Mixing Index Values Research Articles

Experimental and numerical study on the performance index of mixing for low aspect ratio serpentine microchannels

In this work, the effect of a range of Dean numbers (De) varying from 0.01–70 on low aspect ratio (AR = 0.05–0.2) serpentine microfluidic devices was studied experimentally and numerically. It was observed that the AR, the number of circular bumps, and the angular positions of bumps transverse to the flow have a significant influence on the pressure drop and flow features (i.e., the position and shape of flow separation zones). Mixing was exclusively driven by diffusive mechanisms at low De values and at high De values, it was primarily induced by Dean vortices. The lowest mixing index (MI) was observed for De = 1 in all channel types, highlighting the transition region between the diffusion and Dean vortices-dominant mixing regimes. The MI was generally increased by increasing the AR of the channels. However, at high De, Dean vortices became strong enough to induce rapid mixing that was largely independent of the AR and bump placement. A dimensional performance index (PI) was defined as a function of the MI and the pressure drop per unit length. Distinct flow patterns arising from various positioning of bumps resulted in significant variations in the MI and PI values, with different dependencies on De. This underscored the importance of bump positioning based on the operational De range to optimize the mixing performance. Despite minor deviations between the designed and fabricated channels, the use of 3D-printed molds proved effective even at scales close to the resolution of the printer, resulting in mixing patterns consistent with the designed channels. These findings provide valuable insights into optimizing serpentine microchannels for efficient mixing while considering the trade-offs between enhanced mixing and increased pressure drop.

Secondary air induced flow structures and mixing in a fixed bed combustor

Air staging features widely in biomass combustion from small space heaters to industrial-scale moving grate systems. Whilst studies have been conducted into the impact of air staging on emissions and combustion performance, there is little or no insight into the exact flow dynamic and physical mechanisms that are induced by secondary air under such conditions. This paper uses experimental data to validate numerical predictions before investigating the flow field structures and mixing characteristics in a fixed bed air staged combustor. The study utilizes Constant Temperature Anemometry (CTA) experiments to establish boundary conditions for a lab-scale fixed bed combustor. Results from numerical simulations obtained using [Formula: see text] l- 𝜔 model were in good agreement with experimental data. Conditions tested cover five different secondary to total air ratios (Qs/Qt) and two different locations of secondary air injection. Results show that at Qs/Qt = 0.50, 0.25 and 0.18 one strong recirculation zone is induced upstream of the secondary air injection and one downstream. Varying the point of injection of secondary air from h/D = 0.64 to h/D = 0.40 also had an effect on the size of the upstream recirculation zone. Qs/Qt = 0.50 had the highest values of mixing index among all spatially located planes at the upstream of secondary air injection. The overall findings shed light on the possible flow interactions between secondary air and the top layers of fuel bed. They also highlight the significance of secondary air on inducing both upstream and downstream flow structures.

Route of biofuel production from macadamia nut shells: effect of parameters on the particles mixing index in fluidized beds

Pyrolysis of macadamia nut shells (MNS) in a fluidized bed reactor has excellent potential to produce bio-oil. High heat transfer rates and uniform temperature in the fluidized bed can be achieved due to effective gas-solid contact in the reactor. However, binary mixtures can lead to the segregation of particles, which negatively affects heat and mass transfer in such a reactor. Therefore, a 2³ statistical experimental design was used to assess the effects of parameters (i.e., air velocity, particle diameter ratio, and mass fraction of MNS) on the mixing index of the bed of MNS and sand. Among the analyzed factors, only DMNS/DS and V/VMF influenced the mixing index (Im) within a confidence interval of 95%. Based on statistical data analysis, an air velocity 20% above the minimum fluidization and particle diameter ratio (DMNS/DS) smaller than 3 results in uniform particle mixing in the bed (i.e., reaching ideal mixing index values). Moreover, the experimental results indicate that fluidized be used for biofuel production from Macadamia nut Shells.

