Ansys Maxwell Simulation - Evaluation of Eddy currents in a Vessel

[Aliosha86]

TL;DR

Ansys Maxwell Simulation - Evaluation of Eddy currents in a Vessel

Hi, guys, I am developing a simulation in Ansys Maxwell.
I set up my simulation in the "transient mode", and I would like to evaluate the eddy currents that arise in a metal Vessel.
The currents rise to a flat top in 20 ms and then after 20 ms in the flat top, it decreases to zero in 20 ms.
I would like to know your opinion on whether Ansys Maxwell can evaluate eddy currents in such a short amount of time or not.

Cheers,

 

[Baluncore]

I see no reason why it should not be possible.
What is a vessel? A supertanker or a thimble?
What is the source of the changing magnetic field?

 

[Aliosha86]

The vessel is just a cylinder having a thickness of 15 mm. (Basically is link a tank)
I have some coils whose current profile is attached.
Basically I am using the magnetostatic solver just to take advantage of the adaptative meshing techniques and then I import the mesh obtained in the transient model.
Should I improve the mesh with the skin depth method? I do not have a frequency therefore I do not know how to evaluate the depth for meshing the eddy currents.
CHeers,
Alessio

 

[Baluncore]

The vessel is just a cylinder having a thickness of 15 mm. (Basically is link a tank)

Is that a 15 mm outside diameter vessel? What is the wall thickness or inside diameter?
You have not identified the metal.

I notice your current pulse is abut 0.1 second long, that suggests a low frequency of 5 Hz.
The rise time and fall time are about 5 msec, so the high frequency component is about 100 Hz.
Knowing the metal resistivity and magnetic characteristics will allow you to calculate skin depth.

I expect the current will flow at different densities at different depths in the material. That suggests you must model skin depth, not only surface currents.

 

[Aliosha86]

Hi,
thank you very much for your reply. The vessel has an outer diameter of 800 mm and a thickness of 15mm.
The material is stainless steel.
How can I calculate the skin depth and how can I model the skin depth?
I am new to this field, sorry if the questions may be silly.
Cheers,
Alessio

 

[Baluncore]

I am new to this field, sorry if the questions may be silly.

Welcome to the fantastic weird world of stainless steel. Be happy not sorry.

Is that current pulse 100 amp or 100 kA ?
How is that coil arranged relative to the vessel ?
It seems you may be building a big induction cooker.

A current carrying coil, wound around a tank will make a transformer, with the tank as a single secondary turn. From resistivity, diameter and wall thickness you can calculate resistance of the tank wall. Then from that, the current in the wall and the power dissipated. The thermal capacity of the stainless steel alloy will let you calculate the immediate heating of the wall.

The 15 mm thick wall would be easy to model if it was a pure metal. But there are many stainless steels with different magnetic properties. They come in several types.
https://en.wikipedia.org/wiki/Stainless_steel#Electricity_and_magnetismCompared to copper they have higher resistivity, so EM will penetrate the 15 mm more quickly.

You must first identify the alloy number used for the vessel, maybe 304 ? or 316 ?
Depending on the alloy used you may also need to consider cold working and heat treatment which may change things.
From that you can find permeability and resistivity to go in the numerical model or skin effect equations.

So; what is the alloy, and what are you trying to design?

 

[Aliosha86]

Thank you for your reply. Yeah, I am trying to evaluate a big cooker, but it is just for exercise purposes.
The current is 100000 A*turn and the coil is arranged around the tank.
The current varies I expect eddy currents, and moreover forces on the tank.
Up to now, I am using the stainless steel library of Ansys Maxwell without specifying if it is 304 or 316.

Do you think it could be a good approach dividing the vessel, like a matryoshka, in more vessel of little thickness so as to help the evaluation of the eddy current? (like a phantom object).
My aim is then to import the evaluated results of force and eddy current in Ansys Mechanical so as to evaluate the stresses and the heating of the coil.

If I use the skin depth calculator with the built-in values for generic stainless steel and a low frequency I get a depth which is higher than my thickness. Does this mean that the current penetrates in the thickness of the tank and therefore it could be useless evaluating the distribution in the thickness of the VV?

 

[Baluncore]

If I use the skin depth calculator with the built-in values for generic stainless steel and a low frequency I get a depth which is higher than my thickness. Does this mean that the current penetrates in the thickness of the tank and therefore it could be useless evaluating the distribution in the thickness of the VV?

If skin depth is greater than thickness, then yes, the eddy current will flow in the full depth of the wall eventually. But how long it will take for current to diffuse through the conductor is the question. You withhold important numbers and information, like frequency and skin depth, so it is difficult to draw a conclusion.

You will also need to consider the end details of the vessel. If the eddy current can spread at the speed of light around the end of the vessel then it can enter the wall from both sides.

You should be able to simulate and plot current density at depth with time. Depending on parameter values, it could be marginal. I would guess the current diffusion velocity will have a magnitude of between 100 m/sec and 1 km/sec. It actually depends on the refractive index of the stainless steel.
What Ansys Maxwell parameter values are you using for permeability and resistivity of stainless steel?

 

[Aliosha86]

If skin depth is greater than thickness, then yes, the eddy current will flow in the full depth of the wall eventually. But how long it will take for current to diffuse through the conductor is the question. You withhold important numbers and information, like frequency and skin depth, so it is difficult to draw a conclusion.

You will also need to consider the end details of the vessel. If the eddy current can spread at the speed of light around the end of the vessel then it can enter the wall from both sides.

You should be able to simulate and plot current density at depth with time. Depending on parameter values, it could be marginal. I would guess the current diffusion velocity will have a magnitude of between 100 m/sec and 1 km/sec. It actually depends on the refractive index of the stainless steel.
What Ansys Maxwell parameter values are you using for permeability and resistivity of stainless steel?

Well In the Calculate skin depth I am using the default values for stainless steel:

1) Relative permeability: 1
2) Conductivity: 1100000 mhos/m
3) Frequency: 5 Hz

Regarding the meshing would you advise me combining the adaptative meshing obtained in "Magnetostatic" combined with some layers added in the "Skin depth-based refinement" or would you suggest me adding phantom elements?
I do not find online something useful for this purpose.
Cheers,

 

[Baluncore]

I do not find online something useful for this purpose.

I will let you know if I find any guidance.
Let me know if you do.

Regarding the meshing would you advise me combining the adaptative meshing obtained in "Magnetostatic" combined with some layers added in the "Skin depth-based refinement" or would you suggest me adding phantom elements?

It is difficult to answer that question as I do not have Ansys Maxwell. At least now you know what to look out for. I think you will have to try both, and to see what results you can get.

 

[Aliosha86]

Thank you very much! I will try both.
Cheers,