Advective Mass Transfer Research Articles

Dispersion of a passive scalar around a Taylor bubble

The motion of Taylor bubbles in capillaries is typical of many engineering and biological systems, ranging from subsurface flows to small-scale reactors. Although the hydrodynamics of elongated bubbles has been the object of several studies, the case where a solute is transported in the surrounding liquid and surface mass-transfer mechanisms act on the solid wall or the bubble–fluid interface is much less understood. To fill this gap, we investigate the transport problem around a confined Taylor bubble to access the competition between advection, diffusion and surface mass transfer in the different regions of the bubble. With this aim, we derive a one-dimensional advection–diffusion–mass-transfer equation where the transport mechanisms are described through an effective velocity, an effective diffusion coefficient and an effective Sherwood number. Our model generalises the Aris–Taylor dispersion to the case of a Taylor bubble and clarifies the impact of surface mass transfer in the advection and diffusion dominated regimes for both the front and rear menisci. The model recovers the typical Pèclet square relationship of the effective diffusion coefficient, which also depends on the film thickness. Also, when the Pèclet number balances with the Sherwood number, there exist conditions that lead to the formation of hot spots of concentration. We show that the typical shape oscillations of the bubble rear locally enhance superficial mass transfer. Finally, we study the transport problem in the uniform film region where the concentration field can be found analytically.

Enhancement of heat and mass transfer by herringbone microstructures in a simple shear flow

The heat and mass transfer characteristics of a simple shear flow over a surface covered with staggered herringbone structures are numerically investigated using the lattice Boltzmann method. Two flow motions are identified. The first is a spiral flow oscillation above the herringbone structures that advect heat and mass from the top plane to herringbone structures. The second is a flow recirculation in the grooves between the ridges that advect heat and mass from the area around the tips of the structures to their side walls and the bottom surfaces. These two basic flow motions couple together to form a complex transport mechanism. The results show that when advective heat and mass transfer takes effect at relatively large Reynolds and Schmidt numbers, the dependence of the total transfer rate on Schmidt number follows a power law, with the exponent being the same as that in the Dittus–Boelter equation for turbulent heat transfer. As the Reynolds number increases, the dependence of the total transfer rate on the Reynolds number also approaches a power law, and the exponent is close to that in the Dittus–Boelter equation.

Unique biofilm structure and mass transfer mechanisms in the foam aerated biofilm reactor (FABR)

ABSTRACT The foam-aerated biofilm reactor (FABR) is a novel biofilm process that can simultaneously remove carbon and nitrogen from wastewater. A porous polyurethane foam sheet forms an interface between wastewater and aerated water, making it a counter-diffusional biofilm process similar to the membrane-aerated biofilm reactor (MABR). However, it is not clear how biofilm develops the foam interior, and how this impacts mass transfer and performance. This research explored biofilm development within the foam sheet and determined whether advective transport within the sheet played a significant role. Foam sheets with 2-, 4.5- and 9-mm thicknesses were explored. Oxygen, nitrate, nitrite and ammonia profiles in the sheet were measured using microsensors, and biofilm imaging studies were carried out using optical coherence tomography (OCT). On the foam’s aerated side, a dense nitrifying biofilm formed. Beyond the aerobic zone, much less biomass was observed, with a high porosity foam-biofilm layer. The higher effective diffusivity within the foam for the 4- and 9-mm sheets suggested advective transport within the foam channel structures. Using an effective diffusivity factor in conventional 1-D biofilm models reproduced the measured substrate concentration profiles within the foam. Four different practical conditions were modelled. The maximum TN removal efficiency was about 70% and a nitrogen removal flux of 1.25 gN.m−2.d−1. We conclude that mass transfer resistance occurred primarily in the dense, nitrifying layer near the aerated side. The rest of the foam sheet was porous, allowing the advective mass transfer.

