Diesel Spray Simulation Research Articles

Diesel Spray Simulation under Non-evaporating Condition with WAVE and Modified Improved TAB Model

Diesel Spray Simulation under Non-evaporating Condition with WAVE and Modified Improved TAB Model

Effects of different droplet dispersion modeling methods on diesel spray simulation in Eulerian-Lagrangian framework

The realistic prediction of spray dynamics and dispersion is one of the main tasks of the combustion simulations in diesel engines. Usually, droplet dispersion modeling accuracy can be improved by the development of turbulence models and stochastic dispersion models. Although droplet dispersion due to turbulence is an important phenomenon in high-velocity fuel sprays, studies on the effect of these models are relatively few. Among these studies, the influence of a common used stochastic dispersion model on spray dynamics is not well understood and the application of hybrid turbulence models to fuel spray simulation in the Eulerian-Lagrangian framework with two-way coupling is still at starting stage. In the present study, a better understanding of the effect of the stochastic model on spray dynamics can be obtained by using a statistical analysis of stochastic dispersion time and velocity of dispersed droplets. In order to assess the ability of the hybrid model to improve the radial spread and the inhomogenous distribution of small droplets produced by a high-pressure injector, a partially averaged Navier-Stokes (PANS) model is implemented in an open-source code OpenFOAM and is compared with an unsteady Reynolds averaged Navier-Stokes (RANS) model and a large eddy simulation (LES) model. The results show the importance of resolving large-scale turbulent structures on droplet dispersion modeling in high-velocity fuel spray simulation. The common stochastic dispersion model used in RANS developed for homogenous, isotropic turbulent flows leads to the non-physical dynamics and distribution of droplets and hence is not applicable to the droplet dispersion which is dominated by the non-stochastic motion of large-scale structures in free shear layers. Using PANS instead of RANS should improve the droplet dispersion modeling accuracy of high-velocity fuel sprays in the development of diesel engines with an advanced injection system.

A new approach to compute temperature in a liquid-gas mixture. Application to study the effect of wall nozzle temperature on a Diesel injector

Thermal effects on internal combustion engines have been always a hot topic. Its effects also on the injection system are still under research. In this work, a homogeneous model called Eulerian Spray Atomization (ESA) model is used to simulate Engine Combustion Network (ECN) Spray A conditions. A new approach is used for the thermodynamic model in Diesel spray simulations. Experimental values of the liquid enthalpy are directly used to obtain the temperature, without any need of knowing its heat capacity. This allows to accurately study heating and cooling phenomena inside the Diesel nozzle. The results show that two different boundary conditions can be applied at nozzle walls, either fixed temperature or adiabatic walls, and obtain good prediction of injection parameters. Also, the effect of fuel temperature has been investigated. For Spray A conditions, nozzle and spray parameters are the same regardless the fuel temperature, but fluid properties inside the nozzle (density, velocity...) significantly change.

Statistical Approach on Visualizing Multi-Variable Interactions in a Hybrid Breakup Model under ECN Spray Conditions

The Direct Numerical Simulation (DNS) approach to solving the fundamental transport equations down to the smallest scales of motion is favorable should the requirement be a truly predictive solution of fluid dynamic problems, but the simulation run times are unacceptable for most practical industrial applications. Despite the steadily increasing computational capabilities, Reynolds Averaged Navier-Stokes (RANS) based frameworks remain the most commercially viable option for high volume sectors, like automotive. The sub models within RANS simplify the description of key physical phenomena and include several numerical constants. These so-called “tuning constants” introduce multivariable dependencies that are almost impossible to untangle with local sensitivity studies. This paper addresses the prevailing difficulties in setting up an adequate diesel spray simulation which arise from the mentioned multi-variable interactions of these “tuning constants”, by applying a statistical approach named Design of Experiments (DoE). Often combined with an optimizer, DoE is commonly used to find an optimum set of engine parameters for set criteria at reduced experimental effort. In this case, the methodology was applied to determine an optimal set of “tuning constants” for the simulations which best matched experimental data at five conditions taken from the Engine Combustion Network (ECN) database. Multi-variable DoE were run for each condition. Stochastic response models (SPM’s) highlighted crucial simulation sensitivities of the turbulent dissipation constant C2and liquid/gas-phase momentum transfer at injection pressure swings. Further, a comparison of the breakup models which produced matching simulations exhibited patterns which correspond with physical processes. Lastly, it was shown that while a single set of the constants can give reasonable results in the space explored, there is merit in adjusting key constants to suit the operating condition in the search for accuracy.

