Front Propagation Rate Research Articles

Infiltration into Two-Layered Soil: The Green–Ampt and Averyanov Models Revisited

Infiltration into a two-layered soil (continuum scale) is studied by the Green–Ampt (GA) model where the rate of infiltration (wetting front propagation) is a non-monotonic and discontinuous function of time. Both a constant ponding depth and a ponding level falling due to evaporation and infiltration are studied theoretically and in column experiments. The triads of saturated conductivity, porosity and front pressure in the two zones, as well as gravity, control vertical infiltration. Another model on the scale of a straight cylindrical capillary uses the Averyanov regime of a film flow, modified to include a front which is a quarter of a torus, rather than a spherical cap in a standard Washburn–Lukas model. The rate of front propagation is controlled by the Newtonian viscosity, tube-film sizes, surface tension, contact angle and gravity. For both scales, Cauchy problems for ODEs with respect to the front position are solved by computer algebra integration. In the field, pedon description and textural analysis of sedimentation-controlled layering are done. Of particular note is the thick cake deposited from a flash flood water pulse, which has very low permeability and can greatly inhibit infiltration. The infiltration rates measured by tension infiltrometers fluctuate, which is attributed to the mosaic and layered special distribution of hydraulic and capillary properties of the accrued sediments. Falling ponding depth laboratory experiments were conducted in four columns with a coarse substratum (sand) subjacent to a layer of thoroughly homogenized repacked dam silt. Imbibition front stopped at the interface between two texturally contrasting strata and the “waiting time” of the capillary barrier breakthrough was measured, as well as further oscillations of the infiltrations rate.

Self-sustaining smouldering combustion of coal tar for the remediation of contaminated sand: Two-dimensional experiments and computational simulations

This study presents the development and validation of a computational model which simulates the propagation of a smouldering front through a porous medium against unique experiments in coal tar and sand. The model couples a multiphase flow solver in porous media with a perimeter expansion module based on Huygens principle to predict the spread. A suite of two-dimensional experiments using coal tar-contaminated sand were conducted to explore the time-dependent vertical and lateral smouldering front propagation rates and final extent of remediation as a function of air injection rate. A thermal severity analysis revealed, for the first time, the temperature–time relationship indicative of coal tar combustion. The model, calibrated to the base case experiment, then correctly predicts the remaining experiments. This work provides further confidence in a model for predicting smouldering, which eventually is expected to be useful for designing soil remediation schemes for a novel technology based upon smouldering destruction of organic contaminants in soil.

Direct observation of graphene growth and associated copper substrate dynamics by in situ scanning electron microscopy.

This work highlights the importance of in situ experiments for an improved understanding of graphene growth on copper via metal-catalyzed chemical vapor deposition (CVD). Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way. It enables the observation of a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time and nanometer-scale resolution without the need of sample transfer. A strong dependence of surface dynamics such as sublimation and surface premelting on grain orientation is demonstrated, and the influence of substrate dynamics on graphene nucleation and growth is presented. Insights on the growth mechanism are provided by a simultaneous observation of the growth front propagation and nucleation rate. Furthermore, the role of trace amounts of oxygen during growth is discussed and related to graphene-induced surface reconstructions during cooling. Above all, this work demonstrates the potential of the method for in situ studies of surface dynamics on active metal catalysts.

<sup>40</sup>Ar/<sup>39</sup>Ar geochronology of supergene jarosite constraints on weathering front propagation rate in the TuHa basin

Two jarosite samples with visually distinct phases collected from the oxidation zone of the Hongshan Cu-Au deposit in the TuHa Basin are dated by laser ⁴⁰Ar/³⁹Ar incremental heating analysis. To test the age reproducibility of each sample, two aliquots are dated. The results indicate that the dates obtained are reproducible, yielding ages of 7.5 and 8.0 Ma, confirming the suitability of supergene jarosite to ⁴⁰Ar/³⁹Ar geochronology. Combined with published ages, the obtained ⁴⁰Ar/³⁹Ar ages decrease from the top to the bottom of the profile, which records the arrival of a weathering front at that depth, yielding a weathering front propagation rate of 2.92 m/Ma.

