A 50 mm*45 mm*20 mm cell phone charger (the shape of the charger is a rectangular parallelepiped) has a constant surface temperature when plugged into an electrical wall outlet. The surface of the charger is subject to a free convection heat transfer coefficient of h = 4.5 W/m^2.K with the room air having a temperature of 15°C. By assuming steady conditions and neglecting radiation, if the electric power of the charger is 0.816 W, the surface temperature of the charger is: a. 36.85°C O b. 32.14°C О с. 29.16°C ○ d. 39.15°C Clear my choice

A 50 mm45 mm20 mm cell phone charger (the shape of the charger is a rectangular parallelepiped) has a constant surface temperature when plugged into an electrical wall outlet. The surface of the charger is subject to a free convection heat transfer coefficient of h = 4.5 W/m^2.K with the room air having a temperature of 15°C. By assuming steady conditions and neglecting radiation, if the electric power of the charger is 0.816 W, the surface temperature of the charger is: a. 36.85°C O b. 32.14°C О с. 29.16°C ○ d. 39.15°C Clear my choice

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.

Chapter3: Transient Heat Conduction

Section: Chapter Questions

Problem 3.10P: 3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the...

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3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the atmosphere from outer space. If its original temperature is 38°C, the effective average temperature of the atmosphere is 1093°C, and the effective heat transfer coefficient is , estimate the temperature of the shell after reentry, assuming the time of reentry is 10 min and the interior of the shell is evacuated.
2.9 In a large chemical factory, hot gases at 2273 K are cooled by a liquid at 373 K with gas-side and liquid-side convection heat transfer coefficients of 50 and , respectively. The wall that separates the gas and liquid streams is composed of a 2-cm thick oxide layer on the gas side and a 4-cm thick slab of stainless steel on the liquid side. There is a contact resistance between the oxide layer and the steel of . Determine the rate of heat loss from hot gases through the composite wall to the liquid.
3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.
2.3 The shield of a nuclear reactor is idealized by a large 25-cm-thick flat plate having a thermal conductivity of . Radiation from the interior of the reactor penetrates the shield and there produces heat generation that decreases exponentially from a value of at the inner surface to a value of at a distance of 12.5 cm from the interior surface. If the exterior surface is kept at 38°C by forced convection, determine the temperature at the inner surface of the field. Hint: First set up the differential equation for a system in which the heat generation rate varies according to .
2.38 The addition of aluminum fins has been suggested to increase the rate of heat dissipation from one side of an electronic device 1 m wide and 1 m tall. The fins are to be rectangular in cross section, 2.5 cm long and 0.25 cm thick, as shown in the figure. There are to be 100 fins per meter. The convection heat transfer coefficient, both for the wall and the fins, is estimated to be K. With this information determine the percent increase in the rate of heat transfer of the finned wall compared to the bare wall.
The interior wall of a large, commercial walk-in type meat freezer is covered under normal operating conditions with a 2-cm thick layer of ice. One day, a power outage cuts electricity to the refrigeration system of the freezer. Estimate the time required to melt this layer of ice if it has a mass density of 700kg/m3 and a latent heat of fusion of 334 kJ/kg. Consider the air temperature inside the freezer to be 20C with a heat transfer coefficient of 2W/m2K for convection from the freezer surface to air, and clearly state the assumptions made in your calculations.
An electronic device that internally generates 600 mW of heat has a maximum permissible operating temperature of 70C. It is to be cooled in 25C air by attaching aluminum fins with a total surface area of 12cm2. The convection heat transfer coefficient between the fins and the air is 20W/m2K. Estimate the operating temperature when the fins are attached in such a way that (a) there is a contact resistance of approximately 50 K/W between the surface of the device and the fin array and (b) there is no contact resistance (in this case, the construction of the device is more expensive). Comment on the design options.
Steam is being transported in a pipe at a chemical production facility in Gebze, Kocaeli. Theouter diameter of the pipe is 8 cm, and the surrounding air temperature is 20 °C. On the surface of thepipe, the combined heat transfer coefficient is 35 W/m2*K. The surface temperature of the pipe is 150 °C.Steady-state conditions exist.(a) What is the rate of heat loss from the pipe to the surroundings?(b) The power used to generate the steam is obtained from natural gas. Perform an online search forapproximate values of the calorific value of natural gas, as well as the cost of natural gas per cubicmeter, and estimate the financial loss due to heat loss from this steam pipe within a period of oneyear. Indicate your sources clearly in your answer.
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SOLVE STEP BY STEO IN DIGITAL FORMAT A 9-cm-diameter sphere, whose surface is maintained at a temperature of 110°C, is hanging in the center of a room at 20°C. If the convective heat transfer coefficient is 15 W/m²"C, and the emissivity of the surface is 0.8, calculate the total rate of heat transfer from the sphere. Air 20°C D=9 cm 110°C
An aluminium bar of circular cross-section has a diameter of 8 cm and is initially at auniform temperature of 250 ºC. It is suddenly exposed to a convective environment at60 ºC. The surface (convection) heat-transfer coefficient is 1200 W/m2 K. The thermalconductivity, density and specific heat capacity of aluminium are 204 W/m K, 2707 kg/m3and 896 J/kg K, respectively.Calculate:a) the time taken for the centre of the bar to cool to 95 ºC.b) the heat loss per unit length during this period of time.