A parallel volume of fluid-Lagrangian Parcel Tracking coupling procedure for diesel spray modelling

A parallel computing Eulerian/Lagrangian multi-scale coupling procedure for diesel spray simulation is presented. Early breakup of the diesel jet is captured by using a compressible Volume of Fluid (VOF) method. In regions where the phase interface can no longer be sufficiently resolved, separated and small scale liquid structures are described by a Lagrangian Parcel Tracking (LPT) approach, in conjunction with secondary breakup modelling and a turbulence stochastic dispersion model. The coupling of these two descriptions utilises a Region Coupling Method and an efficiently parallelised droplet identification and extraction procedure. This approach enables run-time VOF-LPT field coupling and filters small-scale liquid structures that are suitable candidates for Eulerian-liquid-structure/Lagrangian droplet conversion, preserving their position, mass and momentum. The coupling procedure is initially applied to model the atomisation of a simple liquid jet and the results are compared with that of a statistical coupling approach to demonstrate the performance of the developed coupling procedure. Its application is then extended to simulate a real diesel spray from a nozzle with a sharp entrance. Coupling in-nozzle phenomena such as flow separation, flow detachment and turbulence to the primary and secondary spray atomisation, provides a tool for the prediction of complex spray dynamics.

Computational and Experimental Investigation of Interfacial Area in Near-Field Diesel Spray Simulation

Authors acknowledge that part of this work was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF (TRA2014-59483-R) project.

On Multi-Fluid models for spray-resolved LES of reacting jets

Numerical simulation of sprays can potentially be used for assessing the performance and variability of engines. But today’s simulations only provide average quantities, after calibration. Spray injection can be thought of as a multiscale problem with 4 levels (chamber, mixing layer, drop scale, and nozzle). Because of their complex interactions, increasing predictivity requires simulations to capture more of these scales. To assess the trade-offs in this regard, we perform a review of recent Diesel spray simulations. After highlighting the importance of the mixing layer in driving spray dynamics, we analyze various two-phase flow formalisms and show the potential benefits of a Eulerian spray formulation combined with Large Eddy Simulation (LES) to capture a variable-density mixing layer (VDML). We then present a framework and numerical methods to make this description operative and show how it performs on a realistic autoignition case called Spray A.

Assessment and validation of liquid breakup models for high-pressure dense diesel sprays

Liquid breakup in fuel spray and atomization significantly affects the consequent mixture formation, combustion behavior, and emission formation processes in a direct injection diesel engine. In this paper, different models for liquid breakup processes in high-pressure dense diesel sprays and its impact on multi-dimensional diesel engine simulation have been evaluated against experimental observations, along with the influence of the liquid breakup models and the sensitivity of model parameters on diesel sprays and diesel engine simulations. It is found that the modified Kelvin-Helmholtz (KH)–Rayleigh-Taylor (RT) breakup model gives the most reasonable predicted results in both engine simulation and high-pressure diesel spray simulation. For the standard KH-RT model, the model constant Cbl for the breakup length has a significant effect on the predictability of the model, and a fixed value of the constant Cbl cannot provide a satisfactory result for different operation conditions. The Taylor-analogybreakup (TAB) based models and the RT model do not provide reasonable predictions for the characteristics of high-pressure sprays and simulated engine performance and emissions.

Accelerating Diesel Spray Simulation by Large-scale Parallel Computing with OpenFOAM

Accelerating Diesel Spray Simulation by Large-scale Parallel Computing with OpenFOAM

A RECENT EULERIAN–LAGRANGIAN CFD METHODOLOGY FOR MODELING DIRECT INJECTION DIESEL SPRAYS

The global objective of this work is to show the capabilities of the Eulerian–Lagrangian spray atomization (ELSA) model for the simulation of Diesel sprays in cold starting conditions. Our main topic is to focus in the analysis of spray formation and its evolution at low temperature 255 K (-18°C) and nonevaporative conditions. Spray behavior and several macroscopic properties, included the liquid spray penetration, and cone angle are also characterized. This study has been carried out using different ambient temperature and chamber pressure conditions. Additionally, the variations of several technical quantities, as the area coefficient and effective diameter are also studied. The results are compared with the latest experimental results in this field obtained in our institute. In the meantime, we also compare with the normal ambient temperature at 298 K (25°C) where the numerical validation of the model has shown a good agreement.

