Role of NAPL Thermal Properties in the Effectiveness of Hot Water Flooding

Hot water flooding is a thermal nonaqueous phase liquid (NAPL) recovery technology originally developed in the petroleum industry that has recently been proposed for enhanced recovery of NAPLs in the contaminated subsurface. This technology, however, has received relatively little laboratory or numerical modeling investigation in the contaminant hydrology community. In this study the utility of flooding NAPL contaminated source zones at elevated water temperatures was investigated. Simulations were conducted using 16 different geostatistical representations of an actual field site. Two NAPLs were selected for this study—a light NAPL with hydraulic properties that have moderate temperature dependencies and a dense NAPL with significant viscosity temperature dependency. For these two NAPLs, flooding the source zone with water at elevated temperatures resulted in enhanced NAPL recovery. However, injection of hot water also resulted in accelerated downward movement of coal tar DNAPL due to the reduced viscosity at elevated temperatures. NAPL recovery was also dependent on the source zone architecture with greater NAPL mass recovery when the NAPL was localized in a small volume at high saturations. These results suggest that hot water flooding can significantly speed up the recovery of viscous NAPLs and, as such, is a powerful technique for the remediation of viscous NAPLs.     

Effects of NAPL Contaminants on the Permeability of a Soil‐Bentonite Slurry Wall Material

Soilbentonite slurry walls are designed to inhibit the subsurface movement of contaminants from hazardous waste sites. Although it is generally accepted that high concentrations of organic compounds will adversely affect soilbentonite slurry walls and clay liners, previous research investigating the effects of NAPLs on the conductivity of clay wall materials has been inconclusive. In this study the effects of various organics (benzene, aniline, trichloroethylene, ethylene dichloride, methylene chloride) on the effective conductivity of a typical soilbentonite slurry wall material were studied under two effective stress conditions, 200 and 52kPa. The hydraulic conductivity for the soilbentonite material permeated with water averaged 1.52×10^-8cms^-1. Compared to water, there was little change in conductivity when the sample was permeated with a solution containing a NAPL compound at its solubility limit, except for aniline. However, there was a one to two order of magnitude decrease in conductivity when the sample was permeated with a pure NAPL for all NAPLs tested. When the soilbentonite material was permeated with a water/NAPL/water/NAPL sequence, the conductivity decreased one to two orders of magnitude when a NAPL was introduced following water; however, when water was reintroduced after the NAPL, the conductivity increased to the initial hydraulic conductivity. The conductivity again decreased one to two orders of magnitude when the NAPL was reintroduced. This trend occurred for all NAPLs tested, and the fluid properties of the NAPL compounds alone did not account for the decrease in conductivity compared to water.     

Determination of thermal conductivities of Sn–Zn lead-free solder alloys with radial heat flow and Bridgman-type apparatus

The variations of thermal conductivities of solid phases versus temperature for pure Sn, pure Zn and Sn–9 wt.% Zn, Sn–14 wt.% Zn, Sn–50 wt.% Zn, Sn–80 wt.% Zn binary alloys were measured with a radial heat flow apparatus. The thermal conductivity ratios of liquid phase to solid phase for the pure Sn, pure Zn and eutectic Sn–9 wt.% Zn alloy at their melting temperature are found with a Bridgman-type directional solidification apparatus. Thus, the thermal conductivities of liquid phases for pure Sn, pure Zn and eutectic Sn–9 wt.% Zn binary alloy at their melting temperature were evaluated by using the values of solid phase thermal conductivities and the thermal conductivity ratios of liquid phase to solid phase.     

