Viscous resistance coefficients in porous medium
 

Fluent manual tell us that, for laminar flow through a fibrous mat, we can calculate the viscous resistance coefficients using Blake-Kozeny equation, and ignore the inertial resistance. In my question, the dimension of the geometry is micro scale, so the viscous resistance coefficient is up to e-10. In this condition, I can't obtain the good result compared with the experimental one. However, when I change the the viscous resistance coefficient to a higher value, for example, e10, the results can be well agreement with the experimental one. Hope someone can tell me what leads to this, and how to calculate the viscous resistance coefficient in porous medium. Thanks in advance.

Viscous and Inertial Resistances
 

The viscous and inertial resistances to flow in a porous medium can be calculated from the Ergun equation, which gives the total pressure drop through a porous medium.
Check any book on fluid mechanics for the Ergun equation
Total pressure drop = Viscous loss + inertial loss

dp/dl = [150*mu*(1-e)^2/phi^2*D^2*e^3]*v + [1.75*rho*(1-e)/phi*D*e^3]*v^2

The first term is the viscous loss (proportional to velocity) and the second term is the inertial loss (proportional to velocity squared).

Compare this to the Fluent's expression for momentum sink:

dp/dl = Rv*mu*v + (Ri/2)*rho*v^2

This gives the values for Rv and Ri,


Rv = [150*(1-e)^2/phi^2*D^2*e^3]


Ri = [2*1.75*(1-e)/phi*D*e^3]


Nomenclature:
Rv: Viscous resistance
Ri: Inertial resistance
mu: Viscosity
rho: Density
e: Porosity of the medium
phi: Sphericity of the particles making the medium (can assume 0.75)
D: Diameter of particles making the medium
v: Average velocity

Hello,

always keep in mind, that Fluent requires the input of 1/a, where a is the permeability you get from your Blake-Kozeny equation. In other words, you have to insert the inverse of a for each direction in your porous zone. In your case a=1e-6 --> 1/a = 1e+6 what is exactly the correct value as you found out at least.

cheers

Hi,
You are right.
Resistance is the inverse of permeability. It is normally in the range of 1e+6 to 1e+10 m2.

Dear SRJP

could you please explain from where I can find diffrent values for phi ,Sphericity ? while i compared with some eurgen equation but i didn`t find this term , I am modelling prouse media in kiln and i need to clculate it, thank you in advance ,

maryam

Hi,
The Ergun equation assumes that the bed is filled with uniform sized and shaped particles. The sphericity parameter is used as a conversion factor for non-spherical particles (comparing the surface-volume ratio of those particles to an equivalent spherical particle).

Of course, for fully spherical particle, the sphericity = 1.

Sphericity = (6/Dp)/(Sp/Vp)

Dp = Diameter of the particle
Sp = Surface area of the particle
Vp = Volume of the particle

For 'not so crazy' shapes, like sand particles, you can use sphericity around 0.8 - 0.9.

Complete list of sphericity values can be found in "Perry's Chemical Engineers Handbook", or "Unit Operations of Chemical Engineering by McCage, Smith and Harriot" or similar books.

ceramic membrane
 

Hi,

i am working in modeling of ceramic membrane with 19 channel.i want to know what mesh i must use for best result.then i must export that geometry to fluent and solve it by porous zone and multiphase flow.

thank you so much

It should be noted that the porous domain calculation in Fluent is an approximate one: It just adds a momentum sink in 3 directions due to the inertial and viscous resistance.
The mesh type will depend on the accuracy, convergence and speed of the simulation. If not difficult, better to go with high quality hex meshes.

hi.
I'm trying to model a micro filtration membrane(MF), are these parameters useful for a filter or a membrane?! and, in porous media approuch via fluent, we should set this two parameters for X,Y direction!!! can we set Ri and Rv from this equations for X,Y direction??
best regards

zohreh

hi.
I'm trying to model a micro filtration membrane(MF), are these parameters useful for a filter or a membrane?! and, in porous media approuch via fluent, we should set this two parameters for X,Y direction!!! can we set Ri and Rv from this equations for X,Y direction??
best regards

zohreh

Yes,
You can still use the porous zone model for the membrane, since the membrane is porous anyway.
If the membrane is homegeneous and symmetric, you can use the same resistance values in all directions (x,y and z). If not, you may have to calculate the porosity and permeabilities separately in the three directions. This would be the case on the regular asymmetric ultrafiltration and reverse osmosis membranes.

dear srjp
thanks a lot for your answer, I want to model a micro filter, This filter (membrane) is rectangular (10×5 cm) with the thickness 12e-5. so,I calculate this parameters from Ergun equation which D=1e-6, because of the Micro filter type. is this assumption true??
i think this is very thin filter so there in not flow through in the membrane, in this situation is inertial coefficieant higher in Y direction? or not?when i set y-direction inertial coefficient higher than x-direction got better answer! is this resonable?