Modeling of photocatalytic oxidation reactor for methyl ethyl ketone removal from indoor environment: Systematic model development and validation

This paper describes the development of a model for predicting the performance of ultraviolet photocatalytic oxidation (UV-PCO) reactor for the removal of volatile organic compounds (VOCs) from indoor environment. The proposed model performance was validated at two stages. First, it was validated by comparing its performance with the prediction made by other models (inter-model comparison), and with experimental data from two different scales (pilot and bench) of UV-PCO reactors. A good agreement (with R2 = 0.98) between the developed model prediction and the experimental data was observed.Simulation of light intensity with linear source spherical emission model (LSSE) in bench-scale reactor showed a more uniform distribution than pilot-scale, due to longer distance of the lamp from the media in the bench-scale reactor. Beer-Lambert model demonstrated that light absorbed totally with one media, and adding extra media does not improve the PCO removal performance. Furthermore, Langmuir-Hinshelwood (L-H) rate expression by considering competitive effect of water and by-products with target compound was used. Formaldehyde and acetaldehyde were identified as the main by-products of the Methyl Ethyl Ketone (MEK) PCO reaction.A sensitivity analysis was performed to investigate the impact of three dimensionless parameters—Peclet (Pe), Stanton (St), and Damköhler (Da) numbers on the system performance. Simulation results indicated the UV-PCO process is mainly limited by the reaction rate at low removal efficiency. For higher removal efficiency, both the photochemical reaction rate and advective mass transfer have a significant impact on the system performance. The practical validation of the dimensionless model at two different scales of PCO reactor with same operating condition indicated that the proposed dimensionless model analysis can provide useful advice (qualitatively and quantitatively) for scale-up of PCO for the real application.

Critical Analysis of Hepatic Clearance Based on an Advection Mass Transfer Model and Mass Balance

The relationship between liver blood flow rate and hepatic clearance has been derived and validated based on the constitutive principle of mass conservation using an advective mass transfer model for bivariate concentration, Ci (x, t). Metabolic elimination continuously diminishes along the blood flow path line, corresponding with the concentration gradient. Because of the constraint introduced by constant blood velocity, x and t are not independent variables. This requires the introduction of a new factor called the dwell time, which accounts for the time a drug is retained in the liver. First-order or saturable drug metabolism is dependent on perfusate drug concentration and the dwell time. A new extraction factor is explicitly identified as E = ln Cin/Cout. Because the isolated organ perfusion model described here is validated, it provides a basis for drawing inferences and exploring underlying features, mechanisms, and principles on which this model is based. In the conventional model, these underlying constitutive principles are unavailable. The concentration-time profile of a physiological-based elimination network is found to be inconsistent with the profile described by a pharmacokinetic model. For this reason, among others discussed, physiological-based models are incompatible with pharmacokinetic models and therefore cannot be used to explain systemic pharmacokinetic behavior.

A mobile-immobile model for reactive solute transport in a radial two-zone confined aquifer

A mobile-immobile model (MIM) of reactive solute transport for an injection well was investigated in a radial two-zone confined aquifer system made up of the aquifer formation zone and the wellbore skin zone evolving from mud invasion or extensive well development. Fundamental transport processes such as advection, radial dispersion, first-order reaction, linear sorption and first-order mass transfer between the mobile and immobile domains were considered. The Laplace transform and numerical inverse Laplace methods were used to solve the model while the skin effect on the spatial concentration distribution as well as breakthrough curves (BTCs) were analyzed. The results indicate that a larger dispersivity or smaller effective porosity in the skin zone result in higher values of BTCs at the early injection stage, lower values at the late stage and higher peak values of BTCs. However, an abrupt change for the spatial concentration distribution was observed at the interface of the skin and aquifer formation zones due to the dispersivity change in the skin zone. It was also found that a larger mass transfer coefficient of the skin zone results in smaller values of BTCs at the early injection stage but higher at the late stage. The general conclusion is that the possible impacts of skin on radial solute transport are significant and should be taken into consideration in tracer-injection tests.

