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What is external forced convection? How does it differ from internal forced convection? Can a heat transfer system involve both internal and external convection at the same time? Give an example.

Consider a hot baked potato. Will the potato cool faster or slower when we blow the warm air coming from our lungs on it instead of letting it cool naturally in the cooler air in the room? Explain.

What is the physical significance of the Nusselt number? How is it defined?

When is heat transfer through a fluid conduction and when is it convection? For what case is the rate of heat transfer higher? How does the convection heat transfer coefficient differ from the thermal conductivity of a fluid?

During air cooling of steel balls, the convection heat transfer coefficient is determined experimentally as a function of air velocity to be \(h=17.9 V^{0.54}\) for \(0.5

The top surface of a metal plate \(\left(k_{\text {plate }}=237 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\right)\) is being cooled by air \(\left(k_{\text {air }}=0.243 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\right)\) while the bottom surface is exposed to a hot steam at \(100^{\circ} \mathrm{C}\) with a convection heat transfer coefficient of \(30 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). If the bottom surface temperature of the plate is \(80^{\circ} \mathrm{C}\), determine the temperature gradient in the air and the temperature gradient in the plate at the top surface of the plate.

A 5 -mm-thick stainless steel strip \((k=21 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \rho=\) \(8000 \mathrm{~kg} / \mathrm{m}^{3}\), and \(\left.c_{p}=570 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) is being heat treated as it moves through a furnace at a speed of \(1 \mathrm{~cm} / \mathrm{s}\). The air temperature in the furnace is maintained at \(900^{\circ} \mathrm{C}\) with a convection heat transfer coefficient of \(80 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). If the furnace length is \(3 \mathrm{~m}\) and the stainless steel strip enters it at \(20^{\circ} \mathrm{C}\), determine the surface temperature gradient of the strip at mid-length of the furnace. Hint: Use the lumped system analysis to calculate the plate surface temperature. Make sure to verify the application of this method to this problem.

What is the no-slip condition? What causes it?

What is Newtonian fluid? Is water a Newtonian fluid?

What is viscosity? What causes viscosity in liquids and in gases? Is dynamic viscosity typically higher for a liquid or for a gas?