Water enters a circular tube whose walls are maintained at constant temperature at a specified flow rate and temperature. To compute the rate of heat transfer between the walls and the fluid, using the flow Nusselt number the following temperature difference is used;Â Â
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a.  The difference between the inlet and outlet water bulk temperature |  | b.  The difference between the inlet water bulk temperature and the tube wall temperature |
c.  | The log mean temperature difference |  | d.  The difference between the average water bulk temperature and the tube temperature |
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