Hydrodynamic dispersion due to a variety of flow velocity profiles in a porous-walled microfluidic channel

A reduced-order model for the advective-dispersive mass transfer due to a variety of flow velocity profiles in a porous-walled microfluidic channel is developed in this study. The fluid flow in the microfluidic channel is described by a generalized class of the Poiseuille equation proposed by Amatore et al. (2009), which includes the bluntness parameter for exhibiting the level of bluntness of the velocity profile. This flow formulation represents all the traditional velocity profiles for the purely pressure-driven (Poiseuille) flow, the combined pressure-driven and electro-osmotic flows, and the purely electro-osmotic flow (EOF) in the microfluidic channel. The resulting reduced-order model delivers the mass transfer coefficients, including the hydrodynamic dispersion and the effective advection coefficients, which are functions of the Peclet number and the bluntness parameter. The results reveal that the hydrodynamic dispersion decreases when the flow varies from the pressure-driven to the electro-osmotic. In other words, the smaller the bluntness parameter the smaller the hydrodynamic dispersion. The diffusive, transient, and advective mass transfer regimes can be identified from the ratio of the hydrodynamic dispersion coefficient in the porous-walled microfluidic channel to the one in the nonporous-walled microfluidic channel. It is found that the mass transfer between the microfluidic channel and the porous medium should be included in determination of the hydrodynamic dispersion coefficient due to a variety of flow velocity profiles in a porous-walled microfluidic channel for the transient and the advective regimes. The results also show that the smaller the bluntness parameter the slower the mass transfer. Furthermore, the mass transfer in a porous-walled microfluidic channel is slower than the mass transfer in a nonporous-walled microfluidic channel.

Characteristics of the high-temperature water film evaporation with countercurrent turbulent air flow in the duct

Passive containment with high design pressure often has high evaporation temperature. The evaporation mass transfer with high evaporation temperature along with countercurrent turbulent airflow in a long duct was experimentally investigated. The experiments were performed for a wall surface temperature range of 71–90 °C. The experimental data showed that the high evaporation temperature significantly enhanced evaporation. By comparison, it reveals that the significant prediction deviation of the correlation from Kang and Park (Nuclear Engineering and Design, Vol. 204, pp 347–359, 2001) is mainly due to the cubic power modification. HMTA from Huang and Yang (Annals of Nuclear Energy, Vol. 76, pp 237–242, 2015) also underestimates the Sherwood number at high evaporation temperature but it could be applicable when the modification of the wavy interface and the advective mass transfer are included. A modified correlation for falling water film evaporation with high evaporation temperature is finally developed.

Energy efficient membrane distillation through localized heating

Membrane-based desalination technologies are crucial for generating clean water, but are energy intensive. Localizing heat generation at the liquid-membrane interface could reduce energy consumption while maintaining sufficient mass flux. Here, a locally heated membrane distillation approach is analytically and experimentally evaluated. The measured evaporated mass flux matches the analytical limits considering diffusive and advective mass transfer across a heated porous membrane into stagnant air. Experimentally, thermally-stable silver membranes demonstrate similar mass flux compared to conventional polymer based membrane at the same surface temperature. At high membrane temperatures (∼80 °C), the locally heated membrane distillation demonstrates good efficiency with up to 75% reduction of energy compared to direct contact membrane distillation and airgap membrane destination. Efficiency can be further improved with better thermal design of the supporting structure and permeate heat recovery.

Quantifying the Trade-Offs between Energy Consumption and Salt Removal in Membrane-Free Cation Intercalation Desalination

