Assumption Of Phase Equilibrium Research Articles

Large Eddy Simulation of an Ethanol Spray Flame with Secondary Droplet Breakup

A computational investigation of three configurations of the Delft Spray in Hot-diluted Co-flow (DSHC) is presented. The selected burner comprises a hollow cone pressure swirl atomiser, injecting an ethanol spray, located in the centre of a hot co-flow generator, with the conditions studied corresponding to Moderate or Intense Low-oxygen Dilution (MILD) combustion. The simulations are performed in the context of Large Eddy Simulation (LES) in combination with a transport equation for the joint probability density function (pdf) of the scalars, solved using the Eulerian stochastic field method. The liquid phase is simulated by the use of a Lagrangian point particle approach, where the sub-grid-scale interactions are modelled with a stochastic approach. Droplet breakup is represented by a simple primary breakup model in combination with a stochastic secondary breakup formulation. The approach requires only a minimal knowledge of the fuel injector and avoids the need to specify droplet size and velocity distributions at the injection point. The method produces satisfactory agreement with the experimental data and the velocity fields of the gas and liquid phase both averaged and ‘size-class by size-class’ are well depicted. Two widely accepted evaporation models, utilising a phase equilibrium assumption, are used to investigate the influence of evaporation on the evolution of the liquid phase and the effects on the flame. An analysis on the dynamics of stabilisation sheds light on the importance of droplet size in the three spray flames; different size droplets play different roles in the stabilisation of the flames.

Liquid vaporization under thermodynamic phase non-equilibrium condition at the gas-liquid interface

Liquid vaporization under thermodynamic phase non-equilibrium condition at the gas-liquid interface is investigated over a wide range of fluid state typical of many liquid-fueled energy conversion systems. The validity of the phase-equilibrium assumption commonly used in the existing study of liquid vaporization is examined using molecular dynamics theories. The interfacial mass flow rates on both sides of the liquid surface are compared to the net vaporization rate through an order-of-magnitude analysis Results indicated that the phase-equilibrium assumption holds valid at relatively high pressures and low temperatures, and for droplets with relatively large initial diameters (for example, larger than 10 µm for vaporizing oxygen droplets in gaseous hydrogen in the pressure range from 10 atm to the oxygen critical state). Droplet vaporization under superheated conditions is also explored using classical binary homogeneous nucleation theory, in conjunction with a real-fluid equation of state. It is found that the bubble nucleation rate is very sensitive to changes in saturation ratio and pressure; it increases by several orders of magnitude when either the saturation ratio or the pressure is slightly increased. The kinetic limit of saturation ratio decreases with increasing pressure, leading to reduced difference between saturation and superheat conditions. As a result, the influence of non-equilibrium conditions on droplet vaporization is lower at a higher pressure.

Application of an Equilibrium-Phase Spray Model to Multicomponent Gasoline Direct Injection

An Equilibrium Phase (EP) spray model has been recently proposed for modeling high-pressure diesel fuel injection, which is based on jet theory and a phase equilibrium assumption. In this approach, the non-equilibrium processes of drop breakup, collision and surface vaporization are neglected, assuming that spray vaporization is a mixing-controlled equilibrium process. A liquid-jet model and a gas-jet model are also intro-duced to improve grid-independency. In the current study, the EP model is applied in simulations of multi-hole gasoline direct injection (GDI). The model is validated at ambient densities from 3 to 9 kg/m3 and ambient temperatures from 400 K to 900 K, for two different GDI injectors, i.e. the Engine Combustion Network (ECN) Spray G injector and a GM injector. Iso-octane is used as the surrogate in consistency with available experi-ments. The results show good agreement in terms of liquid/vapor penetration, shape of the vapor envelope, and the velocity evolution along the injector centerline. Then, a 10-component gasoline surrogate fuel is used to demonstrate the capability of this model for multi-component spray simulations, which is essential for engine combustion and emission predictions.

