Charactising the behaviour of supercritical carbon dioxide at high heat fluxes
Keywords: computational fluid dynamics
heat transfer
supercritical CO2

State of the art energy systems require components capable of managing thermal intensities that are continually increasing. Heat fluxes of over 1 MW/m2 are now routinely generated in a broad spectrum of technological fields, ranging from micro-electronics on the low end to the hot stage of a gas turbine and fusion reactor first wall on the high end.
Design of components destined for these environments requires experimental facilities with the capability to recreate these thermal boundary conditions, enabling the assessment of thermal performance and design lifetime. The Oxford Laser Heating Facility (OLAHF) was developed with this aim, using a combination of high-power Vertical Cavity Surface Emitting Laser modules, and is capable of applying up to 24 kW of laser power at a base intensity of 1.15 MW/m2. This research expanded on these capabilities through an experimental study integrating the laser system with compound parabolic concentrators. Experimentally demonstrated heat fluxes of 6 MW/m2 were readily achieved at high transmission efficiencies through the use of standard machining, coating, and polishing techniques. Systems with expanded capabilities of up to 30 MW/m2 have since been deployed at this facility.
The use of supercritical carbon dioxide (sCO2) in microchannel systems to manage extreme thermal loads is a major research focus. However, few studies have been conducted investigating the heat transfer performance of sCO2 in these geometries, and there is no experimental data at high heat fluxes. In particular, there is limited understanding of how the non-uniform thermal boundary conditions characteristic of heat removal systems effects the heat transfer performance of sCO2. This research sought to address these points.
Experimental investigations were undertaken at a range of conditions with resulting heat transfer coefficients determined using a combination of finite element (FE) and computational fluid dynamics (CFD) analyses. Data were obtained for fluid inlet temperatures between 18 and 28C, mass fluxes of 1500 and 3000 kg/m2s, for a constant inlet pressure of 80 bar. Four test pieces were studied and contained arrays of square microchannels with hydraulic diameters 0.75 < D < 1.5 mm. Heat fluxes ranging from 116 to 1000 kW/m2 were applied to only the upper surface of each horizontally-oriented test piece using OLAHF's laser system. To the authors' knowledge, these are the highest thermal loads ever applied to a microchannel sCO2 system. Average heat transfer coefficients were shown to exhibit a significant dependence on applied heat flux and channel hydraulic diameter for both mass flux conditions and for all inlet temperatures.
Results from the above experimental studies were compared to predictions made by analytical and computational methods. The ability of empirical correlations and computational fluid dynamics to accurately predict the experimental behaviour of sCO2 systems was found in general to be modest-to-poor. In an effort to improve the predictive capability of analytical methods, Gaussian processing regression (GRP) models were developed using the experimentally generated data. These provided better agreement across the range of conditions from this research, and showed promise when applied to geometrically similar studies from the literature.