Recent work has predicted that Na-ion batteries with symmetric electrodes can be used for desalination with low energy consumption and high desalination degree from brackish to seawater salinity level.1 This technology, referred to as cation intercalation desalination (CID), shows much promise, but its wide-scale application will be dependent on capital cost. Similar to other electrochemical desalination technologies,2 initial embodiments of the CID concept used ion-exchange membranes (IEMs) to separate desalinated and concentrated streams in electrodes.1,3,4 IEMs in capacitive deionization cells can consume 50% of their expense, and IEMs have a finite lifespan.5 One solution to these challenges is to replace IEMs with chemically inert separators. Since the 1920s, non-selective diaphragms have been used in brine electrolysis. Furthermore, environmental-friendly diaphragms have shown the property of limiting ionic transfer between electrodes. Building on these concepts, we consider a novel CID cell design that omits IEMs by the use porous, fluid-blocking diaphragms as economical and stable alternatives to IEMs. In this study we quantify how the loss of selectivity incurred by using a diaphragm impacts the resultant salt removal and energy consumption levels. We consider two different flow arrangements for these devices, with or without electrolyte recycling, among which we compare energy consumption and salt removal. We also explore the effect of the operating conditions using a reduced-order analytical model. With our analytical model we predict salt removal efficiency (SRE, the ratio of the amount of salt removed relative to that of a perfect diaphragm) and degree of desalination (DoD, defined as the fraction influent concentration removal as a percentage) as a function of two dimensionless numbers: (1) the Peclet number Pe (the ratio of advective mass transfer rate over diffusive mass transfer rate) and (2) the Damköhler number Da (the ratio of reaction rate to the diffusive mass transfer rate). Figures 1a and 1b show contours of SRE in the space of Pe and Da for the non-recycling and recycling cell, respectively. Based on the definition of Pe and Da, these results reveal the influence of flow rate and current density on SRE, thus, operating conditions can be selected to reach a specific efficiency. Using the optimized operating conditions determined from the analytical model, we simulate the two-dimensional charge transport processes within porous electrodes and the diaphragm as source water is desalinated. The two-dimensional model employed here is based on porous electrode theory. To produce 60% DoD and 90% SRE from influent with 500 mM NaCl we use the conditions marked by α and β in Figs. 1a and 1b, respectively, for our two-dimensional simulations: Da=6.31 and Pe=109 for the recycling cell and Da=6.31 and Pe=6.31 for the non-recycling cell. Simulations with these operating conditions and 90% SRE reveals that the diaphragm cell can desalinate 500 mol/m3 solution to 260 mol/m3 (with recycling) and 244 mol/m3 (without recycling) solution (see Figs. 1c and 1d). For these two cases both diaphragm cells consume approximately 4.7 kWh energy when 1 m3 of desalinated water is produced. Figure 1c shows effluent salt concentration as a function of time for the non-recycling cell. Effluent salinity within the negative electrode decreases to a steady value after the process starts For the recycling cell effluent salinity continuously decreases during the charging processes (Fig. 1d). Salt concentration is distributed differently within each cell depending on whether or not effluent recycling is used. This effect is shown in Fig. 1e, where the concentration distribution in the recycling cell (lower) is more uniform than that in the non-recycling cell (upper), because a faster flow speed is used in the recycling cell. The simulated DoD (50%) agrees well with the predicted 60% DoD, and hence the analytical model can be used as a tool for performance predictions. The CID cells simulated here using diaphragms demonstrate the potential to replace IEMs with diaphragms. These findings motivate the future application of membrane-less CID technology in ion separation and/or removal processes. Acknowledgements: We are grateful for funding and support from the College of Engineering and the Mechanical Science and Engineering Department at UIUC.

Heterogeneous oxidation of zinc vapor by steam and mixtures of steam and carbon dioxide

The kinetics of the heterogeneous oxidation of zinc vapor by water vapor were measured in a tube flow reactor for temperatures from 800 to 1100 K, zinc vapor partial pressures up to 0.39 atm, and water vapor partial pressures up to 1.0 atm. The results extend the prior data for oxidation of zinc by water vapor from zinc partial pressures on the order of 0.01 atm to higher values appropriate for fuel production via the Zn/ZnO thermochemical cycle. Measured oxidation rates span 10−7–10−5 mol cm−2 s−1. A second order, reversible reaction rate expression r″=kZn-H2OpZn(g)pH2O-pH2Keq is developed from regression of the data and a numerical model of advective and diffusive mass transfer. The kinetic parameter kZn-H2O is a non-monotonic function of temperature with a negative activation energy for temperatures between 800 and 1050 K, consistent with prior studies. In a second set of experiments, the rate of the heterogeneous oxidation of zinc vapor by mixtures of water vapor and carbon dioxide was measured. The product gas is hydrogen rich due to faster surface reaction kinetics for oxidation with water vapor than with carbon dioxide. We conclude that it is preferable to split water and carbon dioxide in separate reactors rather than co-produce H2 and CO in a single reactor for production of synthesis gas in the Zn/ZnO solar thermochemical redox cycle.

