Chapter 7: Problem 5
Define frontal area of a body subjected to external flow. When is it appropriate to use the frontal area in drag and lift calculations?
Chapter 7: Problem 5
Define frontal area of a body subjected to external flow. When is it appropriate to use the frontal area in drag and lift calculations?
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Get started for freeDuring flow over a given body, the drag force, the upstream velocity, and the fluid density are measured. Explain how you would determine the drag coefficient. What area would you use in calculations?
Wind at \(30^{\circ} \mathrm{C}\) flows over a \(0.5\)-m-diameter spherical tank containing iced water at \(0^{\circ} \mathrm{C}\) with a velocity of \(25 \mathrm{~km} / \mathrm{h}\). If the tank is thin-shelled with a high thermal conductivity material, the rate at which ice melts is (a) \(4.78 \mathrm{~kg} / \mathrm{h} \quad\) (b) \(6.15 \mathrm{~kg} / \mathrm{h}\) (c) \(7.45 \mathrm{~kg} / \mathrm{h}\) (d) \(11.8 \mathrm{~kg} / \mathrm{h}\) (e) \(16.0 \mathrm{~kg} / \mathrm{h}\) (Take \(h_{i f}=333.7 \mathrm{~kJ} / \mathrm{kg}\), and use the following for air: \(k=\) \(0.02588 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \operatorname{Pr}=0.7282, v=1.608 \times 10^{-5} \mathrm{~m}^{2} / \mathrm{s}, \mu_{\infty}=\) \(\left.1.872 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}, \mu_{\mathrm{s}}=1.729 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}\right)\)
Consider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing his body to air flow. The air temperature is \(32^{\circ} \mathrm{C}\), and the fan is blowing air at a velocity of \(5 \mathrm{~m} / \mathrm{s}\). The surrounding surfaces are at \(40^{\circ} \mathrm{C}\), and the emissivity of the person can be taken to be \(0.9\). If the person is doing light work and generating sensible heat at a rate of \(90 \mathrm{~W}\), determine the average temperature of the outer surface (skin or clothing) of the person. The average human body can be treated as a 30 -cm-diameter cylinder with an exposed surface area of \(1.7 \mathrm{~m}^{2}\). Evaluate the air properties at film temperature of \(35^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\).
A \(20 \mathrm{~mm} \times 20 \mathrm{~mm}\) silicon chip is mounted such that the edges are flush in a substrate. The substrate provides an unheated starting length of \(20 \mathrm{~mm}\) that acts as turbulator. Airflow at \(25^{\circ} \mathrm{C}(1 \mathrm{~atm})\) with a velocity of \(25 \mathrm{~m} / \mathrm{s}\) is used to cool the upper surface of the chip. If the maximum surface temperature of the chip cannot exceed \(75^{\circ} \mathrm{C}\), determine the maximum allowable power dissipation on the chip surface.
Air at \(15^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\) flows over a \(0.3\)-m-wide plate at \(65^{\circ} \mathrm{C}\) at a velocity of \(3.0 \mathrm{~m} / \mathrm{s}\). Compute the following quantities at \(x=0.3 \mathrm{~m}\) : (a) Hydrodynamic boundary layer thickness, \(\mathrm{m}\) (b) Local friction coefficient (c) Average friction coefficient (d) Total drag force due to friction, \(\mathrm{N}\) (e) Local convection heat transfer coefficient, W/m² \(\mathbf{K}\) (f) Average convection heat transfer coefficient, W/m² \(\mathrm{K}\) (g) Rate of convective heat transfer, W
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