Pore-Scale Analysis of NAPL Blob Dissolution and Mobilization in Porous Media

A pore-scale analysis of nonaqueous phase liquid (NAPL) blob dissolution and mobilization in porous media was presented. Dissolution kinetics of residual NAPLs in an otherwise water-saturated porous medium was investigated by conducting micromodel experiments. Changes in residual NAPL volume were measured from recorded video images to calculate the mass transfer coefficient, K and the lumped mass transfer rate coefficient, k. The morphological characteristics of the blobs such as specific and intrinsic area were found to be independent of water flow rate except at NAPL saturations below 2%. Dissolution process was also investigated by separating the mass transfer into zones of mobile and immobile water. The fractions of total residual NAPL perimeters in contact with mobile water and immobile water were measured and their relationship to the mass transfer coefficient was discussed. In general, residual NAPLs are removed by dissolution and mobilization. Although these two mechanisms were studied individually by others, their simultaneous occurrence was not considered. Therefore, in this study, mobilization of dissolving NAPL blobs was investigated by an analysis of the forces acting on a trapped NAPL blob. A dimensional analysis was performed to quantify the residual blob mobilization in terms of dimensionless Capillary number (Ca _I). If Ca _I is equal to or greater than the trapping number defined as , then blob mobilization is expected.     

A Fickian diffusion model for the spreading of liquid plumes infiltrating in heterogeneous media

Infiltration of water and non-aqueous phase liquids (NAPLs) in the vadose zone gives rise to complex two- and three-phase immiscible displacement processes. Physical and numerical experiments have shown that ever-present small-scale heterogeneities will cause a lateral broadening of the descending liquid plumes. This behavior of liquid plumes infiltrating in the vadose zone may be similar to the familiar transversal dispersion of solute plumes in single-phase flow. Noting this analogy we introduce a mathematical model for ‘phase dispersion’ in multiphase flow as a Fickian diffusion process. It is shown that the driving force for phase dispersion is the gradient of relative permeability, and that addition of a phase-dispersive term to the governing equations for multiphase flow is equivalent to an effective capillary pressure which is proportional to the logarithm of the relative permeability of the infiltrating liquid phase. The relationship between heterogeneity-induced phase dispersion and capillary and numerical dispersion effects is established. High-resolution numerical simulation experiments in heterogeneous media show that plume spreading tends to be diffusive, supporting the proposed convection-dispersion model. Finite difference discretization of the phase-dispersive flux is discussed, and an illustrative application to NAPL infiltration from a localized source is presented. It is found that a small amount of phase dispersion can completely alter the behavior of an infiltrating NAPL plume, and that neglect of phase-dispersive processes may lead to unrealistic predictions of NAPL behavior in the vadose zone.     

Eutectic solidification patterns: Interest of microgravity environment

The solidification of binary eutectic alloys produces two-phase composite materials in which the microstructure, that is, the geometrical distribution of the two solid phases, results from complex pattern-formation processes at the moving solid–liquid interface. Since the volume fraction of the two solids depends on the local composition, solidification dynamics can be strongly influenced by thermosolutal convection in the liquid. In this contribution, we review our experimental and numerical work devoted to the understanding of eutectic solidification under purely diffusive conditions, which will soon be tested and extended during the microgravity experiment TRANSPARENT ALLOYS planned by the European Space Agency (ESA).     

Nonaqueous Phase Liquid Dissolution in Porous Media: Current State of Knowledge and Research Needs

Our understanding of nonaqueous phase liquid (NAPL) dissolution in the subsurface environment has been increasing rapidly over the past decade. This knowledge has provided the basis for recent developments in the area of NAPL recovery, including cosolvent and surfactant flushing. Despite these advances toward feasible remediation technologies, there remain a number of unresolved issues to motivate environmental researchers in this area. For example, the lack of an effective NAPLlocation methodology precludes effective deployment of NAPL recovery technologies. The objectives of this paper are to critically review the state of knowledge in the area of stationary NAPL dissolution in porous media and to identify specific research needs. The review first compares NAPL dissolutionbased mass transfer correlations reported for environmental systems with more fundamental results from the literature involving model systems. This comparison suggests that our current understanding of NAPL dissolution in smallscale (on the order of cm) systems is reasonably consistent with fundamental mass transfer theory. The discussion then expands to encompass several issues currently under investigation in NAPL dissolution research, including: characterizing NAPL morphology (i.e. effective size and surface area); multicomponent mixtures; scale-related issues (dispersion, flow by-passing); locating NAPL in the subsurface and enhanced NAPL recovery. Research needs and potential approaches are discussed throughout the paper. This review supports the following conclusions: (1) Our knowledge related to local dissolution and remediation issues is maturing, but should be brought to closure with respect to the link between NAPL emplacement theory (as it impacts NAPL morphology) and NAPL dissolution; (2) The role of nonideal NAPL mixtures, and intra-NAPL mass transfer processes must be clarified; (3) Valid models for quantifying and designing NAPL recovery schemes with chemical additives need to be refined with respect to chemical equilibria, mass transfer and chemical delivery issues; (4) Computational and large-scale experimental studies should begin to address parameter up-scaling issues in support of model application at the field scale; and (5) Inverse modeling efforts aimed at exploiting the previous developments should be expanded to support field-scale characterization of NAPL location and strength as a dissolving source.     

