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. In turbulence models 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 Fluent's experience with this issue? What is the recommendation for setting the air properties for this case of heat exchange?
In many articles properties of air is not detailed. But this affect exchanged heat, heat transfer coefficient, ...and so on.
Dear Mladen, in heat exchanger simulation air properties should be introduce as temperature dependent in a model second order approximation (first oder = constant properties) for realistic results. Comsol Multiphysics offers a module for Heat Transfer and user can define physical properties depending on temperature as functions of type a+bt+ct2+... I think in Fluent is possible too. Gianluca
Dear Mladen, if the temperature difference in your case is considerable, the temperature-dependent thermo-physical properties is recommended especially for air. There are some publications presenting the experimental correlations for air at different temperature ranges for different properties of air. You may extract their correlations and interpret them to Fluent using UDF codes. I have attached a sample of paper presenting the correlations for humid air.
Regards,
Emad
When I worked with Rotary Heat Exchagers type Ljungström - we always used tables for temperature dependent air and gas properties in our calculations. That was necessary - otherwise one could miss a lot on the size of the heat exchanger. We even corrected for the water content in the air.
Thank you for your answer Mr. Emad Hasani Malekshah and attached material.
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