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COMSOL Microwaves Error

Posted Oct 6, 2011, 2:18 a.m. EDT RF & Microwave Engineering Version 4.0 2 Replies

Muhammad Mohsin Rehman
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Hi,

I am modelling microwaves oven to heat potato placed in it. It works fine as I worked according to the instructions given in tutorial. But when I changed the material of boundaries of microwaves from copper to soil, temperature of potato does not rise. Can any one suggest, why is it so?

In addition, when I changed the material of plate carrying potato from glass to water, it gives an error mentioning " "Failed to find consistent Initial values", when I made the mesh fine, the simulation continued to work on without either giving error or results. Can you suggest?

Any help will be highly appreciated.

Thanks and regards,
2 Replies Last Post Oct 17, 2011, 3:20 a.m. EDT
Robert Koslover Certified Consultant
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Posted: 1 decade ago Oct 13, 2011, 4:45 p.m. EDT
I haven't tried to reproduce your particular problem, but I have some wild guesses as to what might be happening, or at least impacting the computation.

1. Soil is a relatively poor conductor compared to copper. Soil "walls" would thus be so leaky and lossy that the cavity of your oven would not generate the strong fields inside that it would when made out of metal. And so your potato would cook very slowly indeed.

2. Water couples well to microwaves. Its dielectric constant is very high, however, which means the wavelengths in it are quite short compared to free space. So perhaps you have numeric/mesh-related problem there.

3. And finally, since you are apparently using 4.0, you may simply have specified the problem entirely wrong and not even realized it (personally, I find the 4.0 interface to be maddening). So I suggest you move up to version 4.2 or down to version 3.5a.
I haven't tried to reproduce your particular problem, but I have some wild guesses as to what might be happening, or at least impacting the computation. 1. Soil is a relatively poor conductor compared to copper. Soil "walls" would thus be so leaky and lossy that the cavity of your oven would not generate the strong fields inside that it would when made out of metal. And so your potato would cook very slowly indeed. 2. Water couples well to microwaves. Its dielectric constant is very high, however, which means the wavelengths in it are quite short compared to free space. So perhaps you have numeric/mesh-related problem there. 3. And finally, since you are apparently using 4.0, you may simply have specified the problem entirely wrong and not even realized it (personally, I find the 4.0 interface to be maddening). So I suggest you move up to version 4.2 or down to version 3.5a.
Muhammad Mohsin Rehman
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Posted: 1 decade ago Oct 17, 2011, 3:20 a.m. EDT
Thanks a lot Robert!

I have reformulated the model by making a big box of oil with boundaries of soil and placing microwaves source in one wall of oil.I was able to get the temperature distribution but the temperature was not penetrating much deeper into the oil, instead it was only across the source boundary.(Model is attached ) Can you please look at it and suggest why temperature is not penetrating deep in to the oil.
Secondly, I want to draw a line of temperature versus distance (say along y axis) but could not find a way to do so, how can I do that?

Thirdly, I joined two different boxes of oil and water and tried to simulate them by using Microwaves but could not get the temperature distribution which I was getting in the above case of only oil domain. It gives me error of inconsistent initial values, Last time step is not converged. What could be the reason ?

One more question is why I can't change the frequency to below 1GHz in microwaves heating, is it because of the Microwaves(2.45GHz) operational characteristic?

Many thanks in advance for any of your help.

Thanks a lot Robert! I have reformulated the model by making a big box of oil with boundaries of soil and placing microwaves source in one wall of oil.I was able to get the temperature distribution but the temperature was not penetrating much deeper into the oil, instead it was only across the source boundary.(Model is attached ) Can you please look at it and suggest why temperature is not penetrating deep in to the oil. Secondly, I want to draw a line of temperature versus distance (say along y axis) but could not find a way to do so, how can I do that? Thirdly, I joined two different boxes of oil and water and tried to simulate them by using Microwaves but could not get the temperature distribution which I was getting in the above case of only oil domain. It gives me error of inconsistent initial values, Last time step is not converged. What could be the reason ? One more question is why I can't change the frequency to below 1GHz in microwaves heating, is it because of the Microwaves(2.45GHz) operational characteristic? Many thanks in advance for any of your help.

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