Reductive dechlorination of DNAPL mixtures with Fe(II/III)-L and Fe(II)-C: Evaluation using a kinetic model for the competitions

A kinetic model for the competitions was applied to understand the reductive dechlorination of tertiary DNAPL mixtures containing PCE, TCE, and 1,1,1-TCA. The model assumed that the mass transfer rates were sufficiently rapid that the target compounds in the solution and the DNAPL mixture were in phase equilibrium. Dechlorination was achieved using either a mixture of Fe(II), Fe(III), and Ca(OH)2 (Fe(II/III)-L) or a mixture of Fe(II) and Portland cement (Fe(II)-C). PCE in the DNAPL mixtures was gradually reduced and it was reduced more rapidly using Fe(II)-C than Fe(II/III)-L. A constant total TCE concentration in the DNAPL mixtures was observed, which implied that the rate of loss of TCE by dechlorination and possibly other processes was equal to the rate of production of TCE by PCE dechlorination. On the other hand, 1,1,1-TCA in the DNAPL mixtures was removed rapidly and its degradation rate by Fe(II/III)-L was faster than by Fe(II)-C. The coefficients in the kinetic model (ki, Ki) were observed to decrease in the order 1,1,1-TCA>PCE>TCE, for both Fe(II/III)-L and Fe(II)-C. The concentrations of target compounds in solution were the effective solubilities, because of the assumption of phase equilibrium and were calculated with Rault's Law. The concentration changes observed were an increase and then a decrease for PCE, a sharp and then gradual increase for TCE, and a dramatic decrease for 1,1,1-TCA. The fraction of initial and theoretical reductive capacity revealed that Fe(II)-C had ability to degrade target compounds.

An equilibrium phase spray model for high-pressure fuel injection and engine combustion simulations

High-pressure fuel injection impacts mixture preparation, ignition and combustion in engines and other applications. Experimental studies have revealed the mixing-controlled and local phase equilibrium characteristics of liquid vaporization in high injection pressure diesel engine sprays. However, most computational fluid dynamics models for engine simulations spend much effort in solving for non-equilibrium spray processes. In this study, an equilibrium phase spray model is explored. The model is developed based on jet theory and a phase equilibrium assumption, without modeling drop breakup, collision and finite-rate interfacial vaporization processes. The proposed equilibrium phase spray model is validated extensively against experimental data in simulations of the engine combustion network Spray A and in an optical diesel engine. Predictions of liquid/vapor penetration, fuel mass fraction distribution, heat release rate and emission formation are all in good agreement with experimental data. In addition, good computational efficiency and grid-independency are also seen with the present equilibrium phase model. The examined operating conditions cover wide ranges that are relevant to internal combustion engines, which include ambient temperatures from 700 to 1400 K, ambient densities from 7.6 to 22.8 kg/m3 and injection pressures from 1200 to 1500 bar for diesel sprays.

Calculation of 2-temperature plasma thermo-physical properties considering condensed phases: application to CO2–CH4 plasma: part 2. Transport coefficients

The transport coefficients, namely thermal conductivity, viscosity and electrical conductivity, of CO2–CH4 mixture in and out of LTE are calculated in this paper. The calculation was based on local chemical equilibrium (LCE) and local phase equilibrium assumption. The 2-temperature composition results obtained with consideration of condensed phase in the previous paper (Part I) of this series were used in this calculation. The transport coefficients were calculated by classical Chapman–Enskog method simplified by Devoto. The results are presented for different temperatures (300–30 000 K), pressures (0.1–10 atm), non-equilibrium degrees (1–5), and CH4 molar proportions (0–100%). The influence of condensed graphite, non-LTE effect, mixture ratio and pressure on the composition and thermodynamic properties has been discussed. The results will serve as reliable reference data for computational simulation of CO2–CH4 plasmas.

Calculation of 2-temperature plasma thermo-physical properties considering condensed phases: application to CO2–CH4 plasma: part 1. Composition and thermodynamic properties

As the first part of this series of papers, a new calculation method for composition and thermodynamic properties of 2-temperature plasma considering condensed species under local chemical equilibrium (LCE) and local phase equilibrium assumption is presented. The 2-T mass action law and chemical potential are used to determine the composition of multiphase system. The thermo-physical properties of CO2–CH4 mixture, which may be a possible substitution for SF6, are calculated by this method as an example. The influence of condensed graphite, non-LTE effect, mixture ratio and pressure on the thermo-physical properties has been discussed. The results will serve as reliable reference data for computational simulation of CO2–CH4 plasmas.

