Studies on Pool Boiling Heat Transfer. 6th Report. Critical Heat Flux on Vertical Wires.

Abstract

Critical heat flux (CHF) on vertical wires was measured for water and ethanol under atmospheric pressure. The diameter of wires ranged from 0.3mm to 3mm and their length from 4mm to 200mm. For shorter wires, CHF occurred at the top and increased with decreasing length, while for longer wires, CHF occurred in a lower region of the wires and did not vary with diameter. A physical model for CHF on vertical wires was constructed based on a CHF mechanism, in which CHF is caused by the dry-out of liquid macrolayers formed on a heating surface. In the model it is assumed that coalesced bubbles moving upward along a wire have a liquid macrolayer on their bottom, and the motion of the coalesced bubbles is determined by the balance of force between buoyancy and drag. CHF on the longer wires occured where coalesced bubbles begin a zig-zag or spiral motion. The predicted CHF and the location of CHF were compared with experimental data and fairly good agreement was obtained.