Forced convection heat transfer from banks of helical coiled resistance wires

Forced convection heat transfer from banks of helical coiled resistance wires in a cross flow of air was investigated experimentally, with reference to various operative conditions and several coil geometries. The resistance banks were subdivided in two branches, independently powered by single-phase alternating current. Average convection heat transfer coefficients were determined for each branch. No significant interactions were detected between adjacent coils nor between upstream and downstream coils, possibly because of the high percentage of inter-spaces between wires. The experimentally measured Nusselt numbers were thus correlated by a modified version of the Churchill and Bernstein model, valid for a single cylinder in cross flow. To this purpose, an equivalent wire diameter was defined in order to take into account the influences of diameter and pitch of the coils. The average deviation between measured and calculated values of the Nusselt number is of the order of +/- 7% in the range of Reynolds numbers from 70 to 400.