Theoretical investigation of heat transfer correlations for supercritical organic fluids

Abstract

Investigation on supercritical Organic Rankine Cycle (ORC) to generate electricity from medium and low-temperature energy resources is increasing in the last decade. The main challenge in supercritical organic fluids research is how to predict heat transfer correlation (HTC) at the supercritical pressure conditions. Then, the correlation will be used to design the supercritical heat exchanger (HE). So, the purpose of this study was to investigate the heat transfer/Nusselt number correlations that is suitable for organic fluid such as R-290, R-134a, and R-227ea under the supercritical conditions. A study was needed because the specific correlation for organic fluids does not exist yet. Literature study was performed to identify eight convection heat transfer correlations. The literature study showed that many of the convection HTC were developed from the experimental data of water and CO2 as working fluids. The thermo-physical properties of the fluids in the area around the pseudo critical temperature are changed drastically. Generally, the current HE design is carried out assuming constant fluid properties at the average of inlet and outlet temperatures. Hence, the method to design the heat exchanger must be done in a segmentation manner. The convection HTC is required to calculate the heat transfer surface area and the pipe length. A horizontal double pipe counter flows heat exchanger was used for this purpose. The investigated correlations for organic fluid to predict convection HTC shows a large variation. The result showed that there is no single equation can be used to predict supercritical convection HTC of R-290, R-134a, and R-227ea. The constant coefficients of some important parameter such as the ratio of specific heat, density, thermal conductivity and viscosity at the wall to the bulk temperature were suggested to evaluate and add in the correlations. Furthermore, the convection HTC should be evaluated and validated by the experimental study.