Gas Radiation in Pipe Annulus

[OzzieFlow]

I'm trying to solve the heat transfer in a pipe annulus that transports combustion gas. My question is how to determine the view factors.

A few text books uses the electrical network analogy to solve radiation between 2 surfaces separated by an absorbing and transmitting medium (combuston gas in this case). Mostly a simple example of 2 infinite parallel plates are used to demonstrate the principle, where all the view factors are unity. However, I'm not sure what the typical view factors from the gas to the surfaces should be in the case of a pipe annulus. Any help will be appreciated.

 

[IRstuff]

Is this for school?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[OzzieFlow]

No, not for school. It is for the design of a process heat exchanger. Actually there are 2 annuli. The outer annulus has the combustion gas and an inner annulus has a mixture of steam and CO2. The combustion gas annulus is used to heat the steam/CO2 mixture.

 

[corus]

If the surface of the pipe/annulus can only see gas, then it's 1.

Tara

 

[hacksaw]

Perry's Handbook covers this design calculation in detail with charts of the view factors for two concentric cylinders, hardly a class room assignment. The emissivivity of the products of combustion are also cover.

good luck

 

[OzzieFlow]

Thank you Corus. The "gas" emits as a volume source, and because the areas of the inner and oute pipes differ, I'm not convinced that unity is the answer.

 

[OzzieFlow]

Hacksaw, yes, most Heat Trasnfer textbooks give the view factors for surface-surface geometries and that is easy, but I'm looking for the view factor from the gas volume to the respective surfaces. Not sure if I'm trying to overcomplicate the problem.

 

[IRstuff]

see ione's posting from today: thread391-292725
seems like the specific condition might be covered in:

http://www.me.utexas.edu/~howell/sectionc/C-63.html

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Compositepro]

In most cases the gases would be considered transparent for all practical purposes, unless you have a yellow flame, in which case it is carbon soot particles that are glowing and not the gasses.

 

[OzzieFlow]

I'm starting to think that Corus may actually be right that it is unity, if you consider all the radiation emitted by surface 1 is seen by the gas (transmitted/absorbed) and the same for surface 2. I'll think about it and try to research it more....

Compositepro, the presence of just the H20 and CO2 molecules in a combustion gas (no flame and no soot) can have a significant contribution to the overall heat transfer at elevated temperatures. In our case we can unfortunately not neglect it.

 

[corus]

If the two surfaces can see each other then the gas must be opaque, so the viewfactor would then be zero (from the solid surface to the gas). If it's somewhere inbetween then you'd need the viewfactor between the annuli (see IRStuff's post) and some factor to the gas. If the two surfaces can't see each other because of the gas then the viewfactor to the gas is 1 and the viewfactor between annuli is zero.

Tara

 

[hacksaw]

do a search on radiative exchange with participating media, the view factor approach is only an approximation for us mortals

 

[OzzieFlow]

Thank you for all the posts. I have since looked more into Perry's Handbook, after hacksaw's earlier comments and found some more valuable information in there. I'm currently working my way through it.