Condensation Front Research Articles

Analysis of phase front structures for energetic materials

The structure of a steady wave system is considered which is admitted by the continuumequations for materials that undergo phase transformations with exothermic chemicalreaction. In particular, the dynamic phase front structures between liquid and gas phasesand solid and liquid phases are computationally investigated. With its theoretical basis inone-dimensional continuum shock structure analysis, the present approach estimates themicro-width of waves associated with phase transformation phenomena. For illustrationpurposes, n-heptane is used in the evaporation and condensation analysis and HMX is usedin the melting and freezing analysis of energetic materials of interest. The estimatedthickness of the evaporation–condensation front of n-heptane is on the order of10−2 µm while the HMX melting–freezing front thickness is estimated at1 µm. The present structure analysis is being extended to account for a broader range ofenergetic materials whose phase front thickness measurements are neither available norexperimentally attainable.

Analysis of Nitrogen and Steam Injection in a Porous Medium with Water

We formulate conservation laws governing steam and nitrogen injection in a one-dimensional porous medium containing water. Compressibility, heat conductivity and capillarity are neglected. We study the condensation front and shock waves arising in the flow. We find that there are four possible types of solutions for the initial and boundary conditions of interest. We describe a simple construction in the temperature saturation plane that determines the complete solution for the given conditions. Applications of the theory developed here are in clean up of soil contaminated with nonaqueous phase liquids. We show that a substantial cold gaseous zone develops in all solutions of practical interest, thus counteracting downward migration of the pollutant.

Interaction of liquid movement and steam condensation during steam centrifugation

Steam centrifugation is a mechanical thermal process that decreases the pore liquid in the porous medium by the help of centrifugal pressure and the pressure of injected steam. The steam is used in centrifuges to overcome the capillary forces, which retain the liquid in the lower area of the porous system. Both external forces affect the pore liquid in a different manner. This may lead to a different velocity of the liquid flow and the steam front, resulting in two different states of operation. The steam intrudes the pore and condenses until the bulk is locally heated on boiling temperature. The liquid flow in the porous system can be modeled as a plug flow in a single capillary. The steam intrudes a porous system, which is already partially de-watered, if the acceleration of the pore liquid due to the centrifugal force is more rapid than the penetration velocity of the steam. In this case the pressure difference cannot affect the mechanical de-watering. The pore liquid runs ahead of the condensation front and both processes operate independently. It is intended that the pressurized steam directly acts on the pore liquid and hence interacts with the centrifugal pressure. In this case both external forces are employed to accelerate the liquid in the capillary system and to reduce it to a greater extent. Therefore the relation between steam pressure and centrifugal pressure has to exceed a characteristic value. Then the steam has access to finer pores and finally overcomes the capillary forces. The aim of the experimental and theoretical work is to find the optimal state of operation. Both front velocities can be measured and compared to each other. Furthermore these results are compared to calculations, which can predict the optimal process parameter. With the help of this optimization the steam centrifugation leads to a lower moisture content and operates more economical than a conventional process.

Co-injection of air and steam for the prevention of the downward migration of DNAPLs during steam enhanced extraction: An experimental evaluation of optimum injection ratio predictions

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant, the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In a previous work, a theoretical model was developed to predict the optimum injection ratio of air to steam that would eliminate accumulation of DNAPL ahead of the temperature front and thus minimize the potential for downward migration. In this work, the theoretical model is summarized, and an experiment is presented in order to evaluate the optimum injection ratio prediction. In the experiment, a two-dimensional water saturated sand pack is contaminated with a known mass of TCE (DNAPL). The system is then remediated by co-injecting air and steam at the predicted optimum injection ratio, calculated based on the average contaminant soil concentration in the sand pack. Results for the co-injection of air and steam are compared to results for the injection of pure steam or pure air. Injection at the predicted optimum injection ratio for a volumetric average NAPL saturation, reduced accumulation of the contaminant ahead of the condensation front by over 90%, as compared to steam injection alone. This indicates that the optimum injection ratio prediction is a valuable tool for limiting the spreading of DNAPL during steam-enhanced extraction. Injection at the optimum injection ratio resulted in earlier recovery of contaminant than for steam injection alone. Co-injection of steam and air is also shown to result in much higher recovery rates than air injection alone.

