Hydrodynamic instabilities of near-critical CO2 flow in microchannels:: Lattice Boltzmann simulation

Motivated by systematic CO2 evaporation experiments which recently became available (J. Pettersen, "Flow vaporization of CO2 in microchannel tubes,'' Doctor technicae thesis, Norwegian University of Science and Technology, 2002), the present work constitutes an exploratory investigation of isothermal flow of CO2 near its liquid-vapor critical point through a long 5 mum diameter microchannel. A modified van der Waals constitutive model-with properties closely approximating those of "real'' near-critical CO2-is incorporated in a two-dimensional lattice Boltzmann hydrodynamics model by embedding a dimensionless parameter X, with X-->1 denoting the "real'' fluid. The hydrodynamic phenomena resulting by imposing a constant pressure gradient along a periodic channel are investigated by considering two regimes in tandem: (1) transition from bubbly to annular flow with a liquid film formed at the channel walls and (2) destabilization of the liquid film by the Kelvin-Helmholtz instability. Due to numerical constraints, intrinsic modeling errors are introduced and are shown to be associated with discrepancies in the relative vapor-liquid interfacial thickness, which is expressed by X. The effects of these errors are investigated both theoretically and numerically in the physical limit X-->1. Numerically determined flow patterns compare qualitatively well with direct visualization results obtained by Pettersen. Overall, the characteristics of isothermal near-critical two-phase flow in microchannels can be reproduced by the appropriate modification of the thermophysical properties of CO2. (C) 2004 American Institute of Physics.