×
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

Estimating Heat Transfer Coefficient

Estimating Heat Transfer Coefficient

Estimating Heat Transfer Coefficient

(OP)
With refernce to my previous thread (Wind Adjusted Heat Transfer Coefficient) I need to estimate the heat transfer coefficient on the inside surface of a sealed enclosure.  The air in the enclosure is 20degC hotter than the ambient (0degC with ambient at -20degC).  There is a heater and fan inside the enclosure.

I have run an experiment with this setup and estimated the combined inner and outer surface heat transfer coefficient to be 7W/m^2-K.  However I expect the internal h to be greater than the external h due to forced (probably mixed) convection internally and natural convection externally.  I would like to apply the experimental results to different exterior flow conditions so need to know the interior heat teansfer coefficient as the internal flow will be the same for all external conditions.

I do not have access to thermal simulation software.  My thoughts are to repeat the experiment with external forced convection but the air speed would need to be extremely fast to be confident that the external heat transfer coefficient is in a stable (i.e. will not increase with increased air speed) condition and has reached an asymptote.

The internal geometry is reasonably complicated so that standard and published empirical heat transfer equation do not apply.  I also do not know where on the fan curve the fan is operating so cannot estimate internal flow velocity.

Another way is to assume the external heat transfer coefficient was 4W/m^2-K during the experiment (natural convection), giving me an internal value of 10W/m^-K.  However this is a big assumption, especially if I want to predict the heat transfer if there is wind/ice/rain externally as using h=10 or h=11 internally gives a substantial difference in heat transfer.

Is there a way of estimating the internal heat transfer coefficient?

RE: Estimating Heat Transfer Coefficient

The heat transfer coefficient for steady state one dimensional heat flow is simply derived from -kdT/dx=h(T-Ta). If you can measure the surface temperatures in a region away from other features then you'll know dT/dx, and k for the material. Ta is just the ambient temperature.

corus

RE: Estimating Heat Transfer Coefficient

(OP)
Corus - thanks for the response.

I have been using the following equation to estimate the average values for h for the internal and external boundary layers:

Q = h.A.(T-Ta)

I had been working with results from a previous experiment that gave Ti (internal air temperature) and Ta.  It was not designed for this problem hence the wall temperature (Tw) was not measured.  The heat input (Q) was controlled and the surface area (A) was known.  I had assumed h(internal) and h(external) were the same as I had no way of differentiating bewteen the two, thus eliminting one variable and giving an answer of 7W/m^2-K for h(internal) and h(external).

However I am now designing an experiment that will also give Tw (enclosure wall temperatures).  Therefore I will know Ti, Tw (inner and outer - although as it is thin walled aluminium I expect Tw to be constant through the wall thickness and provide negligible thermal resistance), Ta, Q and A.

If I then take:

h(internal) = Q / A.(Ti - Tw)

and:

h(external) = Q / A.(Tw - Ta)

This will give me the average values for h for the inernal and exteral boundary layers.

Is this correct thinking?

Thanks.

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