Heat Gain for Empty Pipework

[DSB123]

Hi Members,
Anyone out there who can provide a quick way to estimate the temperature profile through an empty pipe which is subject to solar radiation. I know the problem is not steady state heat transfer (since the heat gained by the pipe will heat up the air inside of the pipe thereby modifying the heat flow rate. But over a day's sunlight what can the inside temperature of the pipe wall be considered to achieve?

Thanks in anticipation

 

[prex]

Do you mean the circumferential or the longitudinal profile?
Anyway much data are required to do such an estimate: emissivity of pipe wall, orientation with respect to sun, presence os reflecting surfaces in the proximity, latitude of the site, etc.
It is a type of calculation where it is very difficult even only to guess a temperature.
Moreover you should specify also why you need this information: this could help in assessing the type of estimate, if the calculation effort is proportioned to the goal, and if a reliable result could be obtained. prex

http://www.xcalcs.com

Online tools for structural design

 

[JKEngineer]

I have done similar analyses on, for example, concrete block walls and wet/dry areas of roofs using Cosmos/m in a transient analysis. I have presented a few papers on this. An example of the results is shown at:

http://www.kleinfeldtechnical.com/feaoncmu/index.html

The biggest difference between this and your problem is that the air inside your pipe will be moving under natural convection as the solar loading and air convection outside change. If you are willing to treat the air as a non-fluid, you can approximate the result. A fudge of increasing the thermal conductivity of the air above the real value would approximate the impact of air motion on the amount of heat transfered, but not on the pattern of the transfer.

Alternatively, you could use a CFD package.

Alternatively, you could do a hand estimate of the system at "steady state" assuming the air is all at one temperature in the pipe and the flow of heat around the pipe can be neglected. This would be VERY approximate.

HTH

Jack Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[JKEngineer]

I just realized that my previous post was written as though this was the FEA forum. Oops.

Cosmos/m is an FEA product. It can handle both steady state and transient analyses. I use it mostly for thermal analysis. To do the kind of problem you described would need the solar loading information for the location, time of year, and orientation of the pipe. Most of that is available in the literature or on the web. The problem would then be time sliced and solved in each increment, with the results of the previous increment being the input for the next. This is standard procedure for transient analysis in Cosmos/M.

Describing the solution is almost harder than doing it. :->

Jack Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[DSB123]

Prex,
I am really after the through thickness temperature profile.I know the problem is dependant upon many variables but was looking for a quick easy solution if possible!!!

JKEngineer,
Thanks for the input.Beleive it or not I have come across those who have used the steady state analogy to determine the through thickness temperature profile for a horizontal pipeline sat empty in the sun(i.e. fixed outside temperature and fixed inside temperature). As you describe the problem is time dependant with heat rate and internal(air) temperature varying all the time but my problem is that without access to FE tools I need to approximate the temperature profile for the pipe say over an 8hr period in mid summer.

Thanks

 

[JKEngineer]

DSB123
If I understand your last post correctly, you are interested in the metal temperatures, not the air temperatures. You have a horizontal pipe with sun on the top. That suggests internal convection will not be a big factor, so FEA could be used. Are you concerned about the variation in the profile at different points around the pipe, for examle at say vertical and 15degrees off vertical as a function of the variation in solar loading with angle?

Given the difference in heat transfer characteristics of the air layers inside and out compared to the metal, a steady state approximation might not be too bad if radiation were included in the estimate (both incoming solar and re-radiation from the hot metal.)

The sample I pointed to on my website treated the internal air as a non-fluid. It gave reasonable results. I still use that software.


HTH
Jack Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com