UCLA

In this study, the acoustically coupled combustion instability experiments conducted at the Air Force Research Laboratory (AFRL/RQR) located at Edwards Air Force Base, and our research group at the Energy and Propulsion Research Laboratory (EPRL) at UCLA are compared. The main goal is to investigate a comparison of gaseous methane flames under oscillatory acoustic fields subjected to laminar (Re ranging from 20 to 100) and turbulent (Re of 5300) conditions using two alternative burner configurations of single and coaxial jets. This study also explores the different flow regimes and the parameters causing high-frequency transverse mode instabilities in fuel jet combustion relevant to liquid rocket engines. At the Air Force Research Laboratory, the experiments are focused on turbulent jet combustion under acoustic forcing at pressure nodes and anti-nodes with the following burner configurations: a single methane jet surrounded by a low-velocity oxidizer co-flow and a coaxial jet with annular oxidizer flow and the low-velocity co-flow. In a collaborative effort, our research group at the EPRL at UCLA explores the gaseous laminar reactive methane microjets under acoustic forcing at a configuration closer to the pressure node for two burner configurations of single and coaxial jets. The main difference between the configuration setups at UCLA and AFRL is the absence of low-velocity co-flow at the UCLA EPRL experiments. To study such combustion instabilities, the data are acquired by high-speed schlieren, OH∗ chemiluminescence, and high-speed imaging. Then, the collected data are analyzed using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to quantify the flames dynamics.

Similar dynamical patterns are observed by comparing the single and coaxial jets subjected to laminar and turbulent conditions. For instance, a single jet with Reynolds number 65 has similar phase portraits as the single turbulent jet with Reynolds number 5300. Both exhibit the locking-in of the oscillating flame under the applied acoustic forcing and a peri- odic behavior. Additionally, no natural instability is observed for the laminar and turbulent single jets. The second burner configuration, the coaxial jet, also demonstrates similarities between the POD modes. A similar annular to inner velocity ratio of R = 0.3 is selected for the coaxial jets. UCLA and AFRL reveal similar dynamical characteristics for the acoustically coupled laminar and turbulent fuel jet studies. This kind of understanding could be beneficial in developing reduced order models (ROMs) for such combustion instabilities, which eventually could be used in controlling and predicting such instabilities based on oper- ating conditions or configuration changes. Additionally, these similarities reveal the value of low Reynolds number reactive microjet dynamics in understanding high Reynolds numbers experiments, but at a smaller scale with a lower cost.