Engine Combustion Network Spray Research Articles

Numerical simulation of needle movement nozzle flow coupled with spray for a diesel injector using an Eulerian spray atomization model

This research was performed in the frame of the project Estudio de la interaccion chorro-pared en condiciones realistas de motor (SPRAY WALL) reference TRA2015-67679-c2-1-R from Ministerio de Economia y Competitividad (Spanish Ministry of Economy).

Computational analysis of end-of-injection transients and combustion recession

Mixing and combustion of engine combustion network Spray A after end of injection are modeled using highly resolved multidimensional numerical simulations to explore the physics underlying recent experimental observations of combustion recession. Reacting spray simulations are performed using a traditional Lagrangian–Eulerian coupled formulation for two-phase mixture transport with a Reynolds-averaged Navier–Stokes approach using the open-source computational fluid dynamics code OpenFOAM. Chemical kinetics models for n-dodecane by Cai et al. and Yao et al. are deployed to evaluate the impact of mechanism formulation and low-temperature chemistry on predictions of combustion recession behavior. Simulations with the Cai mechanism show that under standard Spray A conditions, the end-of-injection transient induces second-stage ignition in distinct regions near the nozzle that are initially spatially separated from the lifted diffusion flame, but then rapidly merge with flame. By contrast, the Yao mechanism fails to predict sufficient low-temperature chemistry in mixtures upstream of the diffusion flame during the end-of-injection transient and does not predict combustion recession for the same conditions. The effects of the shape and duration of the end-of-injection transient on the entrainment wave near the nozzle, the likelihood of combustion recession, and the spatiotemporal development of mixing and chemistry in near-nozzle mixtures are also investigated. With a more rapid ramp-down injection profile (ramp-down duration < 400 µs), a weaker combustion recession occurs earlier in time after the start of ramp-down. For extremely fast ramp-down (ramp-down duration = 0), the entrainment flux varies rapidly near the nozzle and over-leaning of the mixture completely suppresses combustion recession. For a slower ramp-down profile with respect to the standard Spray A condition, complete combustion recession back toward the nozzle is observed and combustion recession occurred later in time. Simulations qualitatively agreed with the past experimental and modeling observations of combustion recession with different end-of-injection transients.

High-Resolution X-Ray and Neutron Computed Tomography of an Engine Combustion Network Spray G Gasoline Injector

High-Resolution X-Ray and Neutron Computed Tomography of an Engine Combustion Network Spray G Gasoline Injector

Numerical study of the internal flow and initial mixing of diesel injector nozzles with V-type intersecting holes

A V-type intersecting hole nozzle, in which each hole is formed by the coalescence of a pair of sub-holes, has been developed to improve fuel-air mixing for diesel engines. In this paper, a numerical study has been conducted to examine the effects of the V-type intersecting hole structure on the internal flow of a nozzle and the initial stage of the fuel-air mixing processes. With this aim, a multi-phase flow calculation has been implemented on seven V-type intersecting hole nozzles with impact angles ranging from 20° to 50° and a referenced cylindrical hole nozzle under injection pressures varying from 60MPa to 240MPa. The comparison was made in terms of mass flow, momentum flux, effective velocity, discharge coefficient, area coefficient and velocity coefficient. The three-fluid model, which was validated using X-ray experimental data of an Engine Combustion Network (ECN) Spray A injector, was employed to calculate the associated multi-phase flow. The main results show that use of a V-type intersecting hole in a nozzle eliminates cavitation, leading to higher mass flow and momentum flux. Correspondingly, for these V-type intersecting hole nozzles, the discharge coefficients are insensitive to injection pressure, but decrease with an increasing impact angle, and are 20–30% higher than those of the cylindrical hole nozzle. These higher discharge coefficients mainly result from very high area coefficients that are approximately 0.98 at all injection pressure conditions because of the non-cavitating nature of the in-nozzle flow. Moreover, the impact effect of a V-type intersecting hole results in a fan-shaped jet with a very large spreading angle (25–40°) at the dispersion plane and a relatively narrower angle (approximately 12°) at the impact plane. The jet spreading angles at both planes are wider than that of the cylindrical hole nozzle (approximately 7°), indicating a noticeable improvement in the initial fuel-air mixing. Additionally, an increase in the impact angle of a V-type intersecting hole nozzle promotes initial mixing in terms of yielding a wider jet spreading angle, in spite of a slightly lower effective velocity.

