Mean Bubble Rise Velocity Research Articles

Bubble swarm characteristics in a bubble column under high gas holdup conditions

Local bubble characteristics were measured using a novel monofibre optical probe manufactured for elevated pressures and high gas holdups. The objective of this study is to test the performance of the optical probe, as well as to investigate the impact of operating conditions on bubble characteristics in a bubble column when operating under high gas holdup conditions. Experiments were conducted in a 101.6mm diameter column operating at pressures up to 6.5MPa. A 0.5wt% aqueous ethanol solution was used in this study to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. The probe struggled to detect all the bubbles due to significant bubble size reduction and non-rectilinear bubble rise. However, successfully detected bubbles were deemed representative of the mean bubble rise velocity as indicated by the dynamic gas disengagement technique. Global gas holdup profiles in conjunction with the drift flux analysis showed that pressure does not have a significant impact at elevated liquid flow rates as bubbles tend to follow the convective current of the liquid. Relatively narrow chord length distributions were observed, where 90% of the bubbles were 1.0mm or less. The energy dissipated through the gas–liquid distributor plate was shown to have a significant impact on the initial bubble size generated and high gas holdups were also achieved at atmospheric pressure by varying the open-surface area of the distributor.

Study of bubble-induced turbulence in upward laminar bubbly pipe flows measured with a two-phase particle image velocimetry

In the present study, focusing on characterizing the bubble-induced agitation (turbulence), spatially varying flow statistics of gas and liquid phases in laminar upward bubbly flows (Reynolds number of 750) with varying mean void fraction are investigated using a two-phase high-speed particle image velocimetry. As the flow develops along the vertical direction, bubbles with small-to-moderate void fractions, which were intentionally distributed asymmetrically at the inlet, migrate fast and show symmetric distributions of wall or intermediate peaking. Meanwhile, the mean liquid velocity saturates relatively slowly to a flat distribution at the core region. Despite small void fractions considered, the bubbles generate a substantial turbulence, which increases with increasing mean void fraction. Interestingly, it is found that the mean vertical velocity, bubble-induced normal stress in radial direction, and Reynolds stress profiles match well with those of a single-phase turbulent flow at a moderate Reynolds number (e.g., 104), indicating the similarity between the bubble-induced turbulence and wall-shear-generated turbulence in a single-phase flow. Previously suggested scaling relations are confirmed such that the mean bubble rise velocity and bubble-induced normal stress (in both vertical and radial directions) scale with mean volume void fraction as a power of −0.1 and 0.4, respectively. Finally, based on the analysis of measured bubble dynamics (rise in an oscillating path), a theoretical model for two-phase turbulent (Reynolds) stress is proposed, which includes the contributions by the non-uniform distributions of local void fraction and relative bubble rise velocity, and is further validated with the present experimental data to show a good agreement with each other.

Radial Bubble Distribution in a Fluidized Bed with Vertical Tubes

The radial distribution of bubble characteristics such as the bubble frequency, the bubble size, and the bubble rise velocity, is a key to better understanding the flow patterns of gas (and solids) in bubbling fluidized beds. Therefore, the radial distribution of bubble characteristics is investigated in a fluidized bed with vertical tubes by means of optical probes. Measurements are taken for Geldart B particles at two bed heights, three gas velocities, and three vertical tube bank configurations. While the bubble frequency shows a strong dependence on the radial position in the bed and the orientation of the vertical tubes, the bubble size and the bubble rise velocity, in particular in the lower portions of the bed, are more constant. Typical bubble pathways are shown to move from the column wall to the center axis of the bed with increased bed height. Therefore, in order to have a representative mean bubble size and mean bubble rise velocity, which accounts for the whole cross-section of the bed, an area-number weighted mean is introduced.

On bubble clustering and energy spectra in pseudo-turbulence

Three-dimensional particle tracking velocimetry (PTV) and phase-sensitive constant temperature anemometry in pseudo-turbulence – i.e. flow solely driven by rising bubbles – were performed to investigate bubble clustering and to obtain the mean bubble rise velocity, distributions of bubble velocities and energy spectra at dilute gas concentrations (α ≤ 2.2 %). To characterize the clustering the pair correlation functionG(r, θ) was calculated. The deformable bubbles with equivalent bubble diameterdb= 4–5 mm were found to cluster within a radial distance of a few bubble radii with a preferred vertical orientation. This vertical alignment was present at both small and large scales. For small distances also some horizontal clustering was found. The large number of data points and the non-intrusiveness of PTV allowed well-converged probability density functions (PDFs) of the bubble velocity to be obtained. The PDFs had a non-Gaussian form for all velocity components and intermittency effects could be observed. The energy spectrum of the liquid velocity fluctuations decayed with a power law of −3.2, different from the ≈ −5/3 found for homogeneous isotropic turbulence, but close to the prediction −3 by Lance & Bataille (J. Fluid Mech., vol. 222, 1991, p. 95) for pseudo-turbulence.

