Reduction In Mass Flux Research Articles

Laboratory investigation of flux reduction from dense non-aqueous phase liquid (DNAPL) partial source zone remediation by enhanced dissolution

This study investigated the benefits of partial removal of dense nonaqueous phase liquid (DNAPL) source zones using enhanced dissolution in eight laboratory scale experiments. The benefits were assessed by characterizing the relationship between reductions in DNAPL mass and the corresponding reduction in contaminant mass flux. Four flushing agents were evaluated in eight controlled laboratory experiments to examine the effects of displacement fluid property contrasts and associated override and underride on contaminant flux reduction ( R j) vs. mass reduction ( R m) relationships ( R j( R m)): 1) 50% ethanol/50% water (less dense than water), 2) 40% ethyl-lactate/60% water (more dense than water), 3) 18% ethanol/26% ethyl-lactate/56% water (neutrally buoyant), and 4) 2% Tween-80 surfactant (also neutrally buoyant). For each DNAPL architecture evaluated, replicate experiments were conducted where source zone dissolution was conducted with a single flushing event to remove most of the DNAPL from the system, and with multiple shorter-duration floods to determine the path of the R j( R m) relationship. All of the single-flushing experiments exhibited similar R j( R m) relationships indicating that override and underride effects associated with cosolvents did not significantly affect the remediation performance of the agents. The R j( R m) relationship of the multiple injection experiments for the cosolvents with a density contrast with water tended to be less desirable in the sense that there was less R j for a given R m. UTCHEM simulations supported the observations from the laboratory experiments and demonstrated the capability of this model to predict R j( R m) relationships for non-uniformly distributed NAPL sources.

Mass-removal and mass-flux-reduction behavior for idealized source zones with hydraulically poorly-accessible immiscible liquid

A series of flow-cell experiments was conducted to investigate aqueous dissolution and mass-removal behavior for systems wherein immiscible liquid was non-uniformly distributed in physically heterogeneous source zones. The study focused specifically on characterizing the relationship between mass flux reduction and mass removal for systems for which immiscible liquid is poorly accessible to flowing water. Two idealized scenarios were examined, one wherein immiscible liquid at residual saturation exists within a lower-permeability unit residing in a higher-permeability matrix, and one wherein immiscible liquid at higher saturation (a pool) exists within a higher-permeability unit adjacent to a lower-permeability unit. The results showed that significant reductions in mass flux occurred at relatively moderate mass-removal fractions for all systems. Conversely, minimal mass flux reduction occurred until a relatively large fraction of mass (>80%) was removed for the control experiment, which was designed to exhibit ideal mass removal. In general, mass flux reduction was observed to follow an approximately one-to-one relationship with mass removal. Two methods for estimating mass-flux-reduction/mass-removal behavior, one based on system-indicator parameters (ganglia-to-pool ratio) and the other a simple mass-removal function, were used to evaluate the measured data. The results of this study illustrate the impact of poorly accessible immiscible liquid on mass-removal and mass-flux processes, and the difficulties posed for estimating mass-flux-reduction/mass-removal behavior.

A pragmatic approach for estimation of source-zone emissions at LNAPL contaminated sites

When considering natural attenuation as a remediation strategy at a site contaminated by a light non-aqueous phase liquid (LNAPL), it is important to consider the emission of contaminants from the source zone. A quantification of source-zone emissions is essential both for comparison with down-gradient mass fluxes to provide an estimate of fractional mass flux reduction, as well as for estimating the source lifetime. Because the spatial distribution of LNAPL at a field site is strongly dependent on both the spill circumstances and the heterogeneity of the geologic materials, which can be problematic for in-situ determination, alternative methods for estimating source-zone emissions are needed. In this work, a three-dimensional multiphase flow and transport modelling approach is used to investigate the relationship between the lateral extent of an LNAPL body and the emission of contaminants to groundwater at a contaminated site. For simulations involving an LNAPL release in an aquifer comprised of heterogeneous porosity and permeability distributions that were generated geostatistically, it is shown that a simple linear relationship exists between the lateral extent of the LNAPL body in the capillary fringe and the emission to the aqueous phase. The parameters describing the relationship are found to be linear functions of the groundwater flow velocity and the vertical infiltration rate. This site-specific relationship provides a simple method to estimate contaminant emissions to groundwater at LNAPL contaminated sites.

