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Non-isothermal pipe flow (nipfl)

May 3, 2016, 10:30 am
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Gooday,

I'm using non-isothermal pipe flow model and heat transfer in solids physics in order to simulate a bore hole heat exchanger (BHE). the problem consist of a tube modeling by nipfl and a ground around the tube modeling as a solid heat transfer. The program seems to simulate the problem in a reasonable way but when the thermal resistance (R) is calculated, the obtained R by Comsol is higher almost 30 % compared with R obtained by formula or 2D model. I guess since this module simulate the pipe flow as a 1D problem and there is a gap between the pipe and solid which in graphics shows as a solid part, it adds the value of thermal resistance. Can anyone help or has a suggestion?

In attachment you can find the file.

kind regards,
Amin
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Discrepancy between particle trajectory and streamline?

March 27, 2013, 4:11 pm
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Hello,

I'm trying to teach myself how to use the particle tracing module, so I'm modeling a simple T-junction microfluidic channel. I get an unexpected result, however, when I trace the particles. Since it is a microchannel, I expect the particles to follow streamlines. However, they do not (see attached). I've tried both the Newtonian and massless configurations. This is a 2D, incompressible model, assuming a shallow channel of 30 um. Could anyone lend any insight into what's happening or which results I should trust?

Thanks,
Alisha
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Parachute CFD simulation

May 4, 2016, 3:51 am
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Hello,

I am trying to perform a CFD flow field simulation of a 3D parachute to find its lift and drag. The geometry model is imported from a CAD software as surfaces only (no thickness). I attached one picture of how it looks like. In order to make the simulation, I created the computational domain (box) big enough to neglect the boundary effects (I saw in a post from an airfoil SST model that a recommended value of 100 chord-lengths toward all sides is enough). For the physics, I am using the SST turbulence model with inlet (velocity input), outlet (pressure output) and symmetry (box lateral sides) boundary conditions. I used an interior wall boundary condition on the surfaces of the parachute with no slip condition. For the mesh, I have tetahedrals for the domains and prisms for the boundary layer over the parachute surfaces. After running the simulation, I visualized clearly the vortices created in the interior of the parachute. However, when calculating the surface integrals to determine lift and drag (I saw in a comsol blog to use spf.T_stress to calculate lift and drag at the correspondent axis direction), I am getting nonsense values (my drag is on the opposite direction resulting in a propulsive force). For sanity checks, I was also trying to see how the pressure differs on the parachute surfaces from the inside and outside but I only see the same values on both sides.
My questions would be:
1. Is this approach valid for this type of scenario?
2. How would it be possible to find the pressure difference on the inside and outside surfaces of the parachute? (I am not sure if this has something to do on how the interior wall boundary condition works). From my understanding, I thought this internal wall boundary condition would act as a baffle (very thin surface) for the parachute surfaces (which will therefore have both inside and outside surfaces). Please correct me if I am wrong on this.

I would really appreciate some help with this issues. Thanks.
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Problem in 1D EHD simulation

May 4, 2016, 8:01 am
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Dear all,

I have built a 1D EHD in COMSOL. But it does not work. There always exists an error. Can anybody give me any suggestion? Please help me! My *.mph file is in the attachment. The error is as follows

The following feature has encountered a problem:
Failed to find a solution.
Singular matrix.
There are 249511 void equations (empty rows in matrix) for the variable comp1.P.
at coordinates: (2.5949,0), (2.5948,0), (2.59482,0), (2.59484,0), (2.59486,0), ...
There are 1 degrees of freedom that do not occur in any equation (empty columns in matrix) for the variable comp1.H0.
at coordinates: (0,0), ...
Returned solution is not converged.
- Feature: Stationary Solver 1 (sol1/s1)

Thanks a lot!
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Inlet pressure boundary condition not allowing COMSOL to converge

May 6, 2016, 12:19 am
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Hi,

In my simulation (file attached here), I'm modeling a slight variation of the 2D axi-symmetric 'stirrer in a cylinder' situation, which is on COMSOL model library.
In my variation, there is applied pressure on the top of the cylinder in which the stirrer rotates. Below the cylinder-stirrer setup is a highly impermeable layer (modeled by Brinkmann equation) through which the fluid has to pass.
This is a simulation of a lab setup, so I know that the applied pressure at the top is usually 4 bar. The outlet below the membrane is open to atmosphere (so the fluid has to pass though the cylinder, through the, membrane and then the open outlet) .

However the pressure boundary condition at inlet does not allow comsol to converge.

I've seen this happen even in a simple pipe flow situation where beyond a certain max pressure, COMSOL can't converge.

Please provide me suggestions!
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Comsol not following ideal gas law or conservation of mass

October 13, 2012, 7:58 pm
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Hello,
I am currently working on modeling a high temperature hydrogen heat exchanger. I decided to start as simple as possible and just model the heat transfer into an axi-symmetric tube of hydrogen surrounded by a solid shell.