Cumulative effect of particle properties on mixing of multi-component mixture in a vibrated packed bed

The effect of particle properties that include shape, size, and density in a vibrated packed bed mixer for binary and ternary mixtures is investigated and mixing is characterized using subdomain mixing index. The mixing behavior can be divided into two different regimes based on the dominant segregation mechanism which is dependent on the theoretically calculated value of the segregation parameter ( Z ). The two different regimes are the percolation and buoyancy control regime. The Z value combines the effect of various physical properties of the particle. The subdomain mixing index value is strongly correlated to the segregation parameter. The results indicate that in a binary mixture decreasing the coarse particle’s density reduces the mixing while decreasing the fine particle’s density enhances the mixing. The extent of segregation behavior for nonspherical particles show shape-induced segregation is not a dominant factor but nonspherical particles show better mixing than spherical particles. • A correlation is developed that can predict dominant segregation mechanisms. • Particle shape-induced segregation is an important secondary factor. • Nonspherical particles show better mixing than spherical particles. • Adding a component to the binary mixture does not always improve mixing. • In a ternary mixture, decreasing the intermediate particles' density reduces mixing.

Design and analysis of a novel Bi-layer curved serpentine chaotic micromixer for efficient mixing

Mixing enhancement at the micro-scale is important in microfluidics applications, including biological and chemical analysis systems. Herein, two new designs of chaotic micromixers named Bi-layer Curved Serpentine (BCS1 and BCS2) micromixers were proposed. The performance analyses were performed by solving the steady equations of conservation of mass and momentum along with the convection-diffusion equation numerically at Reynolds numbers of 0.1–200. These micromixers consisted of curved serpentine channels stacked in two layers with different combinations of curved channel radii in each layer. They exhibited efficient homogenization with a comparatively lower pressure drop and a higher mixing cost. The main mixing processes were Dean vortices, extensional flow, and chaotic advection caused by asymmetrical vortices. The BCS1 micromixer, which has a constant but unequal radius of the serpentine channel in the top and bottom layers, showed the best mixing. BCS1 and BCS2 micromixers showed significantly higher mixing indices than the simple serpentine (SS) micromixer. Furthermore, the BCS1 micromixer provided a higher mixing cost by showing similar mixing at a ∼53% lesser pressure drop than a previously proposed serpentine micromixer. BCS1 and BCS2 micromixers showed mixing index values higher than 0.80 at Re ≥ 40 and Re ≥ 60, respectively.

A Digital Twin of the Coaxial Lamination Mixer for the Systematic Study of Mixing Performance and the Prediction of Precipitated Nanoparticle Properties.

The synthesis of nanoparticles in microchannels promises the advantages of small size, uniform shape and narrow size distribution. However, only with insights into the mixing processes can the most suitable designs and operating conditions be systematically determined. Coaxial lamination mixers (CLM) built by 2-photon polymerization can operate long-term stable nanoparticle precipitation without fouling issues. Contact of the organic phase with the microchannel walls is prevented while mixing with the aqueous phase is intensified. A coaxial nozzle allows 3D hydrodynamic focusing followed by a sequence of stretch-and-fold units. By means of a digital twin based on computational fluid dynamics (CFD) and numerical evaluation of mixing progression, the influences of operation conditions are now studied in detail. As a measure for homogenization, the mixing index (MI) was extracted as a function of microchannel position for different operating parameters such as the total flow rate and the share of solvent flow. As an exemplary result, behind a third stretch-and-fold unit, practically perfect mixing (MI>0.9) is predicted at total flow rates between 50 µL/min and 400 µL/min and up to 20% solvent flow share. Based on MI values, the mixing time, which is decisive for the size and dispersity of the nanoparticles, can be determined. Under the conditions considered, it ranges from 5 ms to 54 ms. A good correlation between the predicted mixing time and nanoparticle properties, as experimentally observed in earlier work, could be confirmed. The digital twin combining CFD with the MI methodology can in the future be used to adjust the design of a CLM or other micromixers to the desired total flow rates and flow rate ratios and to provide valuable predictions for the mixing time and even the properties of nanoparticles produced by microfluidic antisolvent precipitation.

Numerical modelling of in-situ alloying of Al and Cu using the laser powder bed fusion process: A study on the effect of energy density and remelting on deposited track homogeneity

