Two-dimensional convection from heated wires at low Reynolds numbers

TLDR

Heat transfer from horizontal wires at very low Reynolds numbers is dominated by free convection, which becomes negligible as Re increases, making wire orientation largely irrelevant except at extremely low velocities. The study measured Nusselt numbers for circular wires in a horizontal airstream over Re 0.01–140, finding that the relation between Nusselt, Re, and temperature loading changes at Re≈44, the vortex‑street threshold, and is independent of turbulence intensity.

Abstract

Measurements of heat transfer from circular wires placed normal to a horizontal airstream have been made in the Reynolds number range 0·01 to 140. The Nusselt number can be related to the Reynolds number and temperature loading by an expression of the form where the values of n, A and B (see table 3) depend on whether the Reynolds number is above or below the value for which a vortex street exists in the wake of the wire. This value of the Reynolds number (R [eDot ] 44) is independent of the intensity and scale of the stream turbulence. The theoretical heat transfer relation based on the Oseen approximation is approached asymptotically as R → 0, provided free convection is negligible.Free convection effects diminish rapidly with increasing Reynolds number so that the orientation of the wire with respect to the vertical has a negligible influence on heat transfer except at very low velocities. For horizontal wires at very low Reynolds numbers, free convection is significant, when, roughly speaking, the Reynolds number is less than the cube root of the Grashof number.