Scale Bubble Research Articles

Sub-Hinze scale bubble production in turbulent bubble break-up

Abstract

Scale characterisation of an oxidised (Hf,Ti)C-SiC ultra-high temperature ceramic matrix composite

A hybrid carbide ultra-high temperature ceramics matrix [(Hf,Ti)C-SiC] reinforced with BN-coated carbon fibres was fabricated and tested for surface oxidation resistance. The UHTC composite showed an average mass ablation rate of 0.0014 g/s after exposure to a high heat flux (∼17 MW/cm2) oxyacetylene flame test for 30 s above 2500 °C. The cross-sectional profile of the oxides scale formed was characterised and analysed. The scale was multicomponent; consisting of oxides of Hf, Ti and Si, as well as HfTiO4 and HfSiO4, which underwent phase separation and immiscibility. Multiple glassy bubbles formed on the scale surface due to the impediment of escaping gases by the glassy layer on the outer scale. The largest pores in the scale and surface bubbles that resisted rupture were the dominant features of the outermost phase-separated layer. Phase separation in the scale top layer improves the resistance to scale rupture.

Experimental analysis of gas phase dynamics in a lab scale bubble column operated with deionized water and NaOH solution under uniform bubbly flow conditions

In this study we present an investigation on the impact of sparger configuration and fluid properties on the local gas phase dynamics for up to 17% total gas holdup. Experiments in a bubble column with 100 mm inner diameter were conducted with deionized water and NaOH solution of up to cNaOH = 32 mmol·l−1. Ultrafast X-ray computed tomography (UFXCT) was applied to obtain local hydrodynamic parameters, such as gas holdup distribution, bubble sizes and average bubble rise velocities. Radial gas holdup profiles show wall peaking, which is attributed to the uniform bubbly flow regime generated by the needle sparger used in this study. The addition of NaOH leads to similar gas holdup values but significant changes on local bubbly dynamics, such as bubble size distribution and axial gas phase velocity. The Sauter mean diameter was found to decrease with increasing concentration of NaOH, whereby the decrease is larger at higher gas flow rate.

Evaluation of different turbulence models on simulation of gas-liquid transient flow in a liquid-ring vacuum pump

To analyze the prediction ability and applicability of different turbulence models to the complicated gas-liquid flow in the liquid-ring vacuum pump, three different turbulence models, namely RNG k-ε, SST k-ω and LES were respectively used to simulate the gas-liquid flow, and the numerical results were compared with the experimental results. The study shows that the trend of pump hydraulic performance with the flow rate and the shape of gas-liquid interface in pump calculated by the three turbulence models have a good agreement with the experimental results. The three models are difficult to capture the small-scale bubbles in liquid-ring, but LES model can capture larger scale bubbles near the gas-liquid interface. The prediction ability of RNG k-ε and SST k-ω models to the complex vortex structures is limited for the influence of Reynolds-averaged treatment, but the LES model can predict the complex channel vortices in impeller passage and the wake vortices of different scales at the impeller outlet. The computation cost of LES model is about three times of the other two models. The RNG k-ε and SST k-ω models have better applicability than LES model if we mainly focus on the distribution of phase and pressure fields in pump.

Effects of bubble coalescence and breakup models on the simulation of bubble columns

This work investigates the effects of coalescence and breakup models on the radial distribution of gas holdup and local bubble size distribution in a laboratory scale bubble column operated at 0.11 m/s, 0.14 m/s, 0.19 m/s and 0.23 m/s. The results quantitatively show the influences of bubble coalescence due to various mechanisms, coalescence efficiency calculated by different models, modification coefficients for coalescence rates, critical energy required for breakup and bubble breakage caused by large scale eddies and viscous shear on numerical simulations. Furthermore, qualitative analysis is performed to analyze the effects of these factors. The performance of various models is evaluated (e.g. Han’s breakup model and Das’s coalescence model). Some optimized combinations of coalescence and breakup models that can be used to accurately predict the local gas holdup and bubble size distribution in laboratory scale bubble columns operated at heterogeneous regime are proposed.

