Chapter 4: Problem 47
How can we use the one-term approximate solutions when the surface temperature of the geometry is specified instead of the temperature of the surrounding medium and the convection heat transfer coefficient?
Chapter 4: Problem 47
How can we use the one-term approximate solutions when the surface temperature of the geometry is specified instead of the temperature of the surrounding medium and the convection heat transfer coefficient?
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Get started for freeA steel casting cools to 90 percent of the original temperature difference in \(30 \mathrm{~min}\) in still air. The time it takes to cool this same casting to 90 percent of the original temperature difference in a moving air stream whose convective heat transfer coefficient is 5 times that of still air is (a) \(3 \mathrm{~min}\) (b) \(6 \mathrm{~min}\) (c) \(9 \mathrm{~min}\) (d) \(12 \mathrm{~min}\) (e) \(15 \mathrm{~min}\)
The temperature of a gas stream is to be measured by a thermocouple whose junction can be approximated as a \(1.2\)-mm-diameter sphere. The properties of the junction are $k=35 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \rho=8500 \mathrm{~kg} / \mathrm{m}^{3}\(, and \)c_{p}=320 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}$, and the heat transfer coefficient between the junction and the gas is \(h=110 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Determine how long it will take for the thermocouple to read 99 percent of the initial temperature difference.
A 6-cm-high rectangular ice block $(k=2.22 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\( and \)\alpha=0.124 \times 10^{-7} \mathrm{~m}^{2} / \mathrm{s}$ ) initially at \(-18^{\circ} \mathrm{C}\) is placed on a table on its square base \(4 \mathrm{~cm} \times 4 \mathrm{~cm}\) in size in a room at $18^{\circ} \mathrm{C}$. The heat transfer coefficient on the exposed surfaces of the ice block is \(12 \mathrm{~W} / \mathrm{m}^{2}\). \(\mathrm{K}\). Disregarding any heat transfer from the base to the table, determine how long it will be before the ice block starts melting. Where on the ice block will the first liquid droplets appear? Solve this problem using the analytical one-term approximation method.
Consider a spherical shell satellite with outer diameter of \(4 \mathrm{~m}\) and shell thickness of \(10 \mathrm{~mm}\) that is reentering the atmosphere. The shell satellite is made of stainless steel with properties of $\rho=8238 \mathrm{~kg} / \mathrm{m}^{3}, c_{p}=468 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\(, and \)k=13.4 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}$. During the reentry, the effective atmosphere temperature surrounding the satellite is \(1250^{\circ} \mathrm{C}\) with a convection heat transfer coefficient of $130 \mathrm{~W} / \mathrm{m}^{2}\(. \)\mathrm{K}$. If the initial temperature of the shell is \(10^{\circ} \mathrm{C}\), determine the shell temperature after $5 \mathrm{~min}$ of reentry. Assume heat transfer occurs only on the satellite shell.
Consider the freezing of packaged meat in boxes with refrigerated air. How do \((a)\) the temperature of air, \((b)\) the velocity of air, \((c)\) the capacity of the refrigeration system, and \((d)\) the size of the meat boxes affect the freezing time?
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