Have someone experience in modeling cyclones?

I have to model a gas-solid cyclone: a gas-solid flow enters the cyclone. The gas exits through one outlet and the solid falls down to another outlet.



<-gas outlet
_|_|_
_| |
inlet _| |
| |
\ /
\___/
| |
<- solid outlet




I'd like to know how to set outlets boundary conditions, because I don't want any solid to exit the cyclone through the gas outlet and, viceversa, I don't want any gas to go through the solid outlet.

Thanks in advance

ap

A friend of mine did his PhD on modelling cyclones - I think dense medium cyclones to be exact - using FLUENT.

The thesis will be under the name of Danield Suasnabar at the University of New South Wales within the Faculty of Engineering and (I think) School of Mining. It was completed in 2000/2001.

That has a lot of details which you probably should look at.

Greg

Thanks Greg.

Do you know if it is possible to have access to that thesis via Internet?

Hi

ap

Doubt it - you'll have to request it through an inter-library loan.

Ok, thanks again.

I ever do my research in cyclone. Our group aim to develop some new cyclones --more collect efficient and lowest pressure loss--used in flue turbulent machine industry.

Though the simplicity of cyclone, it represent some most characteristic appeared frequently in many flow, such as intense rotation which many turbulence models can't cope with, inverse gradient transportation which little model can interprets, PVC phenomena etc. To my known, the CFD can¡¯t simulate thoroughly the flow of cyclone, but some important characteristic can be disclosed by CFD. your opinion is welcome.

Why do you think 'gas-only to gas-outlet' and 'solid-only to solid-outlet' ?

Some solid should exit to the 'gas-outlet' and some gas should exit 'solid-outlet', according to the pressure of each location. I'd like to define 'upper outlet', where gas flow is dominant, and 'lower outlet', where sold flow is dominant. It is natural definition in accordance with real physics, I think.

Sincerely, Jinwook

In answer to your question, I would say that you can use a pressure outlet for the "gas outlet" and simply a wall for the solid outlet. Assuming you are modelling a low concentration of the solid phase to the gas phase you can use Fluents discrete phase options to inject in particles at the inlet. At each boundary you can select whether the particle is reflected or escapes. Hence the bottom wall you set to escape and the particle leaves the domain and the gas stays. At pressure outlet the gas exits and reverse flow is allowed to form depending on your inlet conditons. Also if you set this bounary to reflect the solid particles will reflect back into the domain .

I agree with Jin-Wook Lee on his commments about particles exiting the "gas outlet". Some of the very tiny particles (microns) will follw the flow flow field and exit here. I guess it all depends on your solids concentrations and size.

Also I would add, you should you the RSM turbulence model and also use second order upwind for discretization. Try and mesh using a hex grid as this tends to give more stable and quicker results.

Good Luck.

I know that physically some solid goes through the gas outlet, but this cyclone is part of a circulating fluidized bed loop.

We put a pre-defined amount of solid in the reactor and we want to keep it constant, so we don't want any solid to go out.

If solid goes out, we should insert a sort of solid feed, which is not present in the real reactor.

Some gas in the solid outlet is acceptable.

What are upper outlet and lower-outlet? FLUENT allows Outflow and Pressure outlet BC.

Hi :=)

ap

I can't use Discrete Particle Model because of the high density of solid phase.

The CFB reactor (see my answer to Jin) is modeled using a multi-fluid approach (Eulerian model) together with the kinetic theory for granular flows.

The cyclone is placed at the exit of the reactor and its purpose is to separate solid particles, which are feeded to the reactor again.

Turbulence is described using a modified version of k-eps model. RSM model is not available for multiphase simulations with the Eulerian model.

I could simulate the cyclone indipendently from the reactor, but my goal is to do a simulation using the entire Reactor-Cyclone loop.