Does "Nu" Changes with axial orientation of pipe during external natural convection heat transfer?
"A lumped study of laminar forced convection in pipes with circular
transverse fins at the outer surface"
A. Campo and F. Miralles-Wilhelm, Caracas, Venezuela
may be useful to you.
Yes, Nu number changes depending on the orientation of the axis.
1. When the axis is placed parallel to the gravity:
The Nu changes along the axis, while it is uniform in the circumferential direction. Nu decreases in the upward direction, because the thermal boundary layer becomes thicker with the increasing height.
2. When the axis is placed perpendicular to the gravity.
The Nu is uniform along the axis, while it changes in the circumferential direction. Nu is highest at the bottom surface and the lowest at the top, because the thermal boundary layer develops from bottom to the top in case of the horizontally placed pipe.
Average Nu over the whole pipe surface is larger for the horizontal pipe, since you assume a long pipe.
thank you hiroshi
theoretically the statements are true and it is reported in case plates.
is there any reported research in, natural convection heat transfer from (external) surfaces i.e., solids/hollow structures(except horizontal/vertical plates) regard to relation between nusselt number and Rayleigh Number at different orientations.
if exists suggest me the corresponding literature.
thanks in advance
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