Heat Transfer - Latest Discussions - COMSOL Forums

Newton's law of cooling

Wed, 23 May 2012 19:11:04 +0000

I am trying to model a cilinder at 200 ºC that its cooling following the Newton's Law of cooling.

What I want is to model how the temperature decrease with time. I need help., Someone can help me to model this?

Thank you.

Fixed Temperature Boundary Condition

Tue, 22 May 2012 19:31:03 +0000

Hello, I'm new to COMSOL and am trying to run a non-isothermal flow problem where I have basically a rectangular solid object immersed in a volume of air flowing across the object. I placed a fixed temperature boundary condition on one of the sides of the rectangle, a temperature and velocity boundary condition on the inlet and a pressure boundary condition on the outlet. What I'm trying to accomplish is to determine the heat loss from the object to the air (without radiation for the time being). After I run the scenario, I plot the temperature and to my surprise the temperature on the wall that I placed the fixed boundary condition on is somehow lower than the fixed value I applied. What have I done wrong, or is there something else I need to do to the model?

Thanks

Stationary and Time-dependent study for Joule Heating

Tue, 22 May 2012 02:14:35 +0000

Hello,

I am working on a rather simple simulation of Joule Heating on a Pt resistor deposited on AlN substrate. Although I can perform the stationary study and have some of the results I want, I cannot perform a time-dependent study. The execution fails with error message "Failed to find consistent initial values. Last time step is not converged.".

Any ideas what might be wrong?

Cheers!

-O.

Definition of convective heat transfer coefficient

Mon, 21 May 2012 15:41:58 +0000

Hi,

I have modeled phase change process (PCM) coupling with forced convection boundaries (air). Now I'd like to define and draw variation of convection heat transfer coefficient along the air channel. But I can't find it.

Could you please help me about this problem?

Kind regards,

Rectangular pulse train

Mon, 21 May 2012 09:35:02 +0000

hi Ivar ,
i know this topic has been discussed on this forum but i still have a problem.

i want to generate a rectangular pulse train of pulse width = 28e-9 , frequency = 1 Hz

i used the rect function along with mod function to get an analytic function an1= rect1(mod(t[1/s],1e-9))
but when i plot this for time t = 0 to 1 sec i get zero magnitude , after solving i also found out that heat flux is zero throughout the time . I also tried 'make periodic' option but still not getting the plot.

As discussed earlier on some other thread , i made sure that the solver is selected as strict but still no luck .

i have to model 2 pulses each of 28 ns width in every second (t = 0 to 2 sec ) please help me with this .

Hirendra

forced convection over a rotating cylinder

Mon, 21 May 2012 09:04:13 +0000

Hi to all,

I am trying solve a problem where a hot rotating cylinder is cooled by an incident air jet. I am having problems with rotating the cylinder and making the air jet localized instead of an uniform crossflow.

Can anyone plzzz help..
thanks
with regards
-chendur

time dependent boundary conditions

Sun, 20 May 2012 01:46:11 +0000

hello all..

i am solving a time dependent heat transfer problem..

at the start ,for a rectangular domain boundaries 1 and 2 are insulated and 3 and 4 are assigned with constant temperature..
after some time t>100 my BC changes to all 1 2 3 and 4 boundaries insulated..
how can i do that

thanks in advance
pavan

Temperature decline in autoclave model while rise expected

Sun, 20 May 2012 00:53:59 +0000

Hi there,

I've made a model for an autoclave with 2 blocks in it that should heat up during time. They however first cool down to like 200K and after they start to heat up again. That's not what i suspect and what is realistic. Since the model with results is to big i will upload only the model. It isnt that big so should be computable in short time;). (comsol 4.2a)

See the attachted image.

hope all is clear.

Thanks in advance!

k anisotropic (exact values, function of coordinates, normal dist.)

Sat, 19 May 2012 12:27:41 +0000

I would like some help with COMSOL 3.4.
I want to introduce the heat equation (steady state) to estimate the temperature of a block (3D). The k (conductivity) is anisotropic.

1) If I have the exact values of k for all block points, how can these values be imported?
2) If k is a function of x,y,z, how should the txt be, which concerns the called function?
3) If k takes values from a distribution (e.g. normal) in each point, how should I work using Comsol?

Thanks a lot.

