hw1-solution - W/m 2 K. The surface emissivity of the...

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1. A square isothermal chip is of width w = 5 mm on a side and is mounted in a substrate such that its side and back surfaces are well insulated, while the front surface is exposed to the flow of a coolant at T = 20 ºC. From the reliability consideration, the chip temperature must not exceed T = 85ºC. (1) If the coolant is air and the corresponding convection coefficient is h = 200 W/m 2 K, what is the maximum allowable chip power? (2) If the coolant is a dielectric liquid for which h = 3000 W/m 2 K, what is the maximum allowable power? w = 5 mm Coolant chip Power = q = hA(T s - T ) (1) Power = 200 × (0.005 × 0.005) (85 – 20) = 0.325 W (2) Power = 3000 × (0.005 × 0.005) (85 – 20) = 4.875 W 2. A sphere 4.0 cm in diameter is heated to a temperature of 150ºC and is enclosed in a large room at 20ºC. Air in the room is also at 20ºC. The average convection heat transfer coefficient is 9.8
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Unformatted text preview: W/m 2 K. The surface emissivity of the sphere is 0.65. Calculate the total heat loss due to both convection and radiation. q = hA(T s – T air ) + σɛA(T s 4 – T sur 4 ) = (9.8)(4π × 0.02 2 )(150 – 20) + (5.67×10-8 )(0.65)(4π × 0.02 2 ) × (423 4 – 293 4 ) = 6.40 + 4.56 = 10.96 W 3. A wall 2.0 cm thick is to be constructed from material which has an average thermal conductivity of 1.3 W/m K. The wall is to be insulated with material having an average thermal conductivity of 0.035 W/m K, so that the heat loss will not exceed 1830 W/m 2 . Assuming that the temperature difference across the combined wall is 1270ºC, calculate the thickness of insulation required. ins w o i k x k x T T q " 035 . 3 . 1 02 . 1270 1830 x 0238 . x m 4....
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hw1-solution - W/m 2 K. The surface emissivity of the...

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