Binary Gas Thermal Conductivity Questions

[markdblissmn]

I would like to understand if thermal conductivity is linear between a differing helium% / air% binary gas mixture and also how it changes at varying pressures between 1atm to 10atm. See below image:

http://files.engineering.com/getfil...file=Helium_Thermal_Conductivity_Question.bmp

Mark

 

[EdStainless]

Generic air or good dry clean air?
If you had two pure gases then it would be linear and you could use ideal gas corrections for pressure.
If there is moisture in your air then it would throw things off a little bit.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[TD2K]

I had to wait to get home to get out my copy of Properties of Gases and Liquids.

For your pressure range, the thermal conductivity only increases by about 1% per atm.

There's no simple approach like a mol or mass weighted average and you'd need to reach the section to figure out how to apply it and hopefully find an example to work through. Any chance someone where you works have a copy of this book? This was one of my University textbooks so a few people might have a copy.

$37.25. And I thought they were expensive back then.

 

[markdblissmn]

What chapter or page in the text did you find this information? My father in law has a copy and I want to give him better guidance on where to look.

I really appreciate your help. We are looking to use helium or a combination of helium/air at pressure to cool some parts coming out of a heat treat oven.

Mark

 

[TD2K]

I have an old copy, 3rd edition and it's in chapter 10. Thermal conductivity has a chapter of its own.

 

[markdblissmn]

I found a copy of the book in PDF from 2001 online.

http://www.google.com/url?sa=t&rct=...noHIDg&usg=AFQjCNGJQ5OO2JfcmfeD89mvmXuLzC7dvA

 

[IRstuff]

Which is great, but why helium? While its heat capacity is certainly great, it's a rare gas that's getting rarer, while air is abundantly abundant and only requires the cost of the compression.

TTFN
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[markdblissmn]

Helium has a 5x multiple over air for thermal conductivity. Hydrogen is 7, but is flammable. We need the pressure of 10atm based on testing. We were just wondering if we used 70%air, 30% helium how it would work over 100/0, 80/20, 50/50, etc. The binary mixture will allow us to save on the cost of helium, as yes, it is getting rare and expensive.

We cannot quench in a liquid. Other suggestions?

 

[Compositepro]

Mark, you may be asking the wrong questions. Thermal conductivity of gases varies little with pressure and pretty much linearly with concentration for mixtures. But conductivity is a relatively small factor in heat transfer with fluids compared to convection. Heat transfer by convection depends mainly on the mass flow rate of gas impinging on the surface. So high molecular weight, high velocity, and high pressure are good for heat transfer. High molecular weight is bad for thermal conductivity.

 

[IRstuff]

Helium also has about 5x the specific heat of air. However, it has about 1/7 the molecular weight. People avoid using helium whenever physically possible.

TTFN
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[markdblissmn]

There will be extremely little convection, if any, only conduction, as the part is encased in a sand casting mold. There is no circulation of gases.

 

[IRstuff]

I think you are confused. Unless you are using liquid or solid helium, its cooling mechanism will be by convection

TTFN
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[25362]

I assume no linearity is to be expected. See,

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640023170_1964023170.pdf