New methods for reconstructing geographical effects on dispersal rates and routes from large-scale radiocarbon databases

We introduce a methodology for reconstructing geographical effects on dispersal and diffusion patterns, using georeferenced archaeological radiocarbon databases. Fast Marching methods for modelling front propagation enable geographical scenarios to be explored regarding barriers, corridors, and favoured and unfavoured habitat types. The use of genetic algorithms as optimal search tools also enables the derivation of new geographical scenarios, and is especially useful in high-dimensional parameter spaces that cannot be characterized exhaustively due to computer runtime constraints. Model selection is guided by goodness-of-fit statistics for observed and predicted radiocarbon dates.We also introduce an important additional model output, namely, modelled phylogenies of the dispersing population or diffusing cultural entity, based on branching networks of shortest or 'least cost' paths. These 'dispersal trees' can be used as an additional tool to evaluate dispersal scenarios, based on their degree of congruence with phylogenies of the dispersing population reconstructed independently from other kinds of information.We illustrate our approach with a case study, the spread of the Neolithic transition in Europe, using a database from the literature (Pinhasi, Fort and Amerman 2005). Our methods find support for a geographical model in which dispersal is limited by an altitudinal cut-off and in which there is a climate-related latitudinal gradient in rate of spread. This model leads to a deceleration in front propagation rate with geodesic distance, which is also consistent with models of the propagation of the Neolithic transition under space competition with pre-existing populations of hunter-gatherers. Our genetic algorithms meanwhile searched the parameter space and found support for an alternative model involving fast spread along the northern Mediterranean coast and the Danube/Rhine riverine corridor. Both these models outperformed the geography-free Great Circle distance model, and both also outperformed another, almost geography-free, model that constrains dispersal to land to and near-offshore coastal waters. The adjusted coefficient of determination for modelled and observed radiocarbon dates for first arrival supports the GA-derived model; the shortest path network analysis, however, gives greater support to the model with altitudinal cut-off and latitudinal gradient in dispersal rate, since it produces branching 'dispersal trees' that are more congruent with these archaeological sites' clade memberships (as defined by archaeological material culture).

Molecular dynamics simulation of the burning front propagation in PETN

One of the models of detonation development in condensed explosives under shock loading is the concept of "hot spots." According to this model, the reaction initially starts at various defects and inhomogeneities, where energy is localized during shock wave propagation. In such a region the reaction may start and the heat flux sufficient for the ignition of the adjacent layers of matter may be formed. If the reaction propagates fast enough, the merging of the burning fronts from several hot spots may lead to detonation. So there is an interest in determining the burning propagation rate from the hot spot in various conditions. In this work we investigate the propagation of plane burning front from initially heated layer in PETN single crystal using molecular dynamics method with the reactive force field (ReaxFF). The burning rate depends on the direction in crystal. The kinetics of chemical transformations is considered. The dependence of the burning front propagation rate along [100] direction on the external pressure in the pressure range from normal to 30 GPa is calculated, it is shown that it grows linearly in the considered range from 50 m/s to 320 m/s. The results are compared with the data from experiments and quantum chemical calculations.

(Invited) Wafer Scale Copper Direct Plating on Thin PVD RuTa Layers: A Route to Enable Filling 30 nm Features and Below

Among all the Ru-based substrates, mixed-phase RuTa liners grown by physical vapor deposition have been explored as potential directly-platable diffusion barrier candidates. In order to understand the full-wafer copper direct plating process that occurs on these liners, the effect of the applied waveform, electrical contacts and suppressor chemistry have been investigated. In order to enhance copper nucleation and wafer scale edge-to-center copper front propagation rate, a liner surface cleaning protocol is developed. The copper front propagation across the 300 mm wafer is reported as a function of the RuTa film thickness. An optimized copper direct plating process on RuTa layers as thin as 4 nm is integrated in 90 and 30 nm half pitch single and dual damascene structures. Results in terms of compatibility of the direct plated copper with the following chemical mechanical planarization step complemented with physical and electrical characterization data are reported. The direct filling of ~25 nm sacrificial fin structures is also investigated.

