The power needed to overcome wind and friction drag

Chapter , Problem 11.74

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The power needed to overcome wind and friction drag associated with an automobile traveling at a constant velocity of 25 m/s is 9 kW. (a) Determine the required heat transfer area of the radiator if the vehicle is equipped with an internal combustion engine operating at an efficiency of 21%. (Assume 79% of the energy generated by the engine is in the form of waste heat removed by the radiator.) The inlet and outlet mean temperatures of the water with respect to the radiator are Tm,i 400 K and Tm,o 330 K, respectively. Cooling air is available at 3 kg/s and 300 K. The radiator may be analyzed as a cross-flow heat exchanger with both fluids unmixed with an overall heat transfer coefficient of 400 W/m2 K. (b) Determine the required water mass flow rate and heat transfer area of the radiator if the vehicle is equipped with a fuel cell operating at 50% effi- ciency. The fuel cell operating temperature is limited to approximately 85C, so the inlet and outlet mean temperatures of the water with respect to the radiator are Tm,i 355 K and Tm,o 330 K, respectively. The air inlet temperature is as in part (a). Assume the flow rate of air is proportional to the surface area of the radiator. Hint: Iteration is required. (c) Determine the required heat transfer area of the radiator and the outlet mean temperature of the water for the fuel cellequipped vehicle if the mass flow rate of the water is the same as in part (a). 1