Sulfur chemistry with time-varying oxygen abundance during Solar System formation

Chemical models of solar nebula chemistry are presented which show the influence of progressive water depletion from the inner solar nebula. The main focus of this work is the equilibrium distribution of S resulting from this process. Under canonical solar nebula conditions, H 2S is the dominant S-bearing species in the gas phase and troilite (FeS) is the primary reservoir for S after condensation. As water vapor diffuses out to its condensation front, the equilibrium distribution of S changes significantly. With the removal of water vapor, SiS becomes the most abundant S-bearing gas and MgS and CaS compete with FeS as the main sulfide reservoir. These results allow us to argue that some of the minerals in the enstatite chondrites formed through the heterogeneities associated with the nebular ice condensation front, and that the sulfur abundance in Jupiter reflects a depletion in H 2S that is the result of inner nebula sulfur chemistry under varying oxygen abundance.

Condensation Front Migration in a Protoplanetary Nebula

Condensation front dynamics are investigated in the mid-solar nebula region. A quasi-steady model of the evolving nebula is combined with equilibrium vapor pressure curves to determine evolutionary condensation fronts for selected species. These fronts are found to migrate inwards from the far-nebula to final positions during a period of 10(exp 7) years. The physical process governing this movement is a combination of local viscous heating and luminescent heating from the central star. Two luminescent heating models are used and their effects on the ultimate radial position of the condensation front are discussed. At first the fronts move much faster than the nebular accretion velocity, but after a time the accreting gas and dust overtakes the slowing condensation front.

Steam condensation waves in water-saturated porous rock

We formulate balance laws governing condensation of steam injected into a linear porous medium containing water. Heat losses to the outside are neglected. Longitudinal heat conduction and capillary effects are taken into account. The condensation process is modeled by a rate equation based on a simple heat-transfer model. We study the condensation front as a traveling wave, under the approximation that pressure variations are negligible within the front. We find this traveling wave for the case of injection of high-quality steam using a combination of phase-plane analysis and numerical calculation of orbits.

Jupiter Formed with More Tar than Ice

Elemental abundances in Jupiter determined from Galileo probe measurements are compared to recently revised solar system abundances. When normalized to the abundance of sulfur, the most abundant refractory rock-forming element reliably determined in Jupiter's atmosphere by the Galileo probe, abundances of argon, krypton, and xenon are 1 times solar, the observed oxygen is depleted by a factor of 4, and carbon is enriched 1.7 times. The fairly uncertain nitrogen abundance ranges from 1 to 3 times solar. The oxygen abundance in Jupiter derived from the observed atmospheric water abundance is only a lower limit to the total planetary oxygen because oxygen is also bound to rock-forming elements such as magnesium or silicon sequestered deep in the planet. The sulfur abundance constrains the amount of rock-forming elements on Jupiter. Considering the amount of oxygen bound to silicate rock, the total oxygen abundance on Jupiter of 0.47 times solar system indicates an overall oxygen depletion by about a factor of 2. The hydrogen and helium abundances in the Jovian atmosphere are depleted (0.48 and 0.39 times solar system, respectively). These relative depletions may indicate the extent of hydrogen and helium partitioning from the molecular envelope into Jupiter's metallic layer. A formation scenario for Jupiter is proposed to explain the relative oxygen depletion and, at the same time, the relative carbon enrichment. In essence, the model assumes that at the time of Jupiter's formation, abundant carbonaceous matter was present near 5.2 AU rather than abundant water ice, increasing the surface mass density of solids in the solar nebula accretion disk. Carbonaceous matter, which has high sticking probabilities, was the agent that sped up accumulation of solid matter of proto-Jupiter. This led to runaway accretion of the planet. Major consequences of this scenario are that the water ice condensation front (the snow line) typically placed near 5.2 AU in solar nebula models must be replaced by a carbonaceous condensation/evaporation front (the "tar line") and that the snow line is located farther out in the solar nebula.

