Relative humidity approach

mtalha976mtalha976 Member Posts: 4

Hi ,

I have patched the whole volume with with constant relative humidity 30% by giving mass fraction for h20 by selecting h20+ air in species transport and temperature 303k. I am giving the air flowing through the mouth 100% humidity and body temperature. And my fan velocity is zero with zero mass fraction and temperature 293k (or I have to give 303k temperature?).

After initialization the whole domain is 30% humidity but after calculation by running for some time the humidity is changed ,

Is that change due to mouth air ? Is my approach right to give constant ambient temperature and constant relative humidity.

I am confused in this matter.

Kindly help me


Best Answer


  • DrAmineDrAmine GermanyPosts: 6,918Forum Coordinator

    Change is expected as you are mixing two streams with different RH and it should repsond to any change in pressure, temperature. Do you know the psychrometric chart? Mollier Diagram?

  • mtalha976mtalha976 Posts: 20Member

    Thank your for your reply . Yes i know the charts and diagrm.

    So my approach is correct? I am using the online calculator for mass fraction.

  • mtalha976mtalha976 Posts: 20Member

    furthermore how can i calculate the total flow rate for dpm given the total mass of particles and number of particles?

  • RobRob UKPosts: 8,970Forum Coordinator

    If the mass fraction or temperature changes then yes, I'd expect RH to change. The above sounds reasonable, but double check the RH of exhaled air, it won't necessarily be 100%.

    We use the parcel method for the DPM model, so track the mass passing though a boundary: that's covered in our documentation (you have access), training materials (you can pay to see those) and open literature (look in the library). If the injection is set to a constant size and there is no weighting each parcel weighs the same.

  • mtalha976mtalha976 Posts: 20Member
    edited April 1

    Yes the mass fraction and temperature changes of the mouth air. But not the fan air because of zero velocity and temperature remain same as ambient temperature.

    how can i check that it is 100% for exhaled air because i have given the whole domain RH 30%.

    Parcel method means i have to give total mass at parcel tab in dpm and question is still there how the mass flow rate is calculated for dpm. The diameter of particles is not constant.

  • RobRob UKPosts: 8,970Forum Coordinator

    RH of exhaled breath is probably in the literature. Otherwise initialise the domain at the temperature & mass fraction of the exhaled air and check. Re-initialise before beginning the run.

    If droplets are changing size you'll need to use the DPM reports, check as there's a mass report too.

  • mtalha976mtalha976 Posts: 20Member

    Thank you for clearing my confusion.

    By reports you mean that I can see the injected total mass of droplets which is given by parcel method? or i have to give flowrate which i read somewhere mdot= mass of droplets * time step size. Is that correct?

  • DrAmineDrAmine GermanyPosts: 6,918Forum Coordinator

    The mass flow rate of particles at the injection (mouth) is just the amount of mass you want to add per time. Forget for now the mass of the single particle. Just check the mass flow rate of the particles from the mouth.

  • RobRob UKPosts: 8,970Forum Coordinator

    You set the injected mass via the injection. Use the DPM Reports under Solution - Report Definition to work out what goes where, escaped mass, evaporated mass and mass in domain should give you what you need: note you may need to manipulate the data to work out

    mass in = mass evaporated + mass escaped + mass in domain

  • mtalha976mtalha976 Posts: 20Member

    okay i understand this concept. But I am not able to inject the total mass of droplets , what could be the formula that i have to give to the total flow rate ?

  • DrAmineDrAmine GermanyPosts: 6,918Forum Coordinator

    We are working with Parcel Method. So please have a look into the theory to understand that method ( we are not injecting particles we are injecting representatives of particles each of them have a strength ).

  • DrAmineDrAmine GermanyPosts: 6,918Forum Coordinator

    What do you know about your droplets? Screenshot of your injection panel.

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