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The need for high-density energy in the current growth of Miniaturization of Electromechanical Systems (MEMs) has led to the recent development of the field of micro-scale combustion. Indeed, even with 10% efficiency, a miniaturized liquid-fueled combustion engine will have a higher energy density than the most advanced batteries. Despite remarkable advancements achieved, a lot of progress is still required to fully understand the flame behavior at micro-scale. The experimental set-up consists of a cylindrical reactor with a controlled external temperature profile. An infrared camera monitores continuously the external temperature and an EMCCD camera is used to register the CH* emission from the flame. A controlled high temperature gradient induces the auto-ignition of the mixture inside a channel smaller than the standard quenching diameter. At this scale, different flame behaviors are observed by modifying the flow velocity, from stable flames to Flames with Repetitive Extinction and Ignition (FREI) and weak flames respectively with high, middle and low flow rates. Fuel type, equivalence ratio, and inner diameter play also a key role in these behaviors and transitions. Understanding these different aspects is one of the major issues in applications of micro-scale combustion in different frameworks such as MEMs or fires in industrial facilities. Previous experimental studies were carried out with fuel/air mixtures, but as applications of microcombustion are numerous, a deep understanding of these behaviors in different environments is required. Therefore the present work focuses on the combustion of a fuel/O2 mixture with different diluents (N2, helium, argon, or CO2). As fluid properties change (heat capacity, heat conductivity, transport properties…), we observed a strong impact on flame behaviors such as stabilization position, auto-ignition position along the reactor, and FREI frequencies. A numerical investigation was also conducted using ANSYS Fluent to support and rationalize experimental observations. The impact of the chemical scheme was investigated. Indeed as the thermal losses at the wall are high in the micro-scale combustion because of the high surface-area-to-volume ratio, reactions heat release plays an important role in the combustion behavior.

Experimental and numerical studies of the influence of diluent on characteristic micro-scale combustion flame behaviour