Convergence issues for transient freezing (apparent heat capacity phase change)

Tue, 15 May 2012 23:49:38 +0000

Hello,

I am fairly new to COMSOL and am trying to solve a transient (over 600 seconds) freezing (phase change) problem in a gel (similar thermal properties to water). I am using temperature-dependent thermal conductivity and density (both implemented as cubic-smoothed lookup functions, f(T) ). I am simulating the phase-change with the apparent heat capacity method (also implemented as a cubic-smoothed lookup function, f(T) ).

Across the model, my temperatures vary from 140K to 300K, with fairly high temperature gradients near the cooling interface (order of 40K/mm).

I have a 2D geometry with phase change occurring over <1K, that solves without issue. However, I have a slightly more complicated 3D geometry that I am having problems converging. I can get the solution to converge, treating the phase change as a triangular function over >30K, but the solver gets hung up for any more aggressive phase change.

I would like to reduce the temperature duration over which my phase change is implemented (less than 10K). I am assuming a triangular “pulse” will provide the most gradual change for specific heat, but is this an incorrect assumption and will another function provide less of an issue for convergence? Is there any other settings I can look at to help with convergence? I am using the default settings for the transient solver.

Any assistance you can offer would be greatly appreciated.

Thanks!
Michael

defining a variable for each element

Tue, 15 May 2012 14:47:01 +0000

hi ,
i would like to know how can i define a variable (which i am using in calculating other properties of the material) such that it is solved for each element separately ?
Right now what COMSOL does is solve for a time step and use that value to calculate material properties for all elements together , whereas i would like COMSOL to solve that variable for all elements differently (much like temperature in heat transfer) in each time step.

- Hirendra

Thermal Dispersion in a Poreus Medium Module Problem

Thu, 10 May 2012 17:33:25 +0000

Hi,

I am working on simulation an Aquifer Thermal Energy Storage (ATES) [1] system. In this I am trying to use thermal dispersion, which is part of the heat transfer module. However I cant get it to work. I have tried the manual and the internet, but I cant figure out what the problem is and let alone solve it. Maybe one of you guys can see what the problem is, and how I would be able to solve it?

I've included the error log and the model, which is a 4.0a model.

Greetings Nick

[1] http://www.iftech.co.uk/index.php?option=com_content&task=view&id=21&Itemid=33

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.ht2.k_dispxx@VDN$mod1.py
- Defined as: (((1/max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps))*((mod1.ht2.rho*mod1.ht2.Cp)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2))))+((((((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))^-0.5)*0.5)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2)))*(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2)))>eps))*((-((mod1.ht2.rho*mod1.ht2.Cp)*(((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))))/(max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps)^2))))
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.ht2.k_eqxx@VDN$mod1.py
- Defined as: ((((1/max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps))*((mod1.ht2.rho*mod1.ht2.Cp)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2))))+((((((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))^-0.5)*0.5)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2)))*(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2)))>eps))*((-((mod1.ht2.rho*mod1.ht2.Cp)*(((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))))/(max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps)^2)))))
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.ht2.k_effxx@VDN$mod1.py
- Defined as: (((((1/max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps))*((mod1.ht2.rho*mod1.ht2.Cp)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2))))+((((((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))^-0.5)*0.5)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2)))*(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2)))>eps))*((-((mod1.ht2.rho*mod1.ht2.Cp)*(((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))))/(max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps)^2))))))
Failed to evaluate expression.
- Expression: d(d((-mod1.ht2.k_effxx*mod1.T2x*test(mod1.T2x)-mod1.ht2.k_effyy*mod1.T2y*test(mod1.T2y))*(dvol),{test@3}),mod1.py)

Temperature Sensor Model

Wed, 09 May 2012 03:11:51 +0000

Hello,

I am designing a temperature sensor using an interdigitated capacitor. Using the electrostatics modules, I was successfully able to design the sensor with the starting capacitance value I wanted. However, regarding changing capacitance with temperature is where I run into problems. Thus far, I have been using the joule heating and thermal expansion module. Can anyone tell me if this is the correct module for my goal?

I have had some measure of success using it. However I find that when I remove the surrounding rectangle with air associated with it in the positive z direction and silicon in the negative z direction, the study completes, but with that rectangle, I get an error of singular matrix/solution does not converge and cannot complete the study. Why is this?

Also, without the surrounding rectangle, and without the proper dielectric in place, it seems that I cannot calculate capacitance correctly, and the values are all very small, approaching zero.

I have attached my model.