Modeling combustion under engine combustion network Spray A conditions with multiple injections using the transported probability density function method

In this study, numerical simulations of an n-dodecane spray flame—known as Spray A—with multiple injections (0.5 ms injection/0.5 ms dwell/0.5 ms injection) have been carried out using the transpor...

An equivalent dissipation rate model for capturing history effects in non-premixed flames

The effects of strain rate history on turbulent flames have been studied in the past decades with 1D counter flow diffusion flame (CFDF) configurations subjected to oscillating strain rates. In this work, these unsteady effects are studied for complex hydrocarbon fuel surrogates at engine relevant conditions with unsteady strain rates experienced by flamelets in a typical spray flame. Tabulated combustion models are based on a steady scalar dissipation rate (SDR) assumption and hence cannot capture these unsteady strain effects; even though they can capture the unsteady chemistry. In this work, 1D CFDF with varying strain rates are simulated using two different modeling approaches: steady SDR assumption and unsteady flamelet model. Comparative studies show that the history effects due to unsteady SDR are directly proportional to the temporal gradient of the SDR. A new equivalent SDR model based on the history of a flamelet is proposed. An averaging procedure is constructed such that the most recent histories are given higher weights. This equivalent SDR is then used with the steady SDR assumption in 1D flamelets. Results show a good agreement between tabulated flamelet solution and the unsteady flamelet results. This equivalent SDR concept is further implemented and compared against 3D spray flames (Engine Combustion Network Spray A). Tabulated models based on steady SDR assumption under-predict autoignition and flame lift-off when compared with an unsteady Representative Interactive Flamelet (RIF) model. However, equivalent SDR model coupled with the tabulated model predicted autoignition and flame lift-off very close to those reported by the RIF model. This model is further validated for a range of injection pressures for Spray A flames. The new modeling framework now enables tabulated models with significantly lower computational cost to account for unsteady history effects.

ECN Spray G external spray visualization and spray collapse description through penetration and morphology analysis

Inside a DISI engine, a wide range of pressure and temperature conditions are possible, and with the current evolution of the systems, many of the conditions are subject to be encountered at the moment of injection. Given the great differences between Diesel injectors and GDi fuel injectors, the effects of such conditions on the development of the fuel injected can cause phenomena like flash boiling and spray collapse that fundamentally change the behavior of sprays. In this work, the Spray G injector developed by Delphi for the Engine Combustion Network (ECN) group has been tested in a High Pressure High Temperature Constant Pressure Flow Rig (HPHT - CPFR) in a wide range of experimental conditions capturing the liquid and vapor phases of the spray by means of DBI and Schlieren imaging. The work presents the results obtained by spray visualization through comparisons of parametric variations with special focus on the collapse of the spray that occurs under high ambient temperature and density conditions. Spray collapse has been described by showing the direct increase that can cause in spray penetration and the great closing effect that can produce to the aperture of the spray (spray angle). Several contour comparisons using the raw images and the detected contours have been discussed in order to support and further explain the observed trends.

Modeling soot emissions from wall films in a direct-injection spark-ignition engine

Due to the new challenge of meeting number-based regulations for particulate matter in spark-ignition engines, and the important contributions of wall films to soot emissions, a computational fluid dynamics model was formulated and improved to enable the prediction of soot emissions (mass and number density) from spark-ignition engines, with consideration of multi-component fuel surrogates to represent real gasoline and with full three-dimensional cylinder engine grids. A chemistry mechanism including n-heptane/iso-octane/toluene/polycyclic aromatic hydrocarbon (89 species and 506 reactions) was formulated, and the model was also validated based on available experimental data from laminar premixed flame burners and the SANDIA engine combustion network spray combustion data. Fuel pyrolysis was also found to be a significant process for soot formation in direct-injection spark-ignition engines. The vaporization of the wall films plays a significant role in soot formation, and a grid-independent wall film model was achieved for predicting soot emissions near wall films. The improved models were then applied to study of soot emissions from late-injection strategies in a four-valve single-cylinder gasoline direct-injection spark-ignition engine, and the trends seen are consistent with experimental data.

