The measurement of bubble properties in two-phase dispersions—III: Bubble properties in a freely bubbling fluidised-bed

An improved electroresistivity probe has been used in a freely bubbling gas-fluidised bed of carbon particles to give the distributions of velocity and size of bubbles therein. Bubble properties agree with previous observations but the significant influence of macro-circulation patterns on bubble velocities has been revealed.     

Correction for wall effect in point probe measurements of bubble size

During the measurement of bubble size by a probe inserted into a fluidized bed, parts of some of the bubbles located near the bed boundary are not accessible to the probe. This effect is analyzed theoretically using considerations of geometry and probability and results are given which may be used to correct experimental probe measurements of bubble size for the boundary effect. The procedure is demonstrated using electroresistivity probe measurements taken on a 2 ft-dia. bed of fluidized coke particles.     

Behavior of gas bubbles in a concentric cylindrical airlift column

Using a light transmission optical probe, the effect of superficial gas velocity on bubble properties(bubble size, bubble rising velocity, bubble frequency and local gas holdup) at axial and radial positions was determined in the riser and the downcomer of a concentric cylindrical airlift reactor. The vertical bubble length, the bubble rising velocity and the bubble frequency at axis in the riser increased with increasing superficial gas velocity and the bed height. The radial distribution of the local gas holdup, vertical bubble length and bubble frequency in the riser and the downcomer were found to be non-uniform. The profiles of the local gas holdup, vertical bubble length and bubble rising velocity in the riser were shown as parabolic shapes. The local gas holdup, the vertical bubble length and the bubble frequency in the downcomer changed with superficial gas velocity and the distance from the top of the draft tube.     

气液两相鼓泡床双电阻信号的小波消噪处理

将Manojit Roy等提出的简易小波消噪算法应用于气液两相流中双电阻探针测量信号的消噪处理中发现,该法在消噪的同时也将原始信号中的部分气泡能量去除,造成消噪信号明显失真.今对原算法中尺度阈值的判定方法进行了改进,也即,选取带噪信号求导数据小波分解后尺度-能量分布曲线中相邻两尺度间连线斜率的最大绝对值作为消噪尺度阈值的新判据;结果表明,改进后的算法可以取得较好的消噪效果.     

Bubble size distribution in bubble columns

The bubble size distributions are measured for the air-water system as a function of air velocity at room temperature in two bubble columns. High speed cinephotography and fiber optic probe techniques are used to measure the bubble size. Our limited measurements suggest that the bubble size may be independent of gas velocity in the range 3.6 to 9.2 cm/s and may be dependent on column diameter with smaller bubbles for narrower columns. The bubble size appears to be smaller at the column wall than at distances away from the wall.     

FIBRE OPTIC AND CAPACITANCE PROBES IN TURBULENT FLUIDIZED BEDS

In the present study, local flow properties are investigated in pilot plant scale fluidized bed reactors using both fibre optic and capacitances probes. Measurements are conducted at ambient as well as at 150°C. The system used is air and spent FCC particles (mean particle diameter: 65pm). The static bed height is 1.6 m. Bubbling and turbulent regimes (V = 0.40 and 0.70 m/s) are investigated in two 0.3 and 0.5 m ID columns.

Bubble fraction under the bubbling regime and at room temperature, measured using fibre optic and capacitance probes, are in good agreement. However, in the turbulent regime, fibre optic probes are prone to underestimate the bubble fraction while capacitance sensors tend to overestimate it. These discrepancies between fibre optic and capacitance measurements increase with temperature.

Using capacitance probes, a prevalent flat bubble rise velocity profile is measured. This is assigned to the relatively slow response, to the size and to the geometry of the capacitance probes. Overall this gives an underestimation of the bubble frequency and an overestimation of the bubble contact time and the bubble contact length.

Due to the high fibre optic probe sensitivity, care should be taken in the interpretation of signals. Overestimation of bubble frequency leads to underestimation of both bubble contact times and bubble contact lengths.     

Wall effects in point probe measurements of radial bubble distributions

The point properties of bubbling systems have been calculated for various assumed bubble frequency and size distributions. Flat, linear and parabolic radial frequency distributions were considered for both uniform and non-uniform bubble size distributions.The point bubble frequency distribution, which is equivalent to the distribution measured by a probe inserted into the system, may not be representative of the distribution which actually exists in the bed. Due to a boundary effect, the theoretical probe output for larger bubbles is found to yield a peak in the bubble frequency toward the bed centre, even if the actual distribution is flat, the magnitude of the bias increasing as bubble size increases. Caution in using radial frequency profiles obtained by point probe methods is thus indicated.     

