Abstract
Unique experiments were performed in a homogeneously sparged rectangular 400×200×2630 mm (W×D×H) bubble column with and without liquid co-flow. Bubbles in the range 4–7 mm were produced by needle spargers, which resulted in a very uniform bubble size. Dual-tip optical fibre probes were used to measure horizontal profiles of gas fractions, bubble velocities and bubble chord lengths for superficial gas velocities Usg in the range 0.63–6.25 cm/s and superficial liquid velocities Usl up to 20 cm/s. Images of the bubble column were captured and a Bubble Image Velocimetry technique was adopted to calculate bubble (parcel) velocities. For low gas fractions, when a homogeneous flow regime occurred, both methods agreed very well and the optical fibre probes were found to be rather accurate for our bubbles. A liquid co-flow was found to have a calming effect and to stabilize a homogeneous bubbly flow regime, with less spatial variation in gas fractions and bubble velocities. Bubble chord lengths were almost normally distributed and do not exhibit the theoretical triangular probability density functions. The mean cord lengths were in the range 1.9–3.5 mm and found to increase with Usg and to decrease slightly with increasing Usl, while a liquid co-flow significantly reduced the standard deviation of the chord length distribution.
Highlights
Due to the continuous increase in computational power and demand for more accurate multiphase CFD simulations, there is an obvious need for more precise and detailed experimental data on bubbly flows for development and validation purposes
We report new bubbly flow data acquired in the “LimBuRig” test facility, which is meticulously described in our previous paper (Muilwijk and Van den Akker, 2019)
The main objectives of this paper are to show how: (1) the the gas fraction α is a function of the superficial liquid and gas velocities and to validate the previously proposed correlation for the gas hold-up; (2) the bubble velocity vb is influenced by liquid co-flow, where the optical fibre probe (OFP) and Bubble Image Velocimetry (BIV) methods are compared; and (3) the distribution of the bubble chord length c is a function of the superficial gas velocity and liquid co-flow velocity and how they relate to the mean bubble diameter
Summary
Due to the continuous increase in computational power and demand for more accurate multiphase CFD simulations, there is an obvious need for more precise and detailed experimental data on bubbly flows for development and validation purposes. Euler-Euler CFD simulations, where both liquid and gas phases are modeled as interpenetrating fluids, require proper modeling of two-phase turbulence and of the interfacial forces such as drag, virtual mass, lift, wall lubrication and turbulent dispersion (Van den Akker, 1998; Van den Akker, 1998; Dhotre et al, 2013; Liao et al, 2015; Van Den Akker, 2015) These sub-models dealing with interfacial momentum transfer rates and bubble induced turbulence, are a strong function of (local) bubble size, slip velocity and void fraction. Bubble size distributions play a vital role in setting up CFD simulations and in their validation (Besagni and Inzoli, 2016)
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