Transverse Concentration Gradients Research Articles

Effects of transverse concentration gradients on the vented explosion of methane-air mixtures

Experiments were performed in a 90-L vessel with a side vent to investigate the effects of transverse concentration gradients on the vented explosion of non-uniform methane-air mixture. Controlled by a mass flow meter, methane was injected uniformly into the vessel through 18 nozzles that are evenly distributed at the top to configure a methane-air mixture with an average methane volume fraction of 10.9%. Various transverse concentration gradients were obtained by altering the free diffusion time of the methane in the vessel. The deflagration flame was recorded through a high-speed camera at a frequency of 1000 Hz, and the internal overpressure at different locations was captured through three piezoelectric sensors. Experimental results reveal that larger concentration gradient resulted in lower flame propagation rate, longer time for the burst of the vent cover, and stronger pressure oscillations. Concentration gradient affects the typical flame structure, especially the formation and development of the tulip flame. Under the present experimental conditions, Helmholtz oscillations occurred in all tests. Concentration gradient has a negative correlation with the frequency of Helmholtz oscillations but a positive correlation with the duration of oscillations. In conclusion, the variation of concentration gradient obviously affects flame propagation and pressure evolution during vented explosions, which deserves more attention in the design of explosion protection.

Study on effect of transverse concentration gradients in pipeline on methane explosion characteristics

In this paper, we studied the effects of transverse concentration gradients on methane explosion characteristics using a fully-transparent pipeline and finally came to the following conclusions. The peak explosion overpressure changed in a waveform of “M” along different measuring points, but a set of non-equivalent concentration gradients maximized this peak value. Higher methane concentrations in the Q2 region could cause the secondary peak overpressure near the both ends of the pipe. In this process, the flame outside the pipe tends to form a near-spherical shape. However, at lower methane concentrations in the Q2 region, the secondary peak overpressure was only detected near the outlet. Its occurrence at different sites may relate to the turbulence effect heightened by concentration gradients. Besides, under the same gradient, the smallest propagation speed was measured on the right of the ignition source when the flame propagated fastest on the left, which was attributed to different inhibiting effects of reflected waves on flame propagation.

Effects of confinement on detonation in H2–O2 mixture with transverse concentration gradient

Since hydrogen has wide flammability limit and low ignition energy, it could be easily ignited and be easy for the transition to a detonation, leading to extremely serious impacts in explosion accidents or extremely high combustion effeciency in the propulsion. In the field of explosion accidents and the combustion chamber of propulsion systems, hydrogen mixtures are more likely to be highly non-uniform and a detonation usually propagates in a non-homogeneous medium. The work studies behaviors of detonations in non-homogenous medium by a high-resolution simulations. We widen computational domain and steepen the gradient to weaken the role of transverse wave on cellular detonations propagating in the medium with transverse concentration gradient and to reveal the interaction of longitudinal shock with reaction wave. The results show that characteristics of detonations in nonuniform medium is controlled by coupling role of gradient and confinement. As a domain is sufficient wide, the reflection wave is rather weak so that the detonation takes on galloping propagation, with a single-head mode. As the width is mediate, detonation cell takes on highly irregularity, similar to that of highly unstable detonation. However, in the narrow domain, steepening gradient plays a key role while confinement becomes minor in detonation propagation.

Visualization research of natural gas jet characteristics with ultra-high injection pressure

In this paper, based on a High-pressure direct injection (HPDI) dual-fuel valve, the schlieren imaging method combined with laser-induced fluorescence imaging method (PLIF) are used to investigate the macroscopic structure and concentration distribution characteristics of a high-pressure gas jet under different injection pressure ratio (NPR) and gas injection moment (Ti). The data processing method for macroscopic structure and concentration distribution of gas jet is optimized, and a new concentration partition method for the gas jet is proposed. The results show that comparing the gas jet penetration and volume correlation equations from Hamzehloo A and Vuorinen V, the correlation coefficients of ultra-high and small NPR conditions are quite different due to the different development processes of the shock wave structure. With the improvement of NPR, the influence on gas jet penetration is decreasing, which is the same as the conclusion of previous research. In this paper, the later stage of gas jet is divided into four regions: the Mach disk core region (A), the concentration rapidly decreasing region (B), the low concentration diffusion region (C), and the edge mixing region (D). The A and B regions are highly correlated with the shock wave structure of the under-expanded gas jet. The increase of NPR and Ti has a great promotion effect on the length and concentration distribution of regions A and B but has rarely effect on regions C and D. The transverse concentration gradient in regions A and B presents a Gaussian distribution structure. The average concentration descent gradient (G¯) in region A is about 4 times that in region B. The transverse concentrations of C and D gradually converge under the action of air turbulence. Under different Ti conditions, the length proportion of regions A and D rarely changed, while the length proportion of regions B and C presents a negative correlation. The G¯ in regions A and B are positively correlated with Ti, but the concentration increase rate keeps decreasing. The range of concentration fluctuation regions in regions C and D keeps expanding with the increase of Ti, indicating that the uniformity of gas jet concentration distribution keeps improving.

