Morphologies des flammes et dynamiques de l’écoulement lors de la transition déflagration détonation dans des canaux lisses : effet des propriétés fondamentales de combustion

Context Reactive mixtures within confined spaces have the potential to detonate upon ignition. Small flames can accelerate and transition from deflagration to detonation (DDT), which often poses significant safety risks. The fundamental mechanisms behind DDT remain a complex challenge in combustion science. The specifics of DDT are influenced by various factors, including the mixture properties, geometry and size of the channel. The main objective of this thesis is to contribute to the fundamental understanding of DDT. Challenges The length and time scale involved in DDT span over six orders of magnitude. Resolving them experimentally requires access to ultra-fast cameras and advanced visualization techniques. Characterizing the flame morphologies and shock dynamics during flame acceleration (FA), from ignition to detonation onset (DO), requires innovative experimental setups like the two-directional simultaneous schlieren visualization (2DSV) developed and used in our group. Approach One of the key but unexplored aspects of FA in smooth channels is flame morphology, which is influenced by fundamental combustion properties (FCP). The most relevant FCPs for DDT include the expansion ratio (σ), laminar flame speed (sL), laminar flame thickness (δf), ratio of specific heats (γ), Zel’dovich number (β), Lewisnumber (Le), and sound speed in combustion products (cb). Experiments were conducted using the 2DSV setup to visualize a 10 × 10 mm2, 1 m long, semi-open channel. Three experimental campaigns were carried out to study flame/shock morphology and dynamics during the entire DDT process investigating : (i) the effect of γ at fixed σ, (ii)the effect of Le at fixed σsL, and (iii) the effect of σsL at fixed δf . Modern data analysis techniques were employed to determine the boundaries driving morphological changes as a function of FCP during early (ES) and late (LS)stages of FA. Detailed analysis of representative DO cases was performed, and a preliminary methodology for flame surface reconstruction using 2DV images was proposed to estimates of the flame surface area during the early stages of FA. Finally, an extensive run-up distance to detonation (xDDT) database for smooth channels was compiled from the literature including over 500 data points. Existing models were assessed and new models were proposed. Key findings— In the ES of FA, two flame morphologies were observed : ES1, characterized by inverted (tulip-shaped) flames, and ES2, featuring flat, wrinkled flames. During the LS of FA, when pressure waves (PW) form, two additional morphologies were identified : LS1, with short, symmetrical flames and PW forming relatively far from the flame, and LS2, with long flames and multiple PW generated successively in front of the flame.— The morphologies observed during the ES and LS of FA are interconnected. Most mixtures exhibited a transition from ES1 to LS1 or from ES2 to LS2. However, two mixtures showed an ES1 to LS2 transition, indicating they are in a morphology-limiting stage.— A relationship was found between flame morphology and dynamics : flames transitioning from ES1 to LS1were slower than those transitioning from ES2 to LS2.— The FCP most affecting flame morphology and dynamics were σ, σsL/cb, γ, and the ratio of the channel hydraulic diameter to the flame thickness, Dh/δf.— In all DO cases, the formation of combustion structures (CS) at the corners was crucial for achieving DO.The DO types were successfully mapped as a function of FCP. The best division boundary was obtained withσsL/cb and γ.— The most widely used models for xDDT predictions performed poorly against our database, explaining only≈ 20% of the variance in the experimental data. The models proposed here resulted in significant improvements,now explaining 74 − 84% of the variance.