Circular rotation of different structures on natural convection of nanofluid-mobilized circular cylinder cavity saturated with a heterogeneous porous medium

The incompressible smoothed particle hydrodynamics (ISPH) method is utilized for studying the circular rotations of three different structures, circular cylinder, rectangle and triangle centered in a circular cylinder cavity occupied by A l 2 O 3 nanofluid. The novelty of this work is appearing in simulating the circular rotations of different solid structures on natural convection of a nanofluid-occupied a circular cylinder. The circular cylinder cavity is suspended by heterogeneous/homogeneous porous media. The embedded structures are taken as a circular cylinder, rectangle and triangle with equal areas. The first thermal condition considers the whole structure is heated, the second thermal condition considers the half of the structure is heated and the other is cooled and the third thermal condition considers the quarter of the structure is heated and the others are cooled. The outer boundary of cylinder cavity is cooled. Due to the small angular velocity ω = 3.15 (low rotational speeds), then the natural convection case will be considered only. The results are representing the temperature, velocity fields. The simulations revealed that the presence of the inner hot/cold structures affects on the velocity distributions and temperature field inside a circular cylinder cavity. The triangle shape has introduced the highest temperature distributions and maximum values of the velocity fields compare to other shapes inside a circular cylinder cavity. The homogeneous porous level reduces the maximum values of velocity field by 25% compared to the heterogeneous porous level.

Field investigation and numerical modelling of gas extraction in a heterogeneous landfill with high leachate level

A field gas extraction experiment is carried out at a high-kitchen food large-scale landfill site with high leachate level. The leachate level was decreased to improve the pumping efficiency. Considering the heterogeneity of the municipal solid waste (MSW), the pores in the unsaturated MSW are divided into matrix pores and fractures. A transient dual-porosity model was then developed to analyze the pumping test results. The first and second boundary conditions considering the effect of cover layers of landfills was involved. The results show that the gas flow rate can be increased by 14-37% due to the drawdown of the leachate level. Compared with the single pore model, the dual-porosity model can better predict the field results, indicating that the preferential flow in the landfill caused by the heterogeneity of MSWs is very important. As the pumping pressure increases by a factor of 5, the ratio of fractures to pores w_f can be decreased by a factor of 4.4. This may be due to the fact that the fractures will be compressed when the effective stress was increased as the negative pumping pressure was applied. The pumping pressure and the anisotropy value of the MSWs have the greater influence on the well radius of influence. The proposed model can be used for effective design of the field gas pumping experiments. The obtained gas generation rate, gas permeability of the dual porosity MSWs can be useful for gas transport analysis and gas pumping well design for the high-kitchen food content landfills.

Effects of different permeable lenses on nitrobenzene transport during air sparging remediation in heterogeneous porous media

Air sparging (AS) is considered an effective remediation technology for groundwater contaminated by volatile organic compounds. However, the effects of AS remediation of heterogeneous aquifers with lenses of different permeability are still unclear, which limits the application of AS technology. In this study, the effects of different permeable lenses on nitrobenzene (NB) transport were quantitatively analysed by tracking the temporal and spatial evolutions of the NB concentration and using light transmission visualisation technology to observe airflow. Experimental results showed that the NB outside the airflow zone of the heterogeneous aquifer containing a gravel lens was rapidly removed, which is a special phenomenon. Through moisture content monitoring and colour tracer technology, the bubble-induced water circulation zone in a gravel lens was discovered during AS. At this time, the zone of influence (ZOI) included air flow zone and water circulation zone, while previous studies believed that the ZOI only contained the air flow zone. The presence of water circulation zone in the heterogeneous aquifer with a gravel lens increased the ZOI area and average contaminant removal flux by 5 and 2.3 times, respectively, compared with those in homogeneous aquifer. These findings have modified the conventional cognition about the ZOI and are conducive to an in-depth understanding of the remediation mechanisms and a better design of AS technology in heterogeneous aquifers with different permeable lenses.

On the interplay between electromigration and electroosmosis during electrokinetic transport in heterogeneous porous media

Electrokinetic techniques represent a valuable approach to enhance solute transport, reactant delivery and contaminant degradation in complex environmental matrices, such as contaminated soil and groundwater, and have a great potential for the remediation of many organic and inorganic pollutants. This study investigates the complex interplay between the key electrokinetic transport mechanisms, electromigration and electroosmosis, in physically heterogeneous porous media and its impact on tracer distribution, reactant mixing and degradation efficiency. We perform experiments in a multidimensional setup, considering different types of heterogeneities, injected tracers and reactants, as well as background electrolyte pore water with different chemical composition and pH. We show that EK transport is significantly affected by the physical heterogeneities, due to the interaction between electrokinetic and hydraulic processes, and by the pore water chemistry that plays a key role on the magnitude and spatial distribution of electroosmotic fluxes. The latter affect the overall transport of charged and non-charged species, including the migration velocity of injected plumes, their spatial patterns, spreading and mixing with the background groundwater, and the extent of degradation and the spatio-temporal evolution of reactive zones in the heterogeneous porous media. Process-based numerical modeling allowed us to interpret the experimental observations and to disentangle the coupled effects of physical, chemical and electrostatic processes in the multidimensional, heterogeneous setups. Besides elucidating the mechanisms controlling electrokinetic transport, the results of this study have also important implications for practical field implementation of EK approaches in intrinsically heterogeneous subsurface systems.

