[mactech001]

Dear all,

i've a confusion on fluid-thermal analysis i'm performing. Using CFX, i find that if i increase the flowrate, the heat taken away by the fluid is decreased, because temperature difference between outlet and inlet have decreased. i think this is not logical. but why does CFX gives me such a result?

i used a simple pipe model and the wall of the pipe is heated with heat flux applied. Pipe model is OD=45mm, ID=32mm, length L=100mm. Heated area on pipe is on OD wall, where this area's boundaries are 30mm from both ends of the pipe as shown in attached picture, pipe.jpg. Fluid is WATER.

Meshing of fluid is finer than solid. GGI mesh connection is used. i've used both SST and BSL Reynolds Stress turbulence option for the fluid domain, and for both turbulence models with 10L/min and 15L/min flowrates, the 15L/min flowrate resulted in a lower inlet/outlet temperature difference than the 10L/min result.

I used the following expression to calculate temperature difference:
areaAve(T)@out-areaAve(T)@in

Then heat taken away by fluid:

Qfluid=massFlow()@in*(areaAve(T)@out-areaAve(T)@in)*4181.7

where 4181.7 is specific heat capacity of water.

Does the results makes sense please? Does higher flowrate rejects less heat?

Funny enough, the Wall Heat Transfer Coefficient looks logical where the Wall HTC is higher at higher flowrate. See attached pipe02.jpg (left pic: HTC for 10L/min, right pic: HTC for 15L/min)

Thank you for your kind attention. i really really like to read any comments/suggestions/ideas you may have.