For a uniform-diameter shaft, does hollowing the shaft increase or decrease the critical speed and why? Determine the ratio of the critical speed for a solid shaft of diameter d to the critical speed of a hollow shaft of inner diameter 0.8d and outer diameter d? g

Athin walled concentric tube exchanger is used to cool engine oil from 160°c to 60°c with water that is available at 25°c acting as a coolant. the oil and water flow rates are each at 2 kg/s, and the diameter of the inner tube is 0.5 m and the corresponding value of the overall heat transfer coefficient is 250 w/m2. oc. how long must the heat exchanger be to accomplish the desired cooling? cpwater=4.187 kj/kg-candcpengine el=2.035 kj/kg·°c, oil . 120]