ID 124

Liquid-Gas Equilibrium and Interface Dynamics at Supercritical Pressure

 

Abstract:

Heat and mass transfer and phase equilibrium are studied numerically when a cool hydrocarbon liquid is introduced to a hotter gas at supercritical pressures. The characteristic diffusion times are determined using real-gas properties and equations of state for density and enthalpy together with phase-equilibrium laws. A comparison with expected growth rates of the Kelvin-Helmholtz instability indicates that phase equilibrium is well established before hydrodynamic instabilities become important. Liquid-gas interface dynamics and the diffusion are analyzed for 10 to 150 bar. At supercritical pressures, two phases can appear because mixture critical properties differ considerably from pure species critical properties. For decane and oxygen, diffusion in both phases affects significantly fluid properties in the 20-100 μs before substantial Kelvin-Helmholtz instability growth; i.e., diffusion layers of 10 μm thickness in the liquid and 30 μm in the gas occur. For 10-50 bar, the liquid vaporizes with an energy flux decrease across the interface from the gas into the liquid. At higher pressures, the energy flux summed from heat conduction and energy transport by mass diffusion increases across the interface; consequently, condensation occurs although heat still conducts from gas to liquid. Throughout this phase change, the fixed-mass volume containing both phases decreases and the integrated internal energy increases with time in accordance with the First Law. Although gas enthalpy always exceeds liquid enthalpy, the liquid-solution internal energy at the interface exceeds the gas-mixture internal energy at higher pressures. Globally, the integrated enthalpy does not vary with time.