Uncertainties in coupled thermal–hydrological processes associated with the Drift Scale Test at Yucca Mountain, Nevada

Understanding thermally driven coupled hydrological, mechanical, and chemical processes in unsaturated fractured tuff is essential for evaluating the performance of the potential radioactive waste repository at Yucca Mountain, Nevada. The Drift Scale Test (DST), intended for acquiring such an understanding of these processes, has generated a huge volume of temperature and moisture redistribution data. Sophisticated thermal–hydrological (TH) conceptual models have yielded a good fit between simulation results and those measured data. However, some uncertainties in understanding the TH processes associated with the DST still exist. This paper evaluates these uncertainties and provides quantitative estimates of the range of these uncertainties. Of particular interest for the DST are the uncertainties resulting from the unmonitored loss of vapor through an open bulkhead of the test. There was concern that the outcome from the test might have been significantly altered by these losses. Using alternative conceptual models, we illustrate that predicted mean temperatures from the DST are within 1 °C of the measured mean temperatures through the first 2 years of heating. The simulated spatial and temporal evolution of drying and condensation fronts is found to be qualitatively consistent with measured saturation data. Energy and mass balance computation shows that no more than 13% of the input energy is lost because of vapor leaving the test domain through the bulkhead. The change in average saturation in fractures is also relatively small. For a hypothetical situation in which no vapor is allowed to exit through the bulkhead, the simulated average fracture saturation is not qualitatively different enough to be discerned by measured moisture redistribution data. This leads us to conclude that the DST, despite the uncertainties associated with open field testing, has provided an excellent understanding of the TH processes.

The Effect of Heat Transfer Between Nearby Layers on the Volume of the Steam Zones

Summary The work of Mandl and Volek1 is extended analytically to include the effect on the volume of the steam zone in one layer owing to heat transfer from a second layer undergoing steam injection. The contribution of the heat transfer delays the onset of heat convection across the advancing condensation front (the critical time) and significantly increases the volume of the steam zone even where 30 or more feet separate the layers. The increase in the heat content of the steam zone (or volume) increases with increasing ratio of the sensible to latent heat of the injected steam, and is larger than the increase in the total heat content. The interpretation for this behavior is that heat transfer delays the rate of condensation of the steam vapor, this being more important at poor steam qualities. Results of the simple approach presented here have been confirmed qualitatively through numerical simulation.

A theoretical model of air and steam co-injection to prevent the downward migration of DNAPLs during steam-enhanced extraction

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In this work, a one-dimensional theoretical model is developed to predict the injection ratio of air to steam that will prevent the accumulation of volatile DNAPLs. The contaminated region is modeled as a one-dimensional homogeneous porous medium with an initially uniform distribution of a single component contaminant. Mass and energy balances are combined to determine the injection ratio of air to steam that eliminates accumulation of the contaminant ahead of the steam condensation front, and hence reduces the possibility of downward migration. The minimum injection ratio that eliminates accumulation is defined as the optimum injection ratio. Example calculations are presented for three DNAPLs, carbon tetrachloride (CCl 4), trichloroethylene (TCE), and perchloroethylene (PCE). The optimum injection ratio of air to steam is shown to depend on the initial saturation and the volatility of the liquid contaminant. Numerical simulation results are presented to validate the model, and to illustrate downward migration for ratios less than optimum. Optimum injection ratios determined from numerical simulations are shown to be in good agreement with the theoretical model.

