Learning Forum

[maliuzair]

Hi All, 

I am simulation biomass drying using thermal phase change evaporation-condensation  model (eulerian multiphase).

initial conditions:

Biomass initial volume fraction: 0.6, Temperature: 300 K

Air: initial temperature 300k... gradually increasing with time

after initializing, i check contours of static temperature of biomass. It shows 300 k throughout the domain. 

when i run simulation for 1 time step (or even more), i check the contours. It shows 300 K where biomass is patched and the rest of the domain shows 373 K. Attached pics. 

 

Then I tried with the energy equation turned off in the beginning, and later I turned on the energy equation. The instant I turn ON the energy equation, the warning comes..

Temperature limited to 1.000000e+00 in 15337 cells on zone 2 in domain 3

and the area weighted average of biomass-temperature drops to less than 50 K from 300 K. 

Can someone please address this issue. the temperature should be 300K throughout the domain. I have checked again and again by turning off the evaporation condensation model. when i turn off the model, there is no such problem.

 

Thanks

[DrAmine]

How is the phase change defined? Please attach some Screenshots of phase definition and interactions. Bear in mind thermal phase change ist not proper for multiphase multicomponent cases. Moreover check if you have unphysical backflow 

 

[maliuzair]

Dear Sir

Which model is suitable then for drying, because for Lee model, experimental data is required to adjust the coefficient and there is no calibration required for the thermal phase change model.

 

Here biomass is a mixture of biomass and liquid water. Drag correlation used  is Gidaspow's

[klu]

Hi, 

May I know how the condensation will be triggered? Is there any wall thermal boundary condition that continues to take heat out of flow? I remember that Thermal Phase Change model cannot deal with condensation occurring on walls. Therefore please make sure all walls can have only insulated conditions.

[maliuzair]

Hi KLU,

I am not sure if I understood properly. I am just a beginner. the walls are adiabatic.

Besides I have another simple heat transfer model between air, sand and biomass. Both sand and biomass are granular. Initial conditions are 300 K. I let it run for sometime. But it is behaving abnormally. The temperature shouldn't drop below 300 K.

The boundary conditions are adiabatic walls, velocity inlet for air, and Pressure outlet.

Based on this case and the evaporation model case, what do you suggest. Where is the problem? What am I doing wrong. 

[DrAmine]

Thermal phase change model is not proper for multicomponent flow. You need to drive your bulk phase change by means of relative humdity say partial pressure to saturation pressure. You can even use the Lee model but adjust the saturation temperature to fulfill Antoine equation at RH Equal t0 100%. On the other hand you can make use of the Ddpm Model and get advantage of the more accurate heat and mass transfer for DPM for combustion or droplet or multicomponent particle. 

[maliuzair]

Thank you sir. I will have a look in to your suggestions.

Meanwhile can you also please address the second issue in this post, i.e.  simple heat transfer between air, biomass (granular) and sand (granular). (No mixture)

Thank you and Best regards

[DrAmine]

Check if you have any openings conducting cold flow from the outside due to backflow (at your pressure outlet). Backflow quantities are always stagnation quantities. So if your air is flowing back with 300 K then it would enter with temperature slightly smaller then 300 K depending on compressibility and pressure in neighborhood.  Moreover please move to the most actual release: there were some issues which have been fixed and might affect your solution.

[maliuzair]

Dear Sir,

Yes, the backflow temperature is set as 300 K, but the temperature instantaneously drops just above the bed height at the interface of solids and air after 5-10 time steps.

 

[DrAmine]

Please post the initialization panel here especially temperatures, vof and mass fractions.

[maliuzair]

Dear Sir,

One solid granular phase was patched as 0.4 vof, and the other as 0.2.

I would like to add that I encountered this problem when I used 2 granular phases.

There is no problem when I use 1 granular phase.

Thanks and Best Regards

 

 

 

[DrAmine]

Which heat transfer correlation's have been used? Which release are you using ?

[maliuzair]

Sir,

For gas/solid interactions, GUNN

For solid/solid interactions, TOMIYAMA

Version: ANSYS 17.2

 

Best Regards

[DrAmine]

So tomiyama is only suitable for interaction with fluids. Is you air compresible? Please use the most current releases as we have enhanced a lot of stuff in the previous versions.

[maliuzair]

Sir, the air is in-compressible. 

As far as tomiyama model is concerned, tomiyama has been used in literature for solids-solids. though i ll try with other suitable model.

Can there be anything wrong with the 2 granular phases, since it works perfectly fine with 1 granular phase. What can possibly be wrong?

 

Best regards