Microscopic investigation of the atomization and mixing processes of diesel sprays injected into high pressure and temperature environments

Atomization and mixing of sprays are key parameters to successfully describe and predict combustion in direct-injection engines. Understanding these processes at the conditions most relevant to engines (high pressures and temperatures) is of primary interest. The present work investigates the atomization and mixing processes of sprays injected into environments with progressively higher pressure and temperature. An Engine Combustion Network Spray A single-hole injector is used to inject n-dodecane fuel into an optically accessible combustion chamber. High-speed imaging is performed using long-distance microscopy and diffused back-illumination to resolve ligament structures and droplets in the near-nozzle region. A unique aspect of this study is the application of high-speed imaging of sprays in the near-field under realistic diesel conditions, particularly those near top-dead-center. Imaging showed droplet and ligament dynamics at low pressure and temperature, but not at high pressure and temperature typical of engine operation, indicating diminished effects of surface tension. Relating the ambient conditions to the fuel critical pressure and temperature showed that when the ambient conditions are slightly above the critical point of the fuel, surface tension remains in effect at gas–liquid interfaces. However, for higher pressure and temperature conditions, the surface tension appears to diminish significantly as expected for supercritical fuel–air mixtures.

Characterization of a Set of ECN Spray A Injectors: Nozzle to Nozzle Variations and Effect on Spray Characteristics

The Engine Combustion Network (ECN) is becoming a leading group concerning the experimental and computational analysis of Engine combustion. In order to establish a coherent database for model validation, all the institutions participating to the experimental effort carry out experiments at well-defined standard conditions (in particular at Spray A conditions: 22.8kg/m3, 900K, 0% and 15% O2) and with Diesel injectors having the same specifications. Due to the rising number of ECN participants and also to unavoidable damages, additional injectors are required. This raises the question of injector's characteristics reproducibility and of the appropriate method to introduce such new injectors in the ECN network. In order to investigate this issue, a set of 8 new injectors with identical nominal Spray A specification were purchased and 4 of them were characterized using ECN standard diagnostics. In particular, the measurements include the nozzle hole diameter, the rate of injection, the liquid and vapor penetrations, the auto-ignition delay and the lift-off length. Variations of ambient temperature, oxygen concentration and density have also been performed. In general the results show similar behavior to ECN standard injectors, confirming that this set of new injectors can be integrated into the pool of ECN injectors. However, discrepancies between spray characteristics were observed, although the injector specifications and the boundary conditions were sensibly the same. The sources of variations from injector to injector are analyzed in order to provide new information on the reproducibility of injectors characteristics, and improve the comparison methodology between experimental data and simulation.

High-Speed Characterization of ECN Spray A Using Various Diagnostic Techniques

Diesel spray experimentation at controlled high-temperature and high-pressure conditions is intended to provide a more fundamental understanding of diesel combustion than can be achieved in engine experiments. This level of understanding is needed to develop the high-fidelity multi-scale CFD models that will be used to optimize future engine designs. Several spray chamber facilities capable of high-temperature, igh-pressure conditions typical of engine combustion have been developed, but because of the uniqueness of each facility, there are uncertainties about their operation. The Engine Combustion Network (ECN) is a worldwide group of institutions using combustion vessels, whose aim is to advance the state of spray and combustion knowledge at engine-relevant conditions. A key activity is the use of spray chamber facilities operated at specific target conditions in order to leverage research capabilities and advanced diagnostics of all ECN participants. The first target condition, called “Spray A”, has been defined with detailed ambient and injector conditions. For this paper, we describe results from the constant-volume pre-burn vessel at Eindhoven University of Technology. The executed measurements include a wide range of diagnostics to characterize “spray A” in reacting and non-reacting conditions in great detail. Observations of spray penetration, ignition, liquid length and flame lift-off location by using several high-speed imaging diagnostics are discussed and compared with other ECN participating institutes. Comparison Spray A data from the other participating institutes, as it was presented during the 2nd ECN workshop is gathered from the ECN website database [1]. It can be concluded that the obtained results from the standardized ECN spray diagnostics, show satisfactory similarity, despite of the challenge to reach similar boundary conditions (ambient and injector) in each of the unique facilities. The differences in results are within the measurement deviation and uncertainty or can be explained by the usage of (slightly) different injectors. Combining the results of the different measurement techniques provides an overall (time resolved) overview where the different phases of fuel injection are directly linked and summarized. The presented overview provides a direct input for (CFD) modeling validation.