Experimental water boiling in macroscopic horizontal and slightly tilted tubes: Flow pattern map and associated heat transfer analysis

Direct Steam Generation (DSG) is gaining relevance in Concentrating Solar Thermal (CST) technologies due to its potential to reduce system complexity and costs by eliminating conventional working fluids (molten salts or synthetic oils). However, DSG introduces complex two-phase flow patterns in horizontal or slightly inclined receivers, which can pose operational risks if not properly understood. This study aimed to investigate flow patterns and heat transfer in a small-scale DSG experimental setup, focusing on the influence of tube inclination on flow regime transitions. An experimental facility, ConBo (CONvective BOiling flow loop), was developed, featuring a heating test section (TS) of 1 m length with an internal diameter of 49.3 mm. Water is vaporized using induction heating, and the section can be inclined to evaluate tilt effects. Various flow patterns are observed and recorded with a high-speed image acquisition system, while wall-mounted temperature sensors and internal thermocouples measure thermal distributions. A total of 231 tests were analyzed to classify flow regimes and assess heat transfer coefficients. Results indicate that tube inclination significantly affects flow regime transitions, from stratified to intermittent flows, and strongly influences wall temperature distribution and heat transfer performance. Flow pattern classification enabled the identification of regime transitions under different operating conditions. For tilted cases, a counterintuitive temperature distribution is observed in the TS, with the upper part of the tube being cooler than its bottom, which significantly improves heat transfer for the same section by delaying local dryout at the top. This inverse stratification is explained by a mechanistic model in accordance with the literature. These findings provide a detailed understanding of two-phase flow behavior in DSG receivers. The insights gained can inform the design and optimization of CSP receivers for industrial process heat applications, contributing to more efficient and reliable solar thermal systems. • A novel experimental facility was developed to investigate direct steam generation in CSP conditions in a 50 mm inner diameter macro-scale tube. • Flow regimes were automatically identified from high-speed image acquisition with an overall accuracy of 80% across four flow patterns. • Flow regime transition maps were established for inclined configurations using the Rouhani correlation. • A new heat transfer coefficient correlation was proposed, with a mean absolute percentage error of 10.43%. • An intermittent thin liquid film at the top of the tube was identified, and a dedicated mechanistic model was proposed to explain that the upper wall of the tube is colder than the lower.