Effects of air flowrate distribution and benzene removal in heterogeneous porous media during air sparging remediation

The decrease of remediation effect during air sparging (AS) remediation in heterogeneous porous media has attracted increasing attention. In this study, an improved light transmission visualization method was used to investigate the air accumulation, migration and flowrate distribution in benzene-contaminated heterogeneous porous media during AS. Experimental results indicated that the benzene removal rate in the porous media was mainly controlled by air flowrate distribution which could be used as a major factor to evaluate the remediation effect. Visualization of air migration showed that air accumulation occurred below the geologic heterogeneous interface when ΔPe > 0 kPa (ΔPe: the air entry pressure difference of the media above and below the interface), and the accumulation thickness and length presented exponential decay increases with increasing ΔPe and air injection rates. Air flowrate was monitored by gas flow sensors, and the flowrate distributions were found as Gaussian distribution when ΔPe ≤ 0 kPa, trapezoidal distribution when 0 <ΔPe< 0.3 kPa and fingered distribution when ΔPe ≥ 0.3 kPa. Fingered distribution of air flowrate resulted in extremely nonuniform benzene removal above the interface and reduced the overall benzene removal rate. These findings reveal the reasons for the poor performance of AS remediation in heterogeneous porous media, leading to a better understanding of the remediation mechanisms in heterogeneous aquifer.

Surface heterogeneity mediated transport of hydrochar nanoparticles in heterogeneous porous media

The effects of clay particles (montmorillonite, M) and phosphate (P) on the transport of hydrochar nanoparticles (NPs) in water-saturated porous media (uncoated and aluminum (Al) oxide-coated sands) were explored in NaCl (1-50 mM) solutions. Our results showed that the deposition behaviors of hydrochar NPs affected by M and phosphate were significantly different between pH 6.0 and pH 9.0, especially in Al oxide-coated sand. This can be attributed to their distinct surface characteristics: hydrochar agglomerates with a larger pore size distribution, more carboxylate groups, and less negative charges on the surface at pH 9.0 than those at pH 6.0. In Al oxide-coated sand, block adsorption of hydrochar was alleviated appreciably with the presence of M due to the preferential preoccupies of M on these favorable retention sites. On the contrary, M substantially increased the hydrochar retention on uncoated sand due to the formation of nanoaggregates between hydrochar and M. Differently, phosphate substantially enhanced the transport of hydrochar, even in coated sand, due to the strong phosphate adsorption onto Al oxide on the surface of sand and hydrochar. Our findings will provide useful insights into designing effective strategies for land application of hydrochar while minimizing potential environmental risks. Graphical abstract.

Experimentally based pore network modeling of NAPL dissolution process in heterogeneous porous media

Practical designs of non-aqueous phase liquids (NAPLs) remediation strategies require reliable modeling of interphase mass transfer to predict the retraction of NAPL during processes such as dissolution. In this work, the dissolution process of NAPL during two-phase flow in heterogeneous porous media is studied using pore-network modeling and micromodel experiments. A new physical-experimental approach is proposed to enhance the prediction of the dissolution process during modeling of interphase mass transfer. In this regard, the normalized average resident solute concentration is evaluated for describing the dissolution process at pore-level. To incorporate the effect of medium heterogeneities, a new experimental factor is considered for enhancing corner diffusion modeling. In addition, capillary desaturation curves (CDCs) are predicted during hydraulic flow modeling to estimate initial residual NAPL saturation. The developed network model can predict residual NAPL saturations and mass transfer rate coefficient for a NAPL-water system at different injection rates and fluid saturations. The evaluated mass transfer rate coefficients using the proposed physical-experimental approach show a significant improvement compared to either mechanistic or empirical methods. The proposed approach in this study can be attractive for possible applications in commercial simulators of contaminant transport in porous media.

Pore-scale simulations of concentration tails in heterogeneous porous media

The retention of contaminants in the finest and less-conductive regions of natural aquifer is known to strongly affect the decontamination of polluted aquifers. In fact, contaminant transfer from low to high mobility regions at the back end of a contaminant plume (i.e. back diffusion) is responsible for the long-term release of contaminants during remediation operation. In this paper, we perform pore-scale calculations for the transport of contaminant through heterogeneous porous media composed of low and high mobility regions with two objectives: (i) study the effect of permeability contrast and solute transport conditions on the exchange of solutes between mobile and immobile regions and (ii) estimate the mass of contaminants sequestered in low mobility regions based on concentration breakthrough curves.

Stochastic analytical modeling of the biodegradation of steady plumes

We present a stochastic analytical framework to assess the contaminant concentration of a steady plume undergoing biodegradation. The method is focused on heterogeneous formations, and it embeds both fringe and core degradation. The Lagrangian concentration approach of Fiori (2001) was employed, which is suited for describing the interplay between the large scale advection caused by heterogeneity and the local dispersion processes. The principal scope of the model is to provide a relatively simple tool for a quick assessment of the contamination level in aquifers, as function of a few relevant, physically based dimensionless parameters. The solution of the analytical model is relatively simple and generalizes previous approaches developed for homogeneous formations. It is found that heterogeneity generally enhances mixing and degradation; in fact, the plume shear and distortion operated by the complex, heterogeneous velocity field facilitates local dispersion in diluting the contaminant and mixing it with the electron acceptor. The decay of the electron donor concentration, and so the plume length, is proportional to the transverse pore-scale dispersivity, which is indeed the parameter ruling mixing and hence degradation. While the theoretical plume length is controlled by the fringe processes, the core degradation may determine a significant decay of concentration along the mean flow direction, thus affecting the length of the plume. The method is applied to the crude oil contamination event at the Bemijdi site, Minnesota (USA).