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HW7 solution

# HW7 solution - Problem 3 A 0.1-W small cylindrical resistor...

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Problem 3. A 0.1-W small cylindrical resistor mounted on a lower part of a vertical circuit board is 0.75 cm long and has a diameter of 0.5 cm. The view of the resistor is largely blocked by another circuit board facing it, and the heat transfer through the connecting wires is negligible. The air is free to flow through the large parallel flow passages between the boards as a resul of natural convection currents. If the air temperature at the vicinity of the resistor is 45 0 C, determine the approximate surface temperature of the resistor. T-inf (degC) = 45 (guess) T-surf (C) = 82.5 SI units T-film (K) = 336.9 k = 0.02833 delta= 0.005 nu = 1.93E-05 beta= 0.0029682 Pr = 0.7195 Ra= 2.64E+02 Q calc Ts h=* 22.706759 0.199 110 degC 0.1619 100 deg C Surface Area = 0.0001178 0.109 85 deg C 0.1 82.5 deg C Q calculated= 0.1003153 = hA(Ts-Tinf) *Table 15-1, components on a circuit board Note, the solution to this problem is highly dependent on the choice of equation for h. Using the equation for the horizontal cylinder yields quite a different result: T-inf (degC) = 45 (guess) T-surf (C) = 106.15 SI units T-film (K) = 348.725 k = 0.02919 delta= 0.005 nu = 2.05E-05 beta= 0.0028676 Pr = 0.7166 Ra= 3.66E+02
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