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Abstract

The paper discusses the latest improvements obtained in performing heat transfer calculations by RANS turbulence models when dealing with fluids at supercritical pressure. An algebraic heat flux model (AHFM) is adopted as an advanced tool for calculating the turbulent Prandtl number distribution to be used in the energy equation. Though maintaining a simple gradient approach, the proposed model manages to obtain interesting results when dealing with temperatures spanning from low to supercritical values. This is due to the introduction of a correlation for defining one of the relevant AHFM parameters. As stated in previous works, in fact, single fixed constant values could not be sometimes sufficient for dealing with very different operating conditions such as the ones occurring with supercritical fluids. In order to make the relation suitable for different fluids, a dependence on a non-dimensional quantity which proved to be relevant by parallel work is assumed. Some sensitivity analyses are also performed, showing some interesting capabilities in reproducing a sort of threshold behaviour when working in transition regions. Buoyancy induced phenomena are much better captured than in past attempts, though incomplete accuracy is observed for some boundary conditions.

5. Conclusions

The main goal of the present paper was improving the predictions obtained with RANS turbulence models when dealing with fluids at supercritical pressure. Moving from previous assumptions, a new definition for the 1 Ct3 quantity from the AHFM equation is proposed showing promising results. Instead of considering a constant value as in past works, a relation taking into account the dimensionless enthalpy is adopted. The approach should allow considering the proposed definition even when dealing with different fluids as suggested from parallel works concerning fluid to fluid scaling. As a first conclusion, it must be noted that this approach resulted quite successful in a variety of addressed cases, involving different fluids and operating conditions.