Sébastien Candel

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.Copyright

Large eddy simulations of a transcritical round jet submitted to transverse acoustic modulation

This article reports numerical computations of a turbulent round jet of transcritical fluid (low temperature nitrogen injected under high pressure conditions) surrounded by the same fluid at rest under supercritical conditions (high temperature and high pressure) and submitted to transverse acoustic modulations. The numerical framework relies on large eddy simulation in combination with a real-gas description of thermodynamics and transport properties. A stationary acoustic field is obtained by modulating the normal acoustic velocity at the lateral boundaries of the computational domain. This study specifically focuses on the interaction of the jet with the acoustic field to investigate how the round transcritical jet changes its shape and mixes with the surrounding fluid. Different modulation amplitudes and frequencies are used to sweep a range of conditions. When the acoustic field is established in the domain, the jet length is notably reduced and the jet is flattened in the spanwise direction. Two reg...

Modeling the response of premixed flame transfer functions - Key elements and experimental proofs

A wide variety of analytical models has been proposed to describe the transfer function of premixed flames submitted to flow disturbances. These models generally rely on a kinematic description of the flame reaction surface which is perturbed around its steady state. The response then depends on the type of flow perturbation considered, the mean flow characteristics and flame properties. Systematic comparisons between model predictions and experimental results are however less numerous. This study revisits some of these theoretical descriptions and their underlying hypotheses to delineate their domain of application. The main features of the response of premixed flames to flow disturbances are highlighted. Different mechanisms are identified and their analytical representation is discussed. It is shown that experimental observations provide useful guidelines in the flame transfer function modeling. The cases investigated include laminar and turbulent configurations in conical or inverted V-flame configurations. Effects of swirl, flame root dynamics and confinement are emphasized.