High pressure gas heat transfer coefficients

[swainw]

Hello,
Does anybody have a source for High Pressure gas (air) heat transfer coefficents. (I've searched google and my heat transfer texts, but all are at ambient pressure) I know due to other literature that the ambient transfer coefficient is different from that at High pressure. The basic problem is that I have an accumulator that stores air up to 6000 psi and we are trying to work out transient pressure changes as a result of thermal variances outside the accumulator. These changes need to consider the thermal lag across an air insulation boundry (simlar to a window's air barrier) as the accumulator is housed inside an aluminum frame with only bolt points to the external frame, otherwise it is basically suspended in an air pocket.
For thought, the main reason for the analysis is that we have some pretty dramtic temperature changes over a very short time period and not sure if the air temp, hence pressure, inside the vessel would actually react to the outside temp changes. I would like to do this in either mathcad or excel using finite difference if necessary. Absolute equations are OK to as long as I can determine the temp at the core or any x distance from the outside wall and at the core of the cylinder at any time (x).



Thanks for any assistance in advance.

Bill Swain

 

[davefitz]

The convective heat transfer coeficient has a pressure correction included in the form of the Reynolds number, if defined in long form Re= density*Velocity * length / mu. The physical properities such as mu ( viscosity ) are primarily temperature dependent, while the density can be easily corrected for higher pressures.

 

[swainw]

The velocity of the gas inside both the enclosure and the pressure accumulator is essentially zero. This would give a zero division result. Can you be more explanative.

Thanks,
Bill

 

[chicopee]

Use the dimensionless analysis with Grashof,Reynold, Prandt, an Nusselt numbers to determine the thermal convective heat transfer coefficients.

 

[25362]

In free "natural" convection the Reynolds number is excluded when estimating the HTC. Buoyancy effects are represented by the Grashof number (Gr). The Prandtl number is not strongly dependent on pressure, it would increase at ambient temperature by about 15% when under the pressure you indicate. Anyhow, Perry VI gives all the data you need for air. From books, the usual formula is:

Nu = C(PrGr)^1/4

C = 0.47 for pipes; for horizontal pipes when 10³<(PrGr)<10⁸
C = 0.56 for vertical cylinders with large diameters and the height as the characteristic length for estimating the Gr number, and (GrPr)<(2)(10⁹)
C= 0.54 for plates pointing upwards, and 0.25 if they are pointing downwards, both with the width as the characteristic legth for calculating Gr.

For horizontal pipes where 100<(PrGr)<1000 use
Nu = (PrGr)^0.14.

There are other formulas for inclined plates and vertical plates.

 

[hacksaw]

Most heat transfer coefficients are developed experimentally and summarized in various correlations that are appropriate to your fluid velocity, physical configuration, fluid state, and the driving force behind the fluid motion (force,convective,or some combination). There are dozens of such correlations.

all of the standard heat transfer text in the last 50 years include pressure effects in these correlations, so it is somewhat surprizing that you've not found any references.

you might try checking out Amazon.com. They have many excellent references on the subject of heat transfer and are quite affordable. you will also find a good deal of advice in many of the the more common mechanical and process engineering handbooks that will help you.

 

[swainw]

Thanks everyone, as this will get me going. I have ordered Perry VI as this seems to be the text I am missing.

Thank you,
Bill