Supercritical CO2 Flow and Heat Transfer in Compact Thermal Systems: A Critical Review of Mechanisms, Geometries, and Enhancement Strategies

Supercritical CO2 (sCO2) is a promising working fluid for compact thermal systems owing to its high heat transfer capability and advantageous thermophysical properties close to the pseudocritical region. However, strong property gradients in this regime give rise to complex thermo–hydraulic phenomena, comprising heat transfer deterioration (HTD), buoyancy-driven stratification, thermal acceleration, and flow instability, which restrict performance and predictive accuracy. This review provides a mechanism-based and design-oriented overview of sCO2 transport behaviour in compact configurations, for example, straight and curved tubes, micro/minichannels, printed circuit heat exchangers and enhanced passages. The analysis shows HTD results from the coupled effects of pseudo-boiling-like behaviour, buoyancy, and thermal acceleration, with their relative importance determined by geometry, heating distribution, and operating conditions. Comparative evaluation reveals that no single configuration is consistently optimal. Conversely, performance is defined by trade-offs between heat transfer enhancement, pressure-drop penalty, and thermal uniformity. Advanced geometries and passive enhancement techniques improve mixing and suppress stratification, whilst often increasing hydraulic resistance and sensitivity to flow maldistribution. Moreover, the review identifies limitations in the transferability of predictive correlations across fluids, geometries and heating conditions, emphasising the need to respect their calibration ranges. The findings underline the need for geometry-aware, heating-aware, and mechanism-based predictive models, in conjunction with integrated design approaches that combine local enhancement, flow distribution, and system-level stability. The results provide an integrated framework connecting transport mechanisms, compact-channel design, and system performance, proposing guidance for the development of efficient and stable sCO2 thermal systems.