Advanced Technology and Manufacturing Institute (ATAMI)

In the vicinity of the pseudocritical point, supercritical carbon dioxide (sCO₂) undergoes a steep change in properties from “liquid-like” to “gas-like” as it is heated at a constant pressure. At the same time, there is a large spike in specific heat which can yield high heat transfer coefficients and heat capacity rates. These unique properties have made sCO₂ an attractive working fluid in next generation power and HVAC&R technologies. Microchannel heat exchangers are being used to safely and efficiently utilize the high pressure fluid in these applications. However, prior investigation of heating of supercritical CO₂ has primarily focused on circular, uniformly heated channels at relatively low heat flux for nuclear power applications. Thus, it is unclear if models and correlations developed from large circular tube data can be scaled down to the smaller, non-circular channels, with non-uniform heating. In the present work, a methodology is developed to experimentally characterize heat transfer for multiple parallel microchannels with a hydraulic diameter of 0.75 mm and aspect ratio of 1. Experiments are conducted over a range of heat fluxes (20 ≤ q” ≤ 40 W cm⁻²), mass fluxes (500 ≤ G ≤ 1000 kg m⁻² s⁻¹), reduced pressure (1.03 ≤ P[subscript R] ≤ 1.1), and inlet temperatures (20 ≤ T[subscript in] ≤ 100°C) in a parallel square microchannel test article with a single-wall constant heat flux boundary condition. Local and average heat transfer coefficients are experimentally measured and the results are compared to previously developed correlations. The predictive capabilities for the supercritical models were poor, with the lowest mean absolute percent error (MAPE) of 55.3% for the range of bulk fluid temperatures, heat fluxes, and mass fluxes. Interestingly, subcritical correlations were also investigated and yielded much lower MAPE than 80% of the supercritical correlations even though the effects of variable fluid properties were not taken into account. The subcritical correlations did not incorporate property ratios to account for the variability in fluid properties; in some supercritical correlations it was found to add additional uncertainty for the case of the present study. The effects of buoyancy and flow acceleration were also evaluated. Based on dimensionless criteria, buoyancy was expected to play a role in heat transfer, especially when the bulk fluid temperature is below the pseudocritical temperature. However, the relative importance of flow acceleration was inconclusive. Despite the apparent importance of buoyancy effects, heat transfer was not degraded, as would be expected in larger, circular, uniformly heated tubes. The mixed convection could be inducing a density driven swirling with the stratification of low-density fluid near the top (unheated). This would ultimately improve the heat transfer at the bottom portion of the test section channels. Therefore, the flow geometry and the non-conventional heated boundary could be improving the heat transfer even with buoyancy driven effects under supercritical conditions.