Experimental Study on Acoustic Attenuation of Bubble Wake

When ship sails on the water surface, it will form a wake containing a large number of bubbles for different scales. The existence of bubbles will cause the attenuation of sound waves. In this paper, bubbles with different sizes are generated in the pool by using ceramic tube array. An experimental study on the acoustic attenuation characteristics of sound waves in bubbles was carried out for still water, small bubbles, medium-sized bubbles and large bubbles. Through experimental research, it is found that the existence of bubbles makes the acoustic signal decay obviously. At different frequencies, bubbles decay more than 10dB relative to still water. Large bubbles have the strongest attenuation effect on acoustic signals, averaging over 20dB. The medium and small bubbles are similar to the attenuation of the acoustic signal. The acoustic signal intensity in static water and different scale bubbles is basically the same as the frequency.

Sequential gas-liquid treatment for gaseous toluene degradation by Fenton’s oxidation in bubble reactors

A lab scale bubble reactor (BR) configuration was employed to evaluate the degradation of gaseous toluene by Fenton’s oxidation under semi-batch mode operation (i.e., with continuous gas bubbling in the liquid phase where organics oxidation is carried out). A parametric study was performed for evaluating the effect of Fenton’s process operating parameters, such as temperature and concentration of Fe2+ and H2O2, in the removal of toluene from the gas stream. The maximum amount of toluene transferred (0.041 mol per liter of solution) was reached when the optimal conditions ([Fe2+] = 2.5 mM, [H2O2] = 20 mM, and T = 25 ºC, at pH 3.0) were used after 120 min of reaction, yielding the highest average toluene absorption rate (5.78 × 10−6 mol/L.s). The treatment of the gas stream increased, however, the organic load in the aqueous phase, so a subsequent treatment stage of the liquid was performed. Additionally, a scale-up of the sequential gas-liquid treatment for a bubble column reactor (BCR) was carried out for several cycles, up to almost 20 h. This strategy, making use of the Fenton’s process for treating the toluene gas stream, with intermediate liquid oxidation, allowed to reach more gas treatment stages, while providing a final effluent that is non-toxic (0.0 % of inhibition of Vibrio Fischeri) and biodegradable.

Synergistically applying 1-D modeling and CFD for designing industrial scale bubble column syngas bioreactors.

The reduction of greenhouse gas emissions and future perspectives of circular economy ask for new solutions to produce commodities and fine chemicals. Large‐scale bubble columns operated by gaseous substrates such as CO, CO2, and H2 to feed acetogens for product formations could be promising approaches. Valid in silico predictions of large‐scale performance are needed to dimension bioreactors properly taking into account biological constraints, too. This contribution deals with the trade‐off between sophisticated spatiotemporally resolved large‐scale simulations using computationally intensive Euler–Euler and Euler–Lagrange approaches and coarse‐grained 1‐D models enabling fast performance evaluations. It is shown that proper consideration of gas hold‐up is key to predict biological performance. Intrinsic bias of 1‐D models can be compensated by reconsideration of Sauter diameters derived from uniquely performed Euler–Lagrange computational fluid dynamics.

Equilibrium Solubility Measurement and Modeling of CO2 Absorption in Aqueous Blend of 2-(Diethyl amino) Ethanol and Ethylenediamine

The equilibrium CO2 solubility in an aqueous blend of 2-(diethyl amino) ethanol (DEEA) and ethylenediamine (EDA) was performed at atmospheric pressure with the use of a laboratory scale bubbling absorber. The total concentration of amine blend solution in the range of 1.13–4.90 mol·kg–1 and the EDA mole fraction in the total amine blend in the range of 0.05–0.20 were taken. The experiments for the measurement of CO2 solubility were performed at a temperature range of 303.15–333.15 K and CO2 partial pressure range of 10.13–20.27 kPa. The analysis of CO2 in the liquid phase was accomplished by a direct titration method using HCl solution. The solubility data of CO2 in DEEA+EDA solution of the present study were compared with data available in the literature. The experimental results were applied to develop an empirical mathematical model with an average absolute deviation of 7.03% to predict the CO2 equilibrium solubility in DEEA+EDA solution within the specified range of operating conditions. In addition, ...