Effect of breakup model on large-eddy simulation of diesel spray evolution under high back pressures

A breakup model has been developed and optimized for diesel spray simulation with a large-eddy simulation. Previously, diesel spray analysis has been carried out using a breakup model such as the Kelvin–Helmholtz and Rayleigh–Taylor model. The Kelvin–Helmholtz and Rayleigh–Taylor models are used for high Weber number conditions. However, even when fuel is injected at high pressures, the downstream region of spray corresponds to relatively low Weber number conditions. Hence, a hybrid breakup model that combines the Kelvin–Helmholtz and modified Taylor analogy breakup models (Kelvin–Helmholtz–modified Taylor analogy breakup) has been developed. The Kelvin–Helmholtz and modified Taylor analogy breakup models are used to model the primary and secondary breakup, respectively. The modified Taylor analogy breakup model is more suitable than the Rayleigh–Taylor model to describe secondary breakup. Spray simulations under non-evaporative and evaporative conditions were performed to validate the Kelvin–Helmholtz–modified Taylor analogy breakup model. It is found that the simulation results of Kelvin–Helmholtz–modified Taylor analogy breakup are in good agreement with experimental measurements of non-evaporative and evaporative spray.

Evaluation of combustion models based on tabulated chemistry and presumed probability density function approach for diesel spray simulation

Two combustion models of different complexity have been implemented in a RANS solver in the CFD platform OpenFOAM®. Both models rely on the flame prolongation of ILDM (FPI) method, which allows the use of detailed chemistry mechanisms at relatively low computational costs. The homogeneous auto-ignition (HAI) model, directly using the chemical data from the FPI tabulation, does not take into account subgrid turbulence-chemistry interaction. Therefore, the second, more advanced model combines the FPI method with a presumed conditional moment approach. This auto-ignition-presumed conditional moment (AI-PCM) model accounts for the fluctuations of the mixture fraction and the progress variable caused by the turbulent flow. Both models have been evaluated by means of a parametric study of a single diesel spray at varying initial temperatures and oxygen concentration levels. The results obtained with the CFD models have been compared with experimental data from the engine combustion network (ECN). The comparison of the two models demonstrates the important role of the subgrid turbulence-chemistry interaction on the accuracy of the auto-ignition process and the diesel flame structure, as indicated by the agreement of the AI-PCM predictions with the measured data.

Heavy Fuel Oil Combustion Modelling Using Continuous Thermodynamics

Global shipping uses Heavy Fuel Oil (HFO) as the primary fuel. HFO is a mixture of oil refinery residue and cutter stock, hence it contains large numbers of hydrocarbons with a wide range of molecular weights and dissimilar structures. Conventional thermodynamics modelling techniques are inadequate for HFO vaporisation modelling. The present article summarises continuous thermodynamics vaporization models along with chemical kinetics models for decomposition and polymerisation, and soot burnout models, employed to study the behaviour of HFO spray combustion. A sample of HFO is also characterised in the laboratory using chemical characterisation procedures including sequential elution solvent chromatography, gas-chromatography mass spectrometry and elemental analysis, to obtain the composition and mean molecular weights of HFO fractions required for continuous thermodynamics modelling. Models are implemented via subroutines in a diesel spray simulation in the CFD package STAR-CD. The simulation is applied to a constant volume spray combustion chamber. The present model accounts for the formation and burnout of liquid phase soot in diesel engines. This soot will be emitted as Black Carbon if unburnt. The models are examined for two representative fuel samples; one with the good combustion quality and the other with poor. Good qualitative agreement is shown between the simulations and published experimental data.

A combined 1D3D–CFD approach for reducing mesh dependency in Diesel spray calculations

A 1D3D–CFD coupled spray model is proposed in this work for the simulation of Diesel sprays under non-evaporative conditions and constant injection velocity in time. The basic idea of the model is to reduce the poor estimations of the gas velocity and droplets/gas relative velocity obtained with the standard 3D–CFD Eulerian–Lagrangian spray model, when coarse meshes are used. The coupling has been achieved in the calculation of the momentum source interaction term. General considerations, descriptions and implementation of the model in a commercial CFD code are outlined. Diesel spray simulations performed using the proposed approach have been compared with those obtained with the standard 3D–CFD, 1D models and experimental data. Encouraging results were found in terms of spray evolution when changing meshes and ambient conditions.

