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.
Highlights
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
As illustrated in figures 4 - 8, the results are presented through the measured volumetric void fractions across the pipe section, using sight glass VSG1 for the experiments as tabulated in table 1
After considering the errors due to the experimental uncertainty that amount to ±19.6% of the readings values, the results, suggest that a quasi-flat volumetric void fraction profile is expected across the vertical pipe section for a downward two-phase bubbly flow
Summary
A schematic diagram of the experimental set up is shown in figure 1. The saturation ratio, α (-), was calculated through the application of Eq (1), as defined by Jones et al [11] This is the ratio of the actual dissolved gas concentration Cgas An analysis of the dissolved gas present in the closed loop system has shown that nitrogen is the dominant gas This is a result of a limited oxidation following the system filling with fresh water. A constant typical system pressure of 2.7 Bars (abs) was used for all experiments This was set through the use of a nitrogen gas cylinder connected to a standard cylinder regulator. The bubble distribution in the horizontal pipes was measured through the use of 6 focal planes at a depth of 1,4,8,12,16,19 mm across the vertical sight glass, VSG1 (figure 2).
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