Phase critical angle scattering for measurement of transient nanoscale growth rate of a micron-sized bubble.

We developed phase critical angle scattering (PCAS) to simultaneously measure the spherical and transparent bubble size at the micron scale and transient bubble growth at the nanoscale. The theoretical derivation of PCAS reveals that the phase of the fine structure of critical angle scattering caused by reflection and first-order refraction is highly sensitive to and linearly shifts with bubble diameter growth. Experiments on a single growing bubble are implemented with a Fourier imaging system. The results show that the PCAS technique can measure the tiny bubble growth down to tens of nanometers, providing a promising tool for accurate characterization of bubble dynamics.

The numerical modeling of the vapor bubble growth on the silicon substrate inside the flat plate heat pipe

Inside the flat plate heat pipes for the microelectronics packages, the micro scale boiling is a common phenomenon under the suddenly high heat flux. The internal vast vapor bubble nucleation and growth become a challenge for the stable operations. Due to the lack of the direct observations, the vapor bubble growth in early stage is still unclear. In order to study the initial bubble growth, the molecular dynamics models of water-silicon interfaces which have different sizes of simulation boxes were examined. The vapor bubble diameter growth rates (0.18 ± 0.02 × 106 mm/s) were calculated among the different sizes of simulation boxes. Since the lack of extra energy input, the initial vapor bubble can only expand to 45 nm diameter large (under 100 W/cm2 heat flux at 350 K initial temperature). The growth phenomena of 0.1 μm to 0.3 mm scale bubbles could be further modeled by the Mixture models. The observed bubble (bubble diameter between 10 μm and 0.3 mm) had a diameter growth of 0.71 ± 0.19 mm/s that was close to the Mixture model simulated one (0.75 mm/s). In addition, the ratio of the bubble volume growth rate to the bubble covered bottom area was about 7 ± 0.2 mm/s. By calculating the ratio between the growth rate of observed bubbles and volume of the simulated stable nucleation bubble, the bubble generation rate (3 × 1014 bubble/s mm2) under certain conditions might be estimated. Therefore, the whole process of bubbles growth could be deduced.

MSBE Analysis with Frequency Spectra for Automated Identification of Epileptic Seizure

Objective-This study introduces a reliable automated seizure detection technique based on MSBE (Multi scale bubble entropy) and frequency spectral analysis. Method- This paper aims to develop a novel seizure detection technique that incorporates AM FM model for decomposition of EEG into different sub bands. In our approach, integrated feature set is constructed using multi scale bubble entropy analysis at each sub band and frequency spectral analysis at each electrode. Result-In this paper, an application of bubble entropy with different frequency parameter such as PPF and PSD is provided in order to access its stable and outstanding performance on epileptic seizer detection. The experimental results show that classification accuracy is improved with this algorithm. These finding suggest that extracted features can be used for treatment of epilepsy. Significance- This method provides greater stability and discriminative power, so this technique could be used to detect wider range of seizures.

Single vapour bubble growth under flash boiling conditions using a modified HLLC Riemann solver

Common fuel-oxidizer combinations in orbital manoeuvring systems consist of toxic substances but will be replaced in the future. Liquid oxygen (LOX) is one potential oxidizer but it rapidly superheats under the initial low-pressure conditions in the combustion chamber. Bubble nucleation and growth dominate the efficient disintegration of the liquid jet, which is called flash boiling. An investigation of the small scale bubble dynamics will help to improve existing models for the break-up of the LOX jet and the mixing with the fuel. Direct numerical simulations (DNS) can be used to analyse the underlying processes if the solver handles phase transition effects at extreme ambient conditions. In this paper we use a fully compressible discontinuous Galerkin solver combined to a level-set equation and to a modified HLLC Riemann solver. The main goal of our investigation is to assess closure conditions for the mass transfer models which accurately represent the physics of vapour bubble growth. We couple three models for the estimation of vaporisation mass fluxes to the interface Riemann solver. The Hertz-Knudsen relation and a kinetic relation predict the volumetric expansion well but cannot represent the instantaneous mass flux. A sub-grid scale heat flux model predicts the mass flux qualitatively better but the volumetric bubble expansion matches only at late time intervals. The dependency of the calibrated coefficients on the physical conditions is similar for the different vaporisation models.

