April 27, 2018 at 1:58 amh395523899Subscriber
Why is the mass transfer rate zero, and is there a diffusion process?
Is this diffusion caused by convection?
April 27, 2018 at 7:19 pmraul.raghavSubscriber
Convection and diffusion are different mass transport phenomena. Convection describes the bulk downstream transport of the fluid with a certain velocity u; Diffusion is a mass transfer phenomenon which occurs due to the random molecular motion (Brownian motion) in order to achieve an equilibrium state of uniform spatial distribution. The general transport equation is given by
So make sure you frame your question correctly and carefully. And moreover, it would very difficult to comment on your issue without knowing what you're trying to do or what physics you are trying to model and how you're modeling them. Could you provide us more information about that?
April 30, 2018 at 2:29 amh395523899Subscriber
April 30, 2018 at 2:32 amraul.raghavSubscriber
Can you attach a schematic of your geometry with the boundary conditions you specified? Species transport model with laminar flow sounds like the best approach for the problem. How did you define the salt water in Ansys Fluent? And how did you define the different concentrations at different regions of your geometry?
April 30, 2018 at 5:50 amh395523899Subscriber
The 2d model is 10m long and 20cm wide
Entrance speed 0.1m/s
Pressure outlet 0
Do not consider the temperature
What I am simulating is potassium chloride solution
The concentration is converted to a mass percentage by the degree of mineralization. For example, 1000 mg/L can be considered as a 1000 mg/1000 g potassium sulfide solution.
I set the concentration of the incoming liquid, and then set the concentration of potassium chloride solution in the container
However, there is no potassium sulfide solution in the material library, so I replaced it with hydrogen ions first.
April 30, 2018 at 5:53 am
April 30, 2018 at 2:12 pmh395523899Subscriber
I seem to have found my problem. I have set the speed too high. The actual fluid is already turbulent, so it causes diffusion, although the mass transfer coefficient is zero.
I reduced the speed, controlled the Reynolds number within the laminar flow range, and the mass transfer rate was 0, and there was no diffusion between the liquids.
One problem here is that if the boundary conditions I set are already turbulent, then laminar flow is selected for simulation. What will happen? Does the program simulate the turbulence? Or does it simulate a bunch of wrong results according to laminar flow?
April 30, 2018 at 5:27 pmraul.raghavSubscriber
I thought I understood what you were trying to model but I'm again confused reading your last post. Would you able to answer the following questions?
1. You mentioned that you were trying to model "the diffusion of ions in the presence of low concentrations of salt water in place of high concentrations of salt water". The transfer of the species due to a concentration gradient is a diffusion process. In order to simulate this, you would have to make sure that diffusion dominates. And you can achieve this by setting the inlet velocity very low.
2. When you say you set the mass transfer coefficient zero, what do you mean by it? Is it because you set the mass diffusivity zero?
3. If the velocity is high, convection dominates and the effect of diffusion would small. And if your aim is to model diffusion, you would have to reduce the velocity (depends on your problem) such that the convective effects are small. And by setting the appropriate diffusion coefficient at low enough velocities, you can model diffusion. And again, for diffusion, you would need to have regions of different species concentration.
4. Laminar model numerically solves the Navier Stokes (NS) equation while the turbulence models solve the Reynolds Averages Navier Stokes (RANS) equation. I'm just talking about RANS turbulence models here. There are other turbulence models like LES, DES which I'm not mentioning here just to keep things simple. So if you model a turbulent flow using the laminar model, it would not be able to capture/estimate the fluctuations in the flow which is caused due to turbulence. A turbulent flow has eddies of various length scales and timescales which needs to be resolved or modeled by using turbulence models.
And if you want to learn more about turbulence, I can suggest you some resources which would be of help.
May 7, 2018 at 12:55 pmh395523899Subscriber
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