How can I apply the feature of 'Enclosed cavity' for expansion air by heating?

Posted Jul 14, 2024, 11:11 a.m. EDT Fluid & Heat, Heat Transfer, Structural Mechanics Version 6.2 3 Replies

Hyeontak Kim

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Hello, I'm trying to make a model of sealed crucible filed with air. I set the heat flux on the top of the crucible to heat it and so does the air which is in the crucible. The air get expansion, and the inside wall of the crucible get pressure as load. So I choose the physics :

Heat transfer in solids and fluids --> about the crucible and air / Structure mechanics --> about the crucible / -->Multiphysics - Thermal expansion --> about the crucible

In Structure mechanics, I set the "Boundary load" = P1(T/T1)/(1+3alpha*(T-T1)) T1 = initial temperature / P1 = initial pressure / alpha = coefficient of thermal expansion of crucible The equation of boundary load means the pressure load considered the thermal expansion of the crucible.

COMSOL 6.2 released the feature of "Enclosed Cavity" which can simulate the volume change by pressure load. But when I apply this feature, it doesn't work. I saw some examples about 'enclosed cavity' (Hyperelastic Seal & Biventricular Cardiac Model tutorial models), they have something in common that the volume changes by force but not the air expansion.

How can I apply the 'enclosed cavity' to simulate the pressure load by air expansion? Would it be accurate to proceed the way I originally did?

Best,

Hyeontak Kim

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3 Replies Last Post Jul 18, 2024, 3:14 a.m. EDT

Aaron Dettmann COMSOL Employee

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Posted: 4 months ago

Updated: 4 months ago

If I understand your model correctly, I believe the Enclosed Cavity feature might still be suited for solving your problem.

Note, however, that the Enclosed Cavity feature assumes that the entrapped air inside the cavity has a uniform pressure and temperature in the fluid domain, and that the gas state is governed by a global equation of state. In other words, temperature or pressure gradients cannot be resolved.

Assuming that you know the air temperature (e.g. assuming thermal equilibrium between wall and gas, or solve for a heat transfer equation), you can define your equation of state for the air (e.g. ideal gas).

You can then enter this equation of state either in the Fluid subnode, if you choose User defined from the Compressibility list. Alternatively, you could use the Prescribed Pressure subnode (and delete the Fluid subnode).

If you have any further questions, please don't hesitate to elaborate or share more details about what specifically does not work as expected.

Hyeontak Kim

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Posted: 4 months ago

Updated: 4 months ago

If I understand your model correctly, I believe the Enclosed Cavity feature might still be suited for solving your problem.

Note, however, that the Enclosed Cavity feature assumes that the entrapped air inside the cavity has a uniform pressure and temperature in the fluid domain, and that the gas state is governed by a global equation of state. In other words, temperature or pressure gradients cannot be resolved.

Assuming that you know the air temperature (e.g. assuming thermal equilibrium between wall and gas, or solve for a heat transfer equation), you can define your equation of state for the air (e.g. ideal gas).

You can then enter this equation of state either in the Fluid subnode, if you choose User defined from the Compressibility list. Alternatively, you could use the Prescribed Pressure subnode (and delete the Fluid subnode).

If you have any further questions, please don't hesitate to elaborate or share more details about what specifically does not work as expected.

Thanks for you advice!

I understood that the temperature and pressure should be uniform, but the temperature profile in the crucible is not uniform because of the heat flux on the top crucible surface. I wonder how can I make the temperature of air uniform. I set the Enclosed Cavity as you said(Prescribed Pressure & delete Fluid node) and disabled the Boundary Load[P1(T/T1)/(1+3alpha*(T-T1))]. And I defined the Isothermal Domain for air domain for uniform temperature of air. The results seem to come out well without errors, but I would like to ask if this is the right way. +If I disable the isothermal feature, the results come out well too. Is it okay to not apply the isothermal feature on my model?

Best,

Hyeontak Kim.

Aaron Dettmann COMSOL Employee

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Posted: 4 months ago

I believe your approach of using the Isothermal Domain for the air temperature is good. This feature calculates a global (uniform) temperature for the air domain based on a heat transfer equation, utilizing the temperature distribution of the surrounding walls. The variable for the global temperature is named something like ht.id1.T. This would be the temperature to use in the equation of state that you enter in the Prescribed Pressure node.

If you use only T in the equation of state in the Prescribed Pressure node, you might end up using the boundary temperature, which is probably not what you want.

It is always beneficial to try different modeling approaches and compare the results when possible, as in this case.

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