A wide number of simulations have been carried out (381); some of them have been used to validate the CFD model and the numerical assumptions with experimental results and previous authors. In the case of laminar situations, maximum deviations of 10 % have been obtained in the prediction of average Nusselt numbers and mass flow rate. In figure 2, a comparison between the numerical values of Nu and the mathematical expressions of different authors is showed [1,13,14].

1.1. New correlations for Nu and mass flow rates

Using the CFD simulations and applying a non linear regression based on iterative estimation algorithms (through the SPSS software) different correlations for the cases with asymmetric isofluxes have been obtained. In the case of the Nusselt numbers, the new correlations are based on the expressions of Bar-Cohen and Roshenow [1], but including the effect of the asymmetry (qwc / qwh), being qwc and qwh, the hot and cold wall isofluxes respectively:

Where: Nuh ,fw is the average Nusselt number for the hot wall; Nuc t is the average Nusselt number

for the cold wall. In the case of the mass flow rate, a new correlation for the friction factor has been obtained:

A comparison of these three correlations with the numerically calculated results has shown deviations lower than 6 %. However, some refinements in the equation 2 for situations with low Rayleigh numbers and in equation 3 for high asymmetric cases will be carried in the next future.