Strange behaviour of Microwaves Heating

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Muhammad Mohsin Rehman

Muhammad Mohsin Rehman

November 26, 2011 10:43am UTC

Strange behaviour of Microwaves Heating

Dear all,

I am trying to heat the water layer with microwaves heating. The layer of water is trapped between two thick layers of oil.
When I place the microwaves source directly in front of the layer of water and heat it, it shows some temperature distribution as shown in figure attached.Although this temperature distribution does not seem to be right as according to my perception , water is a good absorber of microwaves and it should absorb most of the energy as a result of which temperature of water should rise significantly. But the found is a very small temperature distribution. Is anyone familiar with such trends and the expected results???


But the real problem comes in when I relocate my source above or below the water layer, it gives me error as follows:


Undefined value found.
- Detail: Undefined value found in the stiffness matrix.
There are 1160 equations giving NaN/Inf in the matrix rows for the variable mod1.E10
at coordinates: (0.0016633,0.00180792), (0.00332659,0.00361583), (0.00382242,0), (0.00714901,0.00361583), (0.00764484,0), ...
There are 290 equations giving NaN/Inf in the matrix rows for the variable mod1.E20
at coordinates: (0.00548572,0.00180792), (0.0016633,0.00514125), (0.00469918,0.00847458), (0.0085216,0.00514125), (0.0191354,0.00333333), ...
There are 290 equations giving NaN/Inf in the matrix rows for the variable mod1.E21
at coordinates: (0.00548572,0.00180792), (0.0016633,0.00514125), (0.00469918,0.00847458), (0.0085216,0.00514125), (0.0191354,0.00333333), ...
There are 871 equations giving NaN/Inf in the matrix rows for the variable mod1.Ez
at coordinates: (0,0), (0.0114673,0), (0.00573363,0), (0.00249494,0.00271187), (0.00498989,0.00542375), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns


I am surprised with the trend as if it is working fine with the source at one location, it should not have any problem for relocation of the source. It could give us unexpected temperature distribution but heating effect should be there at least.

What you people think about it?

Thanks for any suggestions, comments or discussions etc...

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Ivar Kjelberg

Ivar Kjelberg

November 26, 2011 11:59am UTC in response to Muhammad Mohsin Rehman

Re: Strange behaviour of Microwaves Heating

Hi

I would suggest to recheck your BCs probably some of them ore no longer valid, or new ones need to be defined, and this for ALL physics, independently, and combined

--
Good luck
Ivar

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Muhammad Mohsin Rehman

Muhammad Mohsin Rehman

November 26, 2011 12:36pm UTC in response to Ivar Kjelberg

Re: Strange behaviour of Microwaves Heating

Thanks Ivar,

I have rechecked the BCs and have deleted the unnecessary ones but still the problem persists.

By the way, there are three expressions being shown in orange, can it be because of them?
First expression is the density of water (built in material) shown as a function of Temperature and Pressure but I have nothing to do over here as it is predefined, right? (If I just make the expression as function of temperature, it becomes black means OK, what could be the potential cause in this case?)

Secondly, I have defined the E-field for Microwaves Port as Ex=0, Ey=0, Ez=cos(pi*y/0.062)[V/m] (Just the way, it is defined in Microwaves Oven-Potato Tutorial). It is also in orange color, my model is 2D, is it suitable to define Ez even though I am using 2D model? If I remove 'y' from the expression Ez=cos(pi*y/0.062)[V/m], it becomes black as well. But 'y' denotes the position of waveguide over here, is it so? If yes, then I want the position of waveguide to be considered for the simulation, how can I fix it?

Thirdly, the propagation constant of Microwaves port is defined as beta = 2*pi/c_const*sqrt(freq^2-c_const^2/(4*0.062^2)).

Which is also the same as defined in Oven Model. What is c_const over here? I believe it is speed of light, if so there is no other variable then why it is shown in orange? If I replace the c_const by (3*10^8) then it becomes black. I am confused a lot, what to do?

Any of your comments/suggestions will be highly appreciated!

Kind regards,

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Ivar Kjelberg

Ivar Kjelberg

November 26, 2011 2:24pm UTC in response to Muhammad Mohsin Rehman

Re: Strange behaviour of Microwaves Heating

Hi

check the other thread, it gives you the explanations of the orange. Indeed c_const is the speed of light in units m/s, so to cancel it you need something that ends up in s/m, again replace the number 0.062 by (0.062[m])^2.

Again orange is a warning, red is an error.

Then material properties, several from the material library are defined as function of T and/or p or even other dependent variables. Well that depends if you model does solve for p or T, this is not always the case. If you use a T dependent material proerty and you have now definition for the temperature T in you model, then you must define one. In the latest versions, comsol is mostly adding a default T and pa (absolute pressure) to avoid such errors, but still, you need to define them to the actual values, and not always the COMSOL default (20°C and 1[atm] respectively).
Then if there are no T or p, try adding a Definition Parameter of T= 20[degC] or p=[1atm], or what ever value that is relevant for your model

--
Good luck
Ivar

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