Direct steam generation in parabolic trough solar collector: Analytical modelling for prediction of flow pattern

Solar energy generating systems based on parabolic trough collector (PTC) are the most proven and commercially available solar thermal technology for power generation. Direct Steam Generation (DSG) in parabolic trough solar collector has great potential to reduce the capital cost and improve the performance of the solar field as well as the power block, but the flow stratification in the receiver leads to dry spots and consequently higher circumferential thermal gradient in the absorber wall that induce thermal stress and deforms the receiver from the focal plane. Hence, it is important to predict the flow regimes in the absorber tube that can further help to analyze heat transfer rate, temperature variation in the circumference of the absorber wall and stability of the receiver tube. In this study, a detailed mathematical model for two-phase flow boiling has been developed and solved using MATLAB. Flow pattern maps and transition of flow regimes are presented that help to predict different types of flows in the absorber tube. The effect of mass flow rate and pressure on flow regimes in the receiver tube have been evaluated. It is observed that stratified flow and stratified wavy flow exist at low mass velocity whereas bubbly flow occurs at a very high mass velocity. It is also observed that to achieve the annular flow in a DSG collector, the mass velocity should be high. The mathematical model is based on the temperature of the absorber tube rather than the temperature of the fluid as it also helps to evaluate the performance of the collector for any working fluid other than water.