Source-zone characterization of a chlorinated-solvent contaminated Superfund site in Tucson, AZ

An extensive site-characterization project was conducted at a large chlorinated-solvent contaminated Superfund site in Tucson, AZ. The project consisted of several components, including traditional site-characterization activities, tracer tests, laboratory experiments conducted with core material collected from the site, and mathematical modeling. The primary focus of the work presented herein is the analysis of induced-gradient contaminant elution tests conducted in a source zone at the site, investigation of the potential occurrence of immiscible liquid in the saturated zone, characterization of the relationship between mass flux reduction and mass removal, and evaluation of the impact of source-zone management on site remediation. The results of the present study, along with those of prior work, indicate that immiscible liquid is likely present in the saturated zone at the site source zones. Extensive tailing and rebound was observed for the contaminant-elution tests, indicating nonideal transport and mass-transfer behavior. The elution data were analyzed with a source-zone-scale mathematical model, and the results indicated that nonideal immiscible-liquid dissolution was the primary cause of the observed behavior. The time-continuous relationship between mass flux reduction and mass removal associated with the plume-scale pump-and-treat operation exhibited an initial large drop in mass flux with minimal mass removed, followed by a period of minimal mass flux reduction and a second period of large reduction. This behavior reflects the impact of both source-zone and aqueous-plume mass removal dynamics. Ultimately, a greater than 90% reduction in mass flux was achieved for a mass removal of approximately 50%. The influence of source-zone management on site remediation was evaluated by conducting two predictive simulations, one for which the source zones were controlled and one for which they were not. A plume-scale model was used to simulate the composite contaminant concentrations associated with groundwater extracted with the pump-and-treat system, which were compared to measured data. The information generated from this study was used to enhance the site conceptual model, help optimize operation of the pump-and-treat system, and evaluate the utility of source-zone remediation.

Groundwater contaminant flux reduction resulting from nonaqueous phase liquid mass reduction

This work presents a stream tube–based analytical approach to evaluate reduction in groundwater contaminant flux resulting from partial mass reduction in a nonaqueous phase liquid (NAPL) source zone. The reduction in contaminant flux, Rj, discharged from the source zone is a remediation performance metric that has a direct effect on the fundamental drivers of remediation: protection of human health and the environment. Closed form expressions are provided for analyzing remediation performance under conditions of joint spatial variability of both groundwater flow and NAPL content. The performance measures derived here are expressed in terms of measurable parameters. Spatial variability is described within a Lagrangian framework where aquifer hydrodynamic heterogeneities are characterized using nonreactive travel time distributions, while NAPL spatial distribution heterogeneity can be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to evaluate the relationship between reduction in contaminant mass, Rm, and Rj. A portion of the contaminant mass in the source zone is assumed to be removed via in situ flushing remediation, with the initial and final conditions defined as steady state natural gradient groundwater flow through the contaminant source zone. The combined effects of aquifer and NAPL heterogeneities are shown to be captured in a single parameter, reactive travel time variability, which was determined to be the most important factor controlling the relationship between Rm and Rj. It is shown that as heterogeneity in aquifer properties and NAPL spatial distribution increases, less mass reduction is required to achieve a given flux reduction, although the overall source longevity increases. When rate‐limited dissolution is important, the efficiency of remediation, in terms of both mass and flux reduction, is reduced. However, at many field sites the combined effects of field‐scale heterogeneities and site aging will result in favorable relationships between mass reduction and flux reduction.

A Method for Pressure Calculation in Ball Valves Containing Bubbles

A method of analyzing bubbly flow in a ball valve in a hydraulic circuit is presented. The dynamics of a single bubble can be well described by a quasi-static approximation of the Rayleigh-Plesset equation. Hence the presence of bubbles in low volume fractions can be modeled through an effective compressibility of the flow, which is easy to implement in commercial CFD packages. In the sample valve, a volume fraction of 4% air bubbles results in a mass flux reduction of up to 10%, as the bubbles expand due to the pressure drop in the valve and partly block it.

An analytical quantification of mass fluxes and natural attenuation rate constants at a former gasworks site

A new integral groundwater investigation approach was used for the first time to quantify natural attenuation rates at field scale. In this approach, pumping wells positioned along two control planes were operated at distances of 140 and 280 m downstream of a contaminant source zone at a former gasworks site polluted with BTEX- (benzene, toluene, ethyl-benzene, o-, p-xylene) and PAH- (polycyclic aromatic hydrocarbons) compounds. Based on the quantified changes in total contaminant mass fluxes between the control planes, first-order natural attenuation rate constants could be estimated. For BTEX-compounds, these ranged from 1.4e−02 to 1.3e−01 day −1, whereas for PAH-compounds natural attenuation rate constants of 3.7e−04 to 3.1e−02 day −1 were observed. Microbial degradation activity at the site was indicated by an increase in dissolved iron mass flux and a reduction in sulphate mass flux between the two investigated control planes. In addition to information about total contaminant mass fluxes and average concentrations, an analysis of the concentration–time series measured at the control planes also allowed to semi-quantitatively delineate the aquifer regions most likely contaminated by the BTEX- and PAH-compounds.

High-curvature outwardly propagating flames

A model describing the evolution of laminar, high-curvature outwardly propagating flames is presented. The model, valid in the limit of large chemical activation energies, can be useful for simulations of the evolution of flame kernels shortly after ignition when flame curvature is high and significantly modifies the local normal burning speed of the flame. Profiles of temperature across the flame as a function of flame radius are presented. The model predicts are reduction in the outward propagation rate of the flame in proportion to the reduction of reactant mass flux across the flame resulting from flame curvature. The burning speed of the flame is similarly reduced as well.