Soret effects on the diffusion-chemistry interaction of hydrogen-air edge flames propagating in transverse gradient evolving mixing layers

The effects of Soret diffusion (SD) on the hydrogen-air edge flame propagation and the diffusion-chemistry interaction are investigated through simulation facilitated by the numerical code MultiDiffFOAM. The edge flames in this study gradually develop from a flame kernel into a tri-brachial structure in a hydrogen-air mixing layer that temporally evolves due to transverse reactant concentration gradient. We demonstrate that the responses of flame displacement speed Sd to flame curvature K, stretch rate κ and scalar dissipation rate χ are distinctly influenced by SD. For the linear Sd-K and Sd-κ correlations, SD would result in a smaller Markstein length. Moreover, SD is shown to lead to shifting of the Sd-χ curve towards the regime with larger χ. Compared with the weak influences of SD on the tangential diffusion component Sd,t and normal diffusion component Sd,n, the chemical reaction component Sd,r is significantly weakened by SD. The important chemical reactions for edge flame propagation are identified based on sensitivity analysis and their rates are found to be smaller when SD is considered. For the local composition at the flame marker, the mass fraction of H2 is slightly larger and that of H is obviously smaller when SD is considered. The SD flux of H2jH2SD and that of H jHSD are both coupled with the driving force ∇(lnT) along the mixture fraction coordinate. However, the jH2SD is mainly concentrated on the unburnt side while the jHSD is on the burnt side. The analyses on decomposed fluxes of H2 and H along the flame normal direction further suggest that SD would enhance the H2 mass diffusion but weaken the H mass diffusion. Such opposite effects stem from the distribution features that H2 is mainly on the unburnt side while H on the burnt side.

Hydrogen-air explosion with concentration gradients in a cubic enclosure

An inhomogeneous hydrogen-air mixture is induced once hydrogen leakage occurs, and an explosion may be triggered due to an accidental ignition with sufficient strength. The effects of different hydrogen concentration gradients on flame behavior and overpressure were experimentally investigated in a closed cubic vessel with a volume of 0.125 m3. The transverse concentration gradients of hydrogen injected from the top face of the vessel were recorded using five oxygen sensors. A high-speed photography was employed to capture the flame shape evolution and the flame tip velocity was derived. Three piezoelectric pressure transducers were mounted on the top and side walls to measure the pressure-time profiles. The results show that the maximum overpressure in homogeneous mixtures is larger than that in inhomogeneous mixtures for fuel-lean hydrogen-air mixtures. However, the maximum overpressure in inhomogeneous mixtures is equal to or larger than that in homogeneous mixtures for fuel-rich hydrogen-air mixtures. Due to a larger difference between the upward and downward flame speeds, a mushroom-like shape flame was recorded in inhomogeneous hydrogen-air mixtures in fuel-lean to slightly fuel-rich hydrogen-air mixtures. The flame speed in two directions has the almost same value in inhomogeneous fuel-richer mixtures. The average flame speed in inhomogeneous mixtures is significantly larger than that in homogeneous mixtures for fuel-rich hydrogen air mixtures.

