Simulation of CO2 under supercritical pressure in solar collectors by variable Lattice Boltzmann method

Supercritical CO2 is widely used in the HVAC facilities, such as gas coolers and solar collectors. The solar collector is a fluid-filled cavity, and the heat and mass transfer characteristics in the cavity is the key factor that affects the performance of solar collector. Considering that the thermophysical properties of supercritical CO2 change dramatically, this article employs the Lattice Boltzmann Method (LBM) to implement the numerical simulation of supercritical heat and mass transfer. Based on analysis of the simulate results, it is found that the heat transfer in the solar collector is mainly related to Rayleigh number. Consequently, several correlations for the average Nusselt number are derived using Rayleigh number. Furthermore, a new concept named the critical Rayleigh number is proposed for the bifurcation phenomenon that occurs during the simulation. The numerical simulation results and the newly proposed correlations in this article could be used directly to improve and evaluate the performances of solar collectors.