation (k = 0.052 W/m·C) in order to reduce the heat loss by 90 percent, Assuming the outer surface temperature of the metal section still remains at about 80°C, determine the thickness of the insulation that needs to be used.
Consider a house that has a 10-m x 40-m base and a 4-m-high wall. All four walls ofthe house have an R-value of 2.50 m2 · °C/W. The two 10-m x 4-m walls have nowindows. The third wall has four windows made of 0.5-cm-thick glass (k = 0.78 W/m ·°C), 2 m x 2 m in size. The fourth wall has the same size and number of double-panedwindows, with a 1.2-cm-thick stagnant air space (k = 0.026 W/m ·°C) enclosedbetween two 0.5-cm-thick glass layers. The thermostat in the house is set at 22 °C andthe average temperature outside at that location is 2°C during the four months longheating season, Solve for the money this household will save per heating season byconverting the single-pane windows to double-pane windows. Consider that the houseis electrically heated and the price of electricity is $0.10/kWh.