Quantitative study of density-driven convection mass transfer in porous media by MRI

CO2 geologic storage in deep saline aquifers is a promising technology to reduce greenhouse gas emissions and maintain the pace of economic development. The dissolution and convection of CO2 injected into brines are the primary trapping mechanisms to enhance storage efficiency and provide long-term storage security. The spatial mass transfer characteristics of convection in porous media remain poorly understood. In this study, two kinds of analogous fluid pairs were used as the equivalents of CO2-saturated brine and in situ brine. Density-driven convection experiments using magnetic resonance imaging (MRI) technology in heterogeneous porous media were performed at various stages, including the diffusion stage, the unstable trigger stage and the convection development stage. Combined with the spatial evolution of the fingers and the change in MRI mean intensity, the local fluid concentration and overall mass transfer behavior could be quantified. Physical quantities associated with mass transfer by convection, such as the mixing layer thickness and scalar dissipation rate at the diffusion stages, the onset time at the unstable trigger stage, the average velocity of the front finger, the finger number, the transverse concentration gradient of the finger, and the sweep area of the finger at the convection development stage, were also analyzed systematically. We found that convective mixing substantially promotes mass transfer over pure diffusion, and the dissolution rate can be further enhanced by improving the injection method at each stage. The range of Ra in this study covers most CO2 capture and storage (CCS) sites and can provide basic data for engineering applications.

Flame propagation in methane-air mixtures with transverse concentration gradients in horizontal duct

Stratified methane-air mixtures are formed in the early stages of methane leakage in confined spaces, and an explosion may occur if a source of ignition were to appear at the appropriate time and location. However, this has not been elucidated thus far. The aim of this study is to measure the concentration gradient of methane injected from the top surface of a duct, and to investigate the effects of ignition delay (tig), which is the time interval from methane leakage to ignition, on flame behavior and explosion overpressure. Experimental results show that tig significantly affects flame behavior in stratified methane-air mixtures, including the flame shape and speed. The stratified methane-air mixtures cannot be ignited at tig ≤ 3 min. The horizontal flame speed and maximum overpressure (Pmax) first increase as tig increases from 4 min to 15 min and thereafter remain nearly unchanged with further increase in tig. The value of Pmax at tig in the range 15–25 min is approximately that observed for homogeneous methane-air mixtures. Tulip flames are observed at tig ≥ 10 min. Diffusion and convection flames, which follow the leading premixed flame front, are only formed at tig ≤ 6 min.

Propagation of cellular detonation in a gas suspension in the presence of a concentration gradient

Models for the description of detonation flows in inhomogeneous gas particle mixtures are presented. The physical and mathematical model of heterogeneous detonation of aluminum suspensions is based on approaches to the mechanics of heterogeneous media and semi-empirical laws of ignition and combustion of particles. The model is extended for suspensions that are inhomogeneous in particle concentrations. This suggests the solution of an additional equation for the spatial distribution of initial concentrations. The dependence of the integral heat release of chemical reactions and the fraction of unburned particles on the initial composition is determined from the solution of the stationary problem of the structure of the detonation wave and its agreement with empirical data on the rate of normal detonation. The model for describing detonation in mixtures of reacting gases with inert particles is based on an analysis of the state of chemical equilibrium. The problem of the initiation and development of cellular detonation in a flat channel in the presence of a longitudinal or transverse gradient of the concentration of aluminium particles is considered. The limits of changes in concentrations from poor to super stoichiometric compositions are considered for 1 μm particles. When detonation propagates along a channel with a longitudinal positive (negative) concentration gradient, the cellular structures do not change, but the average and peak values of pressure and phase density increase (decrease). With a transverse concentration gradient, cell-like structures with a curved front, asymmetric and strongly elongated cells form in the channel. The results are consistent with studies of similar gas detonation flows.

The effect of concentration gradients on deflagration-to-detonation transition in a rectangular channel with and without obstructions – A numerical study

Explosions in homogeneous reactive mixtures have been widely studied both experimentally and numerically. However, in accident scenarios, mixtures are usually inhomogeneous due to the localized nature of most fuel releases, buoyancy effects and the finite time between release and ignition. It is imperative to determine whether mixture inhomogeneity can increase the explosion hazard beyond what is known for homogeneous mixtures. The present numerical investigation aims to study flame acceleration and transition to detonation in homogeneous and inhomogeneous hydrogen-air mixtures with two different average hydrogen concentrations in a horizontal rectangular channel. A density-based solver was implemented within the OpenFOAM CFD toolbox. The Harten–Lax–van Leer–Contact (HLLC) scheme was used for accurate shock capturing. A high-resolution grid is provided by using adaptive mesh refinement, which leads to 30 grid points per half reaction length (HRL). In agreement with previous experimental results, it is found that transverse concentration gradients can either strengthen or weaken flame acceleration, depending on average hydrogen concentration and channel obstruction. Comparing experiments and simulations, the paper analyses flame speed and pressure histories, identifies locations of detonation onset, and interprets the effects of concentration gradients.

