Destabilisation of homogeneous bubbly flow in an annular gap bubble column

Abstract

Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102 m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014–0.200 m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift‐flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity.

Comprehensive experimental investigation of counter-current bubble column hydrodynamics: Holdup, flow regime transition, bubble size distributions and local flow properties

“…The sparger used in this study has "large" holes (d 0 41 mm), and the shape of the holdup curve is the one typically obtained for a sparger with similar hole openings (i.e., single/multiple nozzles, spider sparger or perforated plate sparger with a hole diameter greater than 1 mm); as expected, no peak can be observed in the holdup curve. Alternatively, when using "fine" spargers, a peak in the holdup curve may appear, as obtained by Al-Oufi (Al-Oufi et al, 2010, 2011, Schumpe and Grund (1986), and Zahradnik et al (1997).…”

“…The value depends mainly on the sparger type. Single and multiple nozzles or perforated plate spargers with a hole diameter greater than 1 mm ("coarse" spargers) usually lead to an early regime transition, whereas "fine" spargers such as porous plates (Al-Oufi et al, 2010, 2011 or needles (Mudde et al, 2009) can maintain a stable homogeneous regime at higher gas flowrates (i.e., Mudde et al (2009) reported a homogeneous regime up to a holdup equal to 0.55). The sparger considered in this study is of the "coarse" type, and the values of U trans agree with the data and observations presented in the literature (i.e., the study of Dargar and Macchi (2006) and Rollbusch et al (2015a)).…”

An experimental study of gas void fraction in dilute alcohol solutions in annular gap bubble columns using a four-point conductivity probe

“…Experiments were conducted in both the OTBC and AGBC and apart from the addition of various alcohols, the experimental conditions and column specifications are described by Al-oufi et al (2010). The liquid phase used in the experiments consisted of tap water to which ethanol and isopropanol were added in various concentrations.…”

“…Figure 2 illustrates the design and geometry of the four-point probe. In principle, the it operates in a similar way to the two-point probe described in Al-Oufi et al (2010).…”

“…Recently Al-Oufi et al (2010) compared void fractions in various geometries of AGBC for tap water with measurements from an OTBC. They fitted their data to Zuber and Findlay's (1965) drift flux model, eq.…”