Frequency Of Bubble Formation Research Articles

Study of bubble induced flow structure using PIV

An experimental investigation of the flow structure induced by a chain of gas bubbles was carried out in a rectangular bubble column using particle image velocimetry (PIV). It is observed that the bubble rising trajectory changes from one dimension to three dimension as liquid viscosity reduces. The variation of bubble rising trajectory associates with the alternation of bubble motions—with or without oscillatory and rotational motion depending the bubble rising trajectory is 3-D or 1-D. The different behaviors of gas bubbles introduce various instantaneous and averaged liquid flow structures. In general, complex fluid velocity fields present in liquid system of low viscosity where free vortex, cross flow, and irregular circular flow can be observed. The liquid pseudo-turbulence measured in terms of turbulence intensity and Reynolds stress is more intense in liquid of low viscosity. The turbulence is also enhanced by the frequency of bubble formation.

Empirical Equations for Bubble Formation Frequency from Downward-Facing Nozzle with and without Rotating Flow Effects

Bubble formation from a downward-facing single-hole nozzle immersed in a still circular water bath has been observed with a high-speed video. An empirical equation is derived for the frequency of bubble formation, f B 0 , as a function of gas flow rate Qg, the inner and outer diameters of the nozzle, d n i and d n o , the acceleration due to gravity, g, and the physical properties of fluids. A cross-flow is imposed on the nozzle by rotating the bath around its vertical axis. The frequency of bubble formation in the presence of the rotating flow, f B , is predicted using /go and the rotating flow velocity, u θ .

Study of different membrane spargers used in waste water treatment: characterisation and performance

In urban waste water treatment, a novel gas sparger based on flexible rubber membrane has been used for the last 10 years. The objective of this present work is to compare two flexible membranes (the new membrane and the old membrane provided by ONDEO-DEGREMONT group) used in waste water treatment. For this purpose, the different membrane properties (hole diameter, pressure drop, critical pressure, deflection at the centerline and elasticity) have been characterized. The bubble generation at the membranes with a single orifice and with four orifices have been studied and their performances have been compared in terms of interfacial area and power consumption. From the experimental and theoretical approach, the new membrane is less elastic (or more rigid) than the old membrane. The bubble diameters generated from the new membrane remain constant with the gas velocity through the orifice, whereas they increase logarithmically for the old membrane. The inverse behaviours are observed in terms of the bubble formation frequency. Moreover, the bubbles generated from the new membrane have significantly larger sizes and lower formation frequencies than those obtained with the old one. From these results, it can be noted that the new membrane has a behaviour comparable to a rigid orifice. No coalescence phenomenon at the bubble formation is observed from the new and the old membranes with four orifices. The interfacial area and the power consumption are evaluated and show slight differences between the interfacial area provided by the old and the new membranes for one value of power consumption.

Gas Absorption during the Burst of an Individual Bubble on the Water Surface

The rate of absorption of carbon dioxide from a bubble to a liquid were investigated using a shallow bubble column with a single nozzle. The overall mass transfer rate was classified into three categories depending on the bubble behavior: bubble formation at the nozzle, bubble rising in the bulk liquid and a bubble bursting (often followed by drifting) on the surface of water in the column. Flow properties including bubble diameter, the frequency of bubble formation and the velocity of bubble when rising were measured to determine the interfacial surface area of the bubbles for each region. As a result, it was possible to accurately measure the respective mass transfer coefficients. The contributions of bubble formation and bubble burst were found to be significant for the mass transfer rate in a shallow bubble column. The mass transfer coefficient of bubble formation was much higher than those for bubble burst and bubble rising.

Wettability Effect on Bubble Formation at Nozzles in Liquid Aluminum

An experimental study was undertaken to determine how wettability affect the volume and formation of gas bubble at nozzles in liquid aluminum. X-ray fluoroscope was used to carry out in-situ observation in the melt. The nozzles were made of steel, silica and alumina to establish different wettability by liquid aluminum. The frequency of bubble formation and the bubbles volume were investigated for low gas flow rate from 0.43 to 12 cm 3 /s and for various diameters of the nozzle. It was shown that bubble volume increased with wettability worsening both for aqueous and metallic systems. A further insight into the mechanism of the bubble formation was obtained by comparison of the bubble behavior at the tip of the injection devices in liquid aluminum and water.

