Heat exchangers for cooling supercritical carbon dioxide and heat transfer enhancement: A review and assessment

Supercritical and trans-critical carbon dioxide cycles are widely used for refrigerators, heat pumps and power plants, of which the heat exchangers are key components. This article presents a critical review of heat exchangers for cooled supercritical carbon dioxide (SCO2) flows, within which CO2 is close to its critical point and thus is likely to experience heat transfer deterioration or enhancement due to the dramatical change of thermo-physical properties. First, SCO2 heat transfer features were highlighted. Second, SCO2 heat transfer characteristics in heat exchangers were analysed in detail, including experiment, CFD simulation, dynamic response, and design of SCO2 heat exchanger. Third, SCO2 heat transfer enhancement was summarized. An approach for representing SCO2 thermo-physical and transport property constants versus temperature was proposed, and the heat transfer enhancement methods which may be potentially applicable to SCO2 heat exchanger design were assessed and selected according to performance evaluation criterion and enhancement efficiency as well as MATLAB optimization. It was demonstrated that a polynomial of Gaussian probability density function can reflect the variation of thermo-physical property constants of SCO2 with temperature mathematically and may be used in SCO2 heat exchanger design and thermal dynamic response analysis. When a SCO2 cooling heat exchanger works at Reynolds numbers Re = 1.9x10(4)-2.55x10(5), the protrusion, rough element and twisted tape insert approaches as well as nano-coating methods may be more suitable heat transfer enhancement methods for SCO2, but numerical and experimental confirmation is desirable. A preliminary guidance to design, optimization and experiment of SCO2 cooling heat exchanger for the project is offered. (C) 2021 The Authors. Published by Elsevier Ltd.