Decarbonising energy systems is a prevalent topic in the current literature on climate change mitigation, but the additional climate burden caused by methane emissions along the natural gas value chain is rarely discussed at the system level. Considering a two-basket greenhouse gas neutrality objective (both CO 2 and methane), we model cost-optimal European energy transition pathways towards 2050. Our analysis shows that adoption of best available methane abatement technologies can entail an 80% reduction in methane leakage, limiting the additional environmental burden to 8% of direct CO 2 emissions (vs. 35% today). We show that, while renewable energy sources are key drivers of climate neutrality, the role of natural gas strongly depends on actions to abate both associated CO 2 and methane emissions. Moreover, clean hydrogen (produced mainly from renewables) can replace natural gas in a substantial proportion of its end-uses, satisfying nearly a quarter of final energy demand in a climate-neutral Europe.

Depuis 2010 et la crise des terres rares, les pays consommateurs de ressources minérales, soucieux de la sécurisation de leurs approvisionnements pour subvenir à leurs besoins stratégiques, tentent d’établir des critères quantitatifs pour évaluer leur criticité. Néanmoins, ces indicateurs manquent souvent de vision à long terme et complète de la chaîne de valeur, de la mine au produit final, qui devient nécessaire face à l’incertitude naissante sur les marchés des matériaux. Cette forte incertitude découle d’une part de la recherche d’autonomie des pays consommateurs dans un contexte de forte hausse de la demande et d’offre contrainte, et d’autre part des stratégies envisagées par les pays producteurs visant à profiter de la manne financière de leurs ressources sans pour autant reprimariser leur économie. Dans ce contexte géopolitique incertain, l’ensemble des producteurs et consommateurs devraient profiter de la dynamique liée aux métaux pour structurer de manière globale les marchés, en intégrant les critères sociaux et environnementaux et en mettant en place une gouvernance mondiale des matériaux.

Low-carbon hydrogen has already been announced by many countries as one of their priorities for achieving carbon neutrality by 2050. In Europe, particularly, a political momentum for hydrogen use has strengthened in recent years. This potential for massive development of low-carbon hydrogen technologies in the coming decades raises questions about the world's future dependence on mineral resources and the associated environmental impacts. The aim of this article is to address this issue by assessing the raw materials consumption associated with hydrogen development in Europe up to 2050. The results are based on an analysis of the European hydrogen development pathways resulting from the H24EU project. They highlight the importance of platinum-group metals in electrolysers deployment. Annual iridium demand for PEM electrolysers installed in the European Union could cause major supply tensions since it could reach 164% of its current global production by 2050. Nickel, cobalt, and copper, already associated with high risks of bottlenecks in the energy transition, are not left out: the development of alkaline electrolysis could contribute to a substantial increase in demand for these strategic materials.

The 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 (Ply). 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's performance and thus to open new industrial applications involving gas-liquid flows such as striping and purification of liquids by a gas. (C) 2019 Elsevier Ltd. All rights reserved.

One of the objective of Engine Combustion Network (ECN), (https://ecn.sandia.gov/) is to provide experimental results with high accuracy in order to validate model and reach new steps in scientific understanding of spray combustion at conditions specific to engines. The ECN community defines different target conditions, experimental diagnostics and post processing methods to facilitate the comparison of experimental and simulations studies performed in different facilities or models. In this context two French laboratories propose two new facilities, based on Rapid Compression Machines to reach the ECN spray A conditions. In this paper, the results of liquid and vapour spray penetration as well as Ignition Delay (ID) and Lift-Off Length (LOL) obtained with these Rapid Compression Machines are compared to the results obtained in the Constant Volume Preburn (CVP) vessel of IFPEN. The specificities of each experimental apparatus allow to bring complementary elements of understanding like confinement effects. In non-reactive condition, the liquid and vapour sprays were characterized by Diffused-Back Illumination and Schlieren technique, and in reactive conditions, the LOL and the ID by OH* chemiluminescence. The analysis of the results with regard to the boundary conditions (temperature, velocity, confinement) make it possible to validate these two new facilities and contribute to enhance the database of ECN, highlighting the confinement effect typical of piston engine operation.

