Guidelines for Heat Transfer Coefficients

This section discusses the recommendations for accurate use of the Standard Wall Functions and the built-in post-processing heat transfer coefficients.

Simcenter STAR-CCM+ always uses Eqn. (1663) to obtain the local surface heat flux when solving for fluid temperatures (and/or solid temperatures in CHT applications). This expression embodies the important boundary layer physics, and it is important that you follow the recommendations to ensure its proper application.

The heat transfer coefficients are Simcenter STAR-CCM+ post-processing results which can be used for comparing to other solutions, visualization, or exporting to other applications such as Abaqus or Nastran. The particular choice of heat transfer coefficient does not affect the simulation results, except in the case when Simcenter STAR-CCM+ is two-way-coupled to another application that solves for the temperature in the solid. The choice of heat transfer coefficient that is applied in an external application does affect the results in that application. In this case, it is important that you choose the correct option, as the choice of heat transfer coefficient that is exported to the external application does affect the Simcenter STAR-CCM+ results.

However, if you are solving for both the fluid and solid temperatures within Simcenter STAR-CCM+ (that is, Conjugate Heat Transfer), your choice of post-processing heat transfer coefficients option has no effect on the simulation results.

In general, standard wall functions give reasonable accuracy for most high-Reynolds-number, wall-bounded flows. However, standard wall functions reach their limitation when the flow conditions differ too much from the ideal conditions that are used to define the functions. This limit can be reached in the following cases:

• Pervasive low-Reynolds-number or near-wall effects (for example, flow through a small gap or highly viscous low-velocity fluid flow).
• Massive transpiration through the wall (such as blowing or suction).
• Severe pressure gradients leading to boundary layer separation.
• Strong body forces (for example, flow near rotating disks or buoyancy-driven flows).
• High three-dimensionality in the near-wall region (for example, Ekman spiral flow or strongly skewed 3D boundary layers).

Thus to get good results when using the standard wall functions, the problem should not contain any of the physics conditions that are outlined above. The $y^{\text{+}}$ of the near-wall cell should be in the inertial sublayer ($30<y^{\text{+}}<150$), and temperature-dependent fluid properties should be used to ensure maximum accuracy. If the near-wall cell lies in the buffer layer ($5<y^{\text{+}}<30$) between the viscous and inertial sublayers, the “All $y^{\text{+}}$” option (default) should be used which smoothly connects the viscous and inertial sublayers.

The differences between the heat transfer coefficient post-processing options are discussed in the following sections:

• Heat Transfer Coefficient
• Local_Heat_Transfer_Coefficient
• Specified_y__Heat_Transfer_Coefficient
• Virtual_Local_Heat_Transfer_Coefficient

For more information on the theory, see Convective Heat Transfer Coefficients.