I am looking forward to hearing from you.
Sincerely yours.

asphaltene precipitation
 

hi,

can i modelling asphaltene precipitation that exist in crude oil by uusing ceramic filter monolit by defult formolation in fluent?

i want to modelling separation of asphaltene from crude oil by 19-channel ceramic filter,but i think it is not define in FLUENT that precipitate asphaltene on surface of ceramic channel base on diffrences between pore size.

i must write UDF for this porpuse or not?

most of oil flow must pass through the membrane wall and most of asphaltene must form a gel-layer and precipitation on inner surface od channel.

please help me as soon as possible

thank you so much

asphalteene
 

hi,

can i modelling asphaltene precipitation that exist in crude oil by uusing ceramic filter monolit by defult formolation in fluent?

i want to modelling separation of asphaltene from crude oil by 19-channel ceramic filter,but i think it is not define in FLUENT that precipitate asphaltene on surface of ceramic channel base on diffrences between pore size.

i must write UDF for this porpuse or not?

most of oil flow must pass through the membrane wall and most of asphaltene must form a gel-layer and precipitation on inner surface od channel.

please help me as soon as possible

thank you so much


http://www.cfd-online.com/Forums/ima...ser_online.gif http://www.cfd-online.com/Forums/ima...reputation.gif http://www.cfd-online.com/Forums/ima...ons/report.gif http://www.cfd-online.com/Forums/ima...c/progress.gifhi,

hello, thank's for this explanation of the viscous resistance equation,
but i have a question : we know that: viscous resistance=1/absolute permeability, but in the multiphase flow: viscous resistance= 1/effective permeabiliy , or viscous resistance=1/absolute permeability, i need an answer please.

Simulating Flow over Porous Medium
 

Hi, I have the data from the flow over a bundle of tubes (in the context of a bundle of tubes at the bed of the channel). I was thinking of using the Ergun equation as in the ANSYS manual to calculate the viscous and inertial resistance, where the the tube diameter = diameter of particle, to create a macroscopic simulation of the flow in FLUENT, however, I haven't been able to obtain a similar velocity profile.

Is this valid? Please give me some advice. Thank you and tell me if I should provide more information.

Is the flow across the pipes or along the pipes? This will change the diameter to be used. Also, ergun equation is only for spherical particles, for cylinders, you may have to use the appropriate sphericity.

Hydraulic conductivity of porous zone
 

Hi,

I am trying to model a 2D porous zone filled with sand lied above a perforated pipe and water will pass through it. I have some queries, hope someone can help me out.

1) Porous zone should have a definite hydraulic conductivity(K). But in fluent, I didn't find any input button for this. Does inertial and viscous resistance represent the inverse of K in fluent? If yes, should I calculate the resistance value by the formula provided above for X and Y direction both or only for y direction since flow is in Y direction?

2)If not, then how can I assign K value?

Thanks in advance.

@Tanjina
Viscous resistance is actually the inverse of permeability value and it's regarding to resistance exerted to flow by porous media. As far as I know, there's nothing about hydraulic conductivity in modeling porous zone, but, if you mean the heat conductivity of the porous material, it should be defined by a UDF,due to anisotropic behavior of porous solid.

two-phase porous zone
 

Thank you very much for your reply. Yup, I also come to know that in fluent, we use permeability . And permeability is related to Hydraulic conductivity.

Do you have any experience about modeling of two phase flow in porous zone? Please let me know.

Hi
you're welcome. Yes, my main focus is porous media in PEM fuel cell, and catalytic reactions.

Hi,

Please have a look on my problem and if you have any comment, please let me know.I am trying to model a 2D porous zone filled with sand placed above a perforated pipe and water will pass through it. I faced a weird problem. When I active multiphase flow, input button for rotation axis direction,inertial resistance and viscous resistance simply disappears!! only porosity parameter remains active. I tried with 3D also, same case happened. But I need to model multiphase flow.

Then I have found something. I have to define porosity from mixture phase. And for which zone I defined porosity, I can assign viscous and inertia resistance for that phase which I assigned in porous zone. For example, I named one zone as water. I defined water zone as porous zone from mixture phase, then I can assign other permeable parameter for water zone by selecting phase-2 ( I assigned water as phase 2). Is this the way to define porous parameter in two phase flow?


