Thermal-hydraulic performance and entropy generation of supercritical carbon dioxide in heat exchanger channels with teardrop dimple/protrusion

Supercritical carbon dioxide (SCO₂) is an excellent candidate for refrigeration system, energy utilization and new power cycle due to its easy-to–reach pseudo-critical point and the advantages of thermo-physical properties. As an efficient and passive thermal control technology, teardrop dimple/protrusion has great potential in heat transfer enhancement and resistance reduction. In this paper, three types of teardrop dimple and protrusions are novelly introduced into the SCO₂ rectangular channels. Numerical calculations have been carried out to obtain the detailed flow structures, friction and heat transfer characteristics, overall thermal performance and amount of entropy generation under full turbulence regions. Moreover, the effects of geometrical structure, eccentricity of teardrop dimple/protrusion, Re, temperature and pressure of SCO₂ are discussed with in-depth analysis. According to the results, complex and different evolution and development patterns of vortices are presented for three different structures. Positive eccentricity dimple (PED) produces the best overall thermal performance, owing to significant heat transfer enhancement and less friction, and the maximum TP reaches 1.17, indicating a 17% improvement compared with the smooth case. The variation of eccentricity has little effect on overall thermal performance and entropy generation for PED structure, while plays a major role in positive eccentricity protrusion (PEP) and negative eccentricity protrusion (NEP) cases. However, the increase of Re is unfavorable to the overall thermal performance. In addition, the temperature and pressure of CO₂ have significant effects on the overall thermal performance for PED cases, and corresponding high-TP and thermal performance deterioration regions have been captured, which is advantageous to fast positioning the specific ranges of temperature and pressure, and whose coupled mode. Specifically, the maximum TP greater than 6 can be obtained near the temperature 305 K and pressure 8 MPa.