Near-critical CO2 liquid--vapor flow in a sub-microchannel. Part II: Flow regimes

The mean-field free-energy LBM is used to investigate the liquid--vapor flow regimes in a two-dimensional 200 nm channel with near-critical CO2 at temperature 25 oC and pressure 6.434 MPa as the working fluid. Flow regimes over vapor qualities ranging from 0.01⩽x⩽0.90, Weber numbers O(10−2)⩽We⩽O(103), and capillary numbers O(10−2)⩽Ca⩽O(10) are investigated. Three major types of flow regimes are encountered -- dispersed flow, bubble/plug flow, and liquid strip flow, each of which encompasses variations of the basic flow regime. The three major flow regimes with all their variations can be further classified into two major categories – regular and irregular. Irregular flow regimes are characterized by a distorted interface, including distorted bubble/slug flow, intermittent strip flow, wavy strip flow, and wispy-strip flow. Flows in which the interface is ordered and symmetric such as bubble/plug and strip flows are classified as regular flow regimes. It is found that the transition from regular to irregular flow regimes occurs at Weber number between 500 and 1000, independent of the vapor quality. Although no experiments exist at the same conditions, comparison of the predicted transition between regular and irregular regimes shows the same qualitative trends as experiments found in the literature.