In-situ alloying using laser powder bed fusion (LPBF) allows for formation of new alloys and complicated components with spatially controlled characteristics. A major issue in in-situ alloying is the macro-segregation of alloying elements, that results in chemical inhomogeneity. The precise control of composition is one of the most important steps in any effective in-situ alloying process. In this study, a mesoscale computational fluid dynamics (CFD) model for simulating single-tracks is developed to advance the in-depth understanding of in-situ alloying of dissimilar metals. Surface tension and recoil pressure driven flows at the free surface, rapid phase change, laser reflections, and other physical phenomena affecting the LPBF process have all been examined. Species conservation is used to trace the mixing of different metals. Simulations are performed by melting a layer of Al powder on a Cu substrate, and the impact of recoil pressure and Marangoni convection driven flows on Al-Cu mixing was addressed. The homogeneity of the simulated track is quantified by defining a mixing index. The effect of different laser energy densities and remelting on the homogeneity of the deposited track is investigated. Numerically, it was observed that the high thermal conductivity of both metals results in a sharp and short melt pool, and rapid solidification of such a melt pool generates a heterogeneous concentration dispersion. At high energy density, a low mixing index value is observed, indicating improved homogeneity in the deposited track. Furthermore, remelted tracks have a low mixing index value, indicating that remelting is an effective strategy for improving homogeneity. This study provides fundamental insights on the mixing of dissimilar metals and aims to support future research activities on the design and development of new alloys using LPBF process.

Development of a mathematical relationship for prediction of mixing quality in industrial convective batch mixers

This study focuses on the effects of bed-height and number of revolutions on the granular mixing performance with an objective to derive a mathematical relationship between mixing performance and number of revolutions. The simulations were carried out in a rectangular box in which three fill-levels and up to 15 blade revolutions were used. Blade forces were observed to have a direct relationship with the fill-level while mixing performance had an inverse relation and decreased with the increase in fill-level or bed-height. Mixing performance increased with the increase in the number of revolutions almost exponentially in the beginning. After around 12 blade-passes, the change in mixing performance was not significant, i.e., the mixing index value reached an asymptotic value. A mathematical relationship was derived for a quick estimate of mixing performance after given number of revolutions. The findings are significant and have direct industrial applications as they can best predict the mixing efficiency and residence times for the mixer. The proposed mathematical relationship can be effectively used for any industrial batch mixer employing blade movement or rotation and reduce the mixing costs by a significant factor.

Analysis of the effect of stirrer and container rotation direction on mixing index (Ip)

The paper discusses the effect of the stirrer and container rotation direction on the mixing index (Ip). The chaos theory is the result of an in-depth study of various problems that cannot be answered by the two previous major theories, namely quantum mechanics and the theory of relativity. Effective mixing of the flow area does not depend on rapid stirring. This study uses a container with a double stirrer, camera, programmable logic controller, tachometer, 6 A adapter, and a computer. DC electric motor (25 V) for turning stirrers and housings. The diameter of the primary and secondary stirrers is Dp=38 mm and Ds=17 mm. The diameter of the container made of transparent plastic is Dw=160 mm and height is 170 mm. Primary stirrer rotation (np)=10 rpm, secondary stirrer rotation (ns)=22.3 rpm, and container rotation (nw)=13 rpm, the angular velocity of the container is Ww=360° while the angular speed of the primary stirrer is Wp=180°. The liquid consists of a mixture of water and paint (white). For dye, a mixture of water and paint (red) is used. For testing the Brookfield viscometer, the viscosity of the liquid and dye is used. The results showed that turning the stirrer in the opposite direction to the container, there will be stretching, bending, and folding around the stirrer, and the smallest mixing index was P2V-b (0.94). In addition, based on the mixing index value above, the highest mixing effectiveness level is obtained, namely: P2V-b, P2S-b, P2B-b, P2V-a, P2B-a, and finally P2S-a. The mixing index is inversely related to the effectiveness level. So the highest effectiveness level is given by the following treatment: 1. Variation rotation (between opposite rotating mixers), 2. Opposite rotation (stirrer rotation opposite direction to the container), 3. Unidirectional rotation (stirrer rotation in the direction of the container)