A model for bubble formation and weeping at a submerged orifice with liquid cross-flow

A new theoretical model to predict bubble frequencies and weeping rates at a submerged orifice with liquid cross-flow has been developed. The model predicts a significant influence of liquid cross-flow velocity on bubble formation frequency, and especially on liquid weeping. Simulated values of weeping rates for different orifice diameters, gas flowrates and liquid cross-flow velocities show good agreement with experimental data.

Dynamics of Pressure Fluctuation in a Bubbling Fluidized Bed at High Temperature

Dynamic behaviors were investigated using ashes of three sizes in an 82-mm-i.d. stainless steel fluidized bed at temperatures up to 1000 °C. Pressure fluctuation signals were analyzed using power spectral density function (PSDF) analysis, chaos analysis, and wavelet analysis. The major frequency of the pressure fluctuation signals is in the range from 20 to 80 Hz in the fixed state, whereas the major frequency ranges from 1 to 9 Hz in the fluidized state. The correlation dimension and Kolmogorov entropy are zero in the fixed state, and then they increase with increasing fluidization number in the transition region. Finally, these two exponents vary little with changing fluidization gas velocity in the fully fluidized state. Through a wavelet transform, the fluctuating pressure signals of the bed can be decomposed into approximations and details at different resolutions. The number of peaks in the five-detail signal represents the bubble formation frequency, which is in agreement with the major frequency obtained from a PSDF analysis of the pressure signals.

Modelling the bubble formation dynamics in non-Newtonian fluids

A theoretical model is developed for modelling the non-spherical bubble formation at an orifice submerged in non-Newtonian fluids under constant flowrate conditions. The equations of motion are, respectively, the radial expansion and vertical ascension of the bubble interface. They are combined with the thermodynamic equations for the gas in the bubble and the chamber below the orifice as well as the fluid rheological equation. In particular, the influence of in-line interactions between bubbles due to the fluid memory effects of the viscoelastic characteristics is taken into account for the first time. The present model is able to compute the instantaneous growing shape of the bubble during its formation and determine the final size of detachment as well as the frequency of bubble formation. The values predicted by this model compare satisfactorily with the experimental results obtained under different operating conditions.

Heat transfer and bubble characteristics from a nozzle in high-pressure bubble columns

Heat transfer and bubble characteristics (including bubble size, bubble rise velocity, and bubble formation frequency) from a nozzle in high-pressure bubble columns are studied. The experiments are conducted at pressures up to 15.2 MPa and temperature at 27°C. As pressure increases, the bubble size and the bubble rise velocity decrease, but the bubble formation frequency increases in the bubbling regime. The effects of pressure on bubble characteristics are more significant at high nozzle gas velocities than at low nozzle gas velocities. The transition from the bubbling to the jetting is identified based on two methods: visualization and power spectra of the signals from a pressure transducer. The transition velocity from bubbling regime to jetting regime decrease with increasing pressure or temperature. In the bubbling regime, the heat transfer coefficient has a significant increase with increasing pressure and nozzle gas velocity. The higher the pressure, the larger the increasing rate of the heat transfer coefficient and the nozzle gas velocity. In the jetting regime, however, the influence of pressure on the heat transfer coefficient is small.

Bubbles in non-Newtonian fluids: Formation, interactions and coalescence

The present work aims at studying the behaviors of air bubbles in non-Newtonian fluids from formation to coalescence. The experimental data reveal that the bubble rise velocity depended upon the injection period and that coalescence took place for short injection periods. The rheological simulation and birefringence visualization prove for the first time that in-line interactions are governed by a dynamical competition between the creation and relaxation of stresses in fluids. The chaos analysis was applied to time series data of bubble passage recorded at different heights in the bubble column. The calculation of several parameters: the largest Lyapunov exponent, the correlation dimension, the power spectrum, and the phase portraits, corroborates the deterministic chaotic mechanism of the coalescence between bubbles. A new theoretical model was developed for describing the non-spherical bubble formation at an orifice. Owing to the knowledge of the interactions, the present model is able to calculate the instantaneous shape of the bubble during its formation and determine the final size of detachment as well as the frequency of bubble formation. The theoretical results compare very satisfactorily with the experimental investigation.

Effect of Parallel Flow on Frequency of Bubble Formation from Single-Hole Nozzle under Micro-Gravity Conditions.