Three-dimensional modelling of steam flush for DNAPL site remediation

Numerical simulation of steam flush for clean-up of non-aqueous phase liquid (NAPL) contaminated groundwater sites involves solution of the multiphase, multicomponent subsurface flow equations. This paper describes techniques for discretizing problems with horizontal wells in a three-dimensiontetrahedral mesh. The effectiveness of non-linear flux limiters for reducing numerical dispersion is discussed. Primary variable selection and thermodynamic state transition rules will also be compared. Some example results for several steam flush scenarios will be presented.     

Significance of Mass–Concentration Relation on the Contaminant Source Depletion in the Nonaqueous Phase Liquid (NAPL) Contaminated Zone

This study develops a new numerical model adopting a generic relation between the nonaqueous phase liquid (NAPL) mass and aqueous-phase NAPL concentration to simulate the relationship between NAPL contaminant mass discharge and contaminant mass reduction in the source zone, which plays a critical role to assist with site management decisions on contaminated zone remediation. The model can accommodate any contaminant mass and concentration relations and applicable to the situations when groundwater flowrate in the NAPL source zone varies in any form temporally. Therefore, the combined effects of mass–concentration relation and groundwater flowrate variations can be examined. It is hypothesized that the NAPL mass–concentration relations reflect the spatial variability of porous media in the subsurface. The developed model is compared with results from field monitoring sites and found to exhibit high flexibility and capability in capturing the observed complex NAPL source zone dynamics. Using six contaminant mass and concentration functions of varying shapes, we show that contaminant mass and concentration relation has pronounced effects on contaminant mass discharge dynamics in addition to the groundwater flowrate temporal variations. In general, the coupled mass–concentration relation and groundwater flowrate variations demonstrate stronger capability in capturing the NAPL source zone dynamics under a wide range of field porous medium conditions reported in the literature than the models that only consider groundwater flux variations. In particular, the concave mass–concentration models can be used in less heterogeneous porous media, while the convex mass–concentration models are more appropriate in more heterogeneous site conditions.

     

Heat and mass transfer during solidification of a binary solution from a horizontal plate

This study deals with the experimental analysis of double diffusive effects on the solidification process of a binary mixture. Solidification experiments from a horizontal plate heat exchanger are performed in a cavity filled with a hypoeutectic NH₄Cl–H₂O solution. Solute redistribution at the interface creates concentration gradients in the fluid phase and a dendritic front growth is expected. We analyze the influence of solute rejection on the equilibrium conditions at the solid–liquid interface, on the solidification front kinetics and on the solid structure. Local front temperature measurements and estimation of the solid fraction of the growing solid allow us to quantitatively analyze the coupled influence of solute rejection and solid fraction on the front growth. A simplified numerical analysis shows that the role played by these two phenomena is very weak in the range of parameters under study.     

Regression and Dimensional Analysis for Modeling Two‐Phase Flow

Numerical models describing multiphase flow phenomena are typically used to predict the displacement of water during the infiltration of nonaqueous phase liquids (NAPLs) into a groundwater system. In this paper, the applicability of regression and dimensional analysis to develop simple tools to bypass these time consuming numerical simulations is assessed. In particular, the infiltration of NAPL through a vertical, homogeneous soil column initially saturated with water is quantified. Two output variables defining the extent of infiltration were considered – the elevation of the NAPL front and the volume of NAPL which had entered the system. Dimensional analysis was initially performed to identify dimensionless terms associated with the underlying relations between these two output variables and the input variables (independent variables and system parameters). Artificial neural network techniques were then employed to develop regression equations for approximating the input–output relationships over a given domain. Application of these equations illustrated the interrelationships among capillary, buoyancy, and viscous forces driving the NAPL infiltration process.     