ANALYSIS OF POSITION AND ROTATION DIRECTION OF DOUBLE STIRRER ON CHAOTIC ADVECTION BEHAVIOR

Turbulent mixing can damage the material molecules because of turbulence. Whereas laminar mixing raises a problem when mixing is carried out on viscous liquids. The mixing mechanism using chaotic flow affects the mixing quality. The aim of the experiment was to determine the position and direction of the double stirrer chaotic mixer. The installation of a chaotic mixer uses a cylindrical tub and two different mixers consisting of a primary mixer (Pp) and a secondary mixer (Ps). Periodically rotate the container and stirrer. The center of the vessel and primary mixer are placed at the same coordinates. For ε=4 cm (Pp to Ps distance), there are three experiments, namely: vessel rotation and directional stirrer (P2S-a), vessel rotation and opposite stirrer (P2B-a), and vessel rotation, both primary and secondary stirrers are directional variations. (P2V-a). Eccentricity 7 cm, there are also three treatments as above: one direction (P2S-b), reverse direction (P2B-b), and variation of direction (P2V-b).
 The video camera recordings are processed digitally. Qualitative data show a pattern of behavior during mixing. Meanwhile, quantitative data is used to determine the level of mixing effectiveness. The results showed that the direction of rotation of the two cylinders had no effect on the effectiveness of chaotic mixing. Based on the number of initial droplets of dye, the treatment that experienced the fastest chaos was P2B-b, at n=2 and r=3.5303. The difference in the number of color droplets does not affect chaotic behavior. The highest mixing efficiency was generated by the lowest P2V-b mixing index value of 0.94. Simultaneously, the direction between the mixer and the container will provide maximum mixing efficiency. Isolated mixing areas (island) and areas of poor mixing occur because of one-way rotation and low eccentricity

Comparative assessment of mixing characteristics and pressure drop in spiral and serpentine micromixers

In this paper, numerical simulations are carried out to analyze the mixing performance and pressure drop of water and ethyl alcohol in a spiral micromixer for Reynolds number in the range of 1 ≤ Re≤100. The characteristics are also compared with those for serpentine and straight micromixers by keeping the total flow length equal. It reveals that at lower Re ( = 1), the values of mixing index are almost same for all the micromixers, For higher Re, spiral micromixer provides better mixing than that of serpentine and straight micromixers, because of the formation of secondary Dean vortices. For straight micromixer, the mixing index decreases monotonically with Reynolds number. However, contrasting features are observed for both spiral and serpentine micromixers at higher regime of Re. The mixing index first decreases with Re and then increases although the values of critical Re beyond which the mixing index increases are different for spiral and serpentine micromixers. The pressure drops for all three micromixers are almost the same at lower values of Re, while the pressure drops for spiral and serpentine micromixers are almost equal and the values are slightly higher than those of straight micromixer at higher Re.

Simulation analysis of mixing quality in T-junction micromixer with bend mixing channel

In this paper, an analysis of the mixing quality of micromixers with bend curvature is proposed by Computational fluid dynamics simulation. Species transport phenomena are described by a set of conservation equations that is the Continuity equation, Navier-Stokes equation, and Convection-Diffusion equation. Reynolds number is taken in the range of 30 to 500. Firstly the mixing quality of T-junction straight and T-junction with bend curvature are compared. At a higher Reynolds number, microchannel T junction with bend curvature gives higher mixing quality (approx. Mi=46%). The results show that as the Reynolds number increases, the mixing index value, as well as pressure drop value also increases. Further, the effect of bend’s curvature is studied for better mixing quality. For this goal, three bend curvatures of the different radius of 1 mm, 2 mm, and 3 mm but with equal lengths were simulated. The results show that as the channel radius increases, mixing quality increases.

Inertial Micromixing in Curved Serpentine Micromixers with Different Curve Angles

Micromixers are of considerable significance in many microfluidics system applications, from chemical reactions to biological analysis processes. Passive micromixers, which rely solely on their geometry, have the advantages of low cost and a less-complex fabrication process. Dean vortices seen in curved microchannels are one of the useful tools to enhance micromixing. In this study, the effects of curve angle on micromixing were experimentally investigated in three curved serpentine micromixers consisting of ten segments with curve angles of 180 ° , 230 ° and 280 ° , at Dean numbers between 12 and 87. To characterize and compare the performance of the micromixers, fluorescence intensity maps and mixing indices were utilized. Accordingly, the micromixer having segments with 280 ° curve angle had significantly higher mixing index values up to the Dean number 60 and outperformed the other two micromixers. This was due to the severe distortion of flow streamlines by Dean vortices and the occurrence of chaotic advection at lower Dean numbers. Beyond the Dean number of 70, no difference was observed in the performance of the micromixers and the mixing index at their outlets had the asymptotic value of 0.93 ± 0.02. Furthermore, the flow behavior of the micromixers was numerically simulated to provide further insight about the mixing phenomena.

Mixing Performance of a Cost-effective Split-and-Recombine 3D Micromixer Fabricated by Xurographic Method.

