Contact US

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!

*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

Heat transfer coefficient change as deltaT change?

Heat transfer coefficient change as deltaT change?

Heat transfer coefficient change as deltaT change?

Hello, i'm a student in electrical engineering and i'm doing a project using COMSOL multiphysiscs tool to anticipate the temperature of a cable being subjected to a specific current.
The test is done in real life and then simulated in COMSOL, to see if its possible to anticipate.
Currently we have some problems understanding the nature of the Heat transfer coefficient, in the Heat fLux module.

My simulation is tuned in so that it fits the cable dimensions etc. and the last parameter tuned was the Heat transfer coefficent[HTC].
This was set to 12 W/m^2*k. The first test was a copper cable which reached 32degC at 50Amps.

For my second test, i tested a aluminium cable which got a lot hotter than my copper cable due the the diffrent size.
At the higher temperatures the simulation became less accurate.
At temperatures around 32degC, the simulation became more accurate.

So from my standpoint is seems that as the deltaT between cable surface and ambient air becomes larger, the Heat transfer coefficient also becomes larger.
Is this correct to assume?

In this picture, the HTC is dependent on the deltaT and the heat flux.
The heat flux is almost constant, except for higher resistance at higher temperatures.
But if the deltaT rise, the HTC should become smaller? when looking at the formula.

So to sum up my question, is the HTC a constant value, or does it go up or down with a deltaT rise?

Thank you in advance, and pardon my bad spelling errors and thermodynamic newness.

Mvh. Jens

RE: Heat transfer coefficient change as deltaT change?

You should be posting in the student forum. But your modeling shows the issue that you are using a convective heat transfer coefficient; has your instructor not covered radiative heat transfer?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Heat transfer coefficient change as deltaT change?

h is normally assigned as the convective heat transfer coefficient, so it is associated with the flowing fluid properties. If the fluid is an ideal gas or a perfect gas then h will vary according to the shear that occurs in the boundary layer near the surface of the solid object being heated or cooled, and so most correlations will vary h as a function of flow properties such as Re , modified with the prandtl number Pr. The fluid properties used are those in the middle of the boundary layer, so as DT increases the properties in the boundary layer slightly increase as well. If the fluid is a liquid close to either the saturation temperature or if it is a supercritical fluid near the critical temperature then the fluid properties vary dramatically in the boundary layer and conventional correlations for h are not valid.

In the 1970's there was an alternate method of computing q and h proposed by E Audiatori in his textbook " the new heat transfer", but it never caught on.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

RE: Heat transfer coefficient change as deltaT change?

The natural convection htc does vary with surface temp. See any college or Uni heat transfer text. The natural convection htc is a function of the Grashof no and the Prandtl no, both computed at the film temp, where Tf = (Ts + Ta) *0.5.

RE: Heat transfer coefficient change as deltaT change?


Thank you all for the answers, it was very informative and helpful.

Kind regards - Jens

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! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close