An investigation of supercritical carbon dioxide in 1.5 mm horizontal channels at elevated thermal loads

The Oxford Laser Heating Facility (OLAHF) has been upgraded with a supercritical carbon dioxide (sCO2) flow circulating loop, capable of providing component level flow rates to test pieces at temperatures and pressures up to 100°C and 100 bar. Near to the critical point, the large variations in sCO2 thermophysical properties can lead to adverse cooling and heat transfer effects. There are currently gaps in the understanding of the effects of larger thermal gradients (i.e. heat fluxes) on the heat transfer performance of sCO2 systems. An initial experimental campaign was performed on an additively manufactured stainless steel test piece containing an array of 11, 1.5 mm square internal channels using the high power laser modules located at OLAHF. Heat fluxes ranging from 116 to 1000 kW/m2 to the upper surface of the horizontally-oriented test piece. Data were obtained for fluid inlet temperatures between 18 and 28°C and mass fluxes of 1500 and 3000 kg/m2s at a constant inlet pressure of 80 bar (inlet Reynolds numbers from 3 x 104 to 8 x 104). Resulting heat transfer coefficients were determined using a combination of ray tracing and finite element (FE) analyses of the test piece at each experimental condition. Average heat transfer coefficients were shown to exhibit a significant dependence on applied heat flux for both mass flux conditions and all inlet temperatures. The heat transfer coefficients were compared to empirical correlations with varied levels of agreement. The data was additionally examined for the potential existence of buoyancy and flow acceleration effects.