A study on the interaction between local flow and flame structure for mixing-controlled Diesel sprays

[EN] A detailed study on the spray local flow and flame structure has been performed by means of PIV and laser-sheet LIF techniques under Diesel spray conditions. Operating conditions were based on Engine Combustion Network recommendations. A consistent comparison of inert and reacting axial velocity fields has produced quantitative information on the effect of heat release on the local flow. Local axial velocity has been shown to increase 50-60% compared to the inert case, while the combustion-induced radial expansion of the spray has been quantified in terms of a 0.9-2.1 mm radius increase. As a result, the drop in entrainment rate has been quantified around 25% compared to the inert case. Streamline analysis also hints at a reduced entrainment under reacting conditions. A 1D spray model under reacting condition has been used, which confirms the modifications obtained in the main flow metrics when moving from inert to reacting conditions. When comparing the flow evolution with the flame structure, little effect of chemical activity on the spray flow upstream the lift-off length has been evidenced, in spite of the presence of formaldehyde in such regions. Only downstream of the lift-off length, as defined by OH LIF, has a strong change in flow pattern been observed as a result of combustion-induced heat release. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.This work was carried out during a scientific visit period by J.M. Garcia-Oliver at IFPEN in 2015, which was funded by the Spanish Ministry of Education, Culture and Sport (Grant PRX14/00192). This study was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF (TRA2014-59483-R) project.García-Oliver, JM.; Malbec, L.; Toda, HB.; Bruneaux, G. (2017). A study on the interaction between local flow and flame structure for mixing-controlled Diesel sprays. Combustion and Flame. 179:157-171. https://doi.org/10.1016/j.combustflame.2017.01.023S15717117

A conceptual model of the flame stabilization mechanisms for a lifted Diesel-type flame based on direct numerical simulation and experiments

This work presents an analysis of the stabilization of diffusion flames created by the injection of fuel into hot air, as found in Diesel engines. It is based on experimental observations and uses a dedicated Direct Numerical Simulation (DNS) approach to construct a numerical setup, which reproduces the ignition features obtained experimentally. The resulting DNS data are then used to classify and analyze the events that allow the flame to stabilize at a certain Lift-Off Length (LOL) from the fuel injector. Both DNS and experiments reveal that this stabilization is intermittent: flame elements first auto-ignite before being convected downstream until another sudden auto-ignition event occurs closer to the fuel injector. The flame topologies associated to such events are discussed in detail using the DNS results, and a conceptual model summarizing the observation made is proposed. Results show that the main flame stabilization mechanism is auto-ignition. However, multiple reaction zone topologies, such as triple flames, are also observed at the periphery of the fuel jet helping the flame to stabilize by filling high-temperature burnt gases reservoirs localized at the periphery, which trigger auto-ignitions

Velocity field analysis of the high density, high pressure diesel spray

In this study, particle image velocimetry (PIV) measurements have been performed extensively on a non-reactive dense diesel spray injected from a single orifice injector, under various injection pressure and steady ambient conditions, in a constant flow chamber. Details of PIV setup for diesel spray measurement without additional seeding are explained first. The measured velocity profiles are compared to those obtained from other similar measurements performed in a different institution, as well as those obtained from a 1D spray model simulation, presenting in both cases a good level of agreement. In addition, the velocity fields under various injection pressures and ambient densities show the dominant effects of these parameters on the behavior of diesel spray. The self-similarity of the transverse cut profiles of axial velocity is evaluated, showing that the measurements are in agreement with the hypothesis of self-similar velocity profiles. Finally, the effect of injection pressure and ambient density on the velocity fluctuations is presented and analyzed as well. While the experimental results presented here could help to understand the complex diesel fuel-air mixing process during injection, they also provide additional spray velocity data for future computational model validation, following the main idea of the Engine Combustion Network.This work was sponsored by "Ministerio de Economia y Competitividad" of the Spanish Government in the frame of the Project "Comprension de la influencia de combustibles no convencionales en el proceso de injeccion y combustion tipo diesel", Reference TRA2012-36932. Additionally, the optical equipment used for the project was purchased with funding from Ministerio de Economia y Competitividad FEDER-ICTS-2012-06.Payri Marín, R.; Viera-Sotillo, JP.; Wang, H.; Malbec, L. (2016). Velocity field analysis of the high density, high pressure diesel spray. International Journal of Multiphase Flow. 80:69-78. https://doi.org/10.1016/j.ijmultiphaseflow.2015.10.012S69788

Study of Low Soot or Soot-Free Leaner Lifted Flame Combustion in a Light Duty Optical Engine