The effect of parallel flow of water on the frequency of bubble formation from a single-hole nozzle placed in a vertical pipe is investigated under terrestrial gravity (1-g) and micro-gravity (μ-g) conditions. The parallel flow means that the water flow in the pipe is parallel to the air flow emitted from the nozzle. The micro-gravity conditions were realized using the drop tower of the Japan Microgravity Center (JAMIC). A force balance equation based on a one-stage model was introduced to predict the frequency of bubble formation under the two gravity conditions. The predicted value could approximate the measured value regardless of the gravity level when the frequency of bubble formation, f B , was relatively low. When the one-stage model was valid, the huoyancy force, surface tension force and drag force acting on the bubble just before detachment from the nozzle tip almost balanced under the terrestrial gravity (1-g) conditions, while the latter two forces balanced under micro-gravity (μ-g) conditions

Effect of cross-flow on the frequency of bubble formation from a single-hole nozzle

Bubble formation from a single-hole nozzle placed vertically upward in a rotating water bath was investigated using a high-speed video camera. Air was used as the working gas. The measured values of the frequency of bubble formation, f B, were compared with those observed in a stationary bath, f B0. The velocity of cross-flow, νθ, affected the bubble formation significantly when it exceeded a critical value, νθc . The ratio of f B to f B0 was unity for νθ≦νθc , but it changed in a complex manner for νθ>νθc . In the latter case, when the air flow rate Q g was relatively low, f B/f B0 became larger than unity irrespective of Q g, and an empirical correlation of f B/f B0 was proposed as a function of νθ and the inner diameter of the nozzle, d ni. As the gas flow rate increased, f B/f B0 decreased monotonically and became smaller than unity, and an empirical correlation of f B/f B0 was derived as a function of Q g, νθ, and d ni. These empirical correlations could approximate the measured values of f B/f B0 within a scatter of −15 to +20 pct.

Gas bubble formation in an oscillatory flow

Production of bubbles from an underwater orifice is a common process that is often difficult to control. A possible solution is to impose a flow around an underwater orifice so that the size of the detached bubbles can be regulated. A simple way of accomplishing this task is to place a coaxial needle in a short tube. At constant water flow rate, a precise control of the bubble size can be achieved regardless of the orientation of the tube with respect to gravity. Since this arrangement may not be practical in some applications, an oscillatory flow in which the net water flux is zero is a better alternative. A series of experiments in which the flow is actuated by a rubber membrane driven by a solenoid show that it is possible to phase lock the bubble formation frequency to the forcing frequency under certain conditions. In the inflow phase of the oscillation, the growing bubble that is attached to the needle tip assumes an oblate shape. As a result, when the flow is reversed, the added mass force pushes the bubble away from the needle in a very efficient manner. Axisymmetric boundary integral simulations have been carried out to simulate this process. A very good agreement is found between experiment and the simulations. [Work supported by NASA.]

Water model study of the frequency of bubble formation under reduced and elevated pressures

Air was injected vertically upward into a water bath through a bottom nozzle or a bottom orifice. The surface pressure was reduced or elevated from an atmospheric pressure in order to change the hydrostatic pressure around the nozzle and orifice. The gas delivery system was designed so that bubbles were generated in the middle and high gas flow rate regimes under a constant flow condition. The frequency of bubble formation, fB, decreased as the surface pressure, Ps, decreased when the volumetric gas flow rate, Qg, was kept constant. The measured fB values were predicted satisfactorily by an empirical equation proposed previously by the present authors. This equation was derived originally to correlate the frequency of bubble formation both in aqueous and molten metal systems under an atmospheric surface pressure. The effect of surface pressure on the frequency of bubble formation was considered in terms of the density of gas, ρg, and the volumetric gas flow rate Qg in the aforementioned empirical equation. These two quantities, ρg and Qg, were evaluated at the nozzle exit by using the hydrostatic pressure there.

Segregation of fuel particles and volatile matter during devolatilization in a fluidized bed reactor—I. Model development

The phenomenology of fuel particles segregation at the surface of fluidized beds during devolatilization is modeled. The beds are assumed to be at incipient fluidization in order to focus the attention on the segregation mechanism caused by the formation of endogenous bubbles of volatiles. These are generated during fuel particle devolatilization immediately after its underbed feeding to a fluidized-bed reactor, and are distinct from exogenous bubbles, which are always present in the bed under aggregative fluidization conditions. Submodels describing the volatile flow pattern, the volatile bubble formation, the vertical motion of the volatile bubble and of the fuel particle are developed and combined in a coherent framework. Results of extensive model computations are presented and discussed. Qualitative and quantitative features of the fuel particle and of the volatile matter segregation processes are assessed by looking at the time required for the fuel particle to become segregated, at the frequency of volatile bubble formation, at the amount of volatiles emitted in the bed prior to particle segregation on the top of it. The proposed framework should complement previous studies on the segregation of fuel particles in freely bubbling fluidized beds based on the effects of exogenous bubbles only.

