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-   -   I Found An Interesting Phenomenon (https://www.cfd-online.com/Forums/cfx/19398-i-found-interesting-phenomenon.html)

 David December 30, 2002 19:50

I Found An Interesting Phenomenon

In tutorial 15, air bubbles have the same pressure field with water. However, according to theories,

diameter of bubble = 4.0*surface tension/ (Pressure in bubble-Pressure in water)

That means air and water have different pressure field. It's reasonable. Here I would like to give you two typical examples. The first one is, for example, every body has experience of baloons. Ballons can be supended in air which has pressure of 1atm, but inside bubble, the pressure is much higher than 1atm. Another example is hot water, hot water can have 1atm, but the water vapour has pressure over 10atm. So my opinion is: for multiphase flow, it is necessary to use different pressure field for each phase like temperature and velocity. That means traditional CFD for multiphase flow should be reconsidered. I post this interesting phenomenon to share with you and hope your opinions.

Happy new year to everybody!

David

 Phil Zwart December 31, 2002 17:33

Re: I Found An Interesting Phenomenon

Hi David,

You make an interesting observation. The assumption of a single pressure field is just that: an assumption. However, it is by far the most common assumption when modelling multiphase flows.

Some multiphase models relax this assumption for densely-packed gas-solid and liquid-solid flows (eg, fluidized beds), choosing instead to account for the effect of particle collisions using other models (typically based on analogies with the kinetic theory of gases).

However, I don't know of any models offhand which relaxes this assumption for gas-liquid flows. Note that the pressure difference will arise naturally in a CFD calculation as you refine the grid to resolve the interface between the fluids.

 David December 31, 2002 21:43

Re: I Found An Interesting Phenomenon

Hi Phil

Thanks for your answer. The driving force for evaporation is the pressure difference between water or other liquids and their vapour. Otherwise if they share the same pressure, they reached saturation state, there is no evaporation. So for gas liquid system, pressure difference is still obvious, they have different pressure fields. I think multiphase should consider different pressure fileds, although in some cases the difference is not significant. Happy new year!

David

 Phil Zwart January 2, 2003 10:19

Re: I Found An Interesting Phenomenon

Hi David,

Happy New Year to you as well.

I don't quite understand your previous post. Evaporation is driven by the temperature difference between the phases, and multiphase models do have independent temperature fields for each phase. Pressure-driven mass transfer is called cavitation, and is usually modelled by considering the difference between the local pressure and saturation pressure rather than tracking individual pressure fields. This is appropriate because cavitation bubbles tend to grow quickly and hence have a "macroscopic" size rather than subgrid size. Note that most cavitation models are still relatively crude though.

Phil

 David January 6, 2003 21:49

Re: I Found An Interesting Phenomenon

Hi Phil,

I agree with you there. But evaporation is similar to mass transfer due to the component concentraion difference between phases which is related to pressure difference directly. For gas/liquid system, the driving force for evaporation is the pressure difference between the gas-liquid interface and the bulk gas phase. Since gas and liquid share the same pressure field, how come the pressure difference and how come the evaporation? I am very intested in such dissussions and hope you and all other opinions.

Regards David

 Robin January 8, 2003 21:31

Re: I Found An Interesting Phenomenon

Hi David,

It is only the partial pressure which would be different, not the static pressure.

Robin

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