Real time light intensity based carbon dioxide feeding for high cell-density microalgae cultivation and biodiesel production in a bubble column photobioreactor under outdoor natural sunlight

Outdoor high cell-density microalgae cultivation is highly challenging due to unavailability of appropriate CO2 feeding strategy under diurnal sunlight intensities. Hence, a novel real time light based CO2 feeding strategy was firstly developed under diurnal simulated sunlight (LED) to test on Chlorella sp. in a 10 L scale bubble column photobioreactor. The strategy yielded a biomass titer of 5.12 g L−1 under simulated sunlight, far higher than existing biomass-density and pH-control based CO2 feeding strategies. In outdoor culturing, the proposed feeding strategy yielded high biomass titers of 6.8 and 9.0 g L−1 in growth-phase of two-stage and single-stage lipid induction studies respectively with same biomass productivity of 0.8 g L−1 day−1. Subsequently, two-stage lipid induction strategy of 6.8 g L−1 titer yielded biodiesel productivity of 120 g L−1 day−1, whereas single-stage strategy of 9.0 g L−1 titer was unable to induce lipid. Moreover, specific light availability affects the lipid production.

Production of biodiesel by autotrophic Chlorella pyrenoidosa in a sintered disc lab scale bubble column photobioreactor under natural sunlight

The main focus of the work is to study Chlorella pyrenoidosa mediated photoautotrophic production of lipid in a bubble column photobioreactor using CO2 as carbon source under natural diurnal outdoor sunlight. The limiting and inhibiting concentrations of CO2 in sparging gas, nitrogen inhibition, reversibility of the CO2 inhibition on growth, and lipid production have been investigated under natural sunlight. A process model coupled with light distribution inside the culture has been developed considering different concentration of dissolved CO2 and urea, repression of nitrogen on lipid production under natural sunlight diurnal in nature in a bubble column reactor. The biomass titer of 4.6 g/L with 10% CO2 has been achieved within 5 days of culture under sunlight. A two stage photoautotrophic lipid production strategy in a sintered disc bubble column photobioreactor under natural sunlight has been developed. 30% (w/w-DCW) lipid within 5 days of lipid induction period has been achieved. The biomass productivity of 0.91 ± 0.01 gm/L/day in growth period with sufficient urea and lipid productivity of 410 ± 12 mg/L/day in last 2 days of urea starvation period have been achieved in outdoor photoautotrophic cultivation under natural sunlight using CO2 as carbon source.

Effect of aeration pattern and gas distribution during scale-up of bubble column reactor for aerobic granulation

This study investigates the influence of aeration pattern and gas bubble distribution on aerobic granulation in pilot bubble columns and advances the understanding on crucial factors to be considered during scale-up. Experiments were conducted in a pilot scale bubble column of 22-L capacity. A laboratory-scale bubble column of 5-L capacity, which could achieve granulation in a short time, was used as the control reactor. The pilot reactor was operated with optimal operational conditions evaluated for the control reactor, but the granulation was initially inhibited; improper aeration pattern was found as the most significant factor in precluding granulation. Aerobic granulation in the pilot-scale reactor was achieved upon modifying the gas distributor design. The effect of gas distributor characteristics (i.e., pore size, free area φ, and relative surface area of the gas distributor to the cross-section of the column R) on the aeration pattern has been thoroughly discussed. A scale-up ratio (S) for the gas distributor has been proposed. Reliability of aeration intensity in terms of superficial air velocity usg, which has been commonly used as an indicator of prevailing hydrodynamics and shear stress in lab-scale granular bubble columns, has been discussed; it was demonstrated that usg could not realistically represent the aforementioned conditions in a larger module. Instead, it was suggested that a greater attention should be paid to bubble size and bubble ascending velocity and thus to the subsequent aeration pattern. Gas holdup has been eventually introduced as a crucial and easy-to-measure controlling parameter for the scale-up of granularreactors.

