Analysis of intermittent flow patterns of water-steam flow in solar thermal power plants with direct steam generation

Solar thermal power plants that use parabolic troughs for direct steam generation depict a commercially available technology for a renewable electricity production. The once-through operation mode is the most cost-effective approach for this type of power plant and, consequently, is mainly considered in the current research. This operation mode preheats and evaporates water and superheats steam within one continuous pipe. Hence, all mass qualities are passed along the pipe and various flow patterns are present. It is presumed that intermittent flow pattern influence the position of the mixture-vapor transition point at the end of evaporation and lead to undesired thermal stresses in the receiver pipe. In order to get an insight in the two-phase flow a numerical analysis is performed with ATHLET. This code provides a one-dimensional mass, momentum and energy balance for both phases water and steam, respectively. The applicability of ATHLET on intermittent flows is discussed and confirmed. In the result an intermittent flow is numerically determined at specific void fraction ranges in the design of the DISS test facility at the Plataforma Solar de Almería, Spain. The occurrence of these unsteady flow pattern diminishes at a pressure P > 60 bar. Furthermore, it is shown that the oscillating nature of the two-phase flow effects the position of the mixture-vapor transition point. For the validation of the numerical results an experiment with a wire-mesh sensor is designed. The obtained results deliver a substantial contribution to understand the two-phase flow behavior in the direct steam generation process.