Simulation of the airflow through a bunch of little balls 2

[HilaryE]

Dear All,

I hope you are well.
I start my work with Ansys Fluent (I did a few airflow analyses long time ago).

I need to model the airflow through a bunch of little balls (see attached draft and picture on it, these balls are for illustration only, the actual balls look somewhat different).
Balls are stored inside 1mx1mx1m container (see attached draft). Each ball has approx. diameter of 10mm.
These balls are inserted into the container after their coating is finished (balls are a product of the food industry, they have softer inside and harder/coated outside).
The balls are conditioned in the container, i.e. the air flows through the container/between balls, to allow some water evaporation from balls and to prevent lumping between balls.
I need to model this airflow process (between balls) and verify whether the airflow is approx. uniform across the container and to check whether there are some stagnation points.
The attached draft presents the location of air inlet and outlet in the container. The air is ‘pushed’ into the container (through inlet hole, see attached draft) and is allowed to flow through the container (between balls) toward the outlet hole. The temperature of air at inlet, inside the container and at outlet is a ‘room’ temperature.
I plan to conduct, at least for a start, two 2D analyses on the planes that correspond to sections A-A and B-B (see attached draft).

I have a few questions:
1. Should I consider the airflow inside the container (especially near inlet) as compressible? What do you think? There are tens of thousands of balls in the container. The actual conditioned product may also have rectangular or pillow like shape (instead of ball/round shape), and can cause (perhaps) considerable resistance to the airflow.
2. I consider using model: viscous laminar, k-epsilon/realizable, non-equilibrium wall functions, and solution method: pressure velocity coupling. Would you apply different models and solution methods?
3. Do you think that 2D simulation will do to effectively simulate the distribution of airflow in selected areas of the container? Is 3D simulation (of a portion of container) necessary? For now, I consider 2D simulation as a good start point (on a plane of section A-A). I want to run a few analyses and see what results I will get from 2D simulations.
4. I wonder what would be the best and simplest way to validate the simulation model. My idea is to locate a few little sensors inside the container (filled with balls). I am not sure what kind of sensors they could be (humidity, air speed, …?). The easiest could be using little thermocouples, but the air temperature is everywhere approx. the same. Any ideas?
5. I know the water content (in %) in the balls. Is it possible to simulate the drop rate of water content (in the ball) at a specific magnitude of airflow?

Please help guys.
Regards

 

[karachun]

1. No one can answer this question without data about air speed and pressure drop. Compressible flows used at speeds greater than 0.3M or when gas volume have big changes, like air compression in diesel engine.
2. Calculate Reynolds number based on average velocity and mean distance between balls. Now you can decide is flow laminar or turbulent. If flow is turbulent then you can try k-e or SST models.
3. No, better variant is to cut small cube, calculate it and then obtain pressure drop coefficients and model whole balls as porous media.

 

[mskds545]

I would also do this in 3D with a porous media model. For spheres, you can use the Ergun equation to calculate the resistance. For rectangles or pillows, you might want to measure it. Consider the impact of packing patterns and density on flow resistance. Rectangular and pillow shapes might have a lot of variation in packing, which will dominate the flow patterns. Even spheres can pack non-uniformly, but to a lesser extent. This would not be included in your model.

-mskds545

 

[HilaryE]

@karachun and mskds545
Thank you for your kind replies. Your responses are very informative. I am grateful.

Please allow me to ask two more questions:
1. I read some papers presenting similar simulations (e.g. airflows through the grains stored in silos). But to be honest, I am still not sure how should I actually develop a porous media (in what way) in Fluent to physically represent a bunch of tens of thousands of little balls and 'voids' between these balls. I would be grateful for any tip.

2. In terms of modeling of the drop rate of water content (in the ball) at a specific magnitude of airflow, I am looking in the literature, what material properties I need to know in order to effectively model it. Is the material heat capacity one of the most important? Would there be any other critical material properties that I should consider?

 

[mskds545]

For question 1, use the porous media model in Fluent. This is pretty straight forward. See

https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node233.htm

from the manual. Also,

https://en.wikipedia.org/wiki/Ergun_equation.

This part should be pretty straight forward, so long as you can find or develop Ergun parameters for the media shapes you plan to use. I can't really help you with question 2. It is certainly possible to lump the mass transfer and include it in the bulk porous media model, but I've not done it myself. I expect this part will be more difficult. If the relative humidity of the air stream isn't changing much from the inlet to the outlet, maybe just look at the drying separately as a function of velocity.

-mskds545

 

[HilaryE]

@mskds545
Thank you for your quick reply. I am grateful.
I will familiarise with the linked material.
Thank you again.

 

[HilaryE]

Dear Guys,

Porous media modeling - input data
Based on received advices, literature review findings and search of other forums, I have concluded that for porous media modeling I need to collect the following input data (before modeling starts):
- The geometry of balls (diameter)
- The geometry of convex and flat tablets - besides balls I also need to conduct the airflow analysis for rectangular tablets of convex and flat shapes, so I will also need the precise CAD models of these.
- The input velocity of the air penetrating these balls.
Would there be anything else that I may have overlooked?