Could you please suggest the solution methods and ranges for porous media modeling in two phase flow. I am just lost. I am using PISO right now.


Thank you very much in advance

First, let me ask you something. Why do you insist on using multiphase model for your problem, if you consider the filled sand zone as porous media? As you know, porous media considers as fluid not solid! So, I don't see any necessity using multiphase model, as long as there's no further complexity in your model. But, it's hard to make any more comment. Please give me an image of the model, if you would.

Hi,

Please find the image attached herewith.

This is my model. Upper red portion is porous zone which I consider filled with sand. For doing this, I used porosity of sand and inertial and viscous resistance of sand. I want to model in such way that porous zone is filled with water also and it will flow towards the lower pipe ( Pink pipe) and water will enter the red zone continuously from the inlet ( topmost line) so that water height remain constant. Since pink pipe is initially filled with air and water will go through it, so I used VOF model(considering the immiscible fluid). In between red and pink box, i used - - - - line as perforated pipe. I didn't use any BC here, except "-" as wall. ( Actually I am not sure whether I have to use porous jump BC here or since I allowed opening, it will works as perforated pipe automatically). I don't know the inlet velocity, so I used pressure inlet.


Please correct me if I am doing something wrong. Thanks in advance.

Sorry for late
Is perforated area part of porous zone or it is a separate zone? Why did you consider the perforated area? Maybe you need to eliminate this zone. Anyway, if the perforated area is part of model, I don't think that a porous jump B.C should be used for this, because it's not a very thin porous media but a perforated area and these two are quite different. One more thing and the foremost, instead of using a pressure in inlet, use mass flow rate. Keep in mind that adjust "Direction specification method " option to " Normal to boundary" . It's important. Is the upper line of porous zone the inlet boundary? If so, maybe you should extend the model and add an inlet boundary as well. This is what I understand from the image.
I hope it helps.

Sorry for late
Is perforated area part of porous zone or it is a separate zone? Why did you consider the perforated area? Maybe you need to eliminate this zone. Anyway, if the perforated area is part of model, I don't think that a porous jump B.C should be used for this, because it's not a very thin porous media but a perforated area and these two are quite different. One more thing and the foremost, instead of using a pressure in inlet, use mass flow rate. Keep in mind that adjust "Direction specification method " option to " Normal to boundary" . It's important. Is the upper line of porous zone the inlet boundary? If so, maybe you should extend the model and add an inlet boundary as well. This is what I understand from the image.
I hope it helps.

Hello. thanks for your reply.

1)Actually it's not a perforated area, just a line by which I wanted to represent the upper surface of a perforated pipe. Because in my model, we assume only upper surface of the pipe is perforated. Now I ask the question again, should I use porous jump BC or this line will work as a perforated pipe automatically? After running the model, there was flow through this perforated line. Please see the attached image which shows the flow after 2.37 sec.

2)What is the advantages of mass flow inlet against the pressure inlet ?

3)Yup, upper line of the porous zone is inlet boundary. I didn't understand your last suggestion. Could you be a little bit more specific that what do you mean by extend the model and add an inlet boundary though there is a inlet already exists?

Thanks a lot again. Any suggestions will be really appreciated.

Hello again
Well, as I told you before, it seems that there's no need to use porous jump B.C. Respecting to mass flow rate, I think it doesn't make sense to use pressure B.C in inlet. If so, how do you inject the water into system? Do you patch it? From the image, it seems that you use a pressure outlet B.C for outlet as well. As far as I understand, a flow of water flows on the top surface of the system (porous zone). Right? Here, you should use mass flow B.C. By saying " extend your model" I meant that you add an inlet to the model and it helps the physics of the model a bit more sensible. But, it's up to you to decide. By the way, the solution is not converged yet. But I guess the problem is due to inappropriate choose of boundary condition.

Hi,

Thanks for your reply.

1) Yup, there is flow of water on the top of surface. I wanted to keep the water height remain constant although the time. So I put 1 in pressure inlet as water -liquid volume fraction. Is it possible to select the mass flow inlet, if I don't know the mass flow rate ? And if I want to remain the water height constant what should I do for mass flow inlet ? Please enlighten me.

2) I patched 'water" in porous zone. And yes, I used pressure outlet.

3) Where should I use another inlet? what will the the BC there? Should I use inlet at the end of porous zone ?

4)So you are saying that perforated pipe defining is ok with my - - - - - - wall? It will work as perforated pipe?

5) for my existing model( I send the image in earlier message), mass flow rate in outlet is showing -250~-350 kg/s. Is it due to wrong BC ?


Please help me by giving any suggestions. I am stuck here for long time.

