Two-phase distribution in the vertical flow line of a domestic wet central heating system

Abstract

The theoretical and experimental aspects of bubble distribution in bubbly two-phase flow are
reviewed in the context of the micro bubbles present in a domestic gas fired wet central heating system. The
latter systems are mostly operated through the circulation of heated standard tap water through a closed loop
circuit which often results in water supersaturated with dissolved air. This leads to micro bubble nucleation at
the primary heat exchanger wall, followed by detachment along the flow. Consequently, a bubbly two-phase
flow characterises the flow line of such systems. The two-phase distribution across the vertical and horizontal
pipes was measured through a consideration of the volumetric void fraction, quantified through photographic
techniques. The bubble distribution in the vertical pipe in down flow conditions was measured to be quasi
homogenous across the pipe section with a negligible reduction in the void fraction at close proximity to the
pipe wall. Such a reduction was more evident at lower bulk fluid velocities.