Multi-species Transport Model Research Articles

Predicting natural attenuation of xylene in groundwater using a numerical model

The aquifer beneath an abandoned refinery in the Lower Rhine area, Germany, was contaminated with a number of different mineral oil products. Groundwater sampling in the area around the former xylene plant revealed that a xylene plume had developed in the underlying groundwater, and moreover, that there is strong evidence for in situ microbial xylene degradation with oxygen, nitrate, sulfate and ferric iron as electron acceptors. In order to prevent further xylene spreading, three pumping wells extracting contaminated water were installed downgradient of the spill zone. The numerical reactive transport code Transport Biochemisty Chemistry (TBC) was applied to this situation to quantify the relation of microbial degradation to xylene removal by the pumping wells. It could be shown that the unamended in situ degradation was an appreciable xylene removal process that contributed to about one-third to the total xylene removal (degradation plus extraction). A further objective of the model application was to predict xylene spreading under regional flow conditions, i.e. without operation of the three pumping wells, to consider the possible effects of natural xylene attenuation. To accomplish this, the model calibrated for the situation with operating wells was transferred to the hydraulic situation of regional flow while retaining the parameters of the biochemical model. It turned out that the xylene plume that is expected to develop downgradient of the source area will be limited to an extension of not more than 1000 m. An interesting feature of the simulations results was that xylene degradation under iron-reducing conditions, which was of minor importance for the situation with operating pumping wells, becomes the dominant degradation mechanism under regional flow conditions. Moreover, iron reduction will be the key process in controlling plume evolution. The model application illustrates that multi-species reactive transport models are needed to adequately transfer reactive processes from one hydraulic situation to another, while single species models are not suited for this predictive task.

Proposal of Universal Dimensionless Parameter for Extinction of Counter Flow Diffusion Flame via Elementary Reaction Model.

In this article a unique universal criterion which describes extinction of counter flow diffusion flame was proposed. A numerical study for extinction of counter flow diffusion flame under various conditions of ambient temperature, oxidizer concentration and fuel concentration was performed. The numerical simulations include detailed chemistry in terms of a set of elementary reactions using up to 102 elementary reactions, as well as multi-species transport model. First, detailed flame structures such as distributions of temperature, species and heat release rate under the prescribed various conditions were clarified. An ultimate aim of the present study is at finding a universal criterion of extinction under the general conditions. To this end, we proposed a new parameter "Reaction Zone Damkohler Number", Dr, which is defined as the ratio of the energy flux to the heat release rate at the reaction zone. The maximum flame temperature was plotted against this Dr and it was found that extinction occurs at nearly constant value of Dr, which implies this parameter can be utilized as a criterion for extinction. Further the relationship between Dr and the usual first Damkohler number D1 was also suggested.

Simulation of the Behavior of Rich Hydrogen-Air Flames Near the Flammability Limit

Abstract This paper presents an investigation of the behavior of rich hydrogen-air flames near the flammability limit. Ignition processes arc simulated using a computational model which involves the solution of the governing equations (for one-dimensional geometries) by implicit methods on an adaptive non-uniform grid, using detailed chemistry and a multi-species transport model. The reaction mechanism consists of 37 elementary reactions and 9 species. Calculations have been performed for different one-dimensional geometries. An investigation of the fundamental flammability limit {intrinsic to the combustion system itself), which is governed only by the physical and chemical processes in the gaseous mixture, is carried out by eliminating external factors such as heat loss, buoyancy, etc, in the calculations. Compulations have been performed for hydrogen-air mixtures of varying hydrogen content. Mixtures containing 75% or less hydrogen are found to be steadily propagating, whereas mixtures containing 82% o...

Numerical Analysis for Microscopic Ignition Process of a Fuel Droplet with a Set of Elementary Reactions.

A numerical study for self-ignition of single fuel droplet (n-heptane) in a high-temperature environment was performed in this article. The numerical simulations include detailed chemistry in terms of a set of elementary reactions using up to 256 elementary reactions, as well as a multispecies transport model. The ignition time was compared to the experimental results and it was shown that both were found to be in good agreement. Further, the computed results (temperature distribution, ignition time, heat release rate and so on) were compared with the results which were obtained under assumption of single-step reaction model and the validity of single-step reaction model for ignition process was examined. It was calculated that the single-step reaction model does not give appropriate results for ignition time and initial heat release distributions. In case of a small droplet, heat release rate in early time becomes much larger than that for large droplet size, thereby retarding ignition process significantly. This may cause no ignitable condition, which we call the ignitable limit. Then, the mechanism of ignition and ignitable limit were also discussed.