Evaporation and condensation fronts in porous media

Flow of a fluid through a porous medium is considered with allowance for heat conduction. Both fronts at which the liquid is transformed into steam or a liquid-steam mixture and fronts with inverse transformations are studied. The evolutionarity conditions of these fronts are considered and a model of their structure is proposed.

Simulation of nanoscale particles elaboration in laser-produced erosive flow

Synthesis of nanoscale particles as a result of vapor condensation under laser vaporisation of materials into high-pressure surrounding atmosphere has been studied. A 2D-hydrodynamic model is developed for a simulation of nanoscale particles formation in a laser-produced erosive flow propagating from the irradiated surface into the ambient gas. The analysis is based on a combined solution of the set of hydrodynamic equations describing the erosive flow and equations of nonequilibrium kinetics of particles nucleation and condensation growth. The detailed structure of the erosive jet is studied: It is shown that nucleation and condensation fronts are separated in space and are formed between the erosive vapor–gas flow and cold ambient gas. The model predicts bimodal particle-size distribution function. The particle size is shown to be independent on the evaporation rate and to increase with the laser beam radius and the ambient gas pressure.

A Fast 3D Interface Simulator for Steamdrives

Summary Here we describe a fast 3D steam drive simulator. We use an interface model, where the single-phase steam zone is separated from the two-phase liquid zone by the steam condensation front (SCF) which constitutes the interface. Steady-state heat balances applied at the interface reduce the steam problem to a problem of gas/oil/water flow. Heat losses are treated by a prescribed conversion of steam to water. The model incorporates gravity, viscous and capillary forces and handles arbitrary permeability distributions and well configurations. We use a multigrid method to solve the pressure equation. The steam zone development is determined by a probabilistic method, which ensures that instability phenomena are properly treated. The oil/water flow problem in the liquid domain is solved as in conventional reservoir simulators. We validate the model with analytical models. Example calculations for a thin medium-viscosity oil field show that a transition zone with a reduced oil viscosity just downstream of the SCF has a pronounced stabilizing effect. This, and the global heat loss effects are the reason for the high displacement efficiency of steam drives.

Theoretical study of multicomponent soil vapor extraction: propagation of evaporation–condensation fronts

Theoretical analysis of the multicomponent soil vapor extraction (SVE) process is presented. It is based on general principles of transport in porous medium and the Raoult's law describing the equilibrium between multicomponent nonaqueous phase liquid (NAPL) and aqueous/gaseous phases. The principal simplifying assumptions of the theory include: isothermal conditions, homogeneous porous medium, radial advective transport of volatile organic compounds, steady-state radial soil gas flow through the contaminated zone, stagnant NAPL and aqueous phases, constant solid–water distribution and gas–water partitioning coefficients. The main emphasis of the study is on the description of propagating composition fronts under local equilibrium conditions. Construction of multicomponent front solutions is carried out by applying the concept of coherent waves, developed by Helfferich and Klein for multicomponent chromatography. It is shown that under typical conditions component volatilities control the sequence of propagating fronts. However, this sequence may change for very small NAPL saturation values (∼10 −3–10 −4 or less) and/or relatively high retardation factors (>10 2). Tailing effect of nonequilibrium (kinetically controlled) gas–water partitioning behind the NAPL zone is also addressed in the presented study. This effect is shown to diminish with the increase of NAPL saturation.

A nucleation experiment in low gravity conditions: Monitoring and collection of Mg and Zn particles

Abstract An experiment on the nucleation and growth of solid particles from the condensation of vapor in low gravity conditions was performed onboard NASA's KC-135 Microgravity Research Aircraft. In addition to the systems to monitor relevant physical parameters, two systems have been developed and used to observe the growth process of the particles and to extract these particles for further microscopic analyses. A dynamic light scattering system is used to measure the in situ mean particle size while a second independent system allows the collection of the nucleated particles directly from the condensation front. These samples have been analyzed in the lab using Scanning Electron Microscopy (SEM). Mean particle sizes are approximately 300 nm and 150 nm for Mg and Zn particles, respectively. Evidence for particle-particle coagulation of Mg into “fractal” like clusters is seen while Zn shows no evidence for any coagulation over the conditions explored.