One-dimensional comparison of numerical approaches on two-phase flow in the membrane electrode assembly of PEMFC

Numerical simulation of two-phase flow is important for the performance evaluation of proton exchange membrane fuel cell (PEMFC). Exchange of water between vapor and liquid phases in the gas diffusion layer (GDL) was considered by Nguyen and co-workers (2000) using the evaporation and condensation rate constants. Hsuen and Yin (2011), in which a pseudo-phase-equilibrium function approximates the vapor/liquid phase equilibrium, proposed another two-phase flow approach. Both methods are capable of predicting the cell performance and the species concentration profiles in the membrane electrode assembly (MEA) without the need to track liquid water front in the GDL explicitly. The present study compares the numerical results of these two approaches in detail. In addition, one-dimensional simplified semi-analytical solution based on the instantaneous phase equilibrium assumption is used to validate the numerical calculations. The evaporation/condensation approach allows the existence of super-saturated vapor near the interface of cathode GDL and membrane. Thus, membrane can be better hydrated near the interface once liquid water emerges in the GDL as compared to the pseudo-phase equilibrium method. Consequently, smaller ohmic resistance and hence better cell performance are observed with Nguyen's prediction once liquid water appears in the GDL.

The diffusion kinetics in isothermal epitaxy of CdxHg1 − x Te solid solutions from an unlimited liquid phase volume

The kinetics of liquid phase epitaxy of CdxHg1 − x Te solid solutions was analyzed in terms of the concept of diffusion-controlled crystallization. The boundary conditions for the boundary-value diffusion mass transfer problem were obtained on the assumption of phase equilibrium at the interface, when component concentrations are coupled by phase equilibrium equations written in terms of the polyassociative solutions model. The thermodynamic model developed for the growth process allowed us to describe the conditions of solid solution growth and predict conditions for the preparation of Cd-Hg-Te layers of the required composition.

Experimental and theoretical study of redistribution of alloying elements in Ni-based weld joints at high temperatures

Ni-based diffusion couples were investigated experimentally at 1000 and 1200 °C. The experimental results were compared with theoretical calculations. Equilibrium calculations needed were obtained using CALPHAD approach. The diffusion processes were simulated by means of DICTRA program. The thermodynamic database TCNI was used for equilibrium calculations. User kinetic database was taken for evaluation of diffusion fluxes. The experimental techniques of light microscopy and analytical electron microscopy (TEM, SEM, EDAX) were used. An additional microhardness measurement was also performed. Some particular aspects of the joining of the Ni-based superalloys are discussed. An experimental evidence of validity of local phase equilibrium assumption is given.

Investigation of density-dependent gas advection of trichloroethylene: Experiment and a model validation exercise

An experiment was conducted to evaluate whether vapor-density effects are significant in transporting volatile organic compounds (VOC's) with high vapor pressure and molecular mass through the subsurface. Trichloroethylene (TCE) was chosen for the investigation because it is a common VOC contaminant with high vapor pressure and molecular mass. For the investigation, a 2-m-long by 1-m-high by 7.5-cm-thick flow cell was constructed with a network of sampling ports. The flow cell was packed with sand, and a water table was established near the lower boundary. Liquid TCE was placed near the upper boundary of the flow cell in a chamber from which vapors could enter and migrate through the sand. TCE concentrations in the gas phase were measured by extracting 25-μl gas samples with an air-tight syringe and analyzing them with a gas chromatograph. The evolution of the TCE gas plume in the sand was investigated by examining plots of TCE concentrations over the domain for specific times and for particular locations as a function of time. To help in this analysis, a numerical model was developed that can predict the simultaneous movements of a gas, a nonaqueous liquid and water in porous media. The model also considers interphase mass transfer by employing the phase equilibrium assumption. The model was tested with one- and two-dimensional analytical solutions of fluid flow before it was used to simulate the experiment. Comparisons between experimental data and simulation results when vapor-density effects are considered were very good. When vapor-density effects were ignored, agreement was poor. These analyses suggest that vapor-density effects should be considered and that density-driven vapor advection may be an important mechanism for moving VOC's with high vapor pressures and molecular mass through the subsurface.

Combined process of reaction, extraction and distillation in a multistage column.

Reactive-extractive distillation in a multistage column is numerically investigated to improve the yield of desired intermediate product in a complex reaction combining reversible and consecutive reactions which are of the first order. On the assumption of phase equilibrium, mass balance equations are formulated in terms of a phase-splitting parameter and solved by use of the modified relaxation method. The optimal value of each decision variable is determined for tentatively chosen parameter values. Then the effect of each parameter on yield is evaluated for fixed values of the remaining parameters. The proposed combined process may have advantages over extractive reaction, reactive distillation and reaction without simultaneous separation singly if the system parameters can be chosen favorably.