水素・空気超音速噴流における着火の数値解析

Ignition of a parallel hydrogen injection into a supersonic hot air stream and ignition of a cold premixed combustible hydrogen/air stream heated by a hot air jet were investigated by a high-resolution TVD LU-SGS numerical algorithm. The present two-dimensional simulation included detailed chemistry in terms of elementary reactions, as well as a multispecies molecular transport model and the Baldwin Lomax algebraic turbulent model. In the case of a cold hydrogen injection into a hot air stream, numerical simulation showed that, with a decrease of the width of fuel jet, ignition point moves upstream quickly. The present results also showed that ignition distance was linearly dependent on the logarithmic function of the width of the fuel jet. Furthermore, the phenomenon could be still observed even for a very low concentration of hydrogen in the fuel jet. In the case of a premixed hydrogen/air stream ignited by a hot air jet, with a decrease of the width of air jet, ignition point moved very slightly downstream. However, there existed an ignition limit for the width of air jet, that is, if the width of the air jet was less than this limit, ignition became impossible. Furthermore, this ignition limit depended strongly on the initial temperatures of air stream and premixed hydrogen/air stream.

Combustion and pressure wave interaction in enclosed mixtures initiated by temperature nonuniformities

The dynamics of one-dimensional combustion waves originating from hot-spot ignition was studied numerically by solving the corresponding conservation equations for mass, species, energy, and momentum. The mathematical model included a multispecies transport model and a detailed reaction mechanism. The numerical solution of the time-dependent conservation equations was based on the method of lines and on a dynamic adaptive grid technique. Due to the stiffness of the resulting system of ordinary differential equations, the BDF-code DASSL was used to integrate the equations. The results of the simulations show that the interaction of combustion and pressure wave can lead to deflagration to detonation transition. Depending on the temperature levels and the shaple of the temperature nonuniformity, ordinary flame propagation, deflagration to detonation transition or to deflagration waves, depending on the initial conditions.

Simulation of ignition and combustion wave propagation in enclosed C 2H 6/O 2/N 2 mixtures

The dynamics of one-dimensional C2H6/Air combustion waves originating from hot-spot ignition was studied numerically by solving the corresponding conservation equations for mass, species, energy and momentum. The model included a detailed reaction mechanism and a multispecies transport model. The numerical solution was based on the method of lines and on a dynamic adaptive grid technique. Due to the stiffness of the resulting differential equations, a BDF-code was used to integrate the system. In this work, hot-spot ignition was studied for a reaction system under high pressure and high temperature conditions. The results show that hot-spot ignition can cause different combustion phenomena depending on the shape of the temperature nonuniformity. A supersonic combustion wave was observed when the velocity of the temperature-gradient based autoignition wave was much higher than the velocity of pressure waves. Detonation wave initiation was the result when an interaction between ignition process and pressure wave was possible. Ordinary flame propagation occurred, when the mixture outside the hot-spot was far away from autoignition.

Analytical Modeling of Calcium Carbonate Deposition for Laminar Falling Films and Turbulent Flow in Annuli: Part II—Multispecies Model

The present study proposes a multispecies transport model to predict calcium carbonate deposition. The model has been applied to predict the deposition flux, the mean fouling layer thickness, and the profile of local fouling thickness along a heated plate of a laminar falling film. Good agreement is found when comparing with experimental data. Similarly, the model is applied to predict the fouling layer in a turbulent annulus flow system and a good agreement of the predicted results is also found with recent experimental data. Finally, solutions in dimensionless forms are presented to show the effects of various dimensionless parameters on calcium carbonate deposition.

Numerical investigation of time-dependent properties and extinction of strained methane and propane-air flamelets

Laminar premixed strained flames are numerically modeled for time-dependent strain rates and pressure. The simulations include detailed chemistry in terms of a set of elementary reactions, as well as a multispecies transport model. The motivation to consider time dependence in modeling of strained flames is the consequence of the concept of strained laminar flamelets in unsteady turbulent combustion, such as occurs in internal combustion engines where the turbulent flow field and pressure are subject to large temporal variations. Nonstationary conservation equations for strained flames have been derived including one-parameter (strain rate or tangential pressure gradient) and two-parameter (tangential pressure gradient and flow field divergence) formulations. These equations then have been used to study the following problems: Extinction limits of methane-air flames at different equivalence ratios in the stationary case, to test the model and the input data used (e.g., different levels of methane oxidation chemistry). Influence of temporally periodical change of the strain rate on the flame front behavior. Actually, a lean methane-air flame with sinusoidally varying strain rate is simulated numerically. Influence of simultaneous application of strain and time-dependent pressure decrease, as occurs in the decompression phase in an Otto engine. A propane-air flame (starting at P = 8 bar) is used to elucidate pressure decrease at strain rate values typical in this environment. Temporal behavior of a strained flame front to determine the characteristic extinction time. Lean and rich propane-air flames are considered during quenching under the influence of a constant strain rate. The results are discussed and compared-as far as possible-with experimental and computational results given in the literature.

