Investigation of convective boiling of quasi-critical fluids focusing on its application on organic Rankine cycles

Abstract

The Organic Rankine Cycle has potential of application in systems involving heat rejection remaining from processes and also in solar energy harvesting. As well as in heat pumps, in certain applications, the Organic Rankine Cycle may be subjected to evaporation pressures near the thermodynamic critical point. However, despite the growing interest in such applications, studies involving pressure drop and heat transfer under convective boiling conditions inside ducts are generally based on results for reduced saturation pressures and temperatures typical of refrigeration applications and most recently of electronic components cooling. Near the critical point, thermodynamic and transport properties present different values and behaviors from those observed at lower pressures, which drastically affects the thermohydraulic behavior of the fluid during evaporation. On contrary to supercritical fluids for which a considerable number of papers is available, it is noticed in literature that there is a shortage of experimental studies involving the convective boiling at near-critical pressures inside minichannels, and most studies used water or carbon dioxide as working fluids. In this context, the present proposal deals with the experimental survey of an extensive database for flow patterns, pressure drop, heat transfer coefficient and critical heat flux involving convective boiling and single-phase flow of R245fa and HFOs R1336mzz (Z) and R1233ze in small diameter channels and different flow inclinations. Based on these results, the thermohydraulic performance of convective boiling and single-phase flow near the critical point will be parametrically analyzed. In addition, pressure drop, heat transfer coefficient and critical heat flux prediction methods from literature will be evaluated for these experimental conditions and, if necessary, new methods will be developed to be used as design and optimization tools. (AU)