Heat transfer investigation of supercritical R134a for trans-critical organic Rankine cycle system

In order to restrict the adverse effects caused by the heat transfer deterioration of the supercritical fluid and to build a cost-effective Trans-critical Organic Ranke Cycle (TORC) system, the heat transfer of supercritical R134a was experimentally investigated in a horizontal ribbed tube for the first time. Knowledge that are important for the design of the supercritical heater and TORC system were explored in detail, including the general features of the heat transfer and effects of heat fluxes, pressures, and mass fluxes. It was found that large heat flux will decrease Nu in most cases, and the effects of pressure differed largely in different working conditions. The working conditions with low heat flux, large mass flue and pressure close to the critical pressure were recommended for the design of the supercritical heater and TORC system. Compared with vertical smooth tubes, the threshold for heat transfer deterioration criterion, Bo, was enlarged at least 7.5 times and the heat transfer was improved around 1.6 times among all the experimental data. Then, different types of previous correlations were evaluated and none of them showed a satisfactory accuracy. Finally, new correlations with better prediction accuracy were built to provide references for the TORC system design.