Learning Forum

[Beb99]

Hey all,

I'm trying to simulate evaporation in a discrete particle and ran into something unusual.

My injection inlet is a discrete phase mixture of a 30% polymer(non-evaporating) and 70% volatile component(liquid, can evaporate) at a temperature of 180C.

The boiling point of the volatile is 85C (at 1 atm) and I've added the volatile in gaseous form too in the material tab and selected it in the evaporating species tab. I'm expecting the volatile liquid to instantaneously evaporate but it is not happening at the boiling point of 85C (Please, note that the temperature of the inlet particle is much higher than its boiling point and the pressure of the domain is not very far from 1atm range).

But, when I raise the boing point to let's say 120C (previously 85C) and keep everything else the same, I'm seeing significant evaporation and when I raise it to even further let's say 300C, every particle in the volatile liquid is evaporated.

Can anyone help me to understand why this is happening and why it is not evaporating in the original condition (BP of 85C) in the first place?

The domain is a cylindrical pipe with dry air at the inlet surface, the discrete particles are injected circumferentially at some distance from the air inlet. The opposite face to the air inlet is the outlet.

Thank you,

Beb99

[Rob]

I assume you set the volume fraction in the injection? Please post a screen shot of the injection settings.

[Beb99]

I'm sending the images of the settings I'm using. Let me know if you need anything specific
Thank you Beb99

[Amine Ben Hadj Ali]

Multicomponent will use cell pressure and vapor pressure information to model vaporization and boiling.

[Beb99]

I'm attaching the images showing the particle temperature, particle boiling point, particle pressure, and particle volatile component mass fraction (starts at 0.7). As you can see that the temperature of the particle is very high compared to the boiling point. So, I'm expecting very high evaporation but not seeing very little. I'm using steady-state as I need properties at the steady-state only. Will it help to do transient? I'm not sure what is it that I'm doing wrong here.

[Amine Ben Hadj Ali]

Again the vapor pressure is used here for the mass transfer. Can you tell the related saturation pressure to that boiling temperature?

[Beb99]

The saturation pressures at different temperatures are as follows:
60C for 0.5e5 Pa
80C for 1e5 Pa
106C for 2e5 Pa
146C for 5e5 Pa
184C for 10bar

For example, at 1e5 Pa, any temp higher than 80C should cause evaporation right???

[Amine Ben Hadj Ali]

Again the Multicomponent mass transfer laws are driven by pressure differences. If you Total Vapor Pressure (in your case only one component is volatile) is now smaller than absolute pressure->Evaporation.

[Beb99]

So, does that mean if I increase the pressure I should see evaporation???

[Beb99]

Also, can you tell me how to get the mass flow rate of the discrete phase through a surface? I know we can do this for the continuum phase easily, but I couldn't find this for the discrete phase.

[Amine Ben Hadj Ali]

You will need to do sampling along the zone surfaces or plane. Is not like an Eulerian Fluid: we have discrete particles.