Thermal simulation question

[Anonymous]

So I'm VERY new to this software and I'm trying to understand what the different heat loads are. Specifically Heat Source versus radiation. 

For background and reference, I am have modeled different ground electrodes designs for spark plugs and I'm trying to study the heat flow characteristics of each. I would think in-cylinder load would be similar to radiation so I used that as a load to study but I originally started with Heat Source and the values are extremely different.

My problem is I don't quite understand what exactly heat source is modeling. I would think that it is similar to applying a load with a torch, for example, but now I'm not exactly sure why the max temperature is so dramatically different between it and a radiation load. 

Sorry if this seems like a basic concept but any guidance on this would be great. 

--Chris

[Mike.Grau]

Hi @Anonymous,

Thank you for asking about that.

Yes, the models will look different depending on what type of load you´ll apply.

I hope this guide here explains a bit more.

This youtube video here has great content about Thermal simulation setup. 

Could you may share your design with me via Public Link and Private Message.

I could help you with the setup.

Thanks,

[Anonymous]

Thanks for the reply Mike, Yes I've seen that link but am still having trouble understanding the differences and choosing what to do with my specific case. 

How can I share the file with you from Fusion 360?

--Chris

[tyler_henderson]

Hi Chris,

Heat Source is used when you need to model heat being transferred to a solid body from some source that is not in your model.  For example, if I have an electronic circuit board and I want to simulate heat being transferred from a chip or processor.  I can just apply a Heat Source load to the face/area where the chip contacts the circuit board.

Radiation is used when you have a solid body that is hot enough to radiate heat to its surroundings.  The radiation does not add heat to other solid bodies around the radiating body, it just removes heat from the radiating body.

In your particular case, it sounds like you want to simulate the heat from the combustion in the cylinder causing the spark plug to heat up, is this correct?  If so, you could simulate this in Fusion Simulation if you know the temperature of the combustion gasses.  If you know that you could add an "Applied Temperature" to all of the faces on the spark plug that are exposed to the combustion gasses.  Then I assume the heat is conducted from the spark plug to the engine block through the threaded portion of the spark plug.  If so, you would add an Applied Temperature to the threaded portion of the spark plug (the operating temperature of the engine block.  Of course this assumes perfect conductivity between the spark plug and the engine block.  A more realistic way to simulate this is to actually model a portion of the engine block around the spark plug then add the engine bock temperature on the outside faces of the engine block geometry.  You would also need Bonded Contact at the spark plug and engine block interfaces with appropriate Thermal Conductance values.

I hope this helps.

[Mike.Grau]

Hi @Anonymous,

Thanks for the update.

You can share your design via Public Link, with the link here.

Please, feel free to share the Public Link via Private Message.

I hope this helps.

Thanks,

[Anonymous]

@tyler_henderson Thanks for the reply. I've tried using the Applied Temperature option and the simulation just shows the entire model at that temperature. 

Here is a public link to the designs. 

http://a360.co/2roETe3

--Chris

[tyler_henderson]

Hi Chris,

I looked at your simulation.  The reason everything is the same temperature/color in Results is because you only have one Thermal Load.  In thermal simulation, heat flows from higher temperature geometry toward lower temperature geometry.  In your simulation there is no place for the heat to go, so the entire model becomes the same temperature as the lone Applied Temperature.  I assume that there is some part of the spark plug attached to the engine block (typically the threaded portion).  So you would want to add an Applied Temperature to that portion which is equal to the engine block temperature (something like 100 deg C).  Then the heat can flow from the 900 deg C electrode to the 100 deg C threads/engine block.

The result looks something like this:

[Anonymous]

@tyler_henderson

Tyler thanks for the feedback. I'm a little concerned that when using the Applied Temperature heat source, the temperature across the ground electrode, (the J shaped nickel body) is uniform. Would the tip furthest from the shell body not be at a higher temperature than the joint location? Similar to heat flow theory through a fin?

--Chris

[Anonymous]

@tyler_henderson

Tyler, another note to make: The model I shared has two designs, one with a solid nickel GW, one with a copper core GW. The copper core design will transfer heat more quickly and efficiently, results in a lower tip temperature, and overall GW part temperature. The applied temperature source does not reflect this at all. 

--Chris

[tyler_henderson]

Hi Chris,

Yes, I did notice the difference in the the two designs.  It's probably not noticeable in the image I posted, but the one with the copper core did transfer more heat to the base of the spark plug.  You can verify that from the higher temperature near the base where the electrode is attached.  I can also see the difference when I create a Slice Plane through the electrodes.  The internal temperature of the copper core is lower than the solid nickel.

[Anonymous]

@tyler_henderson

Tyler,

Unfortunately I don't believe this is an accurate representation of the running temperature of the GW in an engine. When I run a Radiation simulation I get results that are more consistent with what is expected in a real world situation. See my attached screen shot. 

[JoeSiii · Accepted solution]

Hi @Anonymous,

I have a few comments adding to what Tyler stated:

• Radiation, like convection, can transfer heat in either direction (ambient environment to the solid surface, or solid surface to the ambient environment).  Heat flow is always from the higher temperature to the lower temperature.
• Radiation is similar to convection loads in that you specify the temperature of the ambient environment to which, or from which, heat is being transferred.  In convection, there is a coefficient that specifies what amount of heat flows per unit area per degree difference in temperature (between solid surface and ambient environment).  It is a linear function. Radiation, on the other hand, is an exponential (fourth-power) function with regard to temperature.  Instead of a convection coefficient, the heat transfer is determined using the Stefan-Botzmann constant and the emissivity/absorptivity of the solid object's surface.  The emissivity is a measure of how easily a surface emits energy to the surrounding environment via radiation.  The absorptivity is a measure of how easily a surface absorbs radiant energy from the surrounding environment.  The two values are typically assumed to be equal.  A typical value of absorptivity/emissivity assumed for steel, for example, is approximately 0.9.  The value is much lower for aluminum.  Because of the fourth power exponent, radiation is much more significant as the temperature difference increases, and relatively insignificant at small temperature differences.
• In your example, a spark plug is in contact with hot gases, so there will be convective heat flow (based on the temperature difference between the surface and gases, and the convection coefficient).  There will also be radiation from the high temperature environment to the spark plug (based on the emissivity of the materials and the temperature difference).  Because combustion temperatures are relatively high, radiation may be significant.  I therefore recommend that you apply both a convection and a radiation load to the appropriate spark plug surfaces.
• If your model includes a portion of the cylinder head, you can model thermal contact resistance between the sparkplug and the cylinder head.  This technique produces more realistic results than assuming a fixed surface temperature at the plug/head interface.
• Alternatively, you can apply a different convection load to the surfaces of the plug that are in contact with the head (if the head is not in the model).  The ambient temperature would be the head temperature, and the convection coefficient would be a relatively high value to simulate the good conductivity (low thermal resistance) between the plug and head.  If you have any empirical data on typical head versus spark plug temperatures, you can "tune" your assumed convection coefficient (by trial and error) to determine what is a reasonable value to assume. 

I hope that these comments are helpful and don't add to any confusion that remains.

Regards,