Simultaneous imaging of temperature and concentration of ethanol–water mixtures in microchannel using near-infrared dual-wavelength absorption technique

This paper presents a simultaneous imaging method of temperature and ethanol concentration of ethanol–water mixtures in microfluidic channels. The principle is based on the facts that the absorbance at a wavelength of 1905 nm is dependent on the temperature of water and that the absorbance at 1935 nm is independent of the temperature but strongly dependent on the molar concentration of water, which is reciprocal to the molar concentration of ethanol in the mixture. The absorbance images at the two wavelengths were acquired alternately, each at 50 frames per second, by an alternate irradiation system and near-infrared (NIR) camera, and then converted to the temperature and concentration images by a linear regression model. The imaging method was applied to a dilute ethanol–water mixture with an ethanol concentration of 0.43 M and water flowing side by side in a temperature-controlled Y-channel. The concentration images clearly showed differences between the mixture and water streams, and that the transverse concentration gradient between the two streams decreased downstream by mutual diffusion. It was also confirmed that the mutual diffusion coefficient increased as the temperature increased. The temperature images showed that uniform distributions were immediately formed due to heat transfer between the fluid and channel materials.

Assessing intrusion by the capillary during spatially resolved mass spectrometry measurement

The impact of the capillary probe of a spatially-resolved mass spectrometer system (SpaciMS) on the measured reactant conversion is reported using propylene oxidation over Pt/Al2O3 washcoated monoliths. The findings suggest that the invasive nature of SpaciMS depends on its configuration and application. Using monoliths with a range of cell densities (100–600 cells per square inch, CPSI), the concentration profiles of propylene sampled with probes of two different outer diameters (170 and 363μm) are compared with the temperature measured using coherent optical frequency domain reflectometry (c-OFDR). The comparison indicates that flow blockage has a negligible effect if the limiting propylene is depleted in the downstream reactor section. Suction by the probe compensates for the blockage for certain combinations of the channel diameter and probe size. In such cases the profile measured by a probe with the larger outer diameter (363μm) is similar to that measured by a smaller capillary (170μm). The experiments reveal that the axial position of the probe does not influence the flow profile in a 100 CPSI monolith channel, nor does it affect the amount of flow deflected to surrounding channels for a 600 CPSI monolith. Under some conditions the results are impacted by transverse concentration gradients. Their existence complicates the interpretation of the SpaciMS data. The difference between the location of propylene depletion and temperature maximum provides a useful metric for capturing the collective impacts of flow deflection and transverse gradients. The complexity of the flow, transport and reaction suggests that at least three dimensionless groups are needed to bound the operating regions in which the presence of the probe has minimal impact on the measured concentration.

Detonation propagation in hydrogen–air mixtures with transverse concentration gradients

The influence of transverse concentration gradients on detonation propagation in $$\hbox {H}_2$$ –air mixtures is investigated experimentally in a wide parameter range. Detonation fronts are characterized by means of high-speed shadowgraphy, OH* imaging, pressure measurements, and soot foils. Steep concentration gradients at low average $$\hbox {H}_2$$ concentrations lead to single-headed detonations. A maximum velocity deficit compared to the Chapman–Jouguet velocity of 9 % is observed. Significant amounts of mixture seem to be consumed by turbulent deflagration behind the leading detonation. Wall pressure measurements show high local pressure peaks due to strong transverse waves caused by the concentration gradients. Higher average $$\hbox {H}_2$$ concentrations or weaker gradients allow for multi-headed detonation propagation.

Diffusiophoresis of a colloidal cylinder in an electrolyte solution near a plane wall

The diffusiophoretic motion of a circular cylindrical particle in an electrolyte solution prescribed with a transverse concentration gradient, which is arbitrarily oriented with respect to a plane wall parallel to its axis, is analyzed at the quasi-steady state. The thickness of the electric double layers adjacent to the particle and wall surfaces is taken to be small but finite, and the relaxation effect of their diffuse ions is allowed. Through the use of cylindrical bipolar coordinates, the equations of conservation governing this problem are solved, and the translational and angular velocities of the confined cylinder are computed for various cases. The influences of the confining wall and double-layer polarization on the diffusiophoretic mobility of the particle are found to be significant and complicated. The proximity of the plane wall can enhance or reduce the diffusiophoretic velocity and even reverse its direction, because the wall effect of hydrodynamic resistance on the particle is in the competition with that of electrochemical enhancement or retardation, and the wall effect on the chemiphoresis of the particle is in the competition with that on the induced electrophoresis. The direction of the diffusiophoretic motion of a particle near a plane wall is different from that of the prescribed concentration gradient, except when it is oriented parallel or perpendicular to the wall. The effects of the plane wall on the diffusiophoresis of a cylindrical particle are more significant than those of a spherical one at the same separation because of more effective particle–wall interaction surfaces.