液粒凝固过程的动力学理论

在平均场的概念下对液态粒子的凝固过程提出了一个简化的液-固-气-雾(LSGF)数学模型,并在小过冷度的条件下,求出了有关初值问题的一致有效渐近解. 结果表明:整个动力学过程可以分为两个相互联结的时间阶段. (1) 液粒初始温度分布的瞬态过渡阶段. 在这个阶段,凝固尚未正式启动,只是系统内的温度从任意给定的初始分布迅速调整到某一特定空间分布. (2)液粒向固粒转变阶段. 在这一阶段,液-固两相开始分离,相界面逐渐向液粒中心传播,直至液相完全消失. 进而以铜为例,讨论了液态粒子在不同生长条件与物理参数下的凝固时间与凝固过程中的温度分布的演化规律.      

A two-fluid model for reactive dilute solid–liquid mixtures with phase changes

Based on the Eulerian spatial averaging theory and the Müller–Liu entropy principle, a two-fluid model for reactive dilute solid–liquid mixtures is presented. Initially, some averaging theorems and properties of average quantities are discussed and, then, averaged balance equations including interfacial source terms are postulated. Moreover, constitutive equations are proposed for a reactive dilute solid–liquid mixture, where the formation of the solid phase is due to a precipitation chemical reaction that involves ions dissolved in the liquid phase. To this end, principles of constitutive theory are used to propose linearized constitutive equations that account for diffusion, heat conduction, viscous and drag effects, and interfacial deformations. A particularity of the model is that the mass interfacial source term is regarded as an independent constitutive variable. The obtained results show that the inclusion of the mass interfacial source term into the set of independent constitutive variables permits to easily describe the phase changes associated with precipitation chemical reactions.     

Hydrodynamics in a new liquid–solid circulating conventional fluidized bed

A new type of liquid–solid fluidized bed, named circulating conventional fluidized bed (CCFB) which operates below particle terminal velocity was proposed and experimentally studied. The hydrodynamic behavior was systematically studied in a liquid–solid CCFB of 0.032 m I.D. and 4.5 m in height with five different types of particles. Liquid–solid fluidization with external particle circulation was experimentally realized below the particle terminal velocity. The axial distribution of local solids holdup was obtained and found to be fairly uniform in a wide range of liquid velocities and solids circulation rates. The average solids holdup is found to be significantly increased compared with conventional fluidization at similar conditions. The effect of particle properties and operating conditions on bed behavior was investigated as well. Results show that particles with higher terminal velocity have higher average solids holdup.     

Laboratory,Field and Modeling Studies of Radon‐222 as a Natural Tracer for Monitoring NAPL Contamination

The recently developed natural radon tracer method has potential as a rapid, lowcost, nondestructive, and noninvasive method for quantifying NAPL contamination. In the subsurface, radon222 (radon) is produced by the decay of naturally occurring radium226 contained in the mineral fraction of aquifer solids. In groundwater radon occurs as a dissolved gas, with a halflife of 3.83 days. In the absence of NAPL, the radon concentration in groundwater quickly reaches a maximum value that is determined by the mineral composition of the aquifer solids, which controls the rate of radon emanation. In the presence of NAPL, however, the radon concentration in the groundwater is substantially reduced due to the preferential partitioning of radon into the organic NAPL phase. A simple equilibrium model and supporting laboratory studies show the reduction in radon concentration can be quantitatively correlated with residual NAPL saturation. Thus, by measuring the spatial distribution in radon it may be possible to identify locations where residual NAPL is present and to quantify the NAPL saturation. When the basic processes of partitioning, radon emanation from the aquifer solids, and firstorder decay are incorporated into an advective/dispersive transport model, good agreement is obtained with the results of laboratory and field experiments. Model sensitivity analyses shows many factors can contribute to the radon concentration response, including the length of the NAPL zone, NAPL saturation, groundwater velocity, porosity, and radon emanation. Thus, care must be taken when applying the radon method to locate and quantify NAPL contamination in the subsurface.     

