Film Coefficient of Salt Water

[gjacc]

I'm designing an underwater heatsink that needs to dissipate 90W. The basic problem is a horizontal cylinder 4.5 inches Dia X 6.0 inches Long, and sits in salt water at 70 deg F under free convection only. I've found the inside temp. results to be highly dependent on the free convection (film) coefficient of salt water. I've estimated this to be .040 Btu/hr*in^2*degF (33 W/m^2*K).

Is this a good estimate? Does anyone have real world film coeff. of salt water numbers?
Thanks for the replies/feedback,
Greg

 

[Montemayor]

gjaccl:

You're only transferring 307 Btu/hr. This is a laboratory, bench-scale size of heat transfer. By "salt water" I presume you mean sea water; by "free" convection, I presume you mean natural convection - convection generated by the differential temperature change in the sea water as the heat transfer takes place.

Under these conditions the natural convection film coefficient for common water should be accurate enough to design the heat sink.

You say you're designing the heat sink at present. How can you have the sink dimensions already and not have finished the calculations - including the estimation of the film coefficient(s)? This is confusing. Do you mean to state that you have an existing, 4.5 inches Dia X 6.0 inches long heat sink and you want to find out what heat transfer rate you can dissipate using 70 oF sea water as your cold bath?


Art Montemayor
Spring, TX

 

[gjacc]

Montemayor,

I've left off a lot of detail. There are really two heat sources (90W total), and internal structure to conduct heat to the outer cylinder (4.50 inch dia). The cylinder diameter is a rigid parameter, the length less so. My FE thermal analysis gives me an internal cavity temp of 130 degrees. This is marginally too high and I wanted to see if I am being too conservative with the natural (free) convection coefficient of .040 Btu/hr*in^2*degF.

Thanks,
Greg

 

[EdStainless]

If this is in natural seawater then bio-fouling is real worry. You may want to use 70/30 Cu/Ni as an outter sheath to retard bio attachment.

If this is just salt water, then you are probably fine with the values that you have used.

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Corrosion never sleeps, but it can be managed.

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