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 extra-polating 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.

Diesel spray mixture formation is investigated at target conditions using multiple diagnostics and laboratories. High-speed Particle Image Velocimetry (PIV) is used to measure the velocity field inside and outside the jet simultaneously with a new frame straddling synchronization scheme. The PIV measurements are carried out in the Engine Combustion Network Spray A target conditions, enabling direct comparisons with mixture fraction measurements previously performed in the same conditions, and forming a unique database at diesel conditions. A 1D spray model, based upon mass and momentum exchange between axial control volumes and near-Gaussian velocity and mixture fraction profiles is evaluated against the data. The 1D spray model quantitatively predicts the main spray characteristics (average mixture fraction and velocity fields) within the measurement uncertainty for a wide range of parametric variations, verifying that a Diesel spray becomes momentum controlled and has a Gaussian profile. A required input to the model is the jet angle, which is obtained experimentally. Although an expected result for a gas jet, this is the first time that combined datasets of velocity and mixture fraction have been obtained in vaporizing sprays at Diesel conditions (900 K, 60 bar). Finally, these results show that a consistent database can be built using advanced diagnostics performed by different institutions when the boundary conditions are well known as prescribed by the ECN Spray A framework.

The Engine Combustion Network (ECN) community has greatly contributed to improve the fundamental understanding of spray atomization and combustion at conditions relevant to internal combustion engines. In this context, standardized spray experiments have been defined to facilitate the comparison of experimental and simulation studies performed in different facilities and with different models. This operating mode promotes collaborations among research groups and accelerates the advancement of research on spray. In efforts to improve the comparability of the ECN spray A experiments, it is of high importance to review the boundary conditions of different devices used in the community. This work is issued from the collaboration in the ECN France project, where two new experimental facilities from PPRIME (Poitiers) and PRISME (Orleans) institutes are validated to perform spray A experiments. The two facilities, based on Rapid Compression Machine (RCM) design, have been investigated to characterize their boundary conditions (e.g., flow velocity as well as fuel and gas temperatures). A set of standardized spray experiments were performed to compare their results with those obtained in other facilities, in particular the Constant Volume Pre-burn (CVP) vessel at IFPEN . It is noteworthy that it is the first time that RCM type facilities are used in such a way within the ECN. This paper (part 1) focuses on the facilities description and the fine characterization of their boundary conditions. A further paper (part 2) will present the results obtained with the same facilities performing ECN standard spray A characterizations. The reported review of thermocouple thermometry highlights that it is necessary to use thin-wires and bare-bead junction as small as possible. This would help to measure the temperature fluctuations with a minimal need for error corrections, which are highly dependent on the proper estimation of the velocity through the junction, and therefore it may introduce important uncertainties. Temperature heterogeneities are observed in all spray A devices. The standard deviation of the temperature distribution at the time of injection is approximately 5%. We report time-resolved temperature measurement from PPRIME RCM, performed in the near nozzle area during the injection. In inert condition, colder gases from the boundary layer are entrained toward the mixing area of the spray causing a further deviation from the target temperature. This emphasizes the importance of the temperature in the boundary (wall) layer. In reacting condition, the temperature of these entrained gases increases by the effect of the increased pressure, as the RCM has a relatively small volume. Generally, the velocity and turbulence levels are an order of magnitude higher in RCM and constant pressure flow compared to CVP vessels. The boundary characterization presented here will be the base for discussing spray behavior in the part 2 of this paper.

