Aerodynamics issues and configurations in MILD reactors

Moderate or intense low-oxygen dilution (MILD) combustion has broad potential in increasing thermal efficiency with reduced pollutant emissions. The absence of a visible flame front along with a distributed ignition process is the main feature of the reactive process, induced by intensely burned gas recirculation. Beyond such main characteristics, there are other ones such as temperature uniformity and the absence of noise that are also important for several applications of such combustion regimes to mid- to large-scale technologies. Thus, it is essential to analyze the peculiar role of both the external parameters and reactor design choices on the performance of such systems. The present book chapter reports a categorization of the different reactor configurations that can be used for MILD combustion and the peculiarity of some aerodynamics configurations used to stabilize distributed ignition processes. Therefore, several literature findings were reported to discuss the role of the fluid dynamics and particle trajectories on the oxidation process peculiarities of MILD reactors. Internal aerodynamics, in fact, contribute to providing MILD macroscopic characteristics by locally inducing the attainment of low-oxygen contents with high preheating levels through a strong mixing between the burned products and fresh reactants. In particular, the different reactor categories for such systems have been analyzed to stress the distinctive role of the reactor design and geometry configuration on the system performance and characteristics. Parallel jets, reverse flows, and swirl or cyclonic flows, in fact, emphasize the difference between each MILD technological application. To better understand the different configuration effects, the role of design choices such as wall position with respect to the inlet flows and/or main feeding arrangements on the achievement of the MILD regime is separately investigated.