Numerical studies of hot spot ignition in H 2−O 2 and CH 4-air mixtures

This paper presents the results on the simulation of “hot spot” ignition in H 2 −O 2 and CH 4 -air mixtures. The hot spots which are generally considered to be formed in the Ottoengines are caused due to the non-uniformities in temperature or pressure. They generally have the potential to ignite the reacting mixture due to the increased temperature with respect to the surrounding. Since the ignition delay is highly temperature dependent, the hot spot ignites much earlier than its surrounding, leading to space- and time-dependent processes governed by the superposition of chemistry, gas-dybamics and transport. Here, simulation of “hot spot” ignition is performed in one dimensional geometries with multi-step chemistry and multi-species transport model. The governing equations after spatial discretization on an adaptive non-uniform grids are solved numerically by implicit methods. The results show that the model used is able to describe the spatial and temporal development of ignition, propagation, and the transitional phenomena from deflagration to detonations, that are initiated by hot spots.

Simulation of chemically reacting flows in two-dimensional geometries

Globally implicit solutions of the compressible Navier-Stokes equations including detailed chemistry in two space dimensions are performed by a method of lines. This has become possible by the recent development of numerical methods for the solution of stiff differential /algebraic equation systems together with the availability of fast computers with high storage capacity. Computations of ignition processes are performed by solving the corresponding conservation equations (i.e., conservation of total mass, momentum, energy, and species mass) using a detailed reaction mechanism for the ozone-oxygen system (consisting of six elementary reactions) and a multispecies transport model. Thermal ignition is simulated by an additional source term in the energy conservation equation. Spatial discretization on a two-dimensional grid that is adapted statically in two spatial directions leads to a differential/ algebraic equation system which is solved numerically by an implicit extrapolation method to overcome stiffness problems caused by the multiscale character of the system considered. Results are presented for the simulation of a laser-induced thermal ignition in an ozone-oxygen mixture in a cylindrical reaction vessel. However, the method can be generally applied to two-dimensional reactive flows in relatively simple geometries.

Ignition processes in carbon-monoxide-hydrogen-oxygen mixtures

Ignition processes in the carbon monoxide-hydrogen-oxygen system are simulated by solving the corresponding conservation equations (i.e. conservation of mass, energy, momentum and species mass) for one-dimensional geometries using a detailed reaction mechanism and a multi-species transport model. An additional source term in the energy conservation equation allows the treatment of induced ignition, and a realistic model for the destruction of reactive species at the vessel surface is used to treat auto-ignitions in static reactors. Spatial discretization using finite differences and an adaptive grid point system leads to a differential/algebraic equation system which is solved numerically by extrapolation or backward differencing codes. Minimum ignition energies are calculated for various mixture compositions and radii of the external energy source. Ignition limits are computed, and a sensitivity analysis shows the rate-limiting reactions.

Ignition processes in hydrogenoxygen mixtures

Ignition processes in the hydrogenoxygen system were simulated by solving the corresponding conservation equations (i.e., conservation of mass, energy, momentum, and species mass) for one-dimensional geometries using a detailed reaction mechanism and a multispecies transport model. An additional source term in the energy conservation allowed the treatment of induced ignition, and a realistic model for the destruction of reactive species at the vessel surface was used to treat auto-ignitions in static reactors. Spatial discretization using finite differences and an adaptive grid point system led to a differential-algebraic equation system, which was solved numerically by extrapolation or by backward differencing codes. Comparisons with experimental works show that one common reaction mechanism is able to simulate shock-tube-induced ignitions (modeled by treating the reaction system as a homogeneous mixture heated up by the shock wave) as well as the three explosion limits of the hydrogenoxygen system. Minimum ignition energies are calculated for various mixture compositions, pressures, radii of the external energy source, and ignition times, and it is shown that for long ignition times the “uniform pressure assumption” is a quite good approximation for computing minimum ignition energies.

Observation and simulation of laser induced ignition processes in O 2−O 3 and H 2−O 2 mixtures

Oxygen-ozone mixtures are ignited by absorption of laser pulses from a TEA CO2-laser along the axis of a cylindrical vessel. Laser excitation, molecular relaxation of O3, and flame propagation are observed by UV light absorption of O3 perpendicular to the cell axis. The measurements are interpreted by solving the corresponding one-dimensional non-stationary conservation equations. Differences between experimental results and simulations can be attributed to the degree of complexity of the simulations, which are performed only in one spatial dimension assuming uniform pressure and a thermal source term. Besides the O2−O3 system, for the first time the auto-ignition in the O2−H2 system could be simulated, using a detailed reaction mechanism and considering realistic multi-species transport models and thermochemistry.