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Advice on modeling mist/fog

Question asked by Amit Katz on Jan 12, 2015
Latest reply on Jan 15, 2015 by Jared Conway

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In the near future I will be tasked with modeling an ultrasonic wave nebulizer, a device which uses a piezoelectric diaphragm to convert liquid water into mist. I'm wondering what would be the best way to tackle this in software.

My initial idea is to model it as a mixture of two gasses: air and an artificial "mist gas" fluid that would represent the slightly denser mist. After I have a steady-state solution I would go back and do a particle analysis as well. I'm curious if anybody has any insight or advice on this matter. For one thing I am still not sure what kind of properties the "mist gas" will have. From preliminary experiments I have seen that it flows much more viscously than air, but I do not have any derived thermodynamic quantities to work with. Additionally I have seen that water droplets which are dropped onto the nebulizer cause a mist to rapidly form and expand, almost as if they were flash boiling. Again the problem is the numbers, I am not sure how to define a mass flow rate (i.e. how much air is entrained into the mist) or an average velocity.

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Jared Conway Jan 12, 2015 2:57 PM

what are you trying to design and analyze in this case?

what is the goal of the analysis?

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Amit Katz @ Jared Conway on Jan 13, 2015 9:02 AM

I'm trying to refine the design of a nebulizer. The goal is to facilitate mist flow out of the device and minimize condensation on the inside walls.

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Jared Conway @ Amit Katz on Jan 15, 2015 2:29 PM

so how does that translate into how you're going to setup the analysis

we can't predict condensation on the walls, so how will you evaluate the flow to know if that will happen? because that should point you in the direction of how to setup the analysis.

to me, all that you can do is create the normal environment and then create an inflow of the mist (maybe air with certain RH or like you said a gas that is heavier) and then look at the temp of that gas relative to the temp of the walls

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Amit Katz Jan 15, 2015 2:42 PM

The amount of condensate on the walls will depend on the interaction between the two fluids. This is not something that is easy to quantify, but basically the way this thing is design is like a cup within an upside jar. I want to get the mist to just overflow the "cup" without touching the walls of the "jar".

What I'd like advice on is how to find the required thermodynamic properties for the mist such as viscosity, density etc... from actual experiments. I'd like to see if anybody has experience on doing experiments like this with the goal of simulating the flow with CFD afterwards.

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Jared Conway @ Amit Katz on Jan 15, 2015 5:31 PM

does it need experimentation? shouldn't it be a combination of the water vapor and carrier gas?

i don't know if this will be the best forum for getting that info.

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Jared Conway Jan 12, 2015 2:57 PM
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what are you trying to design and analyze in this case?

what is the goal of the analysis?
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- Amit Katz @ Jared Conway on Jan 13, 2015 9:02 AM
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  I'm trying to refine the design of a nebulizer. The goal is to facilitate mist flow out of the device and minimize condensation on the inside walls.
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  - Jared Conway @ Amit Katz on Jan 15, 2015 2:29 PM
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    so how does that translate into how you're going to setup the analysis
    
    we can't predict condensation on the walls, so how will you evaluate the flow to know if that will happen? because that should point you in the direction of how to setup the analysis.
    
    to me, all that you can do is create the normal environment and then create an inflow of the mist (maybe air with certain RH or like you said a gas that is heavier) and then look at the temp of that gas relative to the temp of the walls
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Amit Katz Jan 15, 2015 2:42 PM
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The amount of condensate on the walls will depend on the interaction between the two fluids. This is not something that is easy to quantify, but basically the way this thing is design is like a cup within an upside jar. I want to get the mist to just overflow the "cup" without touching the walls of the "jar".
What I'd like advice on is how to find the required thermodynamic properties for the mist such as viscosity, density etc... from actual experiments. I'd like to see if anybody has experience on doing experiments like this with the goal of simulating the flow with CFD afterwards.
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- Jared Conway @ Amit Katz on Jan 15, 2015 5:31 PM
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  does it need experimentation? shouldn't it be a combination of the water vapor and carrier gas?
  
  i don't know if this will be the best forum for getting that info.
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Advice on modeling mist/fog

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