I would greatly appreciate help understanding this, thank you.

Conjugate, non-isothermal u-pipe, time-dependent study

Wed, 02 May 2012 01:04:23 +0000

I wanted to simulate the heat transfer of fluid in a u-pipe. However, I'm really confused about which module and model should I use?

Initially what I wanted to do is to choose the heat transfer in porous media modulue, however, I found difficulties of solving the problem of directing the fluid from vertical pipe into u-shaped corner by defining a velocity. Based on my shallow experience with Comsol so far, it seems that this modulus only allows the fixed velocity vector to be defined at the input stage, and I found it difficult to resolve for the model.

I have also tried to use the conjugate heat transfer as well as non-isothermal flow... then I started to get more confused because my analysis has to be carried in time-depenent study. Would anyone give me some advices?

Thanks for your advice in advance!

Trouble coupling heat transfer physics for both fluids and solids

Wed, 25 Apr 2012 09:19:20 +0000

Hey all,

So I'm getting stuck with my model where I try to couple three different physics. The three physics are: Electric Currents, Heat transfer in Solids, and Heat transfer in Fluids. Basically what I'm trying to model is a silicon microbridge which is doped, which makes it so that it has a temperature gradient accross it when heated. I am able to couple and run the electric currents and heat transfer in solids physics, where the power dissipated from the ec physics is inputed into the ht physics as the heat source, and then the temperature that is calculated is used to calculate a new resistivity value for the silicon, until the model converges to a value for the temperature at the given voltage.

The problem arises when I try to incorporate the ht2 physics to mimic the effects of natural convection from the sorrounding gases. I set a temperature boundary condition within the ht2 physics which tells the program that the temperature of the microbridge is given by T, which is calculated in the ht physics. However, when I run this, I get the following error:

"Constraing found for variables in different Segregated groups. Try to merge these groups.
-Feature: Stationary Solver 1 (sol1/s1)
-Error: Constraint found for variables in different Segragated groups. Try to merge these groups."

Any ideas? Attached you will find my model. Thanks.

Object rotation in a microwave oven

Sat, 17 Mar 2012 06:43:55 +0000

I am trying to model the temperature distribution in foods rotating in a microwave oven. My problem can be best described as a stationary food object experiencing a time-variant heat source distribution. Can such movement be done in a straightforward manner or is something more complex called for?

Thank you in advance for any help.

Dielectric heating between parallel plate electrodes

Sat, 10 Mar 2012 05:25:06 +0000

Hello all, I'm fairly new to Comsol. What I'm trying to do is model the heating of a dielectric in between two parallel plate electrodes at a frequency of 27.12 MHz. I would appreciate any suggestion as to which physics to use. Thanks in advance.

Variable material properties

Mon, 21 Nov 2011 19:49:08 +0000

Is it possible to make my material properties temperature dependend? I'm talking about a Cp value for example. Also do I have a MATLAB license, so if that has to be incoorporated; that's not a problem. Thanks in advance.

Extruding to 3d

Thu, 30 Jun 2011 00:00:15 +0000

Hi,
2 questions:

1) I am creating a ring in 2d, and it seems perfectly circular, but when I extrude it to 3d, it suddenly gets slightly jagged sides and becomes a sort of polygon. I tried to fillet it in 2d and then extrude, but it makes no difference, the 3d still has edges. Anyone know how to fix this?

2) Is there any way to put a 2d face on a 3d object? For example, on a cylinder, I would like to put a small circle on its top face to model a laser's heat deposition. I think this would let me impose heat transfer conditions on the boundary. Other suggestions are also welcome.
Thanks

Determine Arrenhius damage integral for a solution

Thu, 13 Aug 2009 00:05:07 +0000

I am trying to apply an Arrenhius damage integral DURING the solution of an electrosurgical problem using the Bioheat Transfer Module along with the Conductive Media DC module (T and V are the dependent variables) . The integral is of the form:

Adi = int(A*exp(-E/(R*T),t)

where Adi is the Arrenhius damage integral, int stands for integral and A, E, and R are constants. T is the nodal tissue temperature and t is time.

Is there a way to apply this into the Global Equations by taking the derivative of Adi? Thus making the equation:

Adit = A*exp(-E/(R*T)

I tried this, but received an error that T could not be solved for. Do I need to do something in order for Global Equations to access my dependent variables?

Thanks for any help...I am also interested in any other way that anybody can think of!