Modelling microbial transport in simulated low-grade heap bioleaching systems: The hydrodynamic dispersion model

The hydrodynamic model was developed to describe microbial growth kinetics within heap bioleaching systems. Microbial partitioning between the bulk flowing pregnant leach solution (PLS) and ore-associated phases that exist within the low-grade chalcopyrite ore bed, as a function of microbial transport between these identified phases, was investigated. Microbial transport between the bulk flowing PLS and ore-associated phases was postulated to be driven by the microbial concentration gradient between the phases, with advection and dispersion forces facilitating microbial colonisation of, and transport through, the ore bed. The population balance model (PBM) was incorporated into the hydrodynamic model to estimate mineral dissolution rates as a function of available surface area appropriately. Temporal and spatial variations in microbial concentration in the PLS and ore-associated phases are presented together with model predictions for overall ferrous and ferric iron concentrations, which account for iron concentrations in the bulk flowing PLS and that in the vicinity of the mineral surface. The model predictions for PLS and ore-associated microbial concentrations are validated with experimental data, demonstrating the improvement of this model over the previously presented ‘biomass model’. Based on Michaelis-Menten type kinetics, model-predicted true maximum specific growth rates for Acidithiobacillus ferrooxidans in the PLS and ore-associated phases were found to be 0.0004 and 0.019h−1, respectively. Estimated microbial attachment and detachment rates suggest that microbial growth is more prolific in the ore-associated phases with subsequent transport to the bulk flowing PLS. Sensitivity analysis of the hydrodynamic transport model to changes in the advection mass transfer coefficient, dispersion coefficient and inoculum size are discussed. For the current reactor configuration, increasing the irrigation rate from 2 to 2.5Lm−2h−1, i.e. increasing the advection mass transfer rate, resulted in a significant decrease in microbial retention within the ore bed.

Analytical solutions for a soil vapor extraction model that incorporates gas phase dispersion and molecular diffusion

To greatly simplify their solution, the equations describing radial advective/dispersive transport to an extraction well in a porous medium typically neglect molecular diffusion. While this simplification is appropriate to simulate transport in the saturated zone, it can result in significant errors when modeling gas phase transport in the vadose zone, as might be applied when simulating a soil vapor extraction (SVE) system to remediate vadose zone contamination. A new analytical solution for the equations describing radial gas phase transport of a sorbing contaminant to an extraction well is presented. The equations model advection, dispersion (including both mechanical dispersion and molecular diffusion), and rate-limited mass transfer of dissolved, separate phase, and sorbed contaminants into the gas phase. The model equations are analytically solved by using the Laplace transform with respect to time. The solutions are represented by confluent hypergeometric functions in the Laplace domain. The Laplace domain solutions are then evaluated using a numerical Laplace inversion algorithm. The solutions can be used to simulate the spatial distribution and the temporal evolution of contaminant concentrations during operation of a soil vapor extraction well. Results of model simulations show that the effect of gas phase molecular diffusion upon concentrations at the extraction well is relatively small, although the effect upon the distribution of concentrations in space is significant. This study provides a tool that can be useful in designing SVE remediation strategies, as well as verifying numerical models used to simulate SVE system performance.

The origin of contractional structures in extensional gneiss domes

Abstract The juxtaposition of domains of shortening and extension at different scales in orogens has fueled many debates about driving forces and tectonic interpretations, including timing of deformation. At the orogen scale, gravitational collapse and mass transfer from orogenic plateaux to forelands explain some of these juxtapositions. At a regional scale, structures in gneiss domes are commonly contractional yet are coeval with regional extension and denudation. Here we use three-dimensional numerical experiments to show that crustal flow in orogenic domains does not necessarily conform to plate motion. We document contractional crustal flow associated with the formation of a gneiss dome in an orogenic pull-apart setting where localized extension and crustal thinning focus the exhumation of deep crust. We show that the flow field results in a complex strain pattern in which an extensional strain regime that is collinear with the direction of plate motion is partitioned into the shallow crust, whereas contractional structures and fabrics at a high angle to the direction of imposed transport develop in the deep crust. Advective mass transfer across regions of contrasting yet coeval strain regimes leads to a polyphase tectonic history. We observe structural features remarkably similar to those documented in some natural gneiss domes such as the Montagne Noire, which developed in a dextral pull-apart domain at the southern margin of the French Massif Central.

