Evaluation of Flow Patterns for Direct Steam Generation

Abstract

In solar Direct Steam Generation (DSG) systems, steam is produced directly in the solar field instead of producing it in a steam generator. Liquid water is fed to the solar field and is heated as it flows through the absorber pipes until it starts to evaporate and a two-phase flow develops. The resulting two-phase flow can take various forms, such as slug flow or annular flow, depending on the vapor content and other influencing variables such as the mass flow through the absorber tubes. There are flow patterns that are unfavorable for the operation of a solar thermal system. In particular, a stratified flow with a high vapor content can lead to the formation of dry areas in the upper inner walls of the absorber tube. At these areas, the heat can no longer be sufficiently transferred to the liquid, so that the wall temperature rises and the tubes can become deformed leading to the breakage of the glass cover [1,2,3,4]. In addition, further mechanical loads can be added by the momentum inherent in the slug flow. The annular pattern is a particularly advantageous flow pattern for direct evaporation. In this case, the liquid flows as a ring along the wall through the absorber tube, while the vapor flows inside the liquid. This ensures that the heat is absorbed evenly by the liquid. In the Jordan Optimizing Solar Steam for Industry (JOSSI) project, a measurement campaign was carried out with the aim of recording the two-phase flow patterns within the horizontal absorber tubes and investigating their occurrence under different operating conditions.