Frequency of bubble formation in water and liquid iron

In the present work a device for measuring the frequency of bubble formation was developed and tested in water and liquid iron. Using a pressure sensor, the pressure difference induced by bubble formation from a nozzle inserted in the liquid phase is registered and analysed by a frequency analyser. The test results showed a good reliability and accuracy of the device in comparison with the bubble formation theory.

Magnetic liquid sensor for very low gas flow rate with magnetic flow adjusting possibility

A gas flow rate sensor which is based on the change of magnetic properties of magnetic fluid due to gas bubbles is well known. A vessel filled with magnetic fluid is equipped with an exciting coil and two detecting coils, the voltage difference between the coils, induced by bubbles, being a measure of the flow rate. This type of device does not permit simultaneously adjustment of the flow rate. This paper describes a new type of micro flow sensor, including the components of the experimental equipment used. The bubbling gas flow rate, i.e. the frequency of bubbles, was measured up to 20 Hz. Under the influence of a nonuniform or uniform magnetic field, due to first order magnetic levitation and magnetic pressure effects, supplementary forces of magnetic origin appear beside the gravitational one, which significantly modify bubble formation and emission frequency. Experimental data are presented concerning the characteristics of a model sensor illustrating the efficiency of magnetic adjustment of gas flow rate.

ノズルおよびオリフィスからの気泡生成に関する実験的研究 第２報 気泡生成頻度に及ぼすガス吹込み方向とノズル内乱流遷移の影響

Gas was injected into a bath through a nozzle attached to an immersed top lance. The injection angle was changed from zero (upward injection) to π/2 (horizontal injection) to investigate the effect of injection angle on the frequency of bubble formation. In the case of upward injection, the direction of the inertia force of injected gas and that of the buoyancy force acting on the injected gas is the same, while in the case of horizontal injection, the two directions are perpendicular to each other. The frequency of bubble formationfB was significantly influenced by the injection angle. It increased monotonically as the injection angle increased from 0 to π/2. An empirical equation was derived for correlating measuredfB data. In addition, the effect of turbulence transition in nozzle flow on the frequency of bubble formation was discussed.

ノズルおよびオリフィスからの気泡生成に関する実験的研究 第１報 上向きガス吹込み時の気泡生成頻度

Gas was injected into a bath vertically upward through a nozzle, an orifice, or a lance nozzle. The frequency of bubble formation at the exit of these injection devices was determined by counting the number of bubbles per second from visual images recorded using a high-speed video camera. The measurements were carried out under the condition that the effect of the gas chamber volume on the bubble formation was negligible. An empirical correlation of the bubble frequency was proposed as a function of gas flow rate, inner diameter of the nozzles and orifices, and the physical properties of gas and liquid. This correlation could estimate measured values of the frequency of bubble formation even for a high gas flow rate within a scatter of ±30% regardless of the injection devices.

Bubble formation at a submerged orifice in reduced gravity

We consider gas injection through a circular plate orifice into an ideally wetting liquid which results in the successive detachment of bubbles, each of which is regarded as a separate entity. At normal gravity and at relatively low gas flow rates, the growing bubble is modelled as a spherical segment that touches the orifice perimeter during the whole time of its evolution. If the gas flow rate exceeds a certain threshold value, a second stage of the detachment takes place that follows the first spherical segment stage. In this second stage, a nearly cylindrical stem forms at the orifice that lengthens as the bubble rises above the plate, and this stems feeds an almost spherical gas envelope stituated at the stem upper end. At high gas flow rates, bubble shape resembles that of a mushroom, and its upper envelope continues to grow until the gas supplied through the stem is completely cut off. This second stage always develops when gravity is sufficiently low, irrespective of the gas flow rate. There are two major factors that determine the moment of bubble detachment: the buoyancy force and a force due to the momentum flowing into the bubble with the injected gas. The buoyancy force dominates the process at normal gravity whereas the inflowing momentum force plays the key role under negligible gravity conditions. As gravity fluctuates, the interplay of these forces drastically influences bubble growth and detachment. At sufficiently low gravity, the bubble formation frequency is proportional to gas flow rate whereas the bubble detachment volume is independent of gas flow rate. At normal and moderately reduced gravity conditions, when the gas flow rate grows, bubble formation frequency slightly decreases and bubble detachment volume increases almost linearly. Effects of other parameters, such as the orifice radius, gas and liquid densities and surface tension coefficient are discussed.