Heat transfer for parallel flow over a flat plate with an extremely rough surface was found to follow a mathematical model of the form = 0.04 . where nux is the local value of the nusselt number at a location x measured from the leading edge of the plate. obtain an expression for the ratio of the average convection coefficient to the local convection coefficient.

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]