In this work, the effect of hexogen (RDX) inclusion in a conventional ammonium perchlorate (AP)/hydroxylterminated polybutadiene (HTPB) composite propellant is investigated. To this end, a detailed gas-phase kinetic mechanism for the ternary system AP/HTPB/RDX is proposed. A revised vapour pressure law is used to model RDX evaporation. The combustion model is able to represent the chemical processes within the flame produced by the combustion of pure AP, homogenized AP/HTPB pseudo-propellants, and pure RDX. With this kinetic model, the combustion of a single RDX particle surrounded by a layer of homogenized AP/HTPB binder is simulated in a 2D axisymmetric configuration. It is shown that RDX inclusion significantly alters the combustion of the propellant. A phenomenological description of the flame structure forming above the heterogeneous propellant is proposed. This flame does not conform to the Beckstead-Derr-Price model, usually valid for conventional AP/HTPB propellants. Ambient pressure and RDX particle size are varied to assess the effect of these key parameters on the combustion. Two combustion regimes are identified: the hot and mild regimes. Conditions for the appearance of each combustion regime are determined in terms of ambient pressure and RDX particle size. Novelty and Significance Composite propellants could include nitramine ingredients such as hexogen (RDX) in their formulation to improve their performance. The effect of RDX inclusion in a conventional ammonium perchlorate (AP) / hydroxyl-terminated polybutadiene (HTPB) propellant was experimentally studied in the past [1,2]. However, understanding the fine combustion processes at stake remained out of reach. On the other hand, numerical simulation of the combustion was unachievable, as no gas-phase kinetic mechanism for the ternary system AP/HTPB/RDX was available. This paper first proposes such a kinetic model based on previous work by the authors on pure AP [3] and AP/HTPB combustion [4]. In doing so, a revised vapour pressure law is proposed for RDX combustion. With this mechanism, the flame structure obtained above an AP/HTPB/RDX propellant is computed. RDX inclusion significantly alters the combustion of the AP/HTPB propellant via specific processes, which are highlighted.

Solving the reactive low-Mach Navier-Stokes equations with high-order adaptive methods in time is still a challenging problem, in particular due to the handling of the algebraic variables involved in the mass constraint. We focus on the one-dimensional configuration, where this challenge has long existed in the combustion community. We consider a model of solid propellant combustion, which possesses the characteristic difficulties encountered in the homogeneous or spray combustion cases, with the added complication of an active interface. The system obtained after semi-discretisation in space is shown to be differential-algebraic of index 1. A numerical strategy relying on stiffly accurate Runge-Kutta methods is introduced, with a specific discretisation of the algebraic constraints and time adaptation. High order is shown to be reached on all variables, while handling the constraints properly. Three challenging test cases are investigated: ignition, limit cycle, and unsteady response with detailed gas-phase kinetics. We show that the time integration method can greatly affect the ability to predict the dynamics of the system. The proposed numerical strategy exhibits high efficiency and accuracy for all cases compared to traditional schemes used in the combustion literature.

As the gas turbine designs used today are reaching the limits of their performance, despite various optimization techniques adopted in recent years, the need arises for new types of propulsion devices. Pressure Gain Combustion (PGC) technology is an actively pursued area of innovation for gas turbine cycles that can be implemented in aerospace propulsion and land-based power generation. The advantages of pressure gain combustion, such as higher thermodynamic cycle efficiency and lower specific fuel consumption, make it an attractive alternative to conventional Brayton cycle, which uses constant pressure heat addition. Various pressure gain combustion technologies such as Pulse Detonation Engines (PDE) and Rotating Detonation Engine (RDE) are currently being investigated for propulsion applications, either as standalone propulsive devices or as constant pressure - PGC hybrid engines. Based on specific engine data, this paper aims at assessing the performance of pressure gain combustion in the framework of aircraft propulsion using the in-house simulation tool ‘TRANSEO’.

This study first proposes a revised combustion model for composite propellants containing ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB). It comprises a detailed gas-phase kinetic mechanism, assembled leveraging recent work from the chemical kinetics community. A model for homogenized AP/HTPB decomposition at the burning surface is formulated to perform coupled flame/solid simulations with the proposed gas-phase mechanism. The obtained combustion model is validated against available experimental data on AP/HTPB combustion for propellants containing 60% to 80% mass fraction. Employing this combustion model, 2D heterogeneous combustion of an AP particle surrounded by a layer of homogenized AP/HTPB binder is then studied. These 2D simulations allow to investigate the flame structure and conditions on the propellant surface, such as burning rate. The effect of AP particle size and ambient pressure on the combustion regime is evidenced: conditions leading to a premixed regime or a diffusion-limited regime are identified. Production of chemical species within the flame is finally studied. Significant differences are observed, with respect to a widely used model from the literature, mainly in the formation of end products and NOx species. In particular, it is shown that C4H6 4 H 6 isomerization and NO formation are important chemical steps in the flame.

