A variable turbulent Prandtl number model for simulating supercritical pressure CO2 heat transfer

In order to predict heat transfer deterioration (HTD) of supercritical pressure fluid correctly and to investigate the HTD mechanism, the effect of turbulent Prandtl number (Prt) on numerical simulation was theoretically analyzed. Based on such analysis, a new turbulent Prt model (TWL model) was proposed. Numerical simulations of the supercritical pressure CO2 heat transfer in vertical heated tubes were conducted with the proposed Prt model as well as two other previous Prt models and two constant Prt numbers. The performance of the new Prt model was validated by comparing with 14 reported heat-transfer experimental data, especially for the HTD cases. The comparison showed that a better prediction of wall temperature can be achieved with the proposed Prt model in most of the validations, especially for the HTD cases. When HTD occurs, turbulent mixing was restrained in the buffer layer since an “M” shape of velocity profile is formed under the buoyancy effect. The maximum predicted Prt value with TWL model also appears in the buffer layer, while the maximum predicted Prt value with other models appear in the viscous sub-layer. Such Prt profile restrains the turbulent mixing contribution to heat transfer further. Although the turbulent mixing contribution is still several times higher than molecular conduction contribution in the buffer layer when HTD occurs, it’s not strong enough to diffuse energy from near wall region to the bulk region.