I am running into two problems. First, COMSOL has decided that the conservation of mass is a suggestion at best and need not be followed. As a check on the results I was getting, I checked the mass flow rate per unit area. I found that the exit mass flow rate is about 25% higher than the inlet mass flow rate. Thus something is obviously wrong with this.

Second, although I have experimented with treating the hydrogen as both a fluid and an ideal gas, at the moment I am letting it be an ideal gas. However, COMSOL doesn't seem the ideal gas law any more than it did the law of conservation of mass. When the pressure and temperature at the inlet are used in combination with the specific gas value of hydrogen (4124 j/kg), a rearranged ideal gas equation rho=P/(R*T) says that the density should be about 1.4 kg/m^3. COMSOL however, returns a value 17 thousand times less than that.

Does anyone have any thoughts or ideas on these problems?
Dan
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inertial focusing in microfluidic channel

May 11, 2016, 10:34 pm
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Firstly，I want to modeling inertial focusing in straight and curved microfluidic channels（may the channel geometry seems like more complicated ）,just like
cn.comsol.com/story/download/1...85/Veryst_Mass_Gen_CN2013.pdf;
here is my model scheme:
1、sovle a CFD probelm(steady) without the particle;
2、calculate the force act on the particle induced by the “stress tensor and pressure ”,just like
www.chem.mtu.edu/~fmorriso/cm3...esonSurfacesinComsol5_2015.pdf
(I know there have “drag force” “lift force”etc.,but I CAN NOT find an explict formula of lift force suited complicated channel)
Lift Force(x)=sphint(r,spf.T_stressx); Lift Force(y)=sphint(r,spf.T_stressy);
The sphint(r, expr) operator computes the surface integral of the expression expr on the surface with radius r around the point in which it is evaluated.;r is the radius of the particle.
3、model particle tracing for fluid flow ,and "Force_x" =lift force(x),"Force_y"=lift force(y)

But the force seems like have influence to the particle, as "the particle inserted at a specific location in the straight channel remian at that location"

So， can anyone give some advises？
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Convective heat flux

May 12, 2016, 5:25 am
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Hi,
I'm trying to see the heat conduction from the electric power conduit pipe to the soil surrounding the pipe.
The square part which I attached as model.png is the pipe and inside.
I built two point source inside.
Inside the pipe is just air, so I applied the heat transfer in fluid.
Wanted to apply convective heat flux to the pipe walls, but there was only inward heat flux in my version.
So I just applied the inward heat fluxes to the walls.
However, the result came out like result.png. Even though I applied heat transfer in solid and heat transfer fluid for the analysis, it doesn't read the heat conduction or convection inside the pipe box at all.
Anyone knows the solution? using inward heat flux to realize the convective heat flux or finding the convective heat flux.

Is it just the problem of this version?
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Simulation of evaporation of water drop

August 14, 2015, 8:54 am
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A water drop is resting on a solid substrate and evaporating continuously. I want to stimulate this to know the fluid flow inside the drop while evaporating. I am a beginner.

Please help me.
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A closed pipe

May 15, 2016, 11:37 pm
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I want to model a pipe has only an outlet, In fact, I changed inlet to wall. How can I define boundary condition for this case?

I attached an image,

TX
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Stopping the flow in a pipe after a certain time

May 16, 2016, 9:22 am
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Hi, I am new to COMSOL and I am simulating the expansion of steam in a a condenser from a cylinder. They are connected by a pipe. The expansion is due to pressure difference. I need to run the simulation for about 1 s.

Is there any way I can block the flow of steam from the cylinder to the condenser after a certain time say about 0.5 s and then simulate just the condenser for the remaining 0.5 s?
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constraining the range of dependent variables

December 5, 2012, 8:19 pm
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Hello,

I have implemented an axisymmetric drift-diffusion equation using the equation-based modeling interface in COMSOL 4.3 (incorporating the necessary terms due to the curvilinear coordinates). I have attached a movie of the resulting density profile over time. I am using zero flux boundary conditions. I was wondering if there is a way to define a range (specifically a maximum) for the dependent variable (number density here) so that the peak remains less than a certain value.

Thank you,

Best,
Aryan
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Flow Separation in fluid flow around a sphere

May 18, 2016, 2:37 am
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I am new to COMSOL MULTIPHYSICS. I want to find out at which Reynolds number the flow separation starts in laminar flow.How can I exactly find the point of separation ?
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Problems with STEP FUNCTION

May 18, 2016, 6:06 pm
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Hi everybody,

I have an inlet in a water reactor working in the following way: 10 s on, 80 s off, 10 s on, 80 s off....