Hello again
I'm terribly sorry for the late. I'm too busy and I need to focus on my works. The perforated model is OK and I rant into a model in Fluent specifically defined for perforated model. The model calculates the pressure drop cross the plate. Have a deep look at conditions in porous media in Fluent theory guide. Do have the velocity of the flow? If yes, you can get the mass flow by multiply it by flow density and cross area of the flow.
If you are interest, send the model to me and I will have a look at it.
Good luck

hello guys
 

hello guys ,

I have trouble finding the alpha and beta values for modelling plasma (blood ) flow through a porous media which acts as the filter. All i have is the porosoity report and material properties.
Is it possible to calculate the inertial and viscous co-efficients ?

membrane separation
 

i am working with oily water separation using ceramic membrane
i drew the geometry and mesh and i am stuck on modeling flow through porous media using oily water which has a lot of contaminants
please help me if you have any examples of how to do it and i can see an example that would help
i would appreciate any help
thanks so much

Hello,

I am working on CFD analysis of humidifier. In this two fluids (Air and water) are mixing and the heat and mass transfer will takes place. Water and air are in counter flow. Also at the middle portion of humidifier I have porous media.I am using Ansys-CFX for the simulation. Please help me for this, I don't know how to solve the multiphase problem with porous media in CFX.:confused:

Do anyone have idea about this? how to do the analysis of this in ANSYS-CFX?

Thanks in advance....:)

Thanks for the wonderful info

I completed an experiment where porous material length increases along the height with time. To start with Zero Length porosity but as time passes, part of the material solidifies and becomes Porous.

Average porosity is 11%. Any idea how to I calculated parameters for Fluent ? There is no specific porosity hole size(diameter). It is Zigzag but I have experimental pressure drop results across the two faces.

Thanks in advance.

Shane

I appreciate your answer but how do I get different values of Ri & Rv in different directions using given formulae?

I think Ri and Rv in different directions you can get only in experiment, i think =)

But i have other problem. I have a cylindrical sample with a diameter of 50 mm and a thickness of 3 mm. Porosity 0.3. According to calculation Rv = 7*10^11 and Ri = 4*10^6. For dP = 5.9 kPa i have Q = 0.008 m3/min. BUT in Fluent a have 0.00037 kg/sec -> 0.018 m3/min. 0.018 is not 0.008 !!!!

Whats happend?
Attachment 43659
Attachment 43660

Can you help me? thx a lot!

Anybody helped me?

hi everybody! I've some problem setting the viscous resistance too. hope that someone want to help me :) i'm trying to simulalte the flow through a bundle of fiber, considering the bundle of fiber as an anisotropic porous medium. As the entire bundle is an anular cilynder, I was trying to use the conical/cilyndrical coordinate.
What i have actually is the value of the viscous resistance in axial direction (corresponding for me to 0°), in radial direction (corresponding to 90°) and also in a direction of 30° and 60° . I was wondering if there is something that I can do to iclude this data setting the viscous resistance in the porous zone using fluent.
Using the conical/cilyndrical coordinate system in fluent, I can set three different values of viscous resistance, respectively for : axial, radial, and circumferential direction. the problem is that I don't have the value for the circumferential, and i have more values in more than one angle in the plane given by radial and axial direction. hope I've been clear.
thanks a lot!!!!!

Porous Medium for a Petroleum Engineer
 

Hi There,

Today is my first day here in this web site. I am also rather new to porous medium, did some tutorials and read text, but cannot figure out how I can do my task. Maybe you can help me?
I have a porous cylinder. The cylinder is filled with gas. Outside the cylinder is gas also and the pressure for both gas inside and outside the cylinder is same at 2180 psi. As time passes, the outside pressure is reduced from 2180 to atmospheric pressure with time e.g. 10 hours. I want to know how the pressure inside the porous cylinder changes with time? I think I need inlet. How can I put inlet for this application?
As petroleum engineer, I know porosity, permeability, but do not know how to convert them to what fluent wants (porous cylinder permeability=0.016 milli darcy and porosity=2%)? Can someone tell me what I should do? I appreciate your replies. You can also email me with rahman.ashena.313@gmail.com.

Help on Porous Medium
 

Dear Whoever Can Help


As a petroleum engineer, I have a porous cylindrical rock saturated with gas under pressure 2185 psi. Outside the cylinder is gas with the same pressure at time zero. As time passes, the pressure outside the cylinder is reducing with time. I wanna see how the pressure inside changes. Where should I put the inlet and outlet surfaces? The outlet surface should look like a cylindrical layer around the cylinder? The How can I convert porosity 2% and permeability 0.016 Milli Darcy to fluent?