Question

Heat transfer from a transistor may be enhanced by inserting it in an aluminum sleeve (k = 200 W/m-K) having integrally machined longitudinal fins on its outer surface. The transistor radius and height are r1 = 2 mm and H = 6 mm respectively, while the fins are of length L = r3 - r2 = 10 mm of uniform thickness t = 0.7 mm. The thickness of the sleeve base is r2 - r1 = 1 mm. The contact resistance of the sleeve-transistor interface is Rt,c'' = 10^-3 m^2.K/W. Air at T∞ = 20°C flows over the fin surface providing uniform convection with h = 25 W/m^2-K. Assume 1-D radial heat transfer. a) Sketch the equivalent thermal circuit for heat transfer from the transistor case (r = r1) to the air. Label each resistance. [1] b) Evaluate each resistance. [2] c) If the temperature of the transistor case is T1 = 80 °C, what is the rate of heat transfer from the sleeve?

          Heat transfer from a transistor may be enhanced by inserting it in an aluminum sleeve (k = 200 W/m-K) having integrally machined longitudinal fins on its outer surface. The transistor radius and height are r1 = 2 mm and H = 6 mm respectively, while the fins are of length L = r3 - r2 = 10 mm of uniform thickness t = 0.7 mm. The thickness of the sleeve base is r2 - r1 = 1 mm. The contact resistance of the sleeve-transistor interface is Rt,c'' = 10^-3 m^2.K/W. Air at T∞ = 20°C flows over the fin surface providing uniform convection with h = 25 W/m^2-K. Assume 1-D radial heat transfer.

a) Sketch the equivalent thermal circuit for heat transfer from the transistor case (r = r1) to the air. Label each resistance. [1]
b) Evaluate each resistance. [2]
c) If the temperature of the transistor case is T1 = 80 °C, what is the rate of heat transfer from the sleeve?

Added by Alfonso R.

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