三相循环流化床中气泡大小及其分布的实验研究

用光纤探头技术对三相循环流化床中的气泡大小及其分布进行了系统研究 ,实验测定了操作条件对气泡大小及其分布的影响规律 .实验结果表明 ,三相循环流化床中气泡的大小分布可用对数正态分布表征 ,在实验条件下气泡平均直径在床中心区域较小且沿半径方向由中心向边壁逐渐增大 ,并随表观气速的增大而减小 ,随固含率的增大而增大 ,表观液速对气泡平均直径的影响较小     

Double-sensor conductivity probe applied in a flotation system

This study proposed a new approach for measuring bubble size distribution, bubble mean diameter, Sauter mean bubble diameter, and gas holdup using a double-sensor conductivity probe in an air/water two-phase system bubble column. The results for the two-phase system were compared and calibrated using analyses from bubble images taken by a digital camera from the side of the column wall. Good agreement was observed between the two techniques. The same double-sensor conductivity was used in an air/water/solids three-phase system. The conductivity probe captured the change in bubble dynamic behaviour inside the pulp phase; however, the presence of the solids made it more challenging to measure. As a result, the VisioFroth commercial package, using images taken from the top of the froth layer, could be used in conjunction with the double-sensor conductivity probe to show the dynamic evolution of mineralized bubbles from the pulp zone to the froth zone in a flotation process.     

Testing of a spherical dual-tipped optical fiber probe for local measurements of void fraction and gas velocity in two-phase flows

An alternate and easy method of constructing an optical liber probe tip is presented for the study of different two-phase flow variables such as local time-averaged void fraction, gas velocity and interfacial bubble passage frequency. The proposed probe tip has similar response characteristics to the “U-bend” single fiber probe and is easier to construct than the 90° wedge tipped probe. The signal from the spherical tip seems to be insensitive to changes in bubble velocity as opposed to the 90° wedge and is more advantageous at higher velocities. The signal from the spherical tip has the same time duration as the signal changes from liquid to gas and vice versa for the same bubble velocity. A simplified model is presented to describe the balance of forces around the spherical tip when a bubble is penetrated. The model offers a qualitative explanation of why the non-dimensional response intensity decreases as the bubble velocity increases.     

光电毛细管探头法的改进及搅拌槽内的气含率分布

用光电毛细管探头法测定搅拌槽中的气含率,毛细管喇叭口外气泡和周围液体之间存在一定的相对速度会导致喇叭口内外的局部气含率之间有不容忽视的偏差。本文提出了由毛细管内的气体和液体流量计算搅拌槽中取样微元内的气含率的新方法。改进后的光电毛细管探头法能成功地用于局部气含率的测定。本文利用改进后的测量方法,测定了搅拌槽不同区域内的气含率,给出了气合率空间分布的初步规律。     

Measurement of bubble size distribution using a flying optical probe technique: Application in the highly turbulent region above a distillation plate

A flying optical probe technique has been developed and employed to measure bubble size above sieve trays in an air-water facility simulating cross-flow distillation (CRODIS). Despite the highly turbulent conditions occurring in the flows investigated, successful measurements were made of bubble size distributions at a series of levels above the trays. Using this technique, experiments are carried out on the CRODIS facility covering a range of gas and liquid flowrates and several tray configurations and the results are presented in this paper. Gas flowrate, sieve hole size and weir height were found to exhibit a significant effect on bubble size distribution, whereas liquid flowrate was shown to have negligible influence.     

Local Measurement of Gas-Liquid Bubbly Flow with a Double-Sensor Probe

A double-sensor probe was used to measure local interfacial parameters of a gas-liquid bubbly flow in a horizontal tube. The parameters included void fraction, interfacial concentration, bubble size distribution, bubble frequency and bubble interface velocity. The authors paid special attention to the probe design and construction for minimizing measurement errors. Measures were also taken in the design of sensor ends for preventing corrosions in the flow. This is an effort to improve the current double-sensor probe technique to meet the ever-increasing needs to local varameter measurements in gas-liquid two-phase flows.     

低压沸腾管内汽泡尺寸的测量与研究

沸腾工况汽液两相流汽泡尺寸的研究对于深入理解两相流动特性以及相关模型的发展具有重要意义。文中采用双探头光学探针对受热上升管内汽液两相流汽泡尺寸的径向分布特性进行了测量与研究。此外,通过对汽泡尺寸概率密度数的分析,得出了对应工况的两相流流型,根据理论分析及实验结果,总结了汽泡尺寸径向分布与流型的关系。     

AN ASSESSMENT OF EXPERIMENTAL TECHNIQUES FOR THE MEASUREMENT OF BUBBLE SIZE IN A BUBBLE SLURRY REACTOR AS APPLIED TO INDIRECT COAL LIQUEFACTION

The proper design and control of an indirect coal liquefaction process plant require an accurate knowledge of bubble sizes and size distribution. Of particular importance regarding proper design is the understanding of the complicated dependence of bubble dynamics on bubble column geometry, and of the nature of the solid-liquid phases. The nature of the products and their relative proportions, on the other hand, are seriously influenced by the bubbling characteristics of the reactor because the mass transfer behavior and mixing are dependent on the bubble size and velocity distributions. This report reviews experimental techniques that have been employed to measure bubble sizes in multiphase reactors. The methods, which fall in three different categories, namely, the photographic method, the optical probe, and the electrical conductivity (resistivity) probe, are described in a historical sequence; and in each case, their unique features and design details are discussed. A general methodology of data analysis in each category is given, together with a state-of-the-art technology assessment and, finally, relative merits and demerits of the methods.     