International audiencePrevious experimental data obtained in constant volume combustion vessels have shown that soot-free diffusive flames can be achieved in a Diesel spray if the equivalence ratio at the flame lift-off location is below 2. The so-called Leaner Lifted-Flame Combustion (LLFC) strategy is a promising approach to limit the levels of in-cylinder soot produced in Diesel engines. However, implementing such strategies in light-duty engines is not straightforward due to the effects of charge confinement , non-steady boundary conditions and spray-spray interactions compared to the simplified configuration of a free-jet in a constant-volume combustion vessel. The present study aims at trying to gain a better understanding of the requirements in terms of injector and engine settings in order to reach the LLFC regime in a light-duty engine.Experiments were performed on a 0.5L single-cylinder optical engine. Various injector nozzle geometries were investigated, offering variations in the number of holes (7 to 14), in hole diameter (63µm to 123µm) and in nozzle mass flow rate (240ml/min to 672ml/min). The impact of variations in engine operating conditions was also studied, in terms of EGR rate (0% to 65%), injection pressure (800bar to 1600bar), swirl (1 and 1.5), and boost pressure (1bar to 2.2bar). Engine-out soot emissions were measured with a smoke meter. The combustion process was characterized using a high-speed camera which collected the broadband emission of incandescent soot. An intensified camera recorded once per cycle OH* chemiluminescence at 310nm.The results shows how in-cylinder soot formation is affected by the parametric variations. In particular, for low levels of EGR, the rate of soot formation is very high and mainly driven by in-cylinder thermodynamic conditions. For high EGR levels(e.g. > 65%) the reaction rates are too low to enable soot formation, even if the thermodynamic conditions are favorable

An improved entrainment rate measurement method for transient jets from 10 KHZ particle image velocimetry.

International audienceOne strategy to reduce soot formation in compression–ignition engines is extending the ignition delay to provide more time for mixing. However, vapor–fuel concentration measurements have shown that near-injector mixtures become too lean to achieve complete combustion, leading to a relative increase in unburned hydrocarbon emissions. One potential contributor to over-leaning is an "entrainment wave," which is a transient increase in local entrainment after the end of injection. Although an entrainment wave can be predicted by a one-dimensional (1D) free-jet model, no previous measurements at diesel injection conditions have demonstrated conclusively its existence, nor has its magnitude been verified. Using particle image velocimetry (PIV) in the ambient gases, we measure entrained gas velocity through a diesel jet boundary before, during, and after the injection. The entrainment calculation depends on the definition of the jet boundary, here newly proposed based on the minimum of the radial coordinate and the radial velocity (rνr). Unlike previous formulations, the method is robust even in the presence of axial flow gradients in the ambient gases. Prior to the end of injection, the measured entrainment rates that agree well with non-reacting steady gas-jet behavior, as well as with the 1D free-jet model. After end of injection, the local entrainment rate temporarily increases by a factor of 2, which is similar to the factor 2.5 increase predicted by the 1D model. However, the entrainment wave is more broadly distributed in the experimental data, likely due to confinement and/or other real-jet processes absent in the 1D model

Hydrodynamics of gas-liquid dispersion in transparent Sulzer static mixers SMX TM

International audienceThe gas-liquid hydrodynamics in a Sulzer SMX TM static mixer was investigated in the present work through two different optical techniques: Backlight Shadowgraph Technique (BST) and Particle Image Velocimetry (PIV). 3D printed static mixers were manufactured using transparent plastic in order to provide optical access. The normal-heptane was used as the continuous liquid phase. Three different lengths of mixers and different gaseous nitrogen flow rates were investigated. The flow pattern in an empty tube without the mixing device was used as a reference. Bubble diameter distributions at the inlet and outlet of the SMX mixer were evaluated. The velocity fields inside the mixers were quantified. The gas holdup was also examined. These original results allow to appreciate the SMX static mixer'

Parametric Comparison of Well-Mixed and Flamelet n-dodecane Spray Combustion with Engine Experiments at Well Controlled Boundary Conditions

