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Dimensions STL problems units de-waxing

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Old   August 29, 2013, 10:05
Default Dimensions STL problems units de-waxing
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kevinlee
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Hi everyone,

I am new to flow-3D. Previously I have done some work on PROCAST but there are a few limitations that stop me from using it.

I am trying to model wax flow out of a ceramic shell mould (solid to liquid phase change) with steam melting the wax.

The problem I have now is with the units. I tried to model everything in SI units (kg, m, J, W, s). But the model just doesn't allow the wax to flow. However if I change the magnitude to be similar as eg, grams, centimeters then it works all right. I just don't understand why it behave like that.

And another problem I have is when I try I am importing STL files from solidedge ST6 and when it opens in flow 3D the dimension tend to be different in number and I am aware of the units conversion thing. For example, I drawn a square box 50x50x50 mm in st6. But in flow 3D it appears with extent x- 2.36 ; extent y - 1.97 ; extent z- 2.36. Please help.

Now, I am not sure if the file importing does affect the magnitude of physics I tried to key in. Anyone who had tried to model de-waxing process please help.

Please shed some lights! Thanks in advance.
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Old   August 30, 2013, 12:09
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Hi Kevin,

In some cad packages the units you create in are not the units you are exporting so I would check that.

FLOW-3D does not use units so it is up to the user to ensure the units are consistent. So if you cad file is showing one set of units and when you bring them in they are in inches as it appears with your case then you will have to modify the geometry by using global magnification for each imported stl including fluid regions.

Check all your units to ensure they are consistent. Wax problems have been done before. Use critical solid fraction as 1 and coherent solid fraction as 0. Ensure you have correct heat transfer coefficients defined. Flow3d will calculate them based on flow over a plate and those may not be accurate for what you are doing.

Hope this helps
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Old   August 30, 2013, 12:41
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Hi Melissa,

Thanks for your reply. 50 cm = 1.685 inches. And what I got on my flow3D is 2.362 which just doesn't make sense to me.

And as you said
"Flow3d will calculate them based on flow over a plate and those may not be accurate for what you are doing". So what other packages do you think is more accurate in doing the work since you have done it before? Thanks in advance.


kevin
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Old   August 30, 2013, 14:55
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Hi Kevin,

As for the unit conversion then I am not sure what you cad package is doing as it exports.

Flow3d can model melting of wax. What I meant was that if you don't specify a heat transfer coefficient then flow3d will calculate it using flow over a plate which may not be an accurate correlation for what you are doing so you should define your own heat transfer coefficients. These are sometimes difficult to determine so may have to do some adjustments.

I might have a simple case of wax melting that I can send you but I will have to look for it. Give me until next week.

Melissa
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