Production of Reactive Oxygen Species in the Rhizosphere of a Spartina-Dominated Salt Marsh Systems

This paper reports the presence of a metastable mixture of Fe(II), O2, superoxide and hydrogen peroxide in sediment pore water in organic carbon-rich sediments in Spartina alterniflora-dominated salt marsh systems. Field measurements at two different estuarine sites in South Carolina (one heavily urbanized and a protected research reserve) showed a broad region of reactive oxygen species (ROS) production more than 15 cm below the sediment surface within and immediately adjacent to the rhizospheres of S. alterniflora. Dissolved Fe(II) was positively correlated with hydrogen peroxide indicating a possible abiotic pathway to ROS production (r 2 = 0.94). The null hypothesis was tested that Fe(II) inventories were maintained by protective ligands and thus unreactive with respect to O2 consumption and ROS production. The addition of an Fe-binding ligand, DTPA, resulted in rapid decline of ROS in pore water, indicating that Fe(II) was labile. The half-life of superoxide under the measured solution conditions was calculated and found to be less than a second. The combination of high lability and persistent ROS was interpreted to indicate a high rate of Fe(II) and O2 supply to the pore water. The 224Ra/228Th disequilibrium was measured to determine the potential for advective mass transfer of dissolved oxygen via pore water exchange. The estimated pore water exchange of 54 L m−2 day−1 was significant but could not support the measured production of ROS alone, the direct exchange of O2 from the S. alterniflora root system may have contributed significantly to ROS production in the sediments.

Mass transfer characteristics of the high temperature falling film evaporation over a vertical wall under countercurrent laminar airflow

The falling film evaporation with large heat fluxes often has high wall temperature. To investigate this evaporation mass transfer characteristics for the falling water film under countercurrent laminar airflow, those previous investigations about the water film flow to enhance heat transfer were carefully reviewed and these factors were expressed in an integrated form. Then, experimental studies were carried out. During data reduction, local data was used to obtain local Sherwood number and to reveal the characteristics of local Sherwood number along the flow direction. Results indicated that evaporation could be dramatically enhanced by the transient wavy interface and high advective mass transfer. Results also revealed that the evaporation by high temperature falling water film was obvious deviated from the laminar mass transfer. Finally, the experimental data was fitted to specify the expressed the correlation for evaporation mass transfer.

Importance of advective mass transfer and sediment surface area for streambed microbial communities

Summary Variability of flow and shear stress cause local sorting and a patchy distribution of grain size in streambed sediments. Sediment microbial communities can be affected by the resulting heterogeneity because grain size determines (i) the sediment surface area available for microbial colonisation and (ii) the hydraulic conductivity and hence advective mass transfer of solutes and redox partners. However, the relative influence of the two factors on sediment microbial communities is not clear. We performed an experiment in 12 indoor flumes with three types of sediment: gravel (2–8 mm), a sand–gravel mixture (25 and 75% by mass) and sand (0.2–0.8 mm) in four flumes each. The sediments differed in both advective mass transfer (gravel > sand–gravel mixture = sand) and surface area (gravel < sand–gravel mixture < sand). We added a microbial inoculum to each flume. After 40 days of colonisation, we collected sediment samples to characterise the sediment microbial community by determining the abundance of bacteria and algae, respiration rates and potential enzyme activities. The consistent pattern found across all microbial variables showed that grain size clearly influences sediment microbial communities through advective mass transfer, although an influence of sediment surface area cannot be ruled out: (i) all variables were highest in gravel when expressed per surface area but differed little between sand and the sand–gravel mixture and (ii) all variables were similarly high in gravel and sand or the sand–gravel mixture on a volumetric basis, in spite of a greatly reduced surface area in gravel. Overall, our findings suggest that grain‐size distribution in streams should be taken into account in analyses of streambed microbial communities, especially if it has an effect on advective mass transfer.