Application of the double optic probe technique to distorted tumbling bubbles in aqueous or organic liquid

The optic probe technique is widely used to investigate bubble reactors. To derive values of bubble local velocities and bubble local sizes, a specific signal treatment is usually applied under severe assumptions for bubble path and shape. However, in most industrial reactors, bubble motion is chaotic and no common shape can be assumed.In this work, the reliability of the signal treatment associated with the optic probe technique is examined for distorted and tumbling bubbles. A double-tip optic probe is settled in a glass tank and the rise of bubbles is filmed simultaneously. Several trains of bubbles are studied, interactions between bubbles being gradually increased.Referring to image analysis, several ways to derive mean bubble velocities from optic probe data have been compared. Crenels from front tip and rear tip raw signals are associated and individual bubble velocities are derived. Nevertheless, complete velocity distributions are difficult to obtain, as they depend on the choice of the time within which the bubble is searched on the second tip. Using a simpler approach it is shown that the most probable velocity, calculated through the raw signals inter-correlation, is a correct estimation of the average bubble velocity.Concerning bubble size, bubble chord distributions show too high values due to bubble distortion and deviation. A simplified estimation of bubble mean Sauter diameter, using the most reliable measurements only (i.e., local gas hold-up, local mean bubbling frequency, and most probable bubble velocity), was tested for highly distorted bubbles; this method was validated both in water and cyclohexane.     

气-固流化床的气泡参数测定——直射式光纤探头在流化床研究中的应用

在鼓泡域中,从直射式光纤探头在二维床的测试与图像分析的结果得到气泡的平均直径与平均刺穿长度的关系为:d_b=1.6E[l]气泡的球形度为0.96。通过反射式和直射式光纤探头信号的比较表明,直射式探头的信号便于处理,并可用来测定气泡内的粒子含量。直射式光纤探头在二维床和三维床的测定结果对比表明,两种塔内的气泡行为规律一致但有明显的差异。     

水平管泡状流局部实验参数的概率统计方法

引　言在水平管泡状流相分布的实验研究方面 ,Ｋｏｃａｍｕｓｔａｆａｏｇｕｌｌａｒｉ[1] 对直径为 5 0ｍｍ的水平管中的泡状流做了较详细的测量 ,孙科霞等[2 ,3] 对直径为 2 0ｍｍ的水平管中的泡状流做了测量 .他们在对测量数据的处理中都假定气泡只沿一个方向运动 ,而没有考虑气泡的横向运动 .事实上 ,气泡在管内运动时 ,在各种力的联合作用下 ,不只是沿探针主轴方向运动 .气泡与探针轴向有一入口角 ,探针测得的只是一种表观速度 ,这一表观速度一般不等于气泡实际速度 .Ｄｉａｓ等[4 ]在对双头电导探针测得的垂直管泡状流参数统计时考虑…     

Meßtechniken zur Bestimmung der Eigenschaften von Blasen in Blasensäulen

Techniques for determining the properties of bubbles in bubble columns . Three different methods for the determination of bubble size parameters namely (i) flash-photography followed by image analysis, (ii) use of a conductivity probe with two electrodes and electronic processing of the signals, and (iii) the electro-optical probe with the same signal processing as the conductivity probe are compared and contrasted from the viewpoint of their optimal use, their accuracy, and their scope of application.     

Bubble characteristics of a scaled‐down three‐phase fluidized bed

Experiments were carried out in a 0.29 m diameter column to provide information on bubble characteristics relevant to an industrial three‐phase fluidized bed reactor. Using a specially built electrical conductivity probe, together with a series of computer programs to gather and filter the data, local bubble properties (rise velocities, pierced chord lengths, frequencies, and direction of travel) were measured. The data are compared to existing correlations. Since the operating conditions were within the dispersed bubble regime, uniform bubbling was anticipated. However, the actual system indicated considerable non‐uniformity, with individual bubbles, large swarms of bubbles and particles traveling at all angles. As a result, it is shown that bubble rise velocities are not necessarily a good measure of the bubble properties and must be vectorized to provide a good means of estimating gas hold‐up.