Pore-Scale Study of the Non-Linear Mixing of Fluids with Viscous Fingering in Anisotropic Porous Media

AbstractMixing processes of miscible viscous fluids in anisotropic porous media are studied through a lattice Boltzmann method in this paper. The results show that fluid mixing can be enhanced by the disorder and interfaces induced by viscous fingers. Meanwhile, the permeability anisotropy affects the developments of viscous fingers and the subsequent mixing behaviors significantly. Specifically, as the streamwise (longitudinal) permeability is larger than the spanwise (transverse) one, the mixing process is faster and stronger than that in the contrary case. Furthermore, the anisotropy can lead to different behaviors of the dissipation rates in streamwise and spanwise directions. Generally, the dissipation rate is dominated by the transverse concentration gradients when the longitudinal permeability is higher than the transverse one; on the contrary, as the transverse permeability is higher than the longitudinal one, the contributions to the dissipation rate from longitudinal and transverse concentration gradients are both significant.

Biofilm responses to smooth flow fields and chemical gradients in novel microfluidic flow cells.

We present two novel microfluidic flow cells developed to provide reliable control of flow distributions and chemical gradients in biofilm studies. We developed a single-inlet microfluidic flow cell to support biofilm growth under a uniform velocity field, and a double-inlet flow cell to provide a very smooth transverse concentration gradient. Both flow cells consist of a layer of polydimethylsiloxane (PDMS) bonded to glass cover slips and were fabricated using the replica molding technique. We demonstrate the capabilities of the flow cells by quantifying flow patterns before and after growth of Pseudomonas aeruginosa biofilms through particle imaging velocimetry, and by evaluating concentration gradients within the double-inlet microfluidic flow cell. Biofilm growth substantially increased flow complexity by diverting flow around biomass, creating high- and low-velocity regions and surface friction. Under a glucose gradient in the double-inlet flow cell, P. aeruginosa biofilms grew in proportion to the local glucose concentration, producing distinct spatial patterns in biofilm biomass relative to the imposed glucose gradient. When biofilms were subjected to a ciprofloxacin gradient, spatial patterns of fractions of dead cells were also in proportion to the local antibiotic concentration. These results demonstrate that the microfluidic flow cells are suitable for quantifying flow complexities resulting from flow-biofilm interactions and investigating spatial patterns of biofilm growth under chemical gradients. These novel microfluidic flow cells will facilitate biofilm research that requires flow control and in situ imaging, particularly investigations of biofilm-environment interactions.

From random sphere packings to regular pillar arrays: Analysis of transverse dispersion

We study the impact of microscopic order on transverse dispersion in the interstitial void space of bulk (unconfined) chromatographic beds by numerical simulations of incompressible fluid flow and mass transport of a passive tracer. Our study includes polydisperse random sphere packings (computer-generated with particle size distributions of modern core–shell and sub-2μm particles), the macropore space morphology of a physically reconstructed silica monolith, and computer-generated regular pillar arrays. These bed morphologies are analyzed by their velocity probability density distributions, transient dispersion behavior, and the dependence of asymptotic transverse dispersion coefficients on the mobile phase velocity. In our work, the spherical particles, the monolith skeleton, and the cylindrical pillars are all treated as impermeable solid phase (nonporous) and the tracer is unretained, to focus on the impact of microscopic order on flow and (particularly transverse) hydrodynamic dispersion in the interstitial void space. The microscopic order of the pillar arrays causes their velocity probability density distributions to start and end abruptly, their transient dispersion coefficients to oscillate, and the asymptotic transverse dispersion coefficients to plateau out of initial power law behavior. The microscopically disordered beds, by contrast, follow power law behavior over the whole investigated velocity range, for which we present refined equations (i.e., Eq. (13) and the data in Table 2 for the polydisperse sphere packings; Eq. (17) for the silica monolith). The bulk bed morphologies and their intrinsic differences addressed in this work determine how efficient a bed can relax the transverse concentration gradients caused by wall effects, which exist in all confined separation media used in chromatographic practice. Whereas the effect of diffusion on transverse dispersion decreases and ultimately disappears at increasing velocity with the microscopically disordered chromatographic beds, it dominates in the pillar arrays. The pillar arrays therefore become the least forgiving bed morphology with macroscopic heterogeneities and the engendered longitudinal dispersion in chromatographic practice. Wall effects in pillar arrays and the monolith caused by their confinement impact band broadening, which is traditionally observed on a macroscopic scale, more seriously than in the packings.