Extensive prior art within the Engine Combustion Network (ECN) using a Bosch single axial-hole injector called 'Spray A' in constant-volume vessels has provided a solid foundation from which to evaluate modeling tools relevant to spray combustion. In this paper, a new experiment using a Bosch three-hole nozzle called 'Spray B' mounted in a 2.34 L heavy-duty optical engine is compared to sector-mesh engine simulations. Two different approaches are employed to model combustion: the 'well-mixed model' considers every cell as a homogeneous reactor and employs multi-zone chemistry to reduce the computational time. The 'flamelet' approach represents combustion by an ensemble of laminar diffusion flames evolving in the mixture fraction space and can resolve the influence of mixing, or 'turbulence-chemistry interactions,' through the influence of the scalar dissipation rate on combustion. Both combustion methodologies are implemented in the Lib-ICE code which is an unsteady Reynolds-averaged Navier-Stokes solver with k-ϵ turbulence closure based on OpenFOAM® technology. Liquid length and vapor penetration predictions generally fall within the experimental measurement uncertainty at 7.5% O2, 900 K, and 15.2 kg/m3. Flame liftoff length, cylinder pressure, apparent heat release rate, and ignition delay time from the two computations are compared to experiments under single parametric variation of ambient density 15.2 kg/m3 and 22.8 kg/m3, temperatures of 800, 900, and 1000 K, at 13, 15 and 21% Oxygen and injection pressure of 500, 1000, and 1500 bar. Both models generally provide apparent heat release rate maxima to within the uncertainty. The flamelet model better predicts the sensitivity of lift-off length while the well-mixed model better predicts ignition delay

Engine combustion network: comparison of spray development, vaporization, and combustion in different combustion vessels

Development and mixing of Diesel sprays are long known to be key factors for combustion and pollutant emissions but the related measurements in a real engine is not an easy task. This fact led researchers to simulate engine conditions in special facilities that allow the use of high-fidelity diagnostics. The Engine Combustion Network (ECN) has focused on overcoming the variability from one institution to the next by testing nominally identical Diesel injectors in four different facilities for the first time, including constant-pressure flow and constant-volume preburn chambers. Liquid- and vapor-phase penetration, ignition delay, and lift-off length measurements are compared with similar experimental setups and processing methodologies. The consistency of the data obtained indicates a good level of repeatability between the test rigs employed, and no deviation of the results can be associated with the facility type. Comparison of liquid length measurements via Mie scattering shows that this diagnostic is sensitive to the orientation of the light source. For more repeatable results between facilities, diffused back-illumination imaging is recommended. A novel image processing method has been employed to detect spray boundaries obtained in high-speed schlieren imaging: the method showed high accuracy and robustness to the different schlieren setups employed by the institutions. High-speed broadband chemiluminescence, as schlieren imaging, shows the onset of cool flame, and moreover when the combustion is stabilized, it provides an important reference to define ignition delay and lift-off length. The methodology put in place by the ECN participants in this work allows an important step forward in two directions. The first is to understand the repeatability related to experimental data in high-pressure, high-temperature environments. The second is to advance the understanding of the different diagnostics applied, thereby providing more quantitative measurements that yield to a more suitable datasets for computational fluid-dynamic model evaluation.Bardi, M.; Payri Marín, R.; Malbec, LM.; Bruneaux, G.; Pickett, LM.; Manin, J.; Bazyn, T.... (2012). Engine combustion network: comparison of spray development, vaporization, and combustion in different combustion vessels. Atomization and Sprays. 22(10):807-842. doi:10.1615/AtomizSpr.2013005837S807842221

Arbeitskampfrecht VII : Leistungsstörungen durch Arbeitskampf

In a collaborative effort to identify key aspects of heavy-duty diesel injector behavior, the Engine Combustion Network (ECN) Spray C and Spray D injectors were characterized in three independent research laboratories using constant volume pre-burn vessels and a heated constant-pressure vessel. This work reports on experiments with nominally identical injectors used in different optically accessible combustion chambers, where one of the injectors was designed intentionally to promote cavitation. Optical diagnostic techniques specifically targeted liquid- and vapor-phase penetration, combustion indicators, and sooting behavior over a large range of ambient temperatures—from 850 K to 1100 K. Because the large-orifice injectors employed in this work result in flame lengths that extend well beyond the optical diagnostics’ field-of-view, a novel method using a characteristic volume is proposed for quantitative comparison of soot under such conditions. Further, the viability of extrapolating these measurements downstream is considered. The results reported in this publication explain trends and unique characteristics of the two different injectors over a range of conditions and serve as calibration targets for numerical efforts within the ECN consortium and beyond. Building on agreement for experimental results from different institutions under inert conditions, apparent differences found in combustion indicators and sooting behavior are addressed and explained. Ignition delay and soot onset are correlated and the results demonstrate the sensitivity of soot formation to the major species of the ambient gas (i.e., carbon dioxide, water, and nitrogen in the pre-burn ambient versus nitrogen only in the constant pressure vessel) when holding ambient oxygen volume percent constant