Heat Transfer Coefficient -Air Flow onto perpendicular plane

[Mitch1]

Hi All,

I have a situation were I need to evaluate the heat transfer coefficient of a horizontal surface. The surface has tubes within although I first used "flow across a bank of tubes", this has been untrue due to the air flow coming from above the surface flowing vertically (and perpendicular) to the surface. I cannot find anywhere, books or websites, where there are examples of this. Has anyone evaluated this before or know of any links to work out the Nusselt no. Etc...

Many thanks

 

[Nidum]

I can't visualise the problem from your description . Please describe further and post a clear diagram .

 

[Mitch1]

Hi Nidum, thanks for your reply - yes I can attach a sketch although I am not too sure how to do it on this forum - any ideas? (Apologies)

 

[Nidum]

Use the upload button bottom right in answer box .

 

[Mitch1]

Thanks, rough sketch shows the air being forced into surface, this will create turbulent flow? But can't find anything on how to approach this and it is both linear and turbulent to some extent

Thanks

 

[Nidum]

Look up impingement heat transfer .

 

[Mitch1]

Is that not for just jets onto a heated surface? Rather than air?
Thanks again

 

[Nidum]

Should not be too difficult to start from basics :

(1) Estimate what the flow pattern of air over your plate looks like and then estimate flow velocities at different radial distances from axis .

(2) Having done that determine whether laminar or turbulent flow conditions apply and then determine heat transfer conditions in circular zones at increasing radial distances .

An analytic solution is probably possible but a numerical solution might be more useful if you just want answers to a practical problem .

 

[Mitch1]

Thanks for this guidance - much appreciated

I will look into this as it is a practical problem

Thanks again

 

[CWatters]

If all of the air from the jet exits via the vents then can you work it out by measuring the flow rate and the temperature delta?

 

[Mitch1]

The air isn't coming from the vents it is being extracted there. The air is coming from above the surface being blown downwards

 

[CWatters]

OK but you understand that by measuring the airflow rate and temperature change you can work out the power extracted.

 

[insightful]

As a practical problem, with basically unknown air velocities everywhere, I'm afraid any calculation will be good to only an order of magnitude. I'd start with a heat transfer coefficient of 20 Btu/(hr.ft².^oF) and do some testing on a mock-up.