Hydrodynamics and steel tube wastage in a fluidized bed at elevated temperature

Hydrodynamic measurements were made in a bubbling fluidized bed operated at 550°C at three different excess gas velocities (0.15, 0.40 and 0.75 m/ s ). The bed has a cross-sectional area of 0.2 m×0.3 m with an immersed tube bank consisting of 59 horizontal stainless steel tubes (AISI 304L), 21 of which are exchangeable, thus allowing erosion studies. Capacitance probe analysis was used to determine the mean bubble rise velocity, the mean bubble frequency, the mean pierced bubble length, the mean bubble volume fraction and the mean visible bubble flow rate. Tube wastage was calculated from roundness profiles obtained by stylus profilometry. A redistribution of the bubble flow towards the center of the bed occurs when the excess gas velocity is increased. Measurements along a target tube, situated next to the capacitance probe, usually show greater material wastage at the central part of the tube, since the mean bubble rise velocity and the mean visible bubble flow rate are higher there. It is suggested that the greater material degradation is also an effect of the through-flow of a particle-transporting gas stream in the bubbles. With increasing height above the distributor plate the circumferential wastage profiles for the lowest excess gas velocity show a gradual change from an erosion pattern with one maximum (Type B behavior) to a pattern with two maxima (Type A behavior). Power spectral density distributions of the fluctuating pressure signals show that this is a result of the formation of larger bubbles, when the fluidization regime is changed in the upper part of the bed. At the highest excess gas velocity the bubble flow becomes more constrained due to a more rapid coalescence of the bubbles and the tubes show Type A wastage profiles throughout the bed.

Simulation of the fluid dynamics of a bubbling fluidized bed. Experimental validation of the two-fluid model and evaluation of a parallel multiblock solver

A mesh refinement study and validation of two-fluid model closures were carried out for a bubbling fluidized bed application. The mesh refinement study indicates that a higher degree of mesh refinement is required for atmospheric than for pressurized fluidization. Statistical bubble parameters (bubble frequency, mean bubble rise velocity, mean pierced bubble length and mean bubble volume fraction) were evaluated. The simulated statistical bubble quantities are computed from voidage signals derived from the transient multidimensional solution of two-fluid models. The algorithm for computing these quantities is taken directly from the evaluation program treating the measurement signals. To remedy the long simulation times required to obtain acceptable statistical values, a parallel version of the two-fluid model solver was developed, based on a domain decomposition method for distributed memory computers. A number of problems related to the parallelization are investigated. These are optimal treatment of velocity components on multi-block boundaries, frequency of data exchange at multi-block boundaries, local errors at multi-block boundaries and simulation time requirements.

Hydrodynamics of a pressurized fluidized bed with horizontal tubes: Influence of pressure, fluidization velocity and tube-bank geometry

Measurements of the bubble hydrodynamics were carried out in a cold pressurized bed with horizontal tubes. The mean bubble rise velocity, the bubble frequency, the mean pierced length, the bubble volume fraction, and the visible bubble flow rate were measured using capacitance probes. The absolute gas velocity through the bubbles was measured using Pitot-static pressure probes. The bed expansion ratio was determined by measuring the pressure difference between the freeboard and the bed at different heights and extrapolating the pressure difference down to zero. The fluctuations in the pressure drop over the entire bed height were also measured, and the power spectral density distribution of these fluctuations was calculated. The influence of pressure, fluidization velocity, and tube-bank geometry on the bubble behaviour and gas-flow distribution were studied. The bed has a cross-section of 0.2 m × 0.3 m. It was operated at pressures between 0.1 and 1.6 MPa, at excess gas velocities of 0.2 and 0.6 m/s. Three different tube-bank geometries were used, one with a fairly dense pitch and two with more sparse configurations, and comparisons are also made with previous results obtained without tubes in the bed. The bed material was silica sand with a mean particle diameter of 0.7 mm. For the three tube banks investigated here, all the measured parameters except the mean pierced length consistently increased with increasing excess gas velocity. The mean pierced length increased with increasing excess gas velocity at low pressures, while the velocity effect at high pressures was less obvious. When the pressure was increased, the mean pierced length first increased to a maximum value, at p = 0.4 MPa for the low excess gas velocity and at p = 0.2 MPa for the high excess gas velocity, then decreased again as the pressure was increased further. The bubble frequency and the bed expansion increased as the pressure was increased. The absolute gas velocity through the bubbles as well as the gas velocity relative to the bubbles decreased as the pressure was increased. For the sparse tube banks at the lower excess gas velocity, the mean bubble rise velocity, the bubble volume fraction and the visible bubble flow rate increased as the pressure was increased. For the dense tube bank, however, these parameters showed a maximum at p = 0.4 MPa. At the higher excess gas velocity, these parameters showed a maximum at about p = 0.2–0.4 MPa for all three tube banks. It appears that, at this velocity, the presence of tubes prevents a further increase in these parameters. This behaviour differs significantly from the behaviour without tubes. From the power spectral density distributions of the pressure fluctuations over the entire bed height and from visual observation, it appears that the bed is slugging, or close to slugging, at atmospheric pressure for all the tube configurations. When the pressure is increased, the power spectral density distribution becomes wider as the large bubbles/slugs break down and the bed moves toward a more dispersed bubbling behaviour.

