Problem 167
A vertical double-pane window consists of two sheets of glass separated by a \(1.2-\mathrm{cm}\) air gap at atmospheric pressure. The glass surface temperatures across the air gap are measured to be \(278 \mathrm{~K}\) and $288 \mathrm{~K}$. If it is estimated that the heat transfer by convection through the enclosure is \(1.5\) times that by pure conduction and that the rate of heat transfer by radiation through the enclosure is about the same magnitude as the convection, the effective emissivity of the two glass surfaces is (a) \(0.35\) (b) \(0.48\) (c) \(0.59\) (d) \(0.84\) (e) \(0.72\)
Problem 168
A horizontal \(1.5\)-m-wide, \(4.5\)-m-long double-pane window consists of two sheets of glass separated by a \(3.5-\mathrm{cm}\) gap filled with water. If the glass surface temperatures at the bottom and the top are measured to be \(60^{\circ} \mathrm{C}\) and \(40^{\circ} \mathrm{C}\), respectively, the rate of heat transfer through the window is (a) \(27.6 \mathrm{~kW}\) (b) \(39.4 \mathrm{~kW}\) (c) \(59.6 \mathrm{~kW}\) (d) \(66.4 \mathrm{~kW}\) (e) \(75.5 \mathrm{~kW}\) (For water, use $k=0.644 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \quad \operatorname{Pr}=3.55\(, \)\nu=0.554 \times 10^{-6} \mathrm{~m}^{2} / \mathrm{s}, \beta=0.451 \times 10^{-3} \mathrm{~K}^{-1}$. Also, the applicable correlation is \(\mathrm{Nu}=0.069 \mathrm{Ra}^{1 / 3} \mathrm{Pr}^{0.074}\).)
Problem 169
A vertical \(0.9\)-m-high and \(1.5\)-m-wide double-pane window consists of two sheets of glass separated by a \(2.0-\mathrm{cm}\) air gap at atmospheric pressure. If the glass surface temperatures across the air gap are measured to be \(20^{\circ} \mathrm{C}\) and \(30^{\circ} \mathrm{C}\), the rate of heat transfer through the window is (a) \(16.3 \mathrm{~W}\) (b) \(21.7 \mathrm{~W}\) (c) \(24.0 \mathrm{~W}\) $\begin{array}{ll}\text { (d) } 31.3 \mathrm{~W} & \text { (e) } 44.6 \mathrm{~W}\end{array}$ (For air, use $k=0.02551 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \quad \operatorname{Pr}=0.7296\(, \)\nu=1.562 \times 10^{-5} \mathrm{~m}^{2} / \mathrm{s}\(. Also, the applicable correlation is \)\left.\mathrm{Nu}=0.42 \mathrm{Ra}^{1 / 4} \operatorname{Pr}^{0.012}(H / L)^{-0.3} .\right)$
Problem 170
Two concentric cylinders of diameters \(D_{i}=30 \mathrm{~cm}\) and $D_{o}=40 \mathrm{~cm}\( and length \)L=5 \mathrm{~m}$ are separated by air at 1 atm pressure. Heat is generated within the inner cylinder uniformly at a rate of \(1100 \mathrm{~W} / \mathrm{m}^{3}\), and the inner surface temperature of the outer cylinder is \(300 \mathrm{~K}\). The steady-state outer surface temperature of the inner cylinder is (a) \(402 \mathrm{~K}\) (b) \(415 \mathrm{~K}\) (c) \(429 \mathrm{~K}\) (d) \(442 \mathrm{~K}\) (e) \(456 \mathrm{~K}\) (For air, use $k=0.03095 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \quad \operatorname{Pr}=0.7111\(, \)\left.\nu=2.306 \times 10^{-5} \mathrm{~m}^{2} / \mathrm{s}\right)$
Problem 171
Write a computer program to evaluate the variation of temperature with time of thin square metal plates that are removed from an oven at a specified temperature and placed vertically in a large room. The thickness, the size, the initial temperature, the emissivity, and the thermophysical properties of the plate as well as the room temperature are to be specified by the user. The program should evaluate the temperature of the plate at specified intervals and tabulate the results against time. The computer should list the assumptions made during calculations before printing the results. For each step or time interval, assume the surface temperature to be constant and evaluate the heat loss during that time interval and the temperature drop of the plate as a result of this heat loss. This gives the temperature of the plate at the end of a time interval, which is to serve as the initial temperature of the plate for the beginning of the next time interval. Try your program for \(0.2-\mathrm{cm}\)-thick vertical copper plates of $40 \mathrm{~cm} \times 40 \mathrm{~cm}\( in size initially at \)300^{\circ} \mathrm{C}\( cooled in a room at \)25^{\circ} \mathrm{C}$. Take the surface emissivity to be \(0.9\). Use a time interval of \(1 \mathrm{~s}\) in calculations, but print the results at \(10-\mathrm{s}\) intervals for a total cooling period of \(15 \mathrm{~min}\).
Problem 173
Contact a manufacturer of aluminum heat sinks and obtain its product catalog for cooling electronic components by natural convection and radiation. Write an essay on how to select a suitable heat sink for an electronic component when its maximum power dissipation and maximum allowable surface temperature are specified.
