Chapter 9: Problem 81
Why are finned surfaces frequently used in practice? Why are the finned surfaces referred to as heat sinks in the electronics industry?
Chapter 9: Problem 81
Why are finned surfaces frequently used in practice? Why are the finned surfaces referred to as heat sinks in the electronics industry?
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Get started for freeA \(0.2-\mathrm{m} \times 0.2-\mathrm{m}\) street sign surface has an absorptivity of \(0.6\) and an emissivity of \(0.7\). Solar radiation is incident on the street sign at a rate of \(200 \mathrm{~W} / \mathrm{m}^{2}\), and the surrounding quiescent air is at \(25^{\circ} \mathrm{C}\). Determine the surface temperature of the street sign. Assume the film temperature is $30^{\circ} \mathrm{C}$.
A 12-cm-diameter and 15-m-long cylinder with a surface temperature of \(10^{\circ} \mathrm{C}\) is placed horizontally in air at $40^{\circ} \mathrm{C}$. Calculate the steady rate of heat transfer for the cases of (a) free-stream air velocity of \(10 \mathrm{~m} / \mathrm{s}\) due to normal winds and (b) no winds and thus a free-stream velocity of zero.
During a plant visit, it was observed that a \(1.5\)-m-high and \(1-\mathrm{m}\)-wide section of the vertical front section of a natural gas furnace wall was too hot to touch. The temperature measurements on the surface revealed that the average temperature of the exposed hot surface was \(110^{\circ} \mathrm{C}\), while the temperature of the surrounding air was \(25^{\circ} \mathrm{C}\). The surface appeared to be oxidized, and its emissivity can be taken to be \(0.7\). Taking the temperature of the surrounding surfaces to be \(25^{\circ} \mathrm{C}\) also, determine the rate of heat loss from this furnace. The furnace has an efficiency of 79 percent, and the plant pays \(\$ 1.20\) per therm of natural gas. If the plant operates \(10 \mathrm{~h}\) a day, 310 days a year, and thus \(3100 \mathrm{~h}\) a year, determine the annual cost of the heat loss from this vertical hot surface on the front section of the furnace wall.
Consider a double-pane window consisting of two glass sheets separated by a \(1-\mathrm{cm}\)-wide airspace. Someone suggests inserting a thin vinyl sheet between the two glass sheets to form two \(0.5\)-cm-wide compartments in the window in order to reduce natural convection heat transfer through the window. From a heat transfer point of view, would you be in favor of this idea to reduce heat losses through the window?
A vertical \(1.5-\mathrm{m}\)-high and \(3.0\)-m-wide enclosure consists of two surfaces separated by a \(0.4-\mathrm{m}\) air gap at atmospheric pressure. If the surface temperatures across the air gap are measured to be $280 \mathrm{~K}\( and \)336 \mathrm{~K}\( and the surface emissivities to be \)0.15$ and \(0.90\), determine the fraction of heat transferred through the enclosure by radiation. Arswer: \(0.30\)
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