Influence of pressure and fluidization velocity on the bubble behaviour and gas flow distribution in a fluidized bed

Measurements of the visible bubble flow rate and the through-flow velocity of gas inside bubbles were made in a pressurized fluidized bed, using capacitance and Pitot-static pressure probes, respectively. The influence of pressure and fluidization velocity on the bubble behaviour and gas flow distribution was studied. The bed, of 0.2 × 0.3 m cross-section, was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities between 0.1 and 0.6 m/s. The bed material was silica sand with a mean particle diameter of 0.7 mm. The mean bubble frequency, the mean bubble rise velocity, the mean bubble volume fraction and the visible bubble flow rate were found to increase with both increasing pressure and excess gas velocity. The mean pierced length of bubbles increases with increasing excess gas velocity but decreases, after an initial increase, wtih increasing pressure. With both increasing pressure and excess gas velocity, the bubbles tend to redistribute themselves towards the center of the bed cross-section. The pressure probe measurements show that the through-flow velocity of gas inside the bubbles decreases with increasing pressure. Furthermore, gas flow balance calculations show that the ratio of dense phase superficial gas velocity to minimum fluidization velocity increases with both increasing pressure and excess gas velocity. Pressure drop measurements show a decrease in minimum fluidization velocity with increasing pressure, in accordance with predictons from the Ergun equation.

An investigation of fluidized-bed scaling—capacitance probe measurements in a pressurized fluidized-bed combustor and a cold model bed

In order to investigate the validity of the scaling relationships for fluidized beds suggested by Fitzgerald and Crane, and Glicksman, comparative measurements of the bubble activity have been carried out in a pressurized fluidized bed burning coal, and in a smaller pressurized model bed operated at slightly above room temperature and scaled to match the hot bed. The hot bed, located at New York University, we operated at a pressure of p = 0.79 MPa and a temperature of T = 870°C, while the model bed, at Chalmers University of Technology, was operated at p = 0.24 MPa and T = 27°C. The bubble activity was measured at two corresponding positions in each bed with the aid of capacitance probes. The parameters compared were the mean bubble volume fraction, and the nondimensional forms of the mean bubble frequency, the mean pierced length of bubbles, the mean bubble rise velocity, and the local mean visible bubble flow rate per unit area. The measurements in the model bed were made with three different bed materials: the hot-bed material and an Olivine sand, both sieved down to a size distribution in accordance with the scaling laws, and the full-sized hot-bed material. The results show that in order to get a similar behaviour between the beds, it is important to operate the model bed at the conditions predicted by the scaling laws. Deviations in fluidizing velocity, pressure or particle size distribution from the scaling law values significantly affect the results. When operating at the properly scaled conditions, the local bubble parameters, expressed in dimensionless form, are in good agreement between the two beds.

Mass transfer to large particles in fluidised beds of smaller particles

A theory is proposed for predicting the transfer of a gas through a fluidised bed of small particles to a large particle. It is proposed that non steady-state mass transfer of the gas occurs by two mechanisms: (i) mass transfer of gas in clusters or packets of the smaller particles approaching the large particle; and (ii) gas convection. The theory developed enables prediction of the Sherwood number (N sh, the dimensionless mass transfer coefficient) for a large particle, diameter d: N sh=2ε mf+ 4 g3 mfd(U mfε mf+ uinb )/π D A 1 2 where U mf is the minimum fluidising velocity, ε mf is the bed voidage at U mf-0 u b is the mean bubble rise velocity and D A is the gas diffusivity. This equation is shown to be in excellent agreement with Sherwood numbers determined from combustion experiments in which single large particles of petroleum coke were burned in air fluidised beds over a wide range of operating conditions. It is also shown that predictions using this expression are in close agreement with those from an empirical expression previously proposed by the autho

The local structure of gas fluidized beds —I. A statistically based measuring system

A system is described for measuring the parameters characterizing the local state of fluidization in beds of arbitrary sizes. This system is based on a miniaturized capacitance probe shaped so as not to disturb the local state of fluidization. Based on a statistical analysis of the signal, the mean bubble pulse duration, the number of bubbles striking the probe per unit time and the local mean bubble rise velocity are measured. The latter is measured by using the cross-correlation technique. From these parameters, further characteristics of the local state of fluidization are derived, in particular the local mean pierced length of bubbles, the local bubble volume fraction and the local bubble gas flow.