Ozone mass exchange between the stratosphere and troposphere for background and volcanic sulfate aerosol conditions

A three‐dimensional global chemical/dynamical model is used to study ozone mass exchange between the stratosphere and the troposphere. The ozone budget is calculated in the northern and southern extratropical lowermost stratosphere and in the tropical upper troposphere during both perturbed and background aerosol conditions. The stratospheric ozone mass flux into the troposphere is highest during the spring months in both hemispheres and higher in the northern hemisphere than in the southern hemisphere. During background aerosol conditions the net modeled stratospheric flux of ozone to the troposphere is 792 Tg/yr. The results suggest that the heterogeneous chemical reactions occurring on the surface of sulfate aerosols can have a significant impact on the ozone mass flux across the tropopause, leading to a possible 15% reduction in the annual global ozone mass transported from the stratosphere to the troposphere after a large volcanic eruption. Most of the reduction in ozone mass flux occurs in the northern hemisphere. In the southern hemisphere winter and spring the ozone concentration is already significantly perturbed by the heterogeneous reactions occurring on the surface of polar stratospheric clouds over Antarctica. The further increase in heterogeneous conversion due to the enhanced sulfate aerosol produces a smaller increase in active chlorine (ClO) than in the northern hemisphere, leading to a smaller increase in ozone destruction.

Heat and mass transfer in film absorption in the presence of non-absorbable gases

This paper is concerned with the effect of non-absorbable gases on the combined heat and mass transfer process in film absorption. Some simple models of the phenomenon have been proposed before which have under-predicted the mass flux. Here, a detailed theoretical model has been developed for the film, taking into consideration bulk movements in the vapor and liquid phases in the direction perpendicular to the film flow, which occur to compensate for the molal flux due to concentration gradients in the absorbent and vapor. The energy and diffusion equations were solved analytically for the entrance region in both the liquid and gas phases to give the temperature and concentration distributions, from which heat and mass fluxes may be determined. The reduction in mass flux due to non-absorbable gases is related to the need for the absorbate to diffuse through a thin layer of these gases blocking its access to the interface, before it can be absorbed. This reduction was found to depend most critically on the ratio of resistance to absorbate diffusion between the liquid and the gas. Even a small amount of non-absorbables can inhibit absorption significantly. Particular cases including isothermal absorption have been considered and the effect of the heat of absorption and other process parameters has been studied.

LAMINAR FREE CONVECTION CONDENSATION OF SATURATED VAPORS IN THE NEAR-CRITICAL REGION

A numerical analysis is presented for laminar free convection condensation of saturated vapors of water and carbon dioxide on a vertical flat surface in the reduced temperature range Ts, / Tc = 0.990−0.9998. The heat transfer coefficient can be correlated by the equation containing the correction for the convection and inertia terms in the case of constant physical properties, when the specific heat is evaluated using its average value over the surface temperature Tw to the saturation temperature Ts and the other physical properties are evaluated at the film temperature. The reduction of condensation mass flux or the heat flux at the vapor-liquid interface due to the convection term is expressed as the same function of the phase change number as the case of constant physical properties, when the specific heat is evaluated using Us average value over the temperature Tr = Tw + (2/3) (Ts−Tm)toTs.

Laminar Forced-Convection Condensation of Saturated Vapors in the Subcritical Region.

A numerical analysis is presented for laminar forced-convection condensation of saturated vapors of water and carbon dioxide on a flat surface in the reduced temperature range TS/TC=0.990∼0.999. The heat transfer coefficient in the region TS/TC<0.998 can be correlated by using Fujii and Uehara's equation for constant physical properties when the representative physical properties are evaluated at the film temperature. The reduction of condensation mass flux or the heat flux at the vapor-liquid interface due to the convection term in the film is expressed as a function of the phase change number with the average isobaric specific heat.

Use of swirl for flow control in propulsion nozzles

In certain off-design conditions it is desirable to vary the effective area of turbofan propulsion nozzles, e.g., to aid component matching or to reduce fan noise on approach. It has been suggested that the mass flux reductions associated with swirl could be used to achieve such variations. This paper addresses the questions of whether swirl can achieve the required mass flux reductions and whether this can be done with minimal thrust losses. A quasicylindrical theory is used to analyze the swirling flow, and comparisons are made with variablearea nozzles for specific case studies. For a low-specific thrust engine of high bypass ratio (BPR = 12) the required fan nozzle mass flux reduction cannot be achieved using swirl, and in any case, swirl gives a far faster rate of decay of thrust with fan nozzle mass flux than does a variable-area nozzle. For a ducted fan of BPR -4, a 10% reduction in core nozzle mass flux can be achieved with swirl for slightly less thrust loss than with a variable-area nozzle. The swirl profile, however, needs to be carefully optimized.