• Energy Research

Supercritical heat transfer has already been applied for decades, as it has several benefits such as improved thermal efficiency of the thermodynamic cycle. Accurate knowledge about supercritical heat transfer and pressure drop of the different working fluids is required to design the heat exchangers and other components used in these systems. In literature, supercritical heat transfer of water and CO2 has already been widely investigated, the research involving refrigerants (for their application in low-temperature heat conversion systems) is however rather scarce. This paper gives an overview of the existing research on supercritical heat transfer. An overview of the applications, general characteristics and the main findings for water and other fluids are summarized. Due to the sharp variations in thermophysical properties, heat transfer and pressure drop cannot be accurately predicted on a single-phase based approach only. An in-detail review of the current research and status of knowledge about supercritical heat transfer of refrigerants is presented. The effect of the different investigated refrigerants and operating parameters on heat transfer and pressure drop, both for heating and cooling applications, is discussed. The remaining gaps in literature are highlighted, which include studies involving larger diameter tubes, horizontal flow, cooling heat transfer and pressure drop estimations and creation of a wider database for a more general correlation development and measurements on newer refrigerants (with low Global Warming Potentials) as these will become increasingly important in the near future. In addition, advances in numerical research should focus on development of suitable turbulence models. Overall, further improving the basic understanding of the fluid structure and occurrence of deteriorated heat transfer, as well as forming reliable models for the thermophysical properties are key in future efforts.

Abstract Knowledge about supercritical heat transfer to refrigerants with horizontal flow is scarce and experimental data is mostly lacking. This makes an accurate sizing of heat exchangers impossible. In addition, the majority of the tested geometries in literature are limited to an inner diameter of only 16 mm. In order to close the gap and increase the knowledge about supercritical heat transfer, a novel experimental setup was used to investigate supercritical heat transfer phenomena on larger tube diameters. The test section consists of a horizontal counter current tube-in-tube heat exchanger. R125 flows in the inner tube having an inner diameter of 24.77 mm. The influence of mass flux (226 - 585 kg/m2s), heat flux (7.7 - 21.9 kW/m2) and supercritical pressure (3.7 - 4.1 MPa, corresponding to 1.03 - 1.12 times the critical pressure) on local forced convective heat transfer was investigated. No visible influence of pressure on the convective heat transfer coefficient is noticed, indicating that buoyancy is present and influential in the performed measurements. The applied heat flux does have an effect, with a decrease in the heat transfer coefficient for an increase in heat flux. This effect is more pronounced at higher mass fluxes. At high mass flux, increasing the heat flux with 42% results in a drop in heat transfer coefficient of 46%, while this is only 40% and 26% at medium and low mass flux, respectively. However, there appears to be a limit to this detrimental effect at higher mass fluxes. Finally, heat transfer increases for an increase in mass flux. Increasing the mass flux with 76% results on average in a rise in heat transfer coefficient of 88%, for measurements at medium heat flux. At high heat fluxes, an increase of 71% is observed for a rise in mass flux of 59%. Nine heat transfer correlations are evaluated. The majority underestimates the heat transfer significantly, however, two correlations operate adequately and are proposed as a basis for future correlation development. The unique dataset and insights presented in this work enable a better understanding of the heat transfer in supercritical vapor generators. Future work includes incorporating top and bottom wall temperature measurements, adjusting control and application of the heat flux and investigating refrigerants with a low Global Warming Potential.

Abstract A new concept of liquid-flooded expansion has been proposed as a performance increasing modification of the basic ORC targeted at low-temperature heat sources. However, little research demonstrates the potential of this technology especially experimentally. In this paper, an experimental test facility based on a conventional recuperative ORC system was constructed with an independent liquid flooding loop that enables testing the influence of liquid flooding on a modified single-screw expander as well as on the cycle itself. Experiments were performed at various pressure ratios (3.3–4.1) over the expander and flooding ratios (0–0.3) with R1233zd(E) as the working fluid and a standard lubricant oil as the flooding medium. The data reduction and uncertainty analysis were also discussed in depth. In total, 142 steady-state points were obtained. Compared with the baseline organic Rankine cycle, the maximum improvement of the liquid-flooded expansion on the expander power output can be 9.1%, although at slightly worse expander inlet conditions. The maximum enhancement of the isothermal efficiency of the expander was 9.5%. Results also showed that the expander power output, the net power output and the thermal efficiency were enhanced with the increase of the flooding liquid amount. The potential of an organic Rankine cycle system with liquid-flooded expansion can be further examined if over-expansion losses can be reduced and larger amount of oil can be injected, i.e., with higher pressure ratios and higher flooding ratios. Overall, this study provides insights into performance improvement by means of modifying the cycle thermodynamics itself.