This paper presents experimental and numerical investigations of a novel passive micromixer based on the lamination of fluid layers. Lamination-based mixers benefit from increasing the contact surface between two fluid phases by enhancing molecular diffusion to achieve a faster mixing. Novel three-dimensional split and recombine (SAR) structures are proposed to generate fluid laminations. Numerical simulations were conducted to model the mixer performance. Furthermore, experiments were conducted using dyes to observe fluid laminations and evaluate the proposed mixer’s characteristics. Mixing quality was experimentally obtained by means of image-based mixing index (MI) measurement. The multi-layer device was fabricated utilizing the Xurography method, which is a simple and low-cost method to fabricate 3D microfluidic devices. Mixing indexes of 96% and 90% were obtained at Reynolds numbers of 0.1 and 1, respectively. Moreover, the device had an MI value of 67% at a Reynolds number of 10 (flow rate of 116 µL/min for each inlet). The proposed micromixer, with its novel design and fabrication method, is expected to benefit a wide range of lab-on-a-chip applications, due to its high efficiency, low cost, high throughput and ease of fabrication.

Novel 3-D T-Shaped Passive Micromixer Design with Helicoidal Flows

Laminar fluid flow and advection-dominant transport produce ineffective mixing conditions in micromixers. In these systems, a desirable fluid mixing over a short distance may be achieved using special geometries in which complex flow paths are generated. In this paper, a novel design, utilizing semi-circular ridges, is proposed to improve mixing in micro channels. Fluid flow and scalar transport are investigated employing Computational Fluid Dynamics (CFD) tool. Mixing dynamics are investigated in detail for alternative designs, injection, and diffusivity conditions. Results indicate that the convex alignment of semi-circular elements yields a specific, helicoidal-shaped fluid flow along the mixing channel which in turn enhances fluid mixing. In all cases examined, homogenous concentration distributions with mixing index values over 80% are obtained. When it is compared to the classical T-shaped micromixer, the novel design increases mixing index and mixing performance values by the factors of 8.7 and 3.3, respectively. It is also shown that different orientations of ridges adversely affect the mixing efficiency by disturbing the formation of helicoidal-shaped flow profile.

Pulsatile flow micromixing coupled with ICEO for non-Newtonian fluids

The most biofluids/polymers, which are extensively used in lab-on-a-Chip devices exhibit non-Newtonian characteristics, whose mixing behaviors become crucial for the most engineering applications. For this purpose, the present numerical study investigates the performance of pulsatile flow micromixer coupled with induced-charge electro-osmosis (ICEO) for non-Newtonian fluids, such as power-law and Carreau. The mixing characteristics of Newtonian fluids are also presented and compared to non-Newtonian cases. The straight microchannel geometry is modified using rectangular obstruction with various heights to increase stirring performance for non-Newtonian fluids. The magnitude of the external electric field, the frequency of pulsatile flow and the width of rectangular obstruction are systematically varied for a comprehensive parametric evaluation. Time-dependent patterns of streamline and corresponding concentration maps are illustrated for qualitative presentation. Moreover, mixing index values are plotted versus obstruction width, the frequency of pulsatile flow and imposed electric field strength for quantitative analysis. It is observed that pulsatile flow with ICEO yields reasonable performance for all type of fluids. However, a rectangular obstruction is required to obtain maximum performance of mixing of non-Newtonian fluids for the given time, t = 10 s and length, l = 0.7 mm. It can be reported that rectangular obstruction has a significant effect on mixing performance for current conditions.

Parametric Study of Obstacle Geometry Effect on Mixing Performance in a Convergent-Divergent Micromixer with Sinusoidal Walls

Abstract This study presents a numerical simulation through computational fluid dynamics on mixing and flow structures in convergent-divergent micromixer with a triangular obstacle. The main concept in this design is to enhance the interfacial area between the two fluids by creating a transverse flow and split, and recombination of fluids flow due to the presence of obstacles. The effect of triangular obstacle size, the number of units, changing the position of an obstacle in the mixing channel and operational parameter like the Reynolds number on the mixing efficiency and pressure drop are assessed. The results indicate that at inlet Reynolds numbers below 5, the molecular diffusion is the most important mechanism of mixing, and the mixing index is almost the same for all cases. By increasing the inlet Reynolds number above 5, mixing index increases dramatically, due to the secondary flows. Based on the simulation results, due to increasing the effect of dean and separation vortices as well as more recirculation of flow in the side branches and behind the triangular obstacle, the highest mixing index is obtained at the aspect ratio of 2 for the triangular obstacle and its position at the front of the mixing unit. Also the highest value of mixing index is obtained by six unit of mixing chamber. The effect of changing the position of the obstacle in the channel and changing the aspect ratio of the obstacle is evident in high Reynolds numbers. An increase in the Reynolds number in both cases (changing the aspect ratio and position of the obstacle) leads to pressure drop increases.