Uniqueness conditions in a hyperbolic model for oil recovery by steamdrive

In this paper we study a one-dimensional model for oil recovery by steamdrive. This model consists of two parts: a (global) interface model and a (local) steam condensation/capillary diffusion model. In the interface model a steam condensation front (SCF) is present as an internal boundary between the hot steam zone (containing water, oil and steam) and the cold liquid zone (containing only water and oil). Disregarding capillary pressure away from the SCF, a 2× 2 hyperbolic system arises for the water and steam saturation. This system cannot be solved uniquely without additional conditions at the SCF. To find such conditions we blow up the SCF and consider a parabolic transition model, including capillary diffusion. We study in detail the existence conditions for traveling wave solutions. These conditions provide the missing matching conditions at the SCF in the hyperbolic limit. We show that different transition models yield different matching conditions, and thus different solutions of the interface model. We also give a relatively straightforward approximation and investigate its validity for certain ranges of model parameters.

Hydrodynamics of laser erosive jet generating nanoscale particles

A 2D-hydrodynamical model is developed for simulation of ultrafine particle elaboration by means of the laser-evaporation technique. The general picture of the process can be predicted as follows. An erosive flow of the submerged jet type is formed under laser evaporation of a target material into ambient gas atmosphere. Due to the interaction with surrounding gas, the erosive flow is cooled and the vapour condenses generating nanoscale particles. A thin condensation front is formed between the erosive vapour–gas jet and cold ambient inert gas. The gas flow streamlines intersect the condensation surface twice and accordingly the condensation process is divided into two stages. The initial stage corresponds to the inlet part of the front near the laser irradiation spot, where streamlines are directed inward the erosive jet, homogeneous nucleation and condensational growth of nucleation centres take place here. The formed particles of a condensate are transferred by the inert gas into internal flow area, where the condensation is suspended. At the final stage, the particles intersect the condensation front from within the vapour–gas mixture at an outlet section where they continue to grow. The resulting particle size distribution depends both on ambient gas pressure and laser irradiation conditions.

Improved Dewatering of Coal by Steam Pressure Filtration

The use of steam as an over-pressure forming gas phase has the advantage of a combined mechanical and thermal dewatering. Steam pressure filtration can provide an economical and effective technique, whereas conventional solid-liquid separation processes do not lead to a marketable residual moisture content of the product. A condensation front is built at the cold layers of the filter cake, while removing the capillary water from the porous filter cake. Depending on the filter cake resistance as well as the pressure difference, the condensation front displaces a great amount of cold mother liquid from the filter cake. For further economic drying pressurized air should be applied to the filter cake, utilizing the stored latent heat for the evaporation of a part of the remaining liquid. Pre-dewatering and the presence of inert gases in the filter cake should be prevented.

Propagation of condensation front in steam injection into dry porous media

The front speed and the liquid saturation distribution in the condensate flow region have been examined for one-dimensional injection of dry steam into a lower temperature, dry porous medium with a constant inlet pressure. Existing models for the volumetric viscous and inertial forces are used along with an upstream region with an immobile liquid, which is followed by a two-phase region, a condensation zone, a liquid region, and a downstream noncondensable gas flow region. The down- and upflow (along and against gravity) are examined assuming a quasi-steady behavior. For downflow, the liquid region grows with the condensation front location. The asymptotic front speed is obtained in closed form for both the near-field regime, where the steam flow rate is high (i.e. deviation from a Darcy behavior is significant), and the far-field regime. For upflow, the liquid region gradually disappears, the liquid saturation distribution in the upstream immobile region undergoes two transitions, and correlation expressions are found for the location of these transitions. An experiment is performed, and the experimental results confirm the predictions for both down- and upflows.