A workflow for the assessment and validation of internal aerodynamics and mixture preparation in a representative high-tumble optical engine using Large Eddy Simulation with the commercial code CONVERGETM is proposed. First, the prediction of the aerodynamic movement in the engine is compared to Particle Image Velocimetry (PIV) measurements. The global velocity fields and position of the center of the tumble for the average experimental and simulation cycles are compared, showing a very good match of the global behavior. The cycle to cycle aerodynamic variability is investigated thanks to velocity profiles, showing that the simulated cycles feature comparable velocity fluctuations to the experiments. In a second part, to account for Direct Injection (DI), a Lagrangian spray modeling approach is used to take into account the injection process. Experimental spray penetration data are used for the calibration of the spray models, leading to a faithful representation of the spray. Mixture preparation in the engine is then assessed, comparing the fuel air equivalence ratio of the LES in the central plane to LIF diagnostics. This qualitative comparison shows a good prediction of the evolution of the position of the fuel-rich areas and overall mixing during the compression stroke. The cycle to cycle variability of the mixing process is found comparable to the experiments, though underestimated. An evaluation of this methodology is given and the next steps towards combustion computations are discussed.

The interaction of single droplets with hot solid surfaces is a fundamental process for a wide range of technical applications, such as urea-water solution (UWS) injection in selective catalytic reduction (SCR) systems. Specifically, the chemical evolution and film topology and state of UWS after its deposit on a hot surface are still not well known, although these phenomena are important for the SCR efficiency. In this work, we have experimentally and numerically investigated the behaviour of solid urea after being deposited on a heated surface. The experimental apparatus consists of a metal saucer whose surface can be heated up to 750K, on which a solid urea crystal is gently deposited. The evolution of the deposit is monitored with two cameras, showing top and side views, to record the dynamics and thermal behaviour of the urea. The images have shown four different urea thermolysis regimes. First, urea ball melts with a melting rate increasing with wall temperature (Tw). It forms a sessile droplet at the saucer bottom that maintains its shape for a long time indicating a very small thermolysis rate. Second, at a relatively low Tw, but higher than 406K, some bubbles appear in the molten sessile droplet due to urea thermolysis gaseous products (ammonia and isocyanic acid). These bubbles nucleate at the wall, especially near the meniscus of the liquid urea lens. This phenomenon is similar to nucleate boiling of hydrocarbons liquid film. In the third stage, the urea thermolysis rate reaches a maximum critical value for Tw between 600K and 650K. Then, the images show the formation of a deposit having a very small thermolysis rate. The evolution time to reach this final state has been measured from the images. In the fourth and final stage (Tw>650K), no deposit is formed on the wall and the molten urea droplet levitates and rebounds on the wall very similarly to a hydrocarbon droplet in the Leidenfrost regime. Therefore, Tw=650K may be considered as a Leidenfrost-like temperature for the urea. This temperature has been used to define the Leidenfrost regime for the urea in the map of the spray-wall interaction model. Finally, several CFD simulations have been carried in the third and fourth thermolysis stages. Thereby, the different measured evolution times have been used to check the thermolysis mechanism numerical results and to reveal the solid deposits composition that has proved to be composed of Cyanuric acid and Ammelide.

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.

The understanding of the stabilization process of Diesel spray flames is a key challenge because of its effect on pollutant emissions. In particular, the close relationship between lift-off length and soot production is now well established. However, different stabilization mechanisms have been proposed and are still under debate. The objective of this paper is to provide an experimental contribution to the investigation of these governing mechanisms. Combustion of a Diesel spray issued from a single-hole nozzle (90 mu m orifice, ECN spray A injector) was studied in a constant-volume precombustion vessel using a combination of optical diagnostic techniques. Simultaneous high frame rate (6kfps) schlieren, 355 LIF (excitation at 355 nm and maximum collection at 430 nm) and high-temperature chemiluminescence (collection from 400 nm to 490 nm) or OH* chemiluminescence (collection at 310 nm and frame rate at 60kfps) are respectively used to follow the evolution of the gaseous jet envelope, formaldehyde location and lift-off position. Additional experiments are performed where the ignition of the mixture is forced at a location upstream of the natural lift off position by laser-induced plasma ignition (at 1064 nm). The evolution of the lift-off position until its return to the natural steady-state position is then studied for different ambient temperatures (800 K to 850 K), densities (11 kg/m3 to 14.8 kg/m3) and rail pressures (100 MPa to 150 MPa) using the same set of optical diagnostics. The analysis of the evolution of the lift off position without laser ignition reveals two main types of behaviors: sudden jumps in the upstream direction and more progressive displacement towards the downstream direction. While the former is attributed to auto-ignition events, the latter is studied through the forced laser ignition results. It is found that the location of formaldehyde greatly impacts the return velocity of the lift-off position: if laser ignition occurs upstream of the zone where formaldehyde is naturally present, the lift-off position convects rapidly until it reaches the region where formaldehyde is present and then returns more slowly towards its natural position, suggesting that cool-flame products greatly assist lift-off stabilization. The average return velocity in this second stage depends on the operating conditions. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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.

