Experimental Investigation of Ignition Stability in a Cyclic Constant-Volume Combustion Chamber Featuring Relevant Conditions for Air-Breathing Propulsion

Pressure-gain combustion concepts are developed around the world as solutions to reach the ambitious target of the ultra-efficient aircraft road map for 2050 that requires a 20% reduction of specific fuel consumption of the engine. This reduction can be obtained by increasing the thermodynamic efficiency. Several patterned designs apply the Humphrey deflagration-based constant-volume combustion (CVC) to parallel piston-less combustors. Recently, a proof of concept constant-volume combustor was operated in representative aircraft combustor conditions. This study evidenced reliable operating regimes, but also critical design issues related to some of the most challenging combustion fields: ignition stability, flame propagation in non-perfectly-premixed conditions and trapped residual gases, as well as cycle hysteresis. A lab-scale facility (CV2) was designed to study and further improve our understanding of such CVC phenomena. The facility features the cyclic operation of constant-volume combustion, independently of a specific technology of intake and exhaust systems, at representative aircraft combustor conditions over more than 10 cycles. The results presented in the paper concern the investigation of CVC stability with a variation of the spark-ignition phasing, in direct injection of gaseous propane. The cyclic stable and unstable operating conditions have been characterized successfully by means of time-resolved PIV, pressure evolution measurements, as well as chemiluminescence visualization. A strong correlation between ignition probability and the cumulative probability density function of the local velocity is evidenced.