Convective Heat transfer for a cylinder in a cross flow - With surface roughness

[WelshEngineer87]

Hi all,
I'm currently building a 2D heat transfer model in matlab of a pipe and want to account for the effect of different surface roughness on the external surface of the pipe in contact with external airflow at 90deg to the longitudinal axis of the pipe.

I'm currently using Churchill Bernstein correlation to calculate the nusselt numbers however this method doesn't account for surface roughness. Anyone aware of any equations / correlations that can be used to account for this?

Thanks

 

[goutam_freelance]

Heat transfer coefficient will increase if the roughness height exceeds the laminar sublayer thickness due to so called finning effect. The heat transfer will generally depend on usual variables plus roughness height and distance between rough peaks. A representative graph is given below. (delta=roughness height, d=inside dia) Refer -Heat Transfer by Isachenko, Osipova, Sukomel Chap-8)

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[EdStainless]

With external surfaces you are dealing with heat transfer with air, so getting a roughness that is 'taller' than the lamer flow zone will take some hellacious flow velocities. Start with Reynolds numbers and laminar layer calacs to see if that roughness is even possible (if it comes out at 0.5mm then no chance of material that rough).

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[IRstuff]

To the 1st order, it's still mostly about surface area, so one look at bumps as half-spheres, and the relative improvement in surface area is 2x the base area of the half-sphere, so it takes a lot. If you want to seriously affect the area, then some sort of clip-on or clamped fin structure might make more sense. Pin fins, pictured below are supposedly very efficient, but I didn't find any that suitable for a cylinder, although it seems plausible that one might be able to bend the base to shape.

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