Chapter 13: Problem 32
What are the two methods used in radiation analysis? How do these two methods differ?
Chapter 13: Problem 32
What are the two methods used in radiation analysis? How do these two methods differ?
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Get started for freeA thermocouple shielded by aluminum foil of emissivity \(0.15\) is used to measure the temperature of hot gases flowing in a duct whose walls are maintained at \(T_{w}=380 \mathrm{~K}\). The thermometer shows a temperature reading of \(T_{\text {th }}=530 \mathrm{~K}\). Assuming the emissivity of the thermocouple junction to be \(\varepsilon=0.7\) and the convection heat transfer coefficient to be \(h=120 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the actual temperature of the gas. What would the thermometer reading be if no radiation shield was used?
Consider two infinitely long concentric cylinders with diameters 20 and \(25 \mathrm{~cm}\). The inner surface is maintained at \(700 \mathrm{~K}\) and has an emissivity of \(0.40\), while the outer surface is black. If the rate of radiation heat transfer from the inner surface to the outer surface is \(2400 \mathrm{~W}\) per unit area of the inner surface, the temperature of the outer surface is (a) \(605 \mathrm{~K}\) (b) \(538 \mathrm{~K}\) (c) \(517 \mathrm{~K}\) (d) \(451 \mathrm{~K}\) (e) \(415 \mathrm{~K}\)
\(3 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), and both natural convection and radiation heat transfer occur at the outer shield surface. To keep the ammonia inside the tube in its liquid state, determine the surrounding temperature that would maintain temperatures of the outer tube and the shield at the specified values. 13-68 PtD A hot liquid is being transported inside a long tube with a diameter of \(25 \mathrm{~mm}\). The hot liquid causes the tube surface temperature to be \(150^{\circ} \mathrm{C}\). To prevent thermal burn hazards, the tube is enclosed with a concentric outer cylindrical cover of \(5 \mathrm{~cm}\) in diameter allowing a vacuumed gap in between the two surfaces. The concentric outer cover has an emissivity of \(0.6\) and the outer surface is exposed to natural convection with a heat transfer coefficient of \(8 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) and radiation heat transfer with the surrounding at a temperature of \(20^{\circ} \mathrm{C}\). Determine the necessary emissivity of the inside tube so that the outer cover temperature is below \(45^{\circ} \mathrm{C}\) to prevent thermal burns.
A car mechanic is working in a shop whose interior space is not heated. Comfort for the mechanic is provided by two radiant heaters that radiate heat at a total rate of \(4 \mathrm{~kJ} / \mathrm{s}\). About 5 percent of this heat strikes the mechanic directly. The shop and its surfaces can be assumed to be at the ambient temperature, and the emissivity and absorptivity of the mechanic can be taken to be \(0.95\) and the surface area to be \(1.8 \mathrm{~m}^{2}\). The mechanic is generating heat at a rate of \(350 \mathrm{~W}\), half of which is latent, and is wearing medium clothing with a thermal resistance of \(0.7 \mathrm{clo}\). Determine the lowest ambient temperature in which the mechanic can work comfortably.
How can you determine the view factor \(F_{12}\) when the view factor \(F_{21}\) and the surface areas are available?
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