Dynamic mathematical heat transfer model for two-phase flow in solar collectors

In the present study, a dynamic heat transfer model is developed for two-phase flow through the absorber tube of linear solar collectors, where the heat transfer fluid is water. The model considers that thermophysical properties, such as viscosity, thermal conductivity, densities, and specific heat, depend on temperature; this dependency is reflected in the value of the convective heat transfer coefficient. The governing partial differential equations for the fluid are solved using the finite difference method in an explicit scheme, the heat transfer equation for the absorber tube uses an implicit scheme, whose solution is implemented in C++ compiler. The model is validated with experimental data from a solar collector using a Solar Fresnel Reflector type, with an error related to the steam quality lower than 4.28% at the outlet of the collector and a better fit with the temperature profile through the collector in comparison with previous studies. The results show that as the phase change occurs, increasing the quality of steam in the absorber tube, the collector efficiency decreases. This is due to that the convective heat transfer coefficient of the absorber decreases, since the thermophysical properties of the liquid-steam mixture do not favor heat transfer.