Porous media modeling
Modeling in 3D certainly takes much more time than in 2D. I do not have much time allowed for this modeling.
For this reason, in order to shorten the overall modeling time, I wondered whether it would make sense to apply the following procedure:
Step 1 - I would take a small subset of the box containing balls and do a simulation (with real balls) to get data showing flow rate v's pressure drop. So this part would be done in 3D (I do not have an access to a lab and I cannot get experimental data). I would run airflow simulations over this multiple flow-rates, say 3-5, and get a graph. Using this graph, I could calculate resistance coefficients required to use in a porous model (based on the procedure given in Fluent manual).

Step 2 - Then, I would use porous media modeling with use of 2D model. If something like that makes sense. I would conduct such 2D analysis for the entire cross-section of the container that is filled with the balls. The customer is predominantly interested in general information on how good their process is in an effective penetration of balls and in identification of areas where there might be a potential risk of stagnation point.
What do you think. Does it make sense?

Thank you.
Regards

 

[mskds545]

It's an 80 mm opening in a 1 m wide box, and you're looking for dead spots. I don't know how much insight you'll get with a 2D model. If you're using a porous media model, I wouldn't think the 3D model would take much computational time, and model setup is fast and simple.

You can do a model of the shapes to get the resistance coefficients, as you suggest, but I don't know how you do that in 2D when the shapes are 3D.

In addition to the ball geometry, you'll need to understand how they pack into the bed. The void fraction and orientation of the particles will have an impact on the resistance. Non-uniform packing with rectangular particles will have a huge impact on the flow distribution -- it will probably dominate the overall behavior, making a model that assumes uniform packing moot. Consider that rectangular particles can make a solid wall.

A test could be pretty simple. A plastic tube, a manometer, an orifice, and the outlet end of a shopvac would probably suffice. If you can't do a test, though, see if you can find Ergun parameters in some textbooks to compare to your small explicit model. And do a sensitivity study.

Good luck!

-mskds545

 

[HilaryE]

@mskds545
Thank you for your answer.
I work with tight time constraints. Therefore, I am looking for ways of making the simulation simpler and a calculation time shorter.
I was not aware that the calculation time for 3D porous media model can be relatively short (although I do not know how well my PC will deal with this).

Indeed, this 2D-3D plan is somewhat 'tricky'. I guess that I will run simulation in Step 1 to get coefficients and then in Step 2 I will develop a 3D porous media model and will see how fast the model is calculated.

Some of the products that I need to 'simulate' are of rectangular shape (like pillow or flat). In Step 1, I will arrange these elements randomly (to mimic the reality).

Thank you for your idea on validation of the model, I like it. The company (customer) is located far away, I will not travel there, I need to develop a simple validation method as I will rely on feedback data provided by instructed local people.

 

[HilaryE]

Dear All,
Thank you again for your kind answers to my earlier questions. I started my modeling in CAD software and Ansys. I wondered whether you could give me a few more hints.

Input data (from the 'real' world)
I developed a list of input data that I needed to collect in order to develop a porous media model 'representing' a real scenario The list refers to a two step procedure outlined by me in earlier post dated on 23.03.2020 at 19.21 (GMT). The list includes as follows:
1. Geometry of a container that contains balls
2. Geometry of balls (besides balls, studied element are also rectangular tablets/pills - flat and pillow shape). Container can be filled with one element type at the time (e.g. balls only)
3. Application of random arrangement of balls (in step 1 of procedure i.e. preliminary 3D simulation inside a small box containing 'real' balls)
4. Ball's surface roughness
5. Material of the container (in my opinion not needed unless I would like to conduct a thermal analysis)
6. Material of ball's coating (again in my opinion not needed unless the analysis will involve something more than the airflow study).
7. The inlet - Air velocity/pressure or volumetric air flow rate, room temperature
8. The outlet - room temperature, atmospheric pressure
9. The 'inside' of container - room temperature, atmospheric pressure
Is it the complete list?

Roughness of a ball
Ball's (and rectangular pill's) surface roughness is rather a 'regular' one for tablet-like products of food/pharmaceutical industry. I cannot measure the roughness of studied elements. I wondered whether definition in Ansys of such surface makes a big difference (in particular for rectangular pills) if the surface is not 'extremally' rough or deformed.

Is there any set of ready to use 'standard' (approximated) values of roughness?

Definition of contact between balls
I develop 3D model of the container and balls in CAD software which I prefer over in-built CAD in Ansys. I wondered how to define the contact between balls. How important contact definition is for a non-thermal analysis?

If I need to define contact between balls, would this be a good idea to overlap the balls a little bit and develop one solid from bunch of balls. If so, will that create a problem for meshing (small meshing elements in 'contact' areas between balls v's large meshing elements elsewhere). Will little elements of meshing in areas of balls 'contact' cause 'strange' results.
If I do not overlap balls little bit, what would be the best approach to define contact between balls (I will probably have a few hundreds of balls in a small 3D box - in reality there are a few hundred thousands of them in a container).

Porous media model - alternative
While reading papers on airflow study through a bunch of little elements (such as grains), I can read about porous media modeling only. Is it by far the most popular model for such analyses? Did you came across a reasonable alternative.

Thank you.
Regards.