The efficiency gain of rotating detonation depends on several loss factors related to the chamber geometry, the injection principle, the propellants and their mass flow rates, and the equivalence ratio. Numerical simulation can help quantify these losses, and this work presents a Large Eddy Simulation (LES) of rotating detonation in an annular chamber and its validation against experiments. The simulation captured the mixing processes, the overall dynamics of the detonation, the deflagration, and the burnt gas expansion. The injection device was numerically designed to ensure partial premixing of the propellants before injection into the chamber. The chamber had a length of 110 mm, an outer diameter of 80 mm, and a radial width of 10 mm. The mixture consisted of gaseous CH4 and O2 with an equivalence ratio of 1.2 and a mass flow rate of 160 g/s. Combustion kinetics was modeled using a skeletal mechanism with 62 reactions and 16 species. The boundary conditions were adiabatic slip walls. The results reproduce well the detonation velocity (within 1% deviation) and the pressure variation behind the wave. The simulated OH* chemiluminescence compares well with experimental high-speed imaging of the outlet and side of the chamber. The simulation results indicate that 65% of the propellant mass is well mixed in front of the wave whereas 15% of the mixture is burned by deflagration. They show that CH4 and O2 do not axially stratify because they have similar injection dynamics between periodic perturbations induced by the rotating detonation. Good propellant mixing and low deflagration losses explain the high experimental detonation velocity, about 90% of DCJ, and a high combustion efficiency of 98%. These agreements between the computational and experimental results indicate that the simulation is capable of capturing the physical scales relevant to RDC operation and producing reliable results for RDC design.

In this study, we propose a revised coupled combustion model for ammonium perchlorate (AP), leveraging recent advances in the modeling of ammonia and NOx chemistry. A coupled combustion model relies on three founding bricks: a detailed gas-phase kinetic model, a condensed-phase decomposition model, and a pyrolysis law describing the relationship between the surface temperature and mass flow. The proposed gas-phase kinetic model, is validated against data on species sampling in jet-stirred reactors, laminar flame speed, and ignition delay time. These test cases, rarely used by the solid propellant community, highlight deficiencies in a reference mechanism from the literature. A new model for AP decomposition in the condensed phase is proposed to be used with the gas-phase mechanism. A suitable pyrolysis law is designed using the Zel'dovich–Novozhilov theory to ensure the stability of the coupled combustion model. The methodology employed is described in detail, for others to replicate. Finally, the overall model is applied to simulate the AP laminar flame in a 1D coupled approach. These calculations provide results on the regression rate, surface temperature, temperature sensitivity and species profiles for prescribed initial temperature of AP and ambient pressure. The behavior of the proposed combustion model is presented in comparison with other reference models. The role of gas-phase kinetics in modeling AP combustion is discussed.

Propulsion systems based on the constant-pressure combustion process have reached maturity in terms of performance, which is close to its theoretical limit. Technological breakthroughs are needed to develop more efficient transportation systems that meet today’s demands for reduced environmental impact and increased performance. The Rotating Detonation Engine (RDE), a specific implementation of the detonation process, appears today as a promising candidate due to its high thermal efficiency, wide operating Mach range, short combustion time and, thus, high compactness. Following the first proofs of concept presented in the 1960s, the last decade has seen a significant increase in laboratory demonstrators with different fuels, injection techniques, operating conditions, dimensions and geometric configurations. Recently, two flight tests of rocket-type RDEs have been reported in Japan and Poland, supervized by Professors Kasahara (Nagoya University) and Wolanski (Warsaw University), respectively. Engineering approaches are now required to design industrial systems whose missions impose efficiency and reliability constraints. The latter may render ineffective the simplified solutions and configurations developed under laboratory conditions. This requires understanding the fundamentals of detonation dynamics relevant to the RDE and the interrelated optimizations of the device components. This article summarizes some of the authors’ experimental and numerical work on fundamental and applied issues now considered to affect, individually or in combination, the efficiency and reliability of the RDE. These are the structure of the detonation reaction zone, the detonation dynamics for rotating regimes, the injection configurations, the chamber geometry, and the integration constraints.

The efficiency of a Rotating Detonation Combustor (RDC) strongly depends on the transitory injection process of fresh reactants in the combustion chamber: poor propellant mixing induces losses of combustion efficiency and consequently low detonation speed and unstable detonation propagation. Moreover, dilution of fresh reactants with burnt gases during injection increases the deflagration losses and decreases the pressure gain provided by the detonation. Numerical simulation can help design an efficient injector to reduce these losses. In this study, the modeling strategy previously proposed by ONERA to simulate the transitory injection process is applied to two existing experimental RDC (from Nagoya University and TU Berlin) and one in-development RDC from ONERA. The computational domain represents only one injection element, convenient for a parametric study at low computational cost. A custom initial condition is used to model the expansion process of burnt gases past a detonation wave. The initial condition parameters are discussed and a method is proposed to correctly set them. The TU Berlin RDC is studied in more detail: mixing efficiency up to 70% is obtained, and 5% of deflagration losses are estimated according to the assumptions of the simulation. Based on the numerical results, detonation speed was evaluated at various distances from the injection plane taking into account the heterogeneities of the fresh mixture. The measured speed lies within the predicted range.