One 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 (r upsilon(r\)). 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.

Previous 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 (63 mu m to 123 mu m) 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.

The accumulation of particulate matter in lubricant oil can become an important issue in Diesel engines where large amounts of Exhaust Gas Recirculation (EGR) are used at medium to high load operating conditions. Indeed, the transport and subsequent accumulation of particulate matter in the engine oil can negatively impact the oil lubricant properties which is critical to ensure mechanical durability and limit the vehicle Total Cost of Ownership (TCO) by reducing the servicing intervals. The objective of this investigation was to gain an improved understanding of the underlying mechanisms that are responsible for the accumulation of particulate matter in the lubricating oil, and ultimately provide design guidelines to help limit this phenomenon.The present study presents the development and validation of experimental and numerical tools used to investigate this phenomenon. Several advanced diagnostic techniques were developed and applied on an optically-accessible single cylinder Diesel engine to detect the presence of particulates and quantify their concentration in two particular zones: (1) in the upper part of the cylinder liner where particulate matter is believed to be absorbed into the oil film and (2) in the engine blow-by gases where particulates can be transported via the piston ring-pack to the oil sump reservoir. The accumulation of soot particulates on the surface of the upper part of the cylinder liner was characterized using a modified Laser Extinction Method (LEM), while the concentration of particulates in the engine blow-by gases was measured using a DMS500 soot sensor. In parallel to the experimental study, 3D numerical computations were performed and provided additional information to the experimental results. In the present paper, the various experimental and numerical methods are presented and first validation results are discussed.

This paper presents new measurements of liquid and liftoff lengths, vapor penetration, and ignition delay using the Engine Combustion Network (ECN) 'Spray B' injector in a 2.34 L skip-fired heavy-duty optical engine. The data from the Spray B injector, having three 90-micron holes, are compared with previously existing constant-volume vessel data using both the Spray B injector as well as the ECN Spray A injector, which has a single 90-micron axial hole. The new data were acquired using Mie scattering, OH* chemiluminescence imaging, schlieren imaging, and incylinder pressure measurements. This paper presents data from estimated isentropic-core top-dead-center conditions with ambient densities of 15.2 and 22.8 kg/m(3), temperatures of 800, 900, and 1000 K, and for both non-reacting (0% and 7.5% O-2) and reacting (13, 15, and 21% O-2) injections of n-dodecane at fuel-rail pressures of 500, 1000, and 1500 bar. Particular attention is given to the data analysis process during transient injection, and boundary condition uncertainty. In addition to the hole of interest (#3), measurements from all three Spray B holes demonstrate that the hole-to-hole variations are small, but may indicate that the influence of internal sac flows, as well as orifice geometry, are important. In addition to the ensemble-averaged data presented here, the full dataset including cycle-to-cycle variability has also been published to an online-database available at https://ecn.sandia.gov for computer model validation. A companion paper makes a preliminary comparison of these data to Reynolds-averaged Navier Stokes solutions using Lib-ICE to simulate Spray B with a sector mesh.