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2. (30 points) Computer CPU chips have highly integrated IC circuits that lead to excessive heat generation during operation. Consider a 1 cmx1 cm CPU chip that generates a constant heat flux of qw = 40 W/cm2. Suppose we want to cool the CPU by blowing air over its surface using a fan. The air temperature is To = 20 °C. The maximum air velocity that the fan can generate is 10 m/s. a. What is the average heat transfer coefficient over the CPU surface at the maximum air velocity? b. Is our proposed method sufficient to cool the CPU? The CPU chip will fail if it exceeds a wall temperature of 120 °C. The properties of air at 70 °C p [kg/m³ 1.03 Cp U/kgK] 1009 k [W/mK 0.029 [Pas] 2.05x10-5 v [m2/s] 2x10-5 a m2/s 2.86x10-4 Pr - 0.7 3. (40 points) A heat exchanger is formed from a section of thin-walled copper tubing (total length = 12 m, diameter = 1.5 cm), which is used to reject heat from an internal water flow (flow rate = 0.01 kg/s, inlet temperature = 70 °C) to the ambient (still air at To = 20 °C). It is placed horizontally. The flow is hydrodynamically and thermally fully developed. a. Calculate the inside average heat transfer coefficient hw. You can assume that the wall temperature is close to the fluid temperature of 70 °C, and evaluate the properties at 340 K. This case is neither a constant wall temperature nor a constant heat flux. For a rough approximation, we take the Nusselt number to be the average of the constant temperature and constant flux situations. b. Calculate the outside average heat transfer coefficient ha. Again, you can assume that the wall temperature is approximately 70 °C. c. What is the fluid temperature at the exit? d. What is the total heat energy released from the water to the ambient? The properties of air at 70 °C p [kg/m³ 1.03 Cp U/kgK] 1009 k (W/mK 0.029 Pas 2.05x10-5 v [m2/s 2x10-5 a [m²/s] 2.86x10-5 0.7

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