Aluminum-droplet combustion is studied in solid-propellant flame environment at 1.0 MPa. Al vapor is observed during the combustion process with high-speed Al-PLIF and visible-emission images. A one-dimensional quasi-steady model of aluminum combustion is used to simulate Al-atom concentration and temperature profiles around the droplet. Al-PLIF image simulation has been developed to generate synthetic fluorescence images that reproduce the experimental images. A comparison approach is initiated between experimental and numerical Al-PLIF images and applied to the study of a specific droplet observed during the experiments with a diameter larger than 120 µm. A reasonable consistency is observed between simulated and experimental data, which is promising to provide validation data for the development of more representative aluminum-combustion models.

An unsteady one-dimensional model of solid propellant combustion, based on a low-Mach assumption, is presented and semi-discretised in space via a finite volume scheme. The mathematical nature of this system is shown to be differential-algebraic of index one. A high-fidelity numerical strategy with stiffly accurate singly diagonally implicit Runge-Kutta methods is proposed, and time adaptation is made possible using embedded schemes. High-order is shown to be reached, while handling the constraints properly, both at the interface and for the mass conservation in the gaseous flow field. Three challenging test-cases are thoroughly investigated: ignition transients, growth of combustion instabilities through a Hopf bifurcation leading to a limit cycle periodic solution and the unsteady response of the system when detailed gas-phase kinetics are included in the model. The method exhibits high efficiency for all cases in terms of both computational time and accuracy compared to first and second-order schemes traditionally used in the combustion literature, where the time step adaptation is CFL-or variation-based.

We investigate a model of solid propellant combustion involving surface pyrolysis coupled to finite activation energy gas-phase combustion. Existence and uniqueness of a travelling wave solution are established by extending dynamical system tools classically used for premixed flames, dealing with the additional difficulty arising from the surface regression and pyrolysis. An efficient shooting method allows to solve the problem in phase space without resorting to space discretisation nor fixed-point Newton iterations. The results are compared to solutions from a CFD code developed at ONERA, assessing the efficiency and potential of the method, and the impact of the modelling assumptions is evaluated through parametric studies.

Detonation applied to propulsion could result in a promising increase of the thermodynamic efficiency of the engine cycle. Numerical simulations of the detonation propagating in the Continuous Detonation Wave Rocket Engine (CDWRE) are currently performed but still do not account for realistic injection process. The assumption of an ideal injected premix is generally chosen for convenience to obtain theoretical results. Comparison of the numerical results with experiments is difficult because of the clear difference of the injection configurations. Some physical aspects of the separate injection of the components used in experiments are not clearly assessed. This study is included in a wider numerical project aimed at designing and optimizing a realistic CDWRE. The optimization process is presently focused on the injector. One element of the injection hole pattern is considered assuming that this element is periodically repeated over the injector head. The aim of the work presented here is to model and analyze the refill process of the components in the combustion chamber behind the rotating detonation. The simulation starts just after the passage of the detonation over the considered injection element. This simulation gives information on the way the injected propellants recreate the reactive mixture for the next detonation. In the first step, two-dimensional (2D) computations helped us to set up the methodology and to study the dynamic response of the fresh components injected. A comparison between 2D homogeneous and separate injections is provided. In the second step, three-dimensional (3D) computations have been performed with a separate injection suitable for the CDWRE operation. Some performance parameters are evaluated such as mixing efficiency or filling of the domain.

The reduction of fuel consumption in future propulsive engines is an ambitious target that will be reached only with a technological breakthrough. Two of the possible solutions are being investigated experimentally and numerically by MBDA France, ONERA, PPRIME and SAFRAN Tech. Changing the actual thermodynamical cycle through the use of Constant-Volume Combustion or Rotative Detonation concepts could theoretically enable this target to be reached. Implementing such concepts requires a deep knowledge and control of several basic phenomena that will occur and interact in real engines, such as: mixing processes, ignition, flow effects, dilution by residual burnt gas, etc. This article presents recent studies carried out on these elementary processes that must be considered in high-velocity flows and under non-stationary conditions, whatever the concept (CVC or RDE). The results provide a comprehensive insight into constant-volume combustion and detonation dynamics from simulation and experiments on a reduced scale or full scale prototype, which enables the physical phenomena to be understood and modeled, and the potential of such concepts for future propulsion to be highlighted.

