Fluid to fluid scaling of heat transfer with supercritical pressure fluids: recent considerations and future perspectives

The contribution reports on the ongoing work at the University of Pisa about fluid to fluid scaling with supercritical pressure fluids. The first studies in this regard started in 2006 when relevant non dimensional quantities were introduced for analysing flow instabilities, supposing the existence of similarities between the exceeding of the pseudo-critical temperature in supercritical fluids and the boiling threshold in subcritical ones. Since the beginning interesting features where noticed, e.g. the fact that the non-dimensional density trend ( ) seems to be a function of the dimensionless enthalpy ( ) only, regardless of the supercritical pressure and, in general, of the fluid. After several attempts, a methodology for performing fluid-to-fluid scaling with supercritical fluids was proposed in the frame of the latest work Pucciarelli and Ambrosini (2016) where the proposed technique was validated via RANS calculations. The very feature of the cited methodology is the abandonment of the search of geometrical correspondence in terms of x/D in favour of dimensionless bulk enthalpy thus taking into account the different behaviours and development lengths for the thermal field of the various fluids. The present paper reports further analyses performed adopting more reliable simulation techniques such as LES and DNS. Two operating conditions adopting carbon dioxide were selected as reference case and reproduced with supercritical water: several attempts were performed for the LES case while for the DNS application previous analyses were performed using RANS techniques in order to be sure to obtain a good scaling with the first shot. The calculations returned promising results, for the DNS application good coherence was obtained while some discrepancies were observed in the LES case; nevertheless the main purpose of the present work is try and evaluate the plausibility of the adopted strategy and no perfect matching is requested at this stage.