Convection in Partial vacuum

[craghav]

There's a polynomial equation for Kinematic viscosity at different temperatures
But what if I we pressure and temperature both are changed, for example in partial vacuum
Is it possible to find viscosity of air at 3pa pressure and 200K temperature?

How to find convective heat transfer coefficient of air at low temperatures and low pressures?

Kindly help
Thank you

 

[EnergyMix]

Is this a school assignment (or a home assigment for school)? If not, then please provide some more details about your issue so we can fully understand your problem. Playing "20 questions" when only part of the information is provided is very frustrating.

 

[craghav]

i was asking for a function which will help me to find viscosity at given pressure and temperature

 

[IRstuff]

Generally, at partial vacuum, the transfer is due to mean free path considerations, more than anything else.

TTFN
faq731-376

 

[EnergyMix]

craghav

You didn't answer my question. You didn't provide any information about your problem. As IRStuff indicated, it's not just a simple formula.

When you don't write anything but "I want a function" then I come away with "must be a student who doesn't have a real world problem." Prove me wrong.

 

[craghav]

EnergyMix

Sorry for late response, firstly it is not a school assignment, I like heat transfer so I'm trying to learn and understand the subject more
I'm trying to understand the heat coming out from a Thermos flask, after the partial evacuation of gap in between flasks it might come around 5pa or less, so at low pressure the viscosity of gas trapped inside may change, which in turn changes Coefficient of Convective Heat Transfer... I'm trying to determine the connective heat transfer in partial vacuum

 

[ione]

The dominant heat transfer mechanism is radiation, coupled with conduction. Convection plays a trivial role at such conditions (and it is not possible in an ideal vacuum).

 

[craghav]

Ione, I understand convection is small in this situation, I'm trying to find out how small

 

[IRstuff]

The search engine on this site DOES WORK: thread391-270659, which leads to

https://science.nrao.edu/facilities/alma/aboutALMA/Technology/ALMA_Memo_Series/alma554/abs554,

wherein there is a graph of thermal conductivity of air at subatmospheric pressures. Since the convective coefficient in the conventional model is directly proportional to the thermal conductivity of air, you can calculate the convection coefficient.

TTFN
faq731-376

 

[ione]

If you are looking for some data you can take a glance at the paper in the link below

http://www.iaeng.org/publication/WCE2010/WCE2010_pp1444-1447.pdf

The values of pressure and temperature you’ve quoted in your OP are lower than those reported in the paper above, but you can clearly see the trend of the convection/radiation heat transfer coefficient ratio.

 

[craghav]

How viscosity of Air is effected in reduced pressures? The polynomial function viscosity will give viscosity at any temperature,

http://hpce.iitm.ac.in/website/Manuals/Fluent_6.3/fluent6.3/help/html/ug/node337.htm

But how to find viscosity of air when both temp and pressure are changed? The viscosity of air at partial vacuum

 

[BigInch]

Well, I found this one in my XL function collection.

C = .0001 * (7.77 + .0063 * SG * 28.964) * t ^ 1.5 / (122.4 + 12.9 * SG * 28.964 + t)
X1 = 2.57 + 1914.5 / t + .0095 * SG * 28.964
Y = 1.11 + .04 * X1
u = C * Exp(X1 * (pg / 62.43) ^ Y) 'centipoise
ulb = u * .000672 'LB/FT-SEC

'where
' SG = specific gravity of gas (ref air)
' t = temperature of gas Rº
' pr = reduced pressure of gas
' tr = reduced temperature of gas
' p = absolute pressure psia
' z = compressibility factor
' pg = MASS density of gas lbm/ft3
' u = viscosity of gas in centipoise
' ulb = viscosity of gas in lbs/ft-sec

I have lost the reference, or it's from one of my books in my hard copy library at my home base.

I decided to do a quick search and found these. I haven't looked at them in detail yet, but they seem to be promising.

http://www.pe.tamu.edu/blasingame/d..._075721_(Londono)_Gas_Density_y_Viscosity.pdf

http://www.pe.tamu.edu/blasingame/data/z_web_Archive/091128_KSAN/API_65_

[Viscosity_of_Natural_Gases_(1970)].pdf

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[IRstuff]

I think you are over complicating the problem; see Fig3 :

http://www.bine.info/fileadmin/content/Publikationen/Englische_Infos/projekt_0108_engl_internetx.pdf

TTFN
faq731-376

 

[BigInch]

Who?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[craghav]

BingINch, thank you very much, I'll try to understand how it arrived..

 

[IRstuff]

BI,

Not you, the OP.

TTFN
faq731-376