A CFD study on the effect of the characteristic dimension of catalytic wall microreactors

AbstractA three‐dimensional computational fluid dynamics study of the steam methane reforming (SMR) in microreactors is presented. Emphasis has been made on investigating the effects of the characteristic dimension (d: 0.35, 0.70, 1.40, and 2.80 mm) on the performance of two microreactor geometries: square microchannels and microslits. Results have shown that for both geometries the SMR conversion decreases markedly as d increases. Conversely, the microchannels provide a methane conversion slightly higher than that of the microslits. The different performance of the microreactors is only partially due to the different surface‐to‐volume ratio. Pronounced transverse temperature and concentration gradients develop as the characteristic dimension increases especially for microslits in the first half of the reactor. Therefore, external transport limitations can affect the performance of microreactors for SMR, although the characteristic dimensions are of the order of very few millimeters. © 2011 American Institute of Chemical Engineers AIChE J, 2012

Erythrocyte Concentration Distribution in Sheathed Microfluidic Flows

The combined effect of dissociative and associative forces on erythrocytes flowing in direct, layered contact with cell-free "sheath" flows in microfluidic systems was studied to aid in the design of blood purification devices. A blood stream flowed in direct contact between two fluids that were cell free. The layered flow created a sharp transverse concentration gradient of erythrocytes that promoted lateral movement of cells. This movement was perceived as both dispersive (arising from particle collisions or shear-enhanced movement) and associative (arising from rouleau formation and shear-induced migration, which limited the extent of movement and maintained a higher hematocrit in the central layer). The concentration changes of erythrocytes in the blood and sheath streams were measured to determine the flux of cells between streams. At flows with low wall shear rates, the flux of erythrocytes into the sheath streams was small. When shear was increased, cell flux away from the central layer increased. There was an intermediate shear rate that maximized cellular flux away from the center stream, and a further increase in shear rate showed no change in cell flux. Changing the transverse position of erythrocytes entering the system strongly affects their distribution in the exiting stream even for long fluid residence times. The results clarify microfluidic device design and elucidate how shear and hematocrit affect erythrocyte movement in blood.

Evaluation of the Effects of Porous Media Structure on Mixing-Controlled Reactions Using Pore-Scale Modeling and Micromodel Experiments

The objectives of this work were to determine if a pore-scale model could accurately capture the physical and chemical processes that control transverse mixing and reaction in microfluidic pore structures (i.e., micromodels), and to directly evaluate the effects of porous media geometry on a transverse mixing-limited chemical reaction. We directly compare pore-scale numerical simulations using a lattice-Boltzmann finite volume model (LB-FVM) with micromodel experiments using identical pore structures and flow rates, and we examine the effects of grain size, grain orientation, and intraparticle porosity upon the extent of a fast bimolecular reaction. For both the micromodel experiments and LB-FVM simulations, two reactive substrates are introduced into a network of pores via two separate and parallel fluid streams. The substrates mix within the porous media transverse to flow and undergo instantaneous reaction. Results indicate that (i) the LB-FVM simulations accurately captured the physical and chemical process in the micromodel experiments, (ii) grain size alone is not sufficient to quantify mixing at the pore scale, (iii) interfacial contact area between reactive species plumes is a controlling factor for mixing and extent of chemical reaction, (iv) at steady state, mixing and chemical reaction can occur within aggregates due to interconnected intra-aggregate porosity, (v) grain orientation significantly affects mixing and extent of reaction, and (vi) flow focusing enhances transverse mixing by bringing stream lines which were initially distal into close proximity thereby enhancing transverse concentration gradients. This study suggests that subcontinuum effects can play an important role in the overall extent of mixing and reaction in groundwater, and hence may need to be considered when evaluating reactive transport.