×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Gas Radiation in Pipe Annulus

Gas Radiation in Pipe Annulus

Gas Radiation in Pipe Annulus

(OP)
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.

RE: Gas Radiation in Pipe Annulus

(OP)
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.

RE: Gas Radiation in Pipe Annulus

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

Tara

RE: Gas Radiation in Pipe Annulus

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
  

RE: Gas Radiation in Pipe Annulus

(OP)
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.

RE: Gas Radiation in Pipe Annulus

(OP)
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.

RE: Gas Radiation in Pipe Annulus

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.

RE: Gas Radiation in Pipe Annulus

(OP)
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.

RE: Gas Radiation in Pipe Annulus

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

RE: Gas Radiation in Pipe Annulus

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

RE: Gas Radiation in Pipe Annulus

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources