How to do a thermal analysis?

General Questions
1

Hello all. I am trying to learn prepomax and i have only done a FEW tutorials from FEAnalyst ( a member here) on youtube.

I do not have the ability to draw a pretty picture but i do have a picture i drew in power point.

In the very crude attached image of a cross section of the closure end of a pressure vessel, you will see a shaft that was molded/drilled into some rubber type material. about 1/8th inch diameter (3mm or so). The countersink bore at the top surface is 0.310 inches in diameter (7.62mm). The orange line is the surface of the rubber material. The hole is partially on a curved surface. The rubber is about 1 inch thick. The shaft and bore are molded into the rubber or drilled out depending on purpose.

The purpose of this shaft is to install a pressure sensor at the bottom onto a metal plate, usually aluminum. the Countersunk diameter is meant to only allow enough hot gas and the pressure to go down the shaft to the sensor without eroding the rubber material away.

My goal with a 2d flow analysis, is to see, if i “accidentally” increase the counter sunk bore from 0.310 inch to 0.5 inch, BUT keep the shaft bore the same:

1.how much heat/temp increase happens on the surface of the rubber INSIDE the countersunk bore from increasing its diameter from 0.3 inches to 0.5 inches and how deep does that change go? (blue and red lines on the image)

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the information i have gathered up is 1.5m/s for gas velocity in THIS region alone. Other areas its much different, so pretty slow for the gas flow. For pressure, this vessel is under 1100PSI for 60 seconds, then the last 9 seconds, the pressure begins to drop to 0

Temp of gas in the chamber is 3300K, dynamic viscosity is 1.0x10^-4 Pa*s, gas density is 6.9kg/m3

For material it is SFEPDM

thermal diffusivity is 1.6E-7
density is 1040000g/m3 or 1.04g/cm3
Cp = 1850J/kg*k
k = 0.30W/mk

I am not sure what analysis type to run to answer my question.

thanks for any help. I am new so im going to need some help and guidance.

Solar Collector
2

CHT (Conjugate Heat Transfer) could be helpful here, but you would also need OpenFOAM for the CFD part. Otherwise, you would have to model the effect of gas on the structure using just a convective heat flux load where you specify the ambient temperature and the heat transfer coefficient. The problem is in determining its value. There are some methods described in the literature (based on various flow characteristics), but accurate estimation can be difficult. You could also predict it using CFD, though.

Since the temperature is high, you may also need radiative heat flux where you specify ambient temperature and emissivity.

So either use CFD for more accuracy or simplified heat transfer analysis in PrePoMax with the following features:

  • 2D axisymmetric model
  • thermal material properties of SFEPDM (conductivity, specific heat and density)
  • transient heat transfer step with the step time set to the total time of the process that you want to simulate (69 s)
  • initial (e.g. room) temperature
  • aforementioned heat flux load(s) with amplitude to control time variation

Or you could use temperature-displacement step where you can apply mechanical loads too and analyze the stresses, also due to thermal expansion.

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3

Could this be something the cfdof workbench in freecad can handle? If so, i assume a 2d cross section is enough or do i need to do a full 3d model?

I can get all the material properties we need since a torch test was done on the material

Step 1 should be a 2d model and somehow setting up freecad cfdof?

Thanks

4

At least partially, sure. Ask on their forum. OpenFOAM has a dedicated solver for CHT (not directly supported in CfdOF), but it’s also possible to couple it with CalculiX via preCICE for that. However, I guess you could just prepare the general CFD setup in FreeCAD and then add the extra commands for CHT to the case file.

CfdOF also supports pseudo 2D models (including wedges for axisymmetry): https://forum.freecad.org/viewtopic.php?t=66741

Either CFD with OpenFOAM and then import temperatures to PrePoMax via file-based distribution (you would just need a proper format of the data) or try doing the whole analysis in OpenFOAM as mentioned above.

Or just use PrePoMax itself with a highly simplified approach where convection is modeled as a load with a prescribed heat transfer coefficient. Regardless of the approach, you can use a 2D model.

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5

if i chose to just use prepomax, how would i do this? i have watched some of your videos in the past but i do not remember any of them covering this type of situation.

thanks for the reply and help with this project.

Maybe staying in prepomax is the best option at the moment

6

I have some about heat transfer. For example: https://www.youtube.com/watch?v=RB_tL699XuI

But you will need to switch to transient analysis.

Btw. FreeCAD FEM would be sufficient for this too. It supports all the thermal analysis features of CalculiX that PrePoMax has.

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7

Ill have to try this out and see. Do i need a full 3d model of what i want to analyze ?

thanks again for the guidance

8

Start from a 2D cross-section / profile. Then you can use it directly as plane stress/strain or axisymmetric model in CalculiX or extrude it to a thin 3D layer for pseudo 2D analysis in OpenFOAM.

1 Like
9

Ok, i will try and figure out how to do 2d drawings in freecad. I have only done 3d models in FC but i assume there is a way to do 2d only?

Thanks

10

You just have to create a sketch (using the Sketch or Part wb) and then use Part Make Face on it. Part Design operates only on 3D solids (volumes).

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11

Hi! In my opinion FreeCAD + Elmer FEM suites better for your task. Elmer do can make 2d heat transfer with gas flow. And, in compare with OpenFoam, Elmer runs on Windows without Linux environment and much easier to learn.

examle heating chamber i Elmer FEM

1 Like
12

Yes, Elmer could be yet another option. Also supported in FreeCAD. So you will end up with that software anyway. But please ask about the details on the FreeCAD forum. We have a dedicated section for FEM topics there.

2 Likes
13

Does elmer have a gui in freecad?

Qnd yes, ill switch over to those forums

Thanks

14

Yes, the FEM workbench also supports many Elmer features including the Heat equation: https://wiki.freecad.org/FEM_EquationHeat

2 Likes
15

Sorry for the late reply. Artimes III booster stacking has begun and that means my job has gotten busy with analysis and inspection of the boosters themselves.

Ill look into elmer. It might be something very useful for me.

16

thank you for the video. ill look at it and see if i can apply it to my situation. Im sure it works, i just need to figure it out haha