Longitudinal instability of slurry pipeline flow

This paper deals with the flow of solid/liquid mixtures through long-distance pipelines. Such flows can be destabilized by the formation of local plugs which may impede or even block the flow. Plugs may develop at the interface between regions of different mean concentration. The driving force for the development of such plugs is the existence of local gradients of the axial flux of solids.A mathematical model is developed which describes this mode of plug formation in slurry pipelines. Several assumptions and approximations enable us to reduce the 3D continuity equation of the solid particles to an effective 1D-equation that contains a concentration-dependent flux function. The latter equation is solved numerically.Illustrative calculations lead to the conclusion that the accumulation of material in a plug does not continue without limit but instead levels off at values that are pumpable under most practical conditions, provided that a certain margin of overdesign is in place.     

Measurement of the surface concentration (liquid) of an evaporating multicomponent droplet using pendant droplet method

The surface concentration on the liquid side of the interface of an evaporating multicomponent droplet could be different from the bulk concentration. In this work, surface tension is used as a means to measure surface concentration of an evaporating multicomponent droplet. Surface tension is measured using pendant droplet method that relies on the best fit between theoretical and experimental drop profiles. Surface tension is a surface property, and it exhibits a dependence on concentration. Hence, it is an ideal candidate to track the variation of surface concentration during the evaporation of a multicomponent droplet. This method is used to study the evaporation of ethanol–water and methanol–water droplets. The correctness and applicability of this technique are critically assessed, and important observations are made for single droplet evaporation for these binary mixtures.     

Theoretical and experimental study of the isothermal mechanical behaviour of alloys in the semi-solid state

An isothermal constitutive model for semi-solid alloys based on the concepts of mechanics of continuous media and the theory of mixtures is presented. The model is applicable to semi-solid states obtained either by solidification from liquid state or partial remelting from solid state in which each of the solid and the liquid phases is contiguous. During deformation their behaviours are coupled: the densification of the solid matrix considered as a porous viscoplastic medium saturated with a liquid drives the fluid flow behaviour, and the resulting pressure distribution in the liquid affects in turn the stresses and the densification of the solid. The identification procedure of the model uses two types of mechanical tests: uniaxial compression and drained die pressing (filtration) carried out with A356 alloy. The identification results are then validated using drained triaxial compression.     

Quadratic programming algorithm for wall slip and free boundary pressure condition

Wall slip is often observed in a highly sheared fluid film in a solid gap. This makes a difficulty in mathematical analysis for the hydrodynamic effect because fluid velocity at the liquid–solid interfaces is not known a priori. If the gap has a convergent–divergent wedge, a free boundary pressure condition, i.e. Reynolds pressure boundary condition, is usually used in the outlet zone in numerical solution. This paper, based on finite element method and parametric quadratic programming technique, gives a numerical solution technique for a coupled boundary non‐linearity of wall slip and free boundary pressure condition. It is found that the numerical error decreases with the number of elements in a negative power law having an index larger than 2. Our method does not need an iterative process and can simultaneously gives rise to fluid film pressure distribution, wall slip velocity and surface shear stress. Wall slip always decreases the hydrodynamic pressure. Large wall slip even causes a null hydrodynamic pressure in a pure sliding solid gap. Copyright © 2005 John Wiley & Sons, Ltd.     

A simple phase transition relaxation solver for liquid–vapor flows

Determining liquid–vapor phase equilibrium is often required in multiphase flow computations. Existing equilibrium solvers are either accurate but computationally expensive or cheap but inaccurate. The present paper aims at building a fast and accurate specific phase equilibrium solver, specifically devoted to unsteady multiphase flow computations. Moreover, the solver is efficient at phase diagram bounds, where non‐equilibrium pure liquid and pure gas are present. It is systematically validated against solutions based on an accurate (but expensive) solver. Its capability to deal with cavitating, evaporating, and condensing two‐phase flows is highlighted on severe test problems both 1D and 2D. Copyright © 2016 John Wiley & Sons, Ltd.