Gas hold-up and mass transfer in a pilot scale bubble column with and without internals

Bubble columns are the reactors of choice for conversion of synthesis gas to liquid fuels (F–T synthesis). The F–T synthesis is an exothermic reaction and cooling internals are installed in the bubble columns to remove the heat generated in the reaction. The presence of the internals affects both the gas and liquid flow patterns in bubble column. This change is expected to reflect on the volumetric mass transfer coefficient. In this work, the impact of vertical cooling internals on overall gas hold-up and volumetric mass transfer coefficients is evaluated on 45cm diameter pilot scale bubble column and 19cm diameter lab scale bubble column. It was found that in presence of internals the gas hold-up in both the columns was increased, however no significant impact of presence of internals on volumetric mass transfer coefficient was observed. The gas hold-up and volumetric mass transfer coefficients evaluated in this work are compared with prediction from existing correlations.

Enhanced photo bio-reaction by multiscale bubbles

Based on experimental and computational fluid dynamics simulations, the scale effect between the bubbles and the algae for different sizes in the culture process were investigated in a photobioreactor column under single and combined gas intake conditions, including factors such as the bubble distribution, gas holdup and mass transfer coefficient of different scale bubbles (500 μm microbubbles and 5 mm bubbles). In general, microbubbles could improve mass transfer and photo bio-reaction. For microalgae cultures with large particle sizes or high biomass concentration, the mixing ability of microbubbles was weakened, and therefore, the culture efficiency decreased. However, this can be improved by using millimeter bubbles with the strong turbulent characteristic. Therefore, a multiscale bubble combination intake method was proposed. This method can significantly strengthen both the multiphase mixture and mass transfer processes and was beneficial to improving the growth efficiency of Chlorella.

Chlorine Production by HCl Oxidation in a Molten Chloride Salt Catalyst

A molten salt mixture containing 45 mol % KCl and 55 mol % CuCl2 was investigated as a catalyst for the reaction of HCl with O2 to produce Cl2. The HCl conversion for an HCl:O2 molar feed ratio of 1:2 at 450 °C and a total pressure of 1 atm was 80% at a residence time of less than 1 s in a lab scale bubble column reactor. The equilibrium conversion at this temperature and pressure is 84%. The catalyst system was found to remain stable throughout a continuous 24-h experiment. The use of a mixed transition metal/alkali metal molten salt catalyst for HCl oxidation reduces the volatility of supported chlorides and may avoid the mechanical stability limitations of solid catalysts caused by volume changes between the halide and the oxide.

Equilibrium solubility of CO2 in aqueous binary mixture of 2-(diethylamine)ethanol and 1, 6-hexamethyldiamine

CO2 solubility data are important for the efficient design and operation of the acid gas CO2 capture process using aqueous amine mixture. 2-(Diethylamino)ethanol (DEEA) solvent can be manufactured from renewable sources like agricultural products/residue, and 1,6-hexamethyldiamine (HMDA) solvents have higher absorption capacity as well as reaction rate with CO2 than conventional amine-based solvents. The equilibrium solubility of CO2 into aqueous binary mixture of DEEA and HMDA was investigated in the temperature range of 303.13-333.13 K and inlet CO2 partial pressure in the range of 10.133-20.265 kPa. Total concentration of aqueous amine mixtures in the range of 1.0-3.0 kmol/m3 and mole fraction of HMDA in total amine mixture in the range of 0.05-0.20 were taken in this work. CO2 absorption experiment was performed using semi-batch operated laboratory scale bubble column to measure equilibrium solubility of CO2 in amine mixture, and CO2 absorbed amount in saturated carbonated amine mixture was analyzed by precipitation-titration method using BaCl2. Maximum equilibrium CO2 solubility in aqueous amine mixture was observed at 0.2 of HMDA mole fraction in total amine mixture with 1.0 kmol/m3 total amine concentration. New solubility data of CO2 in DEEA+HMDA aqueous mixtures in the current study was compared with solubility data available in previous studies conducted by various researchers. The study shows that the new absorbent as a mixture of DEEA+HMDA is feasible for CO2 removal from coal-fired power plant stack gas streams.