In Constant Volume Combustion (CVC) and wider combustion applications, unwanted transitions to detonation can occur in highly reactive end-gas with a reactivity gradient present. The objective of this present study was to identify variables encountered in practical applications which influence detonation transition behavior. Different temperature gradients were imposed on stoichiometric H2/C10H22/O2/Ar mixtures with a 5

This study investigates the minimum ignition energy (MIE) measured in a constant volume chamber (CVC), specifically examining its sensitivity to variations in both mixture compositions C3H8/air/residual burned gas (RBG) and flow properties. To gain a comprehensive understanding of the ignition process, the average velocity of the flow is investigated using time-resolved high-speed Particle Image Velocimetry (PIV). The experimental setup involves the examination of inert mixtures followed by reactive mixtures of propane/air/RBG, with PIV techniques employed to identify and track critical parameters such as the fresh gas velocity and the contour of the ignition kernel. By exploring the influence of mixture compositions and flow properties on MIE, this research contributes to the fundamental understanding of ignition phenomena in confined environments.

This work investigates the transition limit between autoignition front and deflagration in hot and premixed conditions. In particular, the objective is to analyze the sensitivity of the combustion regime to the temperature gradient, mixture composition and thermodynamical conditions, which is first analyzed numerically. The results are then compared to experimental data obtained in an optical RCM, fitted with a flat piston and a multi-zone heating system. Temperature gradient after compression is measured in the hot core region using two thin wire thermocouples of about 7.6 mu m diameter. The chemiluminescence records confirm that the dynamics of autoignition fronts is strongly dependent on (i) the gaseous mixture composition and the thermodynamical conditions, and (ii) on thermal gradients in the unburned mixture. In particular, the authors evidence experimental conditions for which a steeper gradient or a change in mixture composition leads to a transition from autoignition to deflagration.

Rapid compression machines (RCM) are well-known tools to study the autoignition phenomenon under engine-relevant conditions. Covering a wide range of pressure and temperature at the top dead center (TDC), it can be employed with different types of mixtures and thermal stratification. Creating a homogeneous hot core region after compression in the combustion chamber is one of the challenges to overcome for RCM studies. The objective of the present work is to characterize from aerodynamic and thermal points of view a new configuration in the optical RCM of Pprime Institute. The latter aims at ensuring a wider adiabatic core region in terms of time and space through the installation of a creviced piston, specifically adapted to the square cross-section cylinder of this particular RCM. For this purpose, the internal flow has been qualified using high-frequency Particle Image Velocimetry with different laser sheet locations. Temperature variation during and after compression is measured at several positions with respect to the cylinder head, using thermocouples with wire diameter of 7.6 µm. It is observed that the piston cavity is able to collect the boundary layer created during compression and maintain a wide region at low velocity after the top dead center. Furthermore, it is demonstrated that different temperature gradient values can be generated and quantified within the adiabatic core region through differential heating of the chamber. This feature is promising for future works devoted to the analysis of combustion regimes. More generally, the thin wire thermocouples are shown to be accurate and reliable sensors to measure temperature in severe and transient pressure and temperature conditions specific to RCM internal flows.

SAFRAN Helicopter Engines has developed the spinning combustion technology in which the burnt gases from one injector travel tangentially along the combustor annulus towards the neighboring injectors. Compared to a conventional design, this arrangement modifies the ignition process, which is a critical phase for aeroengines. In order to understand the ignition process in this technology, experiments and Large-Eddy Simulation (LES) have been performed in a cylindrical combustion chamber where the flow is injected tangentially (named Radius chamber). Three cases are considered with different strain and turbulence levels representative of real combustor flows. Micro calorimetry and the Background-Oriented Schlieren technique allows for detailed temporal measurements of energy deposited in the flame kernel. Pressure measurement and Schlieren imaging are used to study the flame propagation. LES are performed with a 19-species and 184-reactions analytically-reduced chemistry together with the thickened flame approach allowing the description of the first instants of ignition in a quasi-DNS mode and ensuing flame propagation. Both a static and dynamic formulations of the wrinkling factor to describe sub-grid scale chemistry-turbulence interaction are used. Results show that LES is able to capture the flame kernel formation and trajectory as well as the time to reach maximum pressure within an error of 10% when using a dynamic formulation. On the other hand, the static formulation of the wrinkling factor predicts the time for maximum pressure within a maximum error of 20%.

Combustion regime transition of n-decane/O2/Ar mixtures is investigated experimentally in a closed optical vessel with a focus on the transition from autoignition to detonation. A deflagration is initiated in a closed vessel at high pressure (P0 = 3 bar) and temperature (T0 from 420 K to 450 K) to trigger autoignition and possibly detonation in the end-gas. The temperature gradient at the onset of the autoignition plays a crucial role in the transition between autoignition and detonation as it influences the propagation speed of the autoignition front. An analysis via Bradley’s formalism was performed on experimental tests to evidence the influence of the temperature gradient on the transition to detonation. Schlieren visualizations at high frequencies (from 120 kHz to 2 MHz) of the combustion regime transitions are also performed to help the understanding of the transition mechanisms.

