Inception of evaporative dryout for CO2 in milliscale pipe flows

Carbon dioxide is a cost-effective, reliable and environmentally-friendly refrigerant with increasing employment in evaporator design. A clear understanding of the underlying flow physics, coupled with robust prediction of phase change through boiling, is necessary to enable widespread uptake of CO2 as a coolant. In scenarios such as nuclear reactors or thermal management in silicon detectors (e.g., Large Hadron Collider at CERN), employing saturated CO2 in milliscale pipes introduces further uncertainties in the design process, particularly regarding its behaviour at high vapour quality. During the phase change process, the fluid exhibits an abrupt decrease in the heat transfer coefficient. Such a condition, known as the onset of dryout, can lead to potentially catastrophic overheating. Two opposing behaviours are observed in the available literature concerned with the onset of dryout, coined in this study as the δ− and δ+ regimes. The δ− regime exhibits decreasing dryout vapour quality with mass flux, while the δ+ regime, which is more relevant to CO2 in millichannels, yields an increasing dryout vapour quality with mass flux. A detailed experimental campaign was conducted at CERN providing unprecedented insight into the phenomena resulting in the inception of dryout. A new theoretical model based on small perturbation theory was developed to accurately predict the dryout phase in the δ+ regime. This study provides general theory to predict dryout, which is validated with specifically-acquired data and the wider literature; to the authors’ knowledge all comparable theories fail to extend their applicability beyond the proposers own datasets. Our new theory is independent from the saturation temperature and heat flux, enabling future investigations to be conducted at a single value of the aforementioned quantities, while permitting extrapolation of the trends to a general parameter set. This unique versatility provides a new framework for the design of carbon-dioxide evaporators with novel cooling architectures.