A detailed Rayleigh scattering model has been implemented and used to post-process detonation wave numerical simulation results to allow for a direct comparison with previous experimental visualizations of detonations in hydrogen-based mixtures. A quantum chemistry approach has been employed to obtain realistic Rayleigh scattering cross-sections. A database of Rayleigh cross-sections for relevant species was created and validated against available experimental data. Steady one-dimensional as well as unsteady two-dimensional simulations of detonation were used for comparison with experimental Rayleigh profiles and images. We demonstrate that both realistic Rayleigh scattering cross-sections and the characteristics of the imaging system have to be taken into account to accurately reproduce the experimental results. We show how this approach can be applied to estimate the performance and design improved Rayleigh imaging systems. Rayleigh scattering appears to offer some advantages over other techniques for studying the structure of detonation waves both at and behind the front.

Knowledge of H2–N2O mixtures explosive properties is important to the safety of nuclear waste storage and semi-conductor manufacturing processes. The present study provides new experimental data on H2–N2O detonations, and proposes a thermochemical model which is used to numerically simulate detonation propagation. Detonation cell size has been measured in a variety of H2–N2O–Ar mixtures. Even at low initial pressure, these mixtures are very sensitive to detonation with cell size of few millimeters. Using a reduced version of a detailed reaction scheme, 2-D Euler simulations have been used to examine the features of detonation in H2–N2O–Diluent mixtures. A PLIF model has been applied to allow for direct comparison with experimental results. Statistical analysis of the cellular cycle dynamics has been performed.

As a detonation wave propagates from a confined tube to an unconfined space, it undergoes di↵raction, which is characterized by a change of geometry from a quasi-planar wave to a cylindrical or spherical wave [1–4]. Depending on the mixture composition, thermodynamic conditions, detonation velocity at the tube exit, the geometry of the area expansion and tube cross section, the detonation wave can be either extinguished (sub-critical regime) or re-initiated (super-critical regime) [2]. Detonation di↵raction has been extensively studied both experimentally and through numerical simulations. Since Zeldovich et al.’s pioneering work [5], numerous studies have been performed on detonation di↵raction using a variety of experimental geometry and techniques, as summarized by Schultz [4] in 2000. Since then, a number of investigations have been performed, for example by Khasainov et al. [6], Meredith et al. [7], and Nagura et al. [8]. The most advanced experimental investigation is the study of Pintgen [1, 2] who performed simultaneous imaging of the shock front and reaction zone, using schlieren and Planar Laser Induced Fluorescence (PLIF), as well as multi-exposure and stereoscopic chemiluminescence imaging. Jones et al. [10–13] and Oran et al. [14, 15] used numerical simulations to study the di↵raction of detonation and reignition process at an abrupt tube diameter increase (180 angle). Layered and uniform H2-O2(-Ar) mixtures were modeled. The chemical schemes were either 1-step or 2-step models. Arienti et al. [9] and Nagura et al. [8] performed simulations of detonation di↵raction from a tube to an unconfined volume (90 angle). Arienti used a 1-step reaction model characterized by di↵erent activation energies whereas Nagura employed a 2-step scheme to model a stoichiometric H2-air mixture. Khasainov et al. [6] studied the di↵raction of detonation from a tube to a cone. A stoichiometric C2H2-O2 mixture was considered using a 1-step pressure dependent reaction model. Deiterding [16] studied the failure and reignition of di↵racting detonations in a stoichiometric H2-O2 mixture diluted with 70% argon. High-resolution numerical simulations with mesh refinement were performed using Euler equations. The reactivity of the mixture was described using a detailed reaction model composed of 34 irreversible reactions and 9 species. From this literature review, we see that most numerical studies have been performed using an inviscid gas model (Euler equations) and global (1or 2-step) kinetic schemes. The present study aims at performing numerical simulation of detonation di↵raction using a viscous gas model (Navier-Stokes equations) and accurate chemical and thermodynamic models to allow for a more realistic comparison with the experimental data of Pintgen in terms of di↵racting detonation behaviour and structure.

The present work is essentially devoted to the simulation of a laminar strained flame using two approaches: 2D realistic and 1D simplified. The studied case corresponds to a laminar burner that creates an upward-oriented round jet of stoichiometric methane–air mixture impacting on a horizontal metal disk. 2D numerical simulations have been performed using the Fluent® 6.3 software in the axisymmetric configuration. Detailed thermochemical and transport models are applied. Results of the 2D and 1D simulations are analyzed and compared with experimental data on flow velocity obtained by particle image velocimetry (PIV). Limitations of the classical 1D approach are identified and further commented on. Measurement errors due to the particle slip are evaluated by simulating the particle motion with inclusion of the gravity, Stokes drag, and thermophoretic forces.