These last years, aircraft manufacturers and environmental international bodies have set drastic targets concerning turbo-engines pollution and consumption reduction. To reach the objectives of pollutant emission and consumption reductions, innovative solutions such as constant-volume combustion are in development. Characterizing the propagation of a kerosene-air flame in this kind of combustion is necessary. Particularly, the knowledge of fundamental properties like the laminar burning velocity in laminar adiabatic conditions can be useful for the design of efficient innovative turbo-engines. A new spherical combustion chamber developed in Institut PPRIME and the associated post-processing procedure are first validated using a n-decane/air premixed flame, and a good agreement is obtained with the literature. Measurements of laminar burning velocities and Markstein lengths of commercial kerosene/air mixture and kerosene surrogates/air are then performed at various temperatures (T-0 = 400 K to 470 K), pressures (P-0 = 0.1 MPa to 0.5 MPa) and equivalence ratios (Phi = 0.6 to 1.5). Two mono-component surrogates are tested as a representative of commercial kerosene (n-decane and n-dodecane), with two multi-component surrogates: the Dagaut surrogate (n-decane/n-propylbenzene/n-propylcyclohexane) and the MURI2 surrogate (n-dodecane/iso-octane/1,3,5-trimethylbenzene/n-propylbenzene). Additionally, numerical simulations of laminar burning velocities are performed using JetSurF 2.0 and the Luche reduced chemical kinetic mechanism, representing respectively mono-component surrogates and the Luche surrogate. They are able to represent correctly the experimental measurements. A comparison of the different employed surrogates with commercial kerosene is also performed to evaluate the ability of these surrogates to reproduce the laminar burning velocity properties of the commercial kerosene.

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.

The canonical diesel spray A is characterized in an optical Rapid Compression Machine (RCM) at high temperature and density conditions (900 K and 850 K, ρ = 23 kg/m 3 ) using simultaneous high-speed OH* chemiluminescence and two-pulse 355 nm Planar Laser Induced Fluorescence (PLIF). The focus is on the time evolution and the repeatability of the early stages of both cool flame and hot ignition phenomena, and on the time evolution of the fluorescing formaldehyde region in between. In particular, time resolved data related to the cool flame are provided. They show the development of several separated kernels on the spray sides at the onset of formaldehyde appearance. Shortly after this phase, the cool flame region expands at high velocity around the kernels and further downstream towards the richer region at the spray head, reaching finally most of the vapor phase region. The position of the first high temperature kernels and their growth are then characterized, with emphasis on the statistics of their location. These time-resolved data are new and they provide further insights into the dynamics of the spray A ignition. They bring some elements on the underlying mechanisms, which will be useful for the validation and improvement of numerical models devoted to diesel spray ignition.

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.

The transition between different combustion regimes is investigated experimentally for a stoichiometric argon-diluted n-decane/O2 mixture. The focus is put on the influence of initial temperature (T = 420–470 K) and pressure (P = 1.5–3 bar) on the regime transitions. Fast schlieren visualization (≥ 120 kHz) and high-speed pressure and temperature measurements are used to monitor the evolution of the reactive processes inside a combustion chamber of square cross-section (40 mm × 40 mm × 172 mm). Results for P ≥ 2.5 bar and the entire temperature range and for P = 2 bar and T ≥ 440 K show three distinct stages following the adiabatic compression of fresh gases induced by the propagation of a flame into the chamber/test section, namely a cool flame, a main heat release stage, and detonation onset. For P = 1.5 bar, however, only the first two stages of the process are observed in the temperature range studied. A two-stage autoignition phenomenon, typical of large hydrocarbons, occurs systematically in the end gas and generates consecutive reactive fronts. The transition to detonation appears to result from the acceleration of the aforementioned fronts toward the speed of sound in fresh gases. Notably, the compression history plays a key role in setting conditions for detonation onset. Our results are in agreement with classical transition maps available in the literature.

Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM). A radiofrequency igniter with a high energy deposit (305 mJ) is used to investigate the SICI combustion phenomena in lean conditions (? = 0.5), relatively close to the frontiers of the SICI regime. Fluorescence images enable to monitor both the development of the cool flame process and the topology and dynamics of reaction fronts during the second stage of ignition. The results are first analyzed from a phenomenological point of view, bringing insights into the understanding of the both HCCI and SICI combustion processes as they take place in the RCM. Additional data are gathered from double-pulse 355 nm PLIF imaging, with focus on the temporal evolution of the cool flame and on the reaction front propagation during hot ignition. From a more quantitative point of view, an analysis of apparent velocities of the reaction zones is then presented, and large variations of these values are observed depending on the experimental conditions. These local quantities are closely related to the global heat release rate which is a key parameter for practical applications of HCCI and SICI combustion modes. The proposed simultaneous diagnostics finally lead to a better understanding in the local reaction modes, namely deflagration, spontaneous ignition fronts and bulk auto-ignition e.g. volumetric auto-ignition -, which are implied in the combustion processes. The results highlight the complex aerothermal interactions taking place in the RCM vessel, in particular through the pre-ignition thermal stratification. The results suggest the latter strongly affects the HCCI combustion process, but also drives the heat release rate during the second stage of the SICI combustion mode. Furthermore, deflagration fronts are found to be significantly affected by cool flame chemistry, as well as by the large and small scale structures of the fluid flow. (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute.

