Current status of the thermohydraulic behavior of supercritical refrigerants: A review

Abstract

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 CO 2 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. • Extensive review on thermohydraulic behavior of refrigerants. • Remaining gaps highlighted, including horizontal flow, cooling and large diameters. • Accurate fluid-to-fluid scaling laws needed to transfer knowledge from different fluids. • Improvement in numerical turbulence models and measurements required. • Need for a large refrigerant database, covering heat transfer deterioration.