Non-Fickian Transport Under Heterogeneous Advection and Mobile-Immobile Mass Transfer

We study the combined impact of heterogeneous advection and mobile–immobile mass transfer on non-Fickian transport using the continuous-time random walk (CTRW) approach. The CTRW models solute transport in heterogeneous media as a random walk in space and time. Our study is based on a d-dimensional CTRW model that accounts for both heterogeneous advection and mass transfer between mobile and immobile zones, to which we also refer as solute trapping. The flow heterogeneity is mapped into the distribution of advective transition times over a characteristic heterogeneity scale. Mass transfer into immobile zones is quantified by a trapping rate and the distribution of particle return times. The total particle transition time over a characteristic heterogeneity scale then is given by the advective time and the sum of trapping times over the number of trapping events. We establish explicit integro-partial differential equations for the evolution of the concentration and discuss the relation to the multirate mass transfer approach, specifically the relation between the trapping time distribution and the memory function. We then analyze the signatures of anomalous transport due to advective heterogeneity and trapping in terms of spatial moments and first passage times or breakthrough curves. The behaviors for different disorder scenarios are analyzed analytically and through random walk particle tracking simulations. Assuming that advective mass transfer is faster than diffusive, we identify three regimes of distinct transport behaviors, which are separated by the characteristic trapping rate and trapping times. (1) At early times, we identify a pre-asymptotic time regime that is fully determined by advective heterogeneity and which is characterized by superlinear growth of longitudinal dispersion. (2) For longitudinal dispersion, we identify an intermediate regime of strong superlinear diffusion. This regime is determined by the combined effect of advective heterogeneity and trapping. (3) At larger time, the asymptotic trapping-driven regime shows the signatures of diffusion in immobile zones, which leads to both sub- and superlinear dispersion. These results shed some new light on the mechanism of non-Fickian transport and their manifestation in spatial and temporal solute distributions.

Leakage of central venous catheter locking fluid by hemodynamic transport.

Central venous catheters are often filled when not in use with an anticoagulating fluid, usually heparinized saline, known as the locking fluid. However, the use of the locking fluid is associated with known risks because of "leakage" of the lock. A new hypothesis is proposed here to explain the lock fluid leakage: that the leakage is due to advective and diffusive mass transfer by blood flow around the catheter tip in situ. On the basis of previous in vitro experiments, the leakage mechanism has been hypothesized to be fluid motion driven by buoyancy forces between the heavier blood and the lighter locking fluid. The current hypothesis is justified by a simple one-dimensional mass transfer model and more sophisticated three-dimensional computational hemodynamic simulations of an idealized catheter. The results predict an initial, fast (<10 seconds) advection-dominated phase, which may deplete up to 10% of the initial lock, followed by a slow diffusion-limited phase which predicts an additional 1-2% of leakage during a 48 hour period. The current results predict leakage rates that are more consistent with published in vivo data when compared with the buoyancy hypothesis predictions, which tend to grossly overestimate leakage rates.

How accurate is predictive modeling of groundwater transport? A case study of advection, macrodispersion, and diffusive mass transfer at the Forsmark site (Sweden)

[1] We study predictive modeling of groundwater transport that accounts for three mechanisms: mean advection, macrodispersion, and mass transfer. A general methodology is presented and applied to a series of nonsorbing tracer tests along multiple pathways on scales ranging from ca. 70 to 300 m, in a highly heterogeneous aquifer at Forsmark (Sweden). The mean water residence time cannot be predicted well using a simple water balance model. Longitudinal macrodispersivity λL (L) and a mass transfer parameter group ψ(1/T) are extrapolated from the control tracer experiments, to yield accurate predictions of tracer discharge, once the mean water residence time is constrained. A relatively simple modeling framework based on Fickian macrodispersion and diffusion seems to be adequate for reproducing the tracer discharge in this complex and highly heterogeneous porous media.