Hi,
I am modelling an Alkaline water electrolyzer by using multi phase model in porous media. I am little bit confused about volume fraction is that does the gas volume fraction represents here only the undissolved gas (bubbles) or both dissolved gas and undissolved gas (bubbles)?
Please someone can provide me clarity about this or recommend any reference which I can go through.
Thanks!
Regarding multiphase flow models, equations accounting for mass and momentum balances use volume fractions of each individual phase. For a typical two-phase gas (dispersed) – liquid (continuous) mixture, there is a bubble volume fraction and a liquid volume fraction to be considered. Gas dissolved in the liquid is part of the liquid phase. Liquid vapor incorporated to bubbles is bubble phase. Mind you, pressure and temperature changes along flow may result in mass transfer between phases and, consequently, volume fractions could vary.
Take a look at the classical textbook “Multiphase Flow and Fluidization - Continuum and Kinetic Theory Descriptions”, by D. Gidaspow, Academic Press (1994).
Of course you are free to define your own volume fractions, but when comparing with literature look carefully at the definitions used, because they may differ.
Looking at gas volume fraction, it is custom to use the gas phase only (VG), but different choices are made for the liquid phase (reactor volume, liquid volume, dispersion volume (gas-liquid)). In porous media there is also the solid phase volume to consider. If there is also a liquid phase, three fractions are to be considered (the sum of which should be 1, but again it depends on the total volume used). Sometimes the solid phase volume is not considered (gas volume fraction = gas volume / (liquid + gas) volume.)
Again, volume fractions are defined differently, and proper and clear defining is a must.
imo, strictly speaking, the gas volume fraction should represent both, since dissolved gas also occupies a volume, but it depends on the gas flow rate and the gas itself. The local time rate of change of mass in the gas phase can be considered negligible when the gas flow rate is rapid, so that the gas content in bubbles instantaneously present in the reactor is small compared to the total quantity of gas passing through it, and a pseudo-steady state for the gas phase would presume. The local time rate can then be neglected, approximating that the gas volume fraction refers principally to the undissolved gas in the gas stream.