University of Ottawa

MCG 3110

QUESTION 5 (17 marks) A pump, illustrated below, induces the water flow in the hot tub. An electric motor (in yellow) drives the pump. The motor efficiency is such that it dissipates 250W in the form of heat. Assume that the motor external shell is a 30 cm long cylinder, with an outer diameter of 10 cm. We will neglect the fact that one part of the external shell is not exposed to the airflow of ambient air due to the mounting configuration. We will also assume that the motor external shell and fins made out of a material that has a thermal conductivity of 45 W/m.K. a) If the motor operates in air at 35°C and assuming that we can neglect the fins, determine the external shell temperature at steady state for windy day with the wind speed at 0.5 m/s. (7 marks) b) If the motor operates in air at 35°C and assuming there are 40 fins distributed uniformly, that are 2 mm thick (t) and 1 cm long (L) and 30 cm wide (w), determine the external shell temperature at steady state. You can assume that the fin efficiency is 98% and that the convection coefficient is 27 W/m2 .K. Make sure you draw the thermal circuit of the system. (10 marks)