Learning Forum

[Andrea Pinardi]

In Fluent 2023R2 User's Guide (section 7.4.15.5.6. Augmented Heat Transfer), the Convective Augmentation Factor CAF is defined as CAF = Nu_measured / Nu_ideal_flow.

In Fluent's Customization Manual (section 2.3.20.2) the DEFINE_HEAT_FLUX macro, which uses the CAF, is described and the diffusion component of the heat flux (qid) is computed as qid = cid[0] + caf_fac*(cid[1]*C_T(c0,t0) - cid[2]*F_T(f,t)) - cid[3]*pow(F_T(f,t),4), where

• cid is a coefficient array
• caf_fac is the CAF
• C_T is the cell temperature
• F_T is the face temperature

It is unclear to me:

• why the diffusive heat flux is linearised in that way (in particular, why is there a power term?)
• why the CAF is applied to the "variable part" of the heat flux and not to the entire heat flux. Is it because a convective heat flux is necessarily a function of temperature, and therefore only the variable part of the qid is the one related to convection (and thus the only one that needs to be augmented by the CAF)?

[SRP]

Hi,

The linearization here simplifies the incorporation of radiation effects within the overall heat flux calculation. The CAF enhances the convective heat transfer component based on the difference between the measured and ideal Nusselt numbers, indicating the effectiveness of the convection. Convective heat transfer is inherently dependent on the temperature difference between the fluid and the surface. Hence, the CAF modifies the temperature-dependent component of the heat flux to account for enhanced or diminished convective effects.

Thank you.