Experimental calibration of a numerical model describing the alteration of cement/caprock interfaces by carbonated brine

Deep wells provide a possible pathway for CO2 and brine leakage from geologic storage reservoirs to shallow groundwater resources and the atmosphere. The integrity of wellbore cement in these environments is of particular concern, because it is not known if changes in cement properties resulting from reaction with CO2-rich brines will lead to enhanced leakage over the life cycle of the storage reservoir. Assessment of wellbore leakage will ultimately be answered through models that capture both the chemical and physical processes and the uncertainty of key parameters within the wellbore environment. Towards this end, we use the results for 13 core–flood experiments conducted at variable partial pressures of CO2, flow rate, durations, and cement–caprock apertures to constrain a wellbore model that couples chemical processes important to assessing the long-term integrity of wellbore cements in geologic carbon storage environments. X-ray computed microtomography collected prior-to and following the experiments was employed to spatially resolve the interface and the extent of the reaction zones, and time dependent solution chemistry was used to track the chemical alteration over the course of the experiments.In this manuscript we focus on the development of geochemical model that describes the alteration of both the cement and the caprock. In our experiments, chemical alteration of the cement significantly exceeded any dissolution of carbonate minerals within the caprock and fracture geometry played no role on the extent of reaction. The experimental data was used to calibrate a numerical model of wellbore-caprock interfaces coupling reaction-front chemistry, fluid flow and transport of dissolved species. The geochemical model adopts an idealized representation of the cement chemistry in which appropriate equilibrium conditions are enforced at a series of discrete reaction fronts. The equilibrium conditions are coupled by diffusive transport between the fronts, which also determines the rate of front propagation. Despite its simplicity, the calibrated model accurately reproduces the reaction-zone growth and effluent chemistry for the range of experimental conditions considered and allowed key parameters to be confirmed or calibrated. These include the use of portlandite, calcite, and analcime solubility as equilibrium controls at specific reaction fronts within the cement; the use of constant effective diffusivity for each alteration zone; and diffusive growth of the alteration layers.

Experiments and analysis of propagation front under gasification regimes in a packed bed

The paper analyses the results of experiments on the propagation rate in a fuel bed under gasification conditions in a co-current reactor configuration. Experiments using wood chips with different values of moisture content have been conducted under gasification conditions. The influence of air mass flux on the propagation rate, peak temperature and gas quality is investigated. It is observed from the experiments that the flame front propagation rate initially increases as the air mass flux increased, reaching a peak propagation rate, and further increase in the air mass flux results in a decrease in the propagation rate. However, the bed movement increases with the increase in air mass flux. The experimental results provide an understanding on influence of the fuel properties on propagation front. The surface area per unit volume of the particles in the packed bed plays an important role in the propagation rate. It has been argued that the flaming pyrolysis contributes towards the flame propagation as opposed to the overall combustion process in a packed bed. The calorific value of the producer gas generated is nearly the same over the entire range of air mass flux for bone-dry and 10% moist wood.

Wafer Scale Copper Direct Plating on Thin PVD RuTa Layers: A Route to Enable Filling 30 nm Features and Below?

Among all the Ru-based substrates, mixed-phase RuTa liners grown by physical vapor deposition have been explored as potential directly-platable diffusion barrier candidates. In order to understand the full-wafer copper direct plating process that occurs on these liners, the effect of the applied waveform, electrical contacts and suppressor chemistry have been investigated. In order to enhance copper nucleation and wafer scale edge-to-center copper front propagation rate, a liner surface cleaning protocol is developed. The copper front propagation across the 300 mm wafer is reported as a function of the RuTa film thickness. An optimized copper direct plating process on RuTa layers as thin as 4 nm is integrated in 90 and 30 nm half pitch single and dual damascene structures. Results in terms of compatibility of the direct plated copper with the following chemical mechanical planarization step complemented with physical and electrical characterization data are reported. The direct filling of ∼25 nm sacrificial fin structures is also investigated.

Universal Flame Propagation Behavior in Packed Bed of Biomass

This article aims at seeking the universal behavior of propagation rate variation with air superficial velocity (V s ) in a packed bed of a range of biomass particles in reverse downdraft mode while also resolving the differing and conflicting explanations in the literature. Toward this, measurements are made of exit gas composition, gas phase and condensed phase surface temperature (T g and T s ), and reaction zone thickness for a number of biomass with a range of properties. Based on these data, two regimes are identified: gasification—volatile oxidation accompanied by char reduction reactions up to 16 ± 1 cm/s of V s and above this, and char oxidation—simultaneous char oxidation and gas phase combustion. In the gasification regime, the measured T s is less than T g ; a surface heat balance incorporating a diffusion controlled model for flaming combustion gives and matches with the experimental results to within 5%. In the char oxidation regime, T g and T s are nearly equal and match with the equilibrium temperature at that equivalence ratio. Drawing from a recent study of the authors, the ash layer over the oxidizing char particle is shown to play a critical role in regulating the radiation heat transfer to fresh biomass in this regime and is shown to be crucial in explaining the observed propagation behavior. A simple model based on radiation–convection balance that tracks the temperature–time evolution of a fresh biomass particle is shown to support the universal behavior of the experimental data on reaction front propagation rate from earlier literature and the present work for biomass with ash content up to 10% and moisture fraction up to 10%. Upstream radiant heat transfer from the ash-laden hot char modulated by the air flow is shown to be the dominant feature of this model.

