Thermal interactions in laminar heterogeneous flames of non-volatile metal suspensions

The characteristic feature of non-volatile solid fuel suspensions is the absence of a continuous premixed gas flame front. Instead, the reaction is localized to or near the surface of individual particles. This “flame-within-a-flame” structure gives rise to a multi-faceted combustion process, whose dynamics are governed by a range of underlying mechanisms. To understand the combustion of solid fuels, fundamental studies often explore the different “facets” of the process. This approach is used in the present thesis which examines two such mechanisms – front coupling and heat locking – and interprets them via simple formulations based on thermal considerations.Front coupling governs the thermal interaction of reaction fronts formed by the combustion of two or more dissimilar fuels. Experiments were conducted in a novel Hele-Shaw cell apparatus in aluminum-propane-air mixtures, in which aluminum burns with the products of the propane-air front. In each run, the propagation of a quasi-2D flame in a thin high-aspect-ratio long rectangular channel between two parallel windows was observed. The unimpeded line-of-sight of the diagnostics allowed the visualization of the thermally coupled fronts, corresponding to the combustion of propane and aluminum, respectively. The coupling behaviour exhibits a strong dependence on the aluminum front rather than on the propane-air one. High-speed recording also showed several types of frontal instabilities affecting flame coupling/decoupling.Heat locking occurs when particles ignite and burn diffusively at temperatures higher than their surroundings. In low-thermal-diffusivity media, the fast reaction “locks” heat in steep temperature gradients around the particles. This transition into the so-called “discrete regime” leads to a fundamental switch in flame physics. Experiments were performed in a remotely operated module (built by Airbus Defence and Space) aboard sounding rockets and parabolic flights, as part of the Percolating Reactive Waves (PERWAVES) project, in collaboration with the European Space Agency (ESA) and the Canadian Space Agency (CSA). In each test, flames in suspensions of micron-size iron particles in xenon/oxygen gas propagated in transparent tubes. During the flight aboard the ESA MAXUS-9 rocket (April 2017), the flame speed of suspensions in 20%- and 40%-oxygen gas was measured. Despite their particles burning three times faster and at higher temperatures, flames in 40%-oxygen mixtures had the same speed as those in the less reactive 20%-oxygen mixture, confirming that flame combustion is controlled by inter-particle heat diffusion rather than by the rate of chemical reaction, in accordance with discrete flame theory. The frontal structure of the flames close to the propagation limit was investigated in a second sounding rocket campaign aboard the ESA TEXUS-56 in November 2019. High-speed imaging of the flames from the rocket flight and from a prior parabolic flight campaign aboard the National Research Council (NRC) Falcon-20 aircraft in May 2019 indicates that, close to their lean propagation limit, flames may exhibit a percolation-like behaviour

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