The Limerick bubbly flow rig: design, performance, hold-up and mixing pattern

peer-reviewedAs Euler-Euler CFD simulations of bubbly flows suffer from uncertainties due to the many underpinning models, there is an obvious need of accurate experimental data for validation. With this in mind, a new bubbly flow test rig was built to be operated with and without liquid co-flow, with bubble size as uniform as possible in the range 4–7 mm, and with a very even horizontal bubble distribution. We designed the gas sparging system such that we can also produce an essentially bi-modal bubble size distribution. The column consists of two square sections to allow for studying the mixing of two originally separated bubbly flows with either the same or a different bubble size. The bubbles are produced from 2 × 196 needles, bubble sizes are determined with high-speed imaging and with a simple acoustical method, overall volume fractions in the column by means of air chamber pressure measurements. Overall volume fractions are presented as a function of gas and liquid flow rates, with slip velocity mostly increasing with increasing void fraction. First results are obtained on (a) producing bi-model bubble size distributions and the pertinent volume fractions in the column, and (b) flow patterns in the case of unequal aeration

Flow structures in bubbly flows with a tailored bubble size with and without liquid co-flow

The aim of this thesis is to provide experimental data on bubbly flows for validation of Computational Fluid Dynamics simulations. Therefore, a new experimental facility was developed to study the interaction of two vertical, initially separated, parallel bubbly flows in a rectangular channel of 0.40×0.20×2.63 m (W×D×H). The superficial gas and liquid velocities for each inlet were independently controlled and a large variety of unique a-symmetric bubble column configurations was investigated. Two multi-needle gas spargers, each comprising 14×14 needles in a square perforated grid, produced uniform large (4-8 mm) bubbles. Preparatory experiments in a downscaled setup, to study bubble formation rates and spreading rates of a single bubble train, yielded a novel correlation for the bubble diameter as a function of the gas flow rate, liquid co-flow velocity and needle diameter. Operating regimes were identified for which bubbles were formed individually. Bubble formation rates for the multi-needle gas sparger, determined with high-speed imaging and a novel acoustic technique, agreed very well with those for a single needle bubbling under similar conditions. Void fraction measurements in the large test-setup resulted in a new correlation for the gas hold-up. Lateral profiles of the bubble velocity were measured by using optical fibres and compared with parcel velocities obtained by using Bubble Image Velocimetry (BIV). Unique data on chord length distributions was obtained as the bubble size was uniform and accurately described by the developed correlation. The effects of a-symmetric gas sparging and (non-)uniform co-flow on the flow patterns were illustrated by contour plots of the bubble velocity magnitude and root-mean-square velocity fluctuations. An operating map was constructed to predict a-symmetric operating conditions for which both inlets yield equally sized bubbles and equal gas fractions, such that flow patterns appeared without buoyancy driven flow structures showing up. Vortex roll-up and circulatory motions were observed under certain conditions

Experimental investigation on the bubble formation from needles with and without liquid co-flow

We report experiments on bubble formation from needles with and without liquid co-flow, carried out with needles in the range of 0.79 < dn < 2.06 mm, for gas flow rates up to 4.5 cm3/s per needle, and with liquid co-flow velocities up to 0.4 m/s. Bubble sizes and frequencies were obtained by means measuring an acoustic signal in the pressurized chamber upstream, which is validated by high-speed imaging analysis. Bubble contours, bubble growth curves and time return plots were obtained to analyse the bubble formation process. Different bubbling regimes are distinguished and a novel dimensionless pressure ratio is proposed to forecast the emergence of weeping and the transition from constant flow rate bubbling to constant chamber pressure bubbling. A single correlation for the non-dimensional bubble size with and without liquid co-flow was developed and validated with the experimental data obtained in the present study

The Limerick bubbly flow rig: design, performance, hold-up and mixing pattern

As Euler-Euler CFD simulations of bubbly flows suffer from uncertainties due to the many underpinning models, there is an obvious need of accurate experimental data for validation. With this in mind, a new bubbly flow test rig was built to be operated with and without liquid co-flow, with bubble size as uniform as possible in the range 4–7 mm, and with a very even horizontal bubble distribution. We designed the gas sparging system such that we can also produce an essentially bi-modal bubble size distribution. The column consists of two square sections to allow for studying the mixing of two originally separated bubbly flows with either the same or a different bubble size. The bubbles are produced from 2 × 196 needles, bubble sizes are determined with high-speed imaging and with a simple acoustical method, overall volume fractions in the column by means of air chamber pressure measurements. Overall volume fractions are presented as a function of gas and liquid flow rates, with slip velocity mostly increasing with increasing void fraction. First results are obtained on (a) producing bi-model bubble size distributions and the pertinent volume fractions in the column, and (b) flow patterns in the case of unequal aeration