MODELING BOUNDARIES BETWEEN CONVERGING FRONTS IN PREHISTORY

We introduce a modeling framework that can be applied to cases of multiple converging fronts during episodes of population expansion and innovation diffusion, referring to two prehistoric case studies known archaeologically (the spread of pottery-making in Europe, and the spread of farming in southern Africa). We model front propagation using Fast Marching methods, drawing on the analogy with crystallization processes to build compoundly-weighted Voronoi diagrams of a spatially partitioned surface in which the zones of influence of each competing spreading process are determined by their respective front initiation times and propagation rates. We analyze the phase space for the general two-source case, and illustrate the potential of this approach by modeling the evolving interface for the archaeological case studies.

Tracing exhumation and orogenic wedge dynamics in the European Alps with detrital thermochronology: REPLY: Figure 1.

Detrital cooling ages from the pro-foreland and retro-foreland basins of the European Alps record distinctive exhumation trends that correlate with orogenic wedge states inferred from thrust front propagation rates. Periods of rapid hinterland exhumation correlate with relatively slow propagation of deformation toward the foreland and are interpreted to represent subcritical wedge conditions, whereas periods of slow hinterland exhumation correlate with rapid propagation of deformation toward the foreland and indicate supercritical wedge conditions. Similar lag time trends recorded in both the pro-foreland and retro-foreland thus mimic orogenic wedge behavior and suggest that local tectonics and/or climate events do not overprint the regional signal.

A study on the combustion synthesis of titanium aluminide in the self-propagating mode

The combustion synthesis of titanium aluminide (TiAl) starting from elemental powders in the self-propagating mode was investigated. A novel technique involving inductive preheating and ignition was utilized to initiate the reaction in a powder mixture compact. The effects of compact green density, titanium particle size, and mechanical activation of powder mixture on the propagation rate and combustion temperature were evaluated. It was found that mechanical activation of green powder mixture up to 2 h, and using finer titanium particles resulted in higher reaction front propagation rates. The optimum value of relative density with respect to front propagation velocity was determined to be within the range of 65–70%. The maximum combustion temperature was similarly obtained in the above range of relative density, but it was not significantly affected by the titanium particle size. The time-to-ignition was found to be highly influenced by titanium particle size and milling time. The products were characterized by X-ray diffraction and it was found out that the products were solely consisted of the TiAl phase without any other detectable phases.

Book Reviews

Book Reviews

Crystallization of X-ray amorphous Ca-Mg aluminosilicate materials

The crystallization mechanism has been considered for amorphous materials prepared by cooling silicate melts of the batches based on dunite and limestone, as well as those based on MgO, CaO, and SiO2, after deep reducing melting and the removal of the iron-containing metallic moiety from the melt. The specific features in the nucleation of centers generating a heat wave of self-propagating crystallization are revealed, and the possibility of visualizing these centers is demonstrated. The values of crystallization heats released in the process and changes in temperature of the heat wave front are reported, and the rate of the wave front propagation is determined.

Experimental study of surfactant transfer in fluid systems in microgravity conditions

The paper presents the results of the space experiment studying the process of surfactant dissolution from a binary fluid drop in microgravity conditions. The experiment was performed during the flight of the space satellite “Foton M-3” in September 2007. Investigation of the surfactant diffusion was made using a new original setup based on the interferometric method. The experimental cuvette represented a thin Hele-Shaw cell filled with water, which surrounded a drop in the form of a short liquid cylinder with a free lateral surface. The drop consisted of a binary mixture, in which one of the components was the surfactant easily dissolved in water. The use of interferometry made it possible to visualize and investigate evolution of the surfactant distributions and the flow structures in the drop and the surrounding liquid. The characteristic stages of the dissolution process were identified, and the rate of the concentration front propagation was defined. It was shown that in microgravity conditions the process of surfactant diffusion through the interface did not initiate an intensive solutal Marangoni convection as contrasted to the case of terrestrial simulation. The observed phenomenon has its origins in the long-lived fields of surfactant concentration formed near the interface due to the absence of the gravitational mechanisms of motion and large characteristic time of admixture diffusion which is hundreds of times longer than the time of thermal diffusion.

