Learning Forum

[Antonio Acosta Iborra]

We are trying to simulate a particle-gas fluidized bed heated by thermal radiation. For that we are using the multiphase Eulerian model and the  Discrete ordinate (DO) model of radiation. Does anyone know how ANSYS Fluent incorporates the Eulerian model in the Radiative transfer equation (RTE) of the DO model? In other words, which parameters in the RTE (equation (5–75) in the Theory Guide, attached) contain the effect of the particle volume fraction, particle size, particle absorptivity, etc.?

Besides, where can we find the dependency (submodel used by ANSYS Fluent) of the RTE parameters on the particle volume fraction, particle size, particle absorptivity, etc. ?

Thank you very much in advance!

[Rob]

You might want to read the Theory Guide https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_th/flu_th_sec_rad_intro.html   DO Limitations section. 

[Antonio Acosta Iborra]

Hello Rob,

Many thanks for the reply. We have checked the Theory Guide and, effectively, it says that the DO model "is not supported for use with granular (fluid-solid) Eulerian multiphase flows." So our question is now: which ANSYS Fluent radiation models (excepting DO) would be more suitable to determine the radiative heat transfer (mostly infrared radiation but also Sun's radiation) in a gas-solid bed simulated with the Eulerian model?

Again, thank you in advance!

[Rob]

How fluidised is the bed?  If you look at the options which do you think may be best suited?

[Antonio Acosta Iborra]

Our bed of particles is fluidized from below in bubbling regime, so that we have apparent bubbles rising in the bed (large voids with particle volume fraction equal to 0 there) that stir the dense phase (rest of the bed with air-particle dense mixture, having particle volume fraction around 0.55). I would say that perhaps the radiation models P-1 and MC would do, but we are not really sure if they are suitable for this kind of beds. Thank you in advance!

[Rob]

P1 will be cheaper, and most (all) radiation will absorb pretty much as soon as it hits the bed: just make the granular material very high absorpion coefficient. I'm not sure how it'll behave in the bubble cavities. MC will do much the same but costs more. 

[Antonio Acosta Iborra]

Thank you Rob. So what about the other alternatives (Rosseland, DTRM and S2S)? Would any be better for this application?

 

 

[Rob]

DTRM is/was serial only so not commonly used. I'm not sure I've ever seen anyone use Rosseland. 

Read how S2S works and tell me why it's not useful here. Hint, how does it handle the absorption coefficent?

[Antonio Acosta Iborra]

I have checked the S2S model and it seems it only considers gray-diffuse surfaces and non-participate gases. Well, my concern is how the S2S and MC models handle the eulerian solids phases. In the Theory Guide I cannot find specific information about that. Does the S2S and MC neglect the interaction with the solid phase in the eulerian model? If not, what kind of submodel are the S2S and MC using for that interaction? I am sorry about the large amount of questions I am posting...

[Rob]

S2S ignores the fluid absorption (ie non-participating gases - they're fully transparent), so it'll not see the granular phase either. MC sees the absorption coefficient so will transfer all of the radiation energy to heat as soon as it hits the granular phase. 

No problems with questions, they're part of learning how stuff works. 

[Antonio Acosta Iborra]

Thank you very much Rob. So S2S is not applicable to our problem but MC seems an interesting option. What I am note sure is how the MC model in ANSYS Fluent deals with the absorption coefficient of the solids particles and their scattering in the eulerian phase (submodel used of the effective absorption and scattering of the solids phase in the Eulerian model). For example: Is the MC model doing an average of the solids absortivity (and the same for the scattering) weighted with their volumetric fraction? Where can I find such information?

[Rob]

It's a volumetric mixture. There are no extra bits for granular scattering, it's just absorption.  

[Antonio Acosta Iborra]

Thank you Bob. OK. Just a final question (I hope): In the P1 model, is the scattering of radiation by the solids-phase of the Eulerian Model not taken into account? (I am not sure whether Equation 5.57 of the Theory Guide is applied to the Eulerian model or only to a discrete particles model). As always, many thanks in advance!

 

[Rob]

Eulerian should just cause scatter based on the volume fraction and fluid properties as there isn't a distinct bubble/droplet/powder phase as such. 

[Antonio Acosta Iborra]

Ok, thank you Rob 👍