High heat fluxes from nuclear fusion pose a challenge for heat exchange. Supercritical CO₂ and micro-channel heat exchangers offer a way to solve this challenge, however heat transfer correlations for supercritical fluids do not necessarily carry over to higher heat fluxes. This experimental study investigates a promising geometry of S-Shaped fin micro-channel heat exchangers at heat fluxes up to 1 MW/m². Experiments were conducted at the Oxford Laser Heating Facility at conditions representative of the heat fluxes seen on the first wall of a fusion reactor across a range of flow rates and sCO₂ temperatures. Experimental results were calculated using a combination of finite element modelling and a thermal resistance network model. Internal heat transfer coefficients calculated using these methods matched both experimental temperatures and correlations developed for similar geometries. Heat transfer coefficients were consistently seen to be above 5 kW/m²K, with little dependence on applied heat flux. They were, however, strongly dependent on inlet sCO₂ temperature. Separately, computational fluid dynamics simulations (CFD) were performed at the same conditions from the experimental study. Although agreement between the experimental results and CFD was poor, overall results were improved when scaling the CFD-generated heat transfer coefficient profiles in the finite element models. At these elevated heat fluxes, heat transfer coefficients were largely in agreement with correlations in the literature. This enables these designs to be effectively deployed in the blanket of a fusion reactor.

Supercritical CO₂ (sCO₂), employed as a working fluid in sCO₂ power cycle, offers enhanced thermal efficiency and compact system design due to its favorable thermophysical properties. The intersection of thermophysical properties of sCO₂ with the two-phase-like regime indicates the presence of pseudo-boiling phenomenon (PBP). The thicker vapor-like layer formed during PBP reduces convective heat transfer efficiency, leading to the onset of heat transfer deterioration (HTD). Notably, this HTD is closely linked to PBP, significantly impacting the overall performance of the system. In this study, an investigation was conducted to explore the magnitude of HTD by considering the characteristics of the vapor-like layer in a heated tube. A diverse range of mass and heat fluxes and pressures (mass fluxes(200kg/(m²s) 800kg/(m²s)), heat fluxes(25kW/m² 100kW/m²), and pressures(7.5MPa 30MPa)) have been simulated in both vertical and horizontal tubes with 4 mm inner diameter. Computational fluid dynamic simulations were conducted using the Eulerian method with turbulence model that was validated against experimental data. The results show that magnitude of HTD beyond 10MPa can be ignored in cases where the Richardson number is less than 0.01. In other words, the study provides a roadmap for identifying critical threshold of the magnitude of HTD in tube heat exchangers and thermal systems.

Despite the increasing interest in supercritical carbon dioxide (sCO2) cycles in place of Rankine cycles for power generation, there is limited information available on the design and off-design performance of mechanical air-cooled heat rejection systems for sCO2 power cycles. In this work, the conceptual design and thermofluid system modelling of an air-cooled heat exchanger (ACHE) system for a 50 MWe sCO2 CSP plant is presented. The design includes the selection of major components for the ACHE system including the piping, fan, and finned-tube bundle, as well as the development of an in-house thermofluid model of the system using Python. A model of the system was also developed using the commercial code Flownex to simulate off-design conditions. To limit auxiliary power consumption, sCO2 ACHEs require deep finned-tube bundles with forced-draft fans that operate at a relatively high static pressure rise, and a relatively low volume flow rate. The results show that flow bypass combined with fan on-off control is an effective method to maintain a bulk outlet sCO2 temperature. However, to prevent local over-cooling to temperatures below the critical point, fan speed control is required. Additionally, employing fan speed control significantly lowers auxiliary power consumption relative to bypass control.