G0700-3-3 移流スキームがディーゼル噴霧のLES解析に与える影響(エンジンシステム部門一般講演(3))

Large Eddy Simulation (LES) directly solves vortexes larger than a computational grid size. Therefore, LES is expected to make better predictions of unsteady turbulent reacting flow. This study was focused on refine predictions of diesel spray simulation, using KIVALES. Thus, we incorporated central difference method and forth-order Runge-Kutta method for the convection term of momentum equations and calculated evaporating diesel spray and diesel spray flame. The results showed that unsteady behavior in the near nozzle locations was predicted as compared with the conventional simulation results using CIP method. Additionally, good agreement of liquid and vapor penetration length and the heat release rate was obtained between the computational result and the experiments result.

A model for the injection boundary conditions in the context of 3D simulation of Diesel Spray: Methodology and validation

Downstream Inflow Turbulent Boundary Conditions (DITurBC) are presented for the Eulerian–Eulerian Large Eddy Simulation (LES) or Reynolds Average Navier–Stokes (RANS) simulation of Diesel Sprays. These boundary conditions initiate the spray physics close to the nozzle exit, which avoids the difficulties linked to the 3D simulation of cavitation, primary break-up and turbulence in the near-nozzle region. An injector model is combined with mass and axial momentum conservation equations to obtain mean profiles of velocity, volume fraction and droplet diameter at a given distance downstream from the nozzle exit. In order to take into account the unsteadiness of the flow, velocity fluctuations are added to the mean profile. These boundary conditions are assessed by comparison with existing data on injection velocity, spray angle and velocity profiles from numerous experiments.

Evaporating and non-evaporating diesel spray simulation: comparison between the ETAB and wave breakup model

Evaporating and non-evaporating diesel sprays have been simulated using the Enhanced Taylor Analogy Breakup (ETAB) and the wave atomisation and breakup models implemented in STAR-CD and compared with experiments. The model validation was carried out by means of experimental data of sprays under controlled conditions in a constant-volume bomb. The model constants were calibrated for a specific test case low injection pressure, non-evaporating conditions and a small nozzle diameter and subsequently their robustness with respect to changes in boundary conditions was assessed for different injection pressures, evaporating conditions and larger nozzle diameters. The results suggest that, for suitable mesh resolutions with calibrated model constants, excellent agreement with experimental data can be achieved with both models. With these calibrated model parameters held constant, accurate predictions for spray penetrations, droplet diameters and spray angles could be obtained also for different injection configurations and air conditions by both models.

SINGLE-DROP EMPIRICAL MODELS FOR SPRAY IMPACT ON SOLID WALLS: A REVIEW

Simplified impingement models are largely used in numerical simulation of spray impingement on solid walls, since the direct simulation of multiple drop impact is outside of the present computer capabilities. These models are usually semiempirical, based on single-drop impact experimental data and some reasonable constraints to allow extrapolation to different impact conditions. This article presents a comparison of the most commonly used drop impingement models in the present scientific literature and in commercial codes. A classification of the models and a unified scheme is proposed in terms of driving variables and outcomes. Every model is then analyzed and its peculiarities and possible weaknesses are pointed out, especially those related to the physical consistency. Finally, a comparison among the various models for a possible use in Gasoline Direct Injection (GDI) and diesel spray simulation is presented.

9436026 Study on necessary grid-size conditions for diesel spray simulation with KIVA II: Kazuyuki Fukushima (Hokkaido University Graduate School), Kazushige Kikuta, Takemi Chikahisa, Tadashi Murayama (Hokkaida University), pp. 13–16, 10 figs., 5 refs

9436026 Study on necessary grid-size conditions for diesel spray simulation with KIVA II: Kazuyuki Fukushima (Hokkaido University Graduate School), Kazushige Kikuta, Takemi Chikahisa, Tadashi Murayama (Hokkaida University), pp. 13–16, 10 figs., 5 refs