Theoretical study of direct steam generation in two parallel pipes

In this work it was simulated the direct steam generation (DSG) in a system of two horizontal parallel pipes by using the commercial simulator RELAP. The pipes were 500 m long, internal diameter 5 cm, and wall thickness of 10 mm. Pipes were fed with sub-cooled water and the output can be: 1) hot water, 2) a steam-water mixture, 3) overheated steam. The flow was at steady state and pipes were undergoing to uniform and concentrated solar heating. Moreover, the effect of symmetric and asymmetric heating, on the DSG process, was studied. RELAP code allowed predicts flow distribution and inlet pressure. Predictions of inlet pressure and flow distribution were compared against predictions from literature. The pressure drop calculated by RELAP presented good fitting with field data from the Plataforma Solar de Almeria (PSA). The study of flow distribution showed that RELAP is able to get the most stable solutions. It was found that for symmetric solar heating, asymmetric flow distribution solutions may be present in the system. Moreover, for asymmetric heating the mass flow rate is larger in the pipe with smaller heating. This is an unfavorable condition under a practical point of view. The concentrated solar heating does not affect in a considerable way the flow distribution in relation to the uniform solar heating; however, it could affect the flow pattern transition along the pipe (s). (C) 2014 The Authors. Published by Elsevier Ltd.