Session R44: Reacting Flows: Detonations and DDT I

Oblique Detonation Waves Exhibit Micro-jetting

A highly-resolved 2-D Direct Numerical Simulation (DNS) of a wedge-induced Oblique Detonation Wave (ODW) using a detailed 9-species 21-reaction chemical kinetic mechanism for a H2-air mixture is conducted. Adaptive Mesh Refinement (AMR) is leveraged to obtain a sub-micron resolution of ∆x = 0.78μm at the finest level (which spans the ODW front and its transverse waves). The simulation is performed for an extended domain to unravel the different instability patterns observed on the ODW front. We capture, for the first time, the onset of microscopic hypersonic jets far downstream of the wedge-tip that lead to cellular instabilities resembling those of a Normal Detonation Wave (NDW). A "psuedo-cellular" zone devoid of the classical NDW triple-points and the micro-jets is also spotted upstream of the cellular zone. A detailed investigation into the shock structure downstream reveals that a pair of transverse waves are critical to the jetting phenomenon. The absence of one of the pair of transverse waves in the upstream region results in the observed pseudo-cellular "half-cell-like" instabilities. Detailed temporal analyses on the divergence of the velocity field is conducted to elucidate further on the origin of these transverse waves in an ODW.   

Chemical kinetics effects of lean hydrogen-air mixtures in wedge-induced oblique detonations

Oblique Detonation Wave Engines (ODWE) rely on the formation and stability of wedge-induced oblique detonations. However, some unstable configurations are reported in numerical and experimental analyses, which are related to oscillations observed after the formation of triple deck structures. In this work, we aim to compare the effect of chemical kinetics of two kind of numerical computations with prior scenarios from the literature. The simulations are conducted using two different codes, finite elements with one-step Arrhenius reaction rate, and CREAMS, WENO high-order finite differences with CHEMKIN library for detailed chemical schemes of H2.

The impact of the reaction of a lean hydrogen-air mixture in the wedge-induced detonation structure and the stability of the process is analyzed. To achieve this, different parameters are varied (wedge-angle, incident stream Mach number and Zeldovich number) to examine their influence in the transition between oblique shock wave (OSW) and oblique detonation wave (ODW). In particular, the appearance of subsonic regions in the post-shock flow induced by the temperature increase is addressed, which might affect the shock and detonation stability through wave reflection towards the front.   

Numerical simulations of detonation diffraction and re-initiation in ethylene-air mixtures

When a confined detonation propagates over an obstacle, the reaction front experiences an abrupt area change. This leads to detonation diffraction, which allows expansion waves to propagate into the reaction zone. These expansion waves slow the reaction front, causing it to decouple from the leading shock. This may lead to complete detonation quenching or re-initiation, depending on the geometry of the channel and obstacle. An understanding of the fluid dynamics behind this process can lead to improved designs of process safety equipment in transport and storage of hydrocarbon mixtures. We study the diffraction and re-initiation phenomena for ethylene-air mixtures using AMRFCT, a numerical simulation model that solves the multidimensional, reacting flow conservation equations with a chemical-diffusive model (CDM) for conversion of fuel to products with energy release. High-fidelity computations varying the obstacle geometry are used to study the effects of these phenomena on the reaction rates, detonation velocity and cellular structure.   

Methane-air Detonations in 2D and 3D Channels with Complex Chemistry

Formation of gaseous methane detonations is a major safety concern in a wide range of industrial settings and mining operations. Prevention of such industrial accidents requires a thorough understanding of detonation properties in such mixtures, in particular in the context of the mechanisms of detonation formation and propagation at near-limit conditions. In this work, we carry out a systematic numerical investigation of methane-air detonations propagating in 2D and 3D channels using realistic boundary conditions, complete molecular transport, and complex chemistry. A fully compressible block-based adaptive mesh refinement code Athena-RFX++ is employed to achieve high numerical resolution. We present the overall detonation structure and dynamics in methane-air mixtures with the primary focus on the following questions: a) How does detonation structure differ between 2D and 3D? b) How well can modern physico-chemical models represent methane-air detonation properties both on large (cell structure) and small (triple-point collision) scales? c) What are the propagation limits (as a function of channel size and equivalence ratio) in 2D and 3D channels? Finally, comparison with existing experimental data on methane-air detonations is discussed.   

Disturbance energy dynamics in gaseous detonations with varying detonation channel widths

Waves travelling in the transverse direction to a detonation shock front reflect off the walls of the detonation channel and collide with each other creating triple points, which are important to understanding energy dynamics downstream of the front. By analyzing data from direct numerical simulations (DNS) of H2-O2-Ar detonations with varying channel widths, which affects interference and changes the number of triple points produced, this study aims to determine their effects on the disturbance energy budget. The Rayleigh source term, which leads to generation of acoustic waves and the propagating transverse disturbances and is known to be concentrated at triple points is compared between data sets. The individual quantities contributing to the Rayleigh source term differ depending on whether disturbances are assumed to be about a steady base flow or a more physical time-averaged flow. The significance of these contributions is investigated by comparing their magnitudes and the nature of their fluctuations.   

Analysis of planar detonations in the presence of non-thermal termolecular reactions

Hydrogen-based detonations constitute a promising sustainable propulsion system for high-speed vehicles. For conditions associated with a detonation, the reacting mixture may undergo non-thermal reactions prior to reaching a thermal equilibrium state. Highly energetic ephemeral radicals undergo reactive collisions with radicals during thermalization and can be represented with chemically termolecular reactions, which have been previously shown to affect important characteristics in reacting systems such as deflagration-to-detonation transition. In this work, we investigate the effects of non-thermal termolecular reactivity on characteristics of planar detonation waves. We conduct high-fidelity numerical simulations of cellular detonation involving hydrogen combustion in a two-dimensional channel on an adaptively refined grid. Our thermodynamic parameters span different regimes identified through Zel’dovich-von Neumann-Döring (ZND) solutions, in which termolecular reactions were found to have opposing effects on detonation characteristics such as induction zone length. We study the effects of non-thermal reactivity on the kinematics and dynamics of the planar detonating system in these different regimes with a particular focus on cellular structure.   

Tabulated chemistry for detonations in a spatially-filtered framework

Recently, we have developed a spatially-filtered framework for simulating shock-dominated flows which introduced sub-filter scale terms to regularize the governing equations. For detonations, the filtered shock thickness is assumed to be at least an order of magnitude smaller than the induction length, and the mixture composition is not affected by the filtering. To reduce the cost of simulations, the chemistry can be simplified through tabulation. The chemical evolution is reduced to the solution of a progress variable transport equation, which is then used to look up relevant thermochemical quantities in a pre-computed table. Of particular importance is the source term, which describes the rate at which chemical reactions are occurring. If the filtered shock thickness approaches the induction length, the filtering of the chemical reaction zone must also be considered. In this work, the effect of filtering the source term is studied in detail. The source term is filtered a priori and tabulated for a range of filter widths. The tables are then used in simulations with varying grid resolutions. The combined effect of filter width and grid resolution on relevant physical parameters such as the detonation velocity are presented.