Fluid-to-fluid scaling of heat transfer phenomena with supercritical pressure fluids: Results from RANS analyses

A methodology for fluid-to-fluid scaling of predicted heat transfer phenomena with supercritical pressure fluids is being developed with the aid of RANS calculations. The proposed approach rephrases and further develops a previous attempt, whose preliminary validation was limited by the considerable inaccuracy of the adopted turbulence models when applied to deteriorated heat transfer. A recent improvement in the accuracy of heat transfer predictions allowed this further step, also based on the broader experience gained in the mean time in the prediction of experimental data. Four representative experimental data cases related to water and CO2, for which reasonably accurate results have been obtained by RANS turbulence equations, are addressed by changing the working fluid and imposing an approximate invariance of the dimensionless trends of fluid enthalpy at the wall and in bulk. Intrinsic differences in the thermal behaviour of the considered fluids (e.g., in the Prandtl number) are reflected in corresponding changes in the value of a single dimensionless parameter, following an indication coming from simple theoretical considerations. Results of RANS models for different fluids are used as a preliminary support to the validity of the approach, showing an interesting persistence of heat transfer behaviour in dimensionless form in the four addressed cases. The present uncertainties in the proposed methodology are mainly a consequence of the limited accuracy of the adopted simulation models. The obtained indications can be used in planning experimental or better resolved computational analyses (LES, DNS) which may better clarify the promising features of this approach.