Learning Forum

[Abbasraza20000220]

Hi to all.

I am trying to induce moisture over a surface (coupled wall) between two different fluids. I put values in mass fraction of H20 using species transport but after simulation, I was getting nothing on mass fraction of h20 in post processing section but on mole fraction of H20. Why it is like this?

Please Seniors help me.

Regards,

Abbas Raza

[Konstantin]

The answer is likely in the definition of molar and mass fractions. Mass fraction distribution along the surface looks to be constant (the artifact at the plate edge is probably due to the nodal interpolation due to post-processing). But why molar fractions should be constant? Find the relation between mass and molar fractions (an appropriate text book or web search should get you one), and you'll see that it's a function of molar weights. So if you have more than two species, and the other species vary along the surface, then the molar fraction will vary as well. n

[Abbasraza20000220]

In simple words, mass fraction of a component in a given mixture is equal to the ratio of mass of a component to thetotal mass of mixture. Similarly, mole fraction is equal to ratio of moles of a component to the total moles of mixture. In this case, I am giving input as a mass fraction (H20) but mass fraction (H20) in post processing is showing nothing. I also gave input as a mole fraction but result is the same.nRegards,nAbbas Razan

[Konstantin]

please clarify mass fraction showing nothing. What is nothing, is it zero or do you mean something else? From your plot, mass fraction on the wall is 0.2, right? So it has a constant non-zero distribution. Based on the information provided, it's impossible to say whether this is correct or not, I'll let you dig deepre into this. nBack to the mole fraction distribution, denoting mass fraction as w_i, mole fraction x_i will benx_i =(w_i/M_i)/(SUM_j (w_j/M_j)nwhere M_j are respective molecular weights of species. So if you have more than two species and other species distributions vary along the wall, then the denominator is not necessarily constant along the wall and mole fraction of H2O doesn't have to be constant when the mass fraction is constant.

[Abbasraza20000220]

Sorry I could not deliver my point. nMy point is that the colour of contour along the surface is red ( completely ) while selecting mass fraction of H20. In contrast, on selecting moles fraction 0f H20, the colour of contours is showing variation from blue to the green. by looking at contours of both, its appearing moles of h20 are changing but mass fraction 0f h20 is constant. from this, I am just thinking that mass fraction part (Contour color distribution) should be changed a little bit.nin this case, I have air & h20 mixture. nRegards,nAbbas Razan

[Konstantin]

Thank you. If it's only two species, then the mole fraction should be constant if mass fraction is a constant for the 1st species. Can you plot both mass and mole fractions with Node Values off and attach both plots? This will show true cell center values at the adjacent cells as they are calculated by the solver and remove any post-processing interpolation artifacts. Also, is this R16 you are using, right? n

[Konstantin]

Also #2: what is the species boundary condition on wall-part-air-part-lid?n

[Abbasraza20000220]

I take mass fraction on wall-part-air-lid is 0.2 Yes, it is Ansys 16.1nPlease have a look in the attached screenshots. This is what you are asking from me?nnRegards,nAbbas Razan

[Konstantin]

I was more interested in seen a contour plot of mass fraction with node values off, but this is also good. So cell centered values of adjacent to the wall cells show a mass fraction variation which is consistent with mole fraction distribution, as it should be. So what you were observing is a post-processing artifact. In general, when Fluent post-processes with node values on, it interpolates from cell centers to the nodes. If a quantity is defined at a boundary, e. g. no-slip velocity or mass fraction of species as in your case, then this exact distribution is displayed instead of interpolated values. Apparently, R16, which is very old version, displays interpolates mole fraction values, but shows exact mass fraction. This was made consistent in newer releases. n

[Abbasraza20000220]

Thanks a lot for your response.nI need to know more things for having better understandings.n1- As I am giving boundary condition mass fraction of H20 and i have to observe moisture content distribution across the surface. In other words, moisture content from one face to adjacent (from one side to other side) I will see either mass fraction of h20 or mole fraction of h20. Both contours are serving the same purpose for depicting moisture content? or I have to focus only mass fraction. Given the fact, I am giving input in mass fraction of h20.n2- In Ansys Fluent, while I am inputting mass fraction of H20 in boundary condition, Fluent takes this input in kg/kg or in g/kg? nRegards,nAbbas Razan

[Rob]

Mass fraction is just that, so units will be consistent. Note, for UDFs we assume the input is in SI so kg, m, K etc. nMass and mole fraction are equivalent but won't give the same numerical value. Given the comment re the interpolation I'd update to the current release to avoid any confusion. n