The effect of liquid co-flow on gas fractions, bubble velocities and chord lengths in bubbly flows. Part I: uniform gas sparging and liquid co-flow

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 distributio

The effect of liquid co-flow on gas fractions, bubble velocities and chord lengths in bubbly flows. Part II: asymmetric flow configurations

This paper describes the effects of uniform and non-uniform liquid co-flow on the bubbly flow in a rectangular column (with two inlets) deliberately aerated unevenly. The two vertical bubbly streams, comprising uniform bubbles, started interacting downstream of the trailing edge of a splitter plate. This study quantifies the emergence of buoyancy driven flow patterns as a function of the degree of a-symmetric gas sparging and (non-)uniform liquid co-flow by using Bubble Image Velocimetry (BIV) and dual-tip optical fibre probes. Without liquid co-flow, small differences in the gas fraction of the left and right inlet had a large effect on the mixing pattern, whereas a liquid co-flow stabilized a homogeneous flow regime and the flow pattern was less sensitive to gas fraction differences. Void fractions, bubble velocities and hord lengths were measured at two fixed position in the flow channel, whereas BIV provided a global overview of the flow structures. A correlation was developed to predict (a-symmetric) operating conditions for which the gas fraction of the left and right inlet are balanced, such that the bubble motion is governed by advection and no buoyancy driven flow structures arise. The data obtained is highly valuable for CFD validation and development purposes

Effect of post-heat-treatment on thermal and physical characteristics of NiTi tubes produced via conventional drawing and laser powder bed fusion

NiTi is one the metal alloys exhibiting shape memory and superelasticity which makes it suitable for a wide range of engineering and biomedical applications. Nickel-rich NiTi requires heat treatment and cold work as post-processing steps to enable appropriate phase production for final applications. In the current work, NiTi tubes (50.8%Ni-49.2%Ti) produced using Laser Powder Bed Fusion (PBF-LB) and conventional drawing were heat treated at a constant temperature of 500 °C for 10, 30, and 60 min. Differential scanning calorimetry results indicate that the austenite finish temperatures for the tubes were set at 25.4 °C and 26.62 °C after 60 min of heat treatment for the PBF-LB and conventionally drawn tubes respectively. The enthalpy of phase transformation for PBF-LB tubes increased from 2.89 J/g to 17.14 J/g while for conventionally drawn tubes it increased from 5.41 J/g to 16.08 J/g. The EDX measurements on as-built and heat-treated tubes indicated no loss of nickel during the heat-treatment process. The thermal expansion results show an unstable evolution of CTEs in as-built NiTi tubes which were stabilized via heat-treated with resulting CTE ≈11.4 × 10−6/ºC. The hardness decreased in the heat-treated tubes as compared to the as-built tubes. The hardness decreased by 12.6% for PBF-LB and 12.4% for the conventionally drawn tubes respectively

Analysis of nitinol actuator response under controlled conductive heating regimes

In the last few decades, Nitinol (NiTi) actuators have created a massive impact at the commercial level due to their application in various engineering and medical fields. In this paper, an experimental analysis study is presented on commercially manufactured nitinol tubes for performance enhancement. As received tubes were super-elastic at room temperature with Af temperature of 1.7°C. The nitinol tubes were heat treated at 500°C for different time ranging from 30 min to 60 min to raise the Af temperature. Metallography was performed on pristine and heat-treated samples to analyse the changes in the physical properties. XRD analysis revealed the crystalline structure present in the tubes (as received and heat treated) was nitinol cubic (110) while nitinol cubic (211) at room temperature. Moreover, dilatometry was performed which showed thermal expansion coefficients very close as noted in the literature as 11.4x10-6/°C. In the last section of this paper, the actuation force of the tubes was experimentally measured and analysed using different springs attached to the tubes connected to a conductive heating stage. A full factorial Design of Experiments (DoE) was used based on factors of time, temperature, and spring constant. For a surface temperature of 125°C and a spring constant of 2.39 kN/m, 131 N force was attained from the tube. The maximum actuation force of 145 N was obeserved for surface temperature of 145°C at an exposure time of 60 s with k = 2.39 kN/m