Question

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;  

                                                           

 

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

Answer 1

option - C

it is said that water enters a tube whose walls are maintained at a constant temperature. When tube wall temperature is kept constant, unlike constant heat flux case, the analysis becomes difficult because the temperature difference between the wall and water temperature varies along the length of the tube. Inlet water temperature being at low compared to wall temperature; water gets heated slowly as they flow along the length of the pipe and temperature increases. Nu =3.66

Heat transfer =

where = heat transfer coefficient.

   = wall area.

   = Logarithmic mean temperature.

 

So heat transfer is proportional to logarithmic mean temperature difference.