Different Flame Patterns Linked With Swirling Injector Interactions in an Annular Combustor

It is known from cold flow experiments that linear arrays of injectors may feature different types of aerodynamic patterns (see for example ASME-GT2013-94280, ASME-GT2014-25094). There are however no indications on what can happen under hot fire conditions since most experiments have been carried out in the absence of reaction or in single injector configurations. It is now possible to investigate this issue by making use of a recently developed annular combustion chamber. This device designated as MICCA is equipped with multiple swirling injectors and its side walls are made of quartz providing full optical access to the flame region thus allowing detailed studies of the combustion region structure and dynamics. Experiments reported in this article rely on direct observations of the flame region through light emission imaging using two standard cameras and an intensified high speed CMOS camera. The data gathered indicate that interactions between successive injectors give rise to patterns of flames which exhibit an alternate geometry where one flame has a relatively low expansion angle while the next spreads sideways. This pattern is then repeated with a period which corresponds to twice the injector spacing. Such arrangements arise when the angle of the cup used as the end-piece of each injector exceeds a critical value. Effects of mass flow rate, equivalence ratio, and injector offset are also investigated. It is shown that the angle which defines the cup opening is the main control parameter. It is also found that when this angle exceeds a certain value and when the laminar burning velocity is fast enough, the flame pattern switches in an unsteady manner between two possible configurations. This unsteady behavior is characterized using high-speed imaging. It is finally shown that these alternating flame patterns lead to alternating heat release rate distributions and inhomogeneous heat transfer to the chamber walls featuring a helicoidal pattern.