-Fist of all, I have built a PIECEWISE (pw1) function (0<t<10 = 1, 10<t<90 = 0)
-Then, using this piecewise function I have built an ANALYTIC function (an1) with a periodic extension 0-90, and plot parameters 0-900. When I plot it, everything is OK.
- Finally I have created a variable for the inlet velocity: 0.265[m/s]*an1(t[1/s])

When I compute my model, the inlet works only in the first 10 seconds, but after 90 seconds it doesn´t start working agains as an inlet, there is no velocity.

Why????

Thank you so much

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Coupling transport of a species to surface reactions

July 6, 2012, 9:09 am
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Hello,

I am trying to make a model that has laminar flow, transport of a dilute species, and surface reactions. I want the laminar flow and transport of a dilute species to distribute the chemical species throughout the model, and then for there to be reactions that consume that species on the surface of a sphere. I am having a very hard time trying to couple the species that is in the transport of dilute species to the species that is being consumed in the surface reactions. Is there any way to do this? Or should I just try to add the consumption of the species in the transport, which I could do, but I could not have the reactions occur only of the surface of a sphere.
Essentially I want there to be chemical reaction on the surface of a sphere, but I need those reactions to alter the space around it. I need the chemical species to decrease its concentration as it gets closer to the sphere.
Any help would be much appreciated.
Thank you!

Kelley
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Comsol not following ideal gas law or conservation of mass

October 13, 2012, 2:58 pm
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0
0
Hello,
I am currently working on modeling a high temperature hydrogen heat exchanger. I decided to start as simple as possible and just model the heat transfer into an axi-symmetric tube of hydrogen surrounded by a solid shell.

I am running into two problems. First, COMSOL has decided that the conservation of mass is a suggestion at best and need not be followed. As a check on the results I was getting, I checked the mass flow rate per unit area. I found that the exit mass flow rate is about 25% higher than the inlet mass flow rate. Thus something is obviously wrong with this.

Second, although I have experimented with treating the hydrogen as both a fluid and an ideal gas, at the moment I am letting it be an ideal gas. However, COMSOL doesn't seem the ideal gas law any more than it did the law of conservation of mass. When the pressure and temperature at the inlet are used in combination with the specific gas value of hydrogen (4124 j/kg), a rearranged ideal gas equation rho=P/(R*T) says that the density should be about 1.4 kg/m^3. COMSOL however, returns a value 17 thousand times less than that.

Does anyone have any thoughts or ideas on these problems?
Dan
↧

constraining the range of dependent variables

December 5, 2012, 4:19 pm
$
0
0
Hello,

I have implemented an axisymmetric drift-diffusion equation using the equation-based modeling interface in COMSOL 4.3 (incorporating the necessary terms due to the curvilinear coordinates). I have attached a movie of the resulting density profile over time. I am using zero flux boundary conditions. I was wondering if there is a way to define a range (specifically a maximum) for the dependent variable (number density here) so that the peak remains less than a certain value.

Thank you,

Best,
Aryan
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Discrepancy between particle trajectory and streamline?

March 27, 2013, 12:11 pm
$
0
0
Hello,

I'm trying to teach myself how to use the particle tracing module, so I'm modeling a simple T-junction microfluidic channel. I get an unexpected result, however, when I trace the particles. Since it is a microchannel, I expect the particles to follow streamlines. However, they do not (see attached). I've tried both the Newtonian and massless configurations. This is a 2D, incompressible model, assuming a shallow channel of 30 um. Could anyone lend any insight into what's happening or which results I should trust?

Thanks,
Alisha
↧

Simulation of evaporation of water drop

August 14, 2015, 3:54 am
$
0
0
A water drop is resting on a solid substrate and evaporating continuously. I want to stimulate this to know the fluid flow inside the drop while evaporating. I am a beginner.

Please help me.
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serval questions of heat transfer in my project

May 2, 2016, 6:52 pm
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Hello
As the figure shows, I am using the fllowing models
(Heat Transfer>Conjugate Heat Transfer>Laminar Flow
Stationary situation
Surface-to-Surface Radiation), and try to find the temperature distribution of the air and the spheres.
I simplified the original object into two spheres,since its shape was very complex. The latter sphere has a boundary heat source.
I have some questions to solve:
1.Since the object was simplified, so the real convective area was changed, i need to modify the convective heat transfer coefficient, but I can't find the modify interface in comsol.
2.The Surface-to-Surface Radiation was considered, and the model is periodic, so I need to define the periodic boundaries as perfect mirrors (the default insulated conditions are just fine to the convective heat transfer and conduction heat transfer)， but I can't find the interface in comsol,too.
3. Since the original object was simplified into two spheres, so the heat conduction was incorrect，because the spheres are not contacted.I want to find the ht.gardT between the spheres and use the Fourier's law of heat conduction to define extra heat source on spheres, is it right?
thank you
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