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When analyzing heat transfer with Ansys Fluent, it is useful to look at heat transfer coefficients (HTC) values at wall boundaries to determine how effective the energy transfer is between the fluid and the solid bodies. Fluent has four different values for HTC that are available. They are:
- Surface Heat Transfer Coefficient
- Wall Function Heat Transfer Coefficient
- Wall Adjacent Heat Transfer Coefficient
- Y+ Based Heat Transfer Coefficient.
Summary of Heat Transfer Coefficient Values
The Wall Function Heat Transfer Coefficient is the only one that doesn’t depend on the surface heat flux and thus will work for cases where the wall is set to adiabatic. As its name implies, it uses the wall functions that are used to determine the near-wall flow behavior to calculate the coefficient. The wall functions are set in the turbulence panel and designed to work best when the first mesh cell has a y+ value between 30 and 80.
The Surface Heat Transfer Coefficient uses the surface heat flux value, the wall temperature, and reference temperature specified in the model settings. This value is highly dependent on how well the reference temperature is defined and how much the flow temperature varies.
The Y+ Based Heat Transfer Coefficient uses the reference Y+ value that is specified in the model settings panel and defaults to 300 to determine where to get the flow temperature. It also uses the surface heat flux value and the surface temperature.
The Wall Adjacent Heat Transfer Coefficient uses the surface heat flux, the surface temperature, and the temperature of the closest adjacent cell. Using this cell temperature can make the value highly dependent on the mesh density near the wall.
Choosing the Right Heat Transfer Coefficient Value
The Wall Function HTC should be used in cases where the adiabatic wall boundary condition is used. It needs to be combined with the wall function setting in the viscous model panel. The near-wall mesh also needs to be coarse enough so that the y+ values at the wall boundaries are in the 30-80 range.
The Surface HTC should be used when the flow temperatures do not vary significantly throughout the flow domain so that the reference temperature used in the calculation reflects the fluid temperature. The wall boundary thermal conditions should also be a fixed heat flux, a fixed temperature, or a coupled wall.
The Y+ Based HTC should be used when the fixed y+ value that is defined in the reference values task page corresponds with the values in core region of the flow field. By default, the y+ reference value is 300. The flow domain needs to extend far enough away from the walls so that this location does not fall outside of the core flow. As with the Surface HTC value, the wall boundary thermal conditions should also be a fixed heat flux, a fixed temperature, or a coupled wall.
The Wall Adjacent HTC should be used when the mesh is coarse enough so that the mesh cell that is used to determine the flow temperature is at the outer edge of the boundary region, where the flow temperature is still varying. If the mesh is too refined, then the temperature that is used will not represent the core temperature, and the HTC value will be inaccurate. As with the two previous coefficients, the wall boundary thermal conditions should also be a fixed heat flux, a fixed temperature, or a coupled wall.
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