I calculate Heat exchanger air-water. the air is heated, the air at the entrance is about 20 C.
Ansys offers the ability to define air properties (density, thermal conductivity, dynamic viscosity as constant or temperature dependent).
In Navier-Stokes's equations these properties are indirectly introduced through constituent relationships. For turbulent flow dinamics viscosity is important ...
in my case, the difference in the results for constant density and temperature-dependent density is about 11% pressure drop, and for temperature (calculated through exchanged heat) is about 2%. The values of other air properties are kept constant.
I have not made a variant with the other temperature dependent properties of the air.
Is there anybody with experience with this issue in Fluent ? What is the recommendation for setting the air properties for this heat change case?
In many articles air properties is not detailed. But this affect exchanged heat, heat transfer coefficient and so on.
I made the calculation of air variants with:
1) constant air density and density = f (T). The difference in pressure is about 11% higher, in temp. About 0.5 ° C less (less than 1% calculated over the converted heat).
2) Density = f (T) and thermal conductivity = f (T): the pressure is higher in relation to the previous variant about 0.8%, the temperature is higher by 1 ° C than the previous variant
3) Density = f (T) and thermal conductivity = f (T) and dynamic viscosity = f (T): the pressure is higher than the previous variant by 0.3% and the temp. Light breeze 0.1 ° C
4) When I set cp=f(T) the program sent error - floating point
I did not find out why,and the formula for cp itself checked several times. Have somebody this issue (error)?
Hi Dear Mladen
You can assume constant with good accuracy
good luck
In my case difference in results for constant and variable physical properties is not neglible. I must take this in account.
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