Three-dimensional numerical study of heat transfer and mixing enhancement in a circular pipe using self-sustained oscillating flexible vorticity generators

In this paper, heat transfer and mixing performances are studied using three-dimensional numerical simulations of fluid-structure interactions. To this aim, a multifunctional heat exchanger/reactor geometry is investigated, consisting of a circular pipe where five arrays of four equally spaced trapezoidal vortex generators are inserted and inclined in a reversed position opposite to the flow direction with an angle of 45° with respect to the pipe wall. A periodic rotation of 45° is applied to the tabs arrays. Two cases are numerically studied: one using flexible vortex generators (FVG) that deform due to fluid forces applied on the structures and the other using conventional non deformable rigid vortex generators (RVG). For the FVG configuration, the tabs oscillate without addition of any external source of energy except that of the fluid flow itself, leading to a passive but dynamic way to perform vortex formation to disturb the flow. Both flow regimes are laminar with a constant Reynolds number of 1500. The flow structures are analyzed using the proper orthogonal decomposition (POD) technique and the effect of tabs oscillation on vortices creation, suppression and dislocation is highlighted.The effect of self-sustained free elastic tabs oscillation on heat transfer and mixing performances is numerically investigated by comparing the FVG with its corresponding RVG configuration. The Nusselt number comparison shows that the free tabs oscillation can improve the overall heat transfer of about 118% with respect to an empty pipe whereas it is about 97% for the RVG study. Finally, to assess the mixing performance, the transport of a passive scalar initially divided into two different concentrations in the pipe is numerically analyzed through the mixing index value. The FVG configuration shows a drastic improvement of the mixture quality at the exit of the pipe with an increase of 195% with respect to the RVG case, leading to much shorter and compact mixers and reactors.

Distributive mixing of carbon nanotubes in poly(caprolactone) via solution and melt processing: Viscoelasticity and crystallization behavior versus mixing indices

Different mixing processes give rise to significant differences in the agglomeration and deagglomeration of nanoparticles and how they are mixed and distributed within a polymeric matrix. Here poly(caprolactone), PCL, was compounded with carbon nanotubes, CNTs, at a weight fraction of 0.5% (volume fraction, Φ = 0.0027), using both solution and melt processing methods. Microscopy, image analysis, and thermogravimetric analysis, TGA, were used to define and characterize the distributions of the CNT-rich domain sizes and to obtain quantitative mixing indices at different scales of examination. The presence of the CNTs led to the shear-induced crystallization of PCL under conditions at which pure PCL would not crystallize and the microstructures obtained upon shear-induced crystallization were documented via X-ray diffraction, differential scanning calorimetry, and rheological characterization. Shear-induced crystallization occurred at a faster rate (smaller induction times) when the nanocomposites exhibited greater mixing index values. The shear-induced crystallization behavior was found to be a more sensitive indicator of the distributive mixing states of the CNTs in comparison with the widely used linear viscoelastic rheological material functions. The demonstrated methodologies can be used as complementary tools for processing and development of nanocomposites to achieve consistent and reproducible microstructures at various scales of examination and functional properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2254–2268.

Mixing characteristics of binary mixtures in a spout-fluid bed

ABSTRACTMixing characteristics of binary mixtures in a flat-bottom cylindrical spout-fluid bed using glass beads and air are reported in this work. Experiments were carried out to investigate the mixing characteristics for binary mixtures at three flow conditions, i.e., only spouting, only fluidization and spout-fluidization. The experiments were performed at different gas velocities, diameter ratios of binary mixtures and three different bed arrangements. Mixing index was determined for fluidized bed and static bed conditions. It was found that, in all cases, lowest-diameter ratio mixture gave good mixing index values. For all flow conditions, mixing index for large–small bed arrangement was increasing with time, whereas for small–large bed arrangement, the mixing index deteriorated with time. However, in both cases, the mixing index reached almost a constant value. For well-mixed bed arrangement and spout-fluidization flow condition, segregation and re-mixing were observed.