Spatiotemporal measurements of flame stretch and propagation rates for lean and rich CH 4/air premixed flames interacting with a turbulent-wake

A 1.5 m long turbulent-wake combustion vessel with a 0.15 m × 0.15 m cross-sectional area is proposed for spatiotemporal measurements of curvature, strain, dilatation and burning rates along a freely downward-propagating premixed flame interacting with a parallel row of staggered vortex pairs having both compression (negative) and extension (positive) strains simultaneously. The wanted wake is generated by rapidly withdrawing an electrically-controlled, horizontally-oriented sliding plate of 5 mm thickness for flame–wake interactions. Both rich and lean CH 4/air flames at the equivalence ratios ϕ = 1.4 and ϕ = 0.7 with nearly the same laminar burning velocity are studied, where flame–wake interactions and their time-dependent velocity fields are obtained by high-speed, high-resolution DPIV and laser-tomography. Correlations among curvature, strain, stretch, and dilatation rates along wrinkled flame fronts at different times are measured and thus their influences on front propagation rates can be analyzed. It is found that strain-related effects have significant influence on front propagation rates of rich CH 4/air (diffusionally stable) flames even when the curvature weights more in the total stretch than the strain rate does. The local propagation rates along the wrinkled flame front are more intense at negative strain rates corresponding to positive peak dilatation rates but the global propagation rate averaged along the rich flame front remains constant during all period of flame–wake interaction. For lean CH 4/air (diffusionally unstable) flames, the curvature becomes a dominant parameter influencing the structure and propagation of the wrinkled flame front, where both local and global propagation rates increase significantly with time, showing unsteady flame propagation. These experimental results suggest that the theory of laminar flame stretch can be applicable to a more complex flame–wake interaction involving unsteadiness and multitudinous interactions between vortices.

OH production by transient plasma and mechanism of flame ignition and propagation in quiescent methane–air mixtures

Transient plasma induced production of OH is followed in a quiescent, stoichiometric CH 4–air mixture using the planar laser induced fluorescence technique. Ignition and subsequent flame propagation, for both the transient plasma and traditional spark ignition, are observed with a high speed camera (2000 fps). The transient plasma is generated using a 70 ns FWHM, 60 kV, 800 mJ pulse. OH production was confirmed throughout the chamber volume; however, the mean number density was found to decay below 1.3 × 10 14 cm −3 near 100 μs. Nonetheless, ignition induced by transient plasma was decidedly faster than by spark ignition. Using the high speed camera, ignition initiated by transient plasma was found to occur along the length of the anode at approximately 1 ms, leading to the formation of a wrinkled, cylindrically-shaped flame. Analysis of the flame front propagation rates shows that flames ignited by transient plasma propagate essentially at the speed consistent with well accepted literature values for the stoichiometric methane–air mixture. This supports the notion that residue plasma, if any, has little effect on flame propagation.

Analysis of the edge fracture process in oscillation for polystyrene melts

The occurrence and development of the edge fracture phenomenon in oscillatory flow was investigated. Large-amplitude oscillatory shear experiments were performed with polystyrene melts of different molecular weights at 170 °C in parallel-plate geometry. Based on Tanner and Keentok’s criterion for the formation of the edge fracture in steady-state rotational flow, an equation was derived for the oscillatory flow. This equation can be used to predict the onset conditions in terms of the critical deformation amplitude γ c and the critical frequency ω c. A very good agreement with the experimental observation was found. Within the critical parameter range, the kinetics of the fracture front propagation was studied by detailed visual observations and by analysing the effect of the fracture process on the measured rheological data. An empirical equation was found that allows the description of the time development of the fracture front, to be characterised by a single parameter b. This finding was also theoretically confirmed, by assuming a linear relationship between the driving force for the fracture formation and the fracture front propagation rate.