This study focuses on the thermodiffusive instabilities formation of spherical premixed flames. The conditions under which these instabilities develop must be characterized for a better understanding and a better control of this regime. The instabilities are described by the emergence of wrinkling and cells overall the flame front surface. Thus, when instabilities occur, the flame front surface becomes larger, increasing the consumption rate of fresh gases. The flame front is more and more unstable, new cells are generated and the flame front speed is faster: instabilities are self-sustained and the flame front is self-accelerated. When combustion takes place in constant volume chamber, the combustion process induces a pressure increase that favours the instabilities.

The aim of this study is to experimentally investigate the combustion regime transitions of a single-component kerosene surrogate (n-decane). For this purpose, a deflagration is generated by a spark in a constant-volume vessel with a length-to-width aspect ratio of 4.3. By consuming the unburnt gas, the flame behaves as a piston that compresses the end gas. The pressure and temperature of the end gas increase with the flame propagation until autoignition conditions are reached. Ultrafast schlieren visualizations are set up to monitor the dynamics of the reactive processes, whereas the time-pressure evolution associated with non-dimensional (0D) numerical models is used to characterize the unburnt gas thermodynamic conditions. Once the thermodynamic conditions needed to initiate the autoignition reactions are reached in the end gas, a transition between deflagration (flame speed of approximately ten m/s) and autoignition fronts is observed (propagation velocity ˜200 m/s). This transition occurs for various fuel equivalence ratio values (0.75 – 1). For the strongest thermodynamic conditions, once the velocity of this autoignition front reaches the speed of sound, a new reactive front propagating up to 1800 m/s is observed, thus indicating the transition to detonation combustion mode. Parametric studies indicated that the occurrence of both transitions was a function of the pressure, temperature and equivalence ratio for n-decane fuel. A small initial temperature variation (approximately 40 K) could change the phenomenology of the constant-volume combustion from deflagration to detonation through the autoignition process, even for lean mixtures. The results obtained using our experimental setup show that the transition between autoignition and detonation is due to the acceleration of the autoignition front towards the speed of sound in the unburnt gas.

The level-set methods have been widely used during the past decades and have become very common to track interfaces in numerous applications as for instance image processing, robotics and fluid mechanics. The level-set approach for combustion applications has been first introduced by Williams [1] who derived the well known G-equation. In this context, the level-set function G(x, t) is a scalar field for which the isovalue G0 corresponds to the surface that separates fresh and burned gases. This scalar field is often defined as the signed distance d to the flame front. Then, considering G0 = 0 so that G < 0 in fresh gas and G > 0 in burned gas gives the following definition of the flame front: G(x, t) = 0. Accordingly, differentiation of G leads to the G-equation: ∂G

Impact of civil aviation on the environment is today a major issue with the expectation of air traffic to soar in coming years. Environmental international bodies as ACARE (Advisory Council for Aviation Research and Innovation in Europe), in partnership with the main international aeronautical groups, require more and more drastic objectives in terms of pollutant emission and consumption reduction [1]. To reach these objectives, innovative solutions with technological breakthrough are in development, as constant-volume combustion which could provide a 10 % to 20 % consumption reduction in comparison to current engines [2-3]. In the case of constant-volume combustion dedicated to aeronautical propulsion, a spark ignition combustion of kerosene-air premixture may be used [4]. To produce power, high frequency cycles are required. Hence air admission has to be as short as possible, implying high velocities at ignition point that can reach several tens of meters per second [5-6]. The ignition process is critical and must generate an ignition kernel able to expand in the overall chamber. This ignition process depends on several parameters: initial temperature, initial pressure, equivalence ratio or dilution of the mixture, aerodynamic conditions and also the history of these parameters when the kernel is moving.

The need of measuring laminar burning velocity (LBV) to characterize the fundamental behavior of a premixed flame has involved the development of several experimental methodologies to perform the measurements. One of the most commonly used method is the generation of a spherical flame in closed combustion chamber. The spherical flame, initiated in a homogeneous and quiescent mixture with an electrical discharge deposit, propagates uniformly in overall directions. From this experimental set-up, several authors have defined different methodologies to extract the LBV. These methodologies are mainly separated into two classes: the confined flame and the unconfined flame. In the first one, the LBV is extracted from the pressure signal obtained during combustion process, while in the second one, the LBV is deduced from the visualization of the spherical flame propagation by using the flame radius time-evolution of the flame front. The aim of this study is to compare different methods of LBV measurements with the use of two well-known mixtures: a methane/air premixed flame at normal temperature and pressure (NTP) conditions, and a n-decane/air premixed flame at initial temperature T0 = 400 K and initial pressure P0 = 0.1 MPa.