Experimental investigation of scale effects on ignition progress in scramjet combustors

The design of large-scale scramjet combustors encounters challenges due to nonlinear scale effects resulting from geometric scaling of combustors. This study investigated the transient ignition scale effects caused by geometric scaling through experiments conducted on ethylene-fueled combustors with a geometric similarity ratio of 2:1 under inflow Mach number of 2.52. Schlieren imaging and CH* chemiluminescence diagnostics were employed to systematically analyze the spatiotemporal flame evolution characteristics. The results indicate that the ignition process consists of two distinct phases: cavity ignition and global flame establishment. Higher ignition energy substantially reduces ignition time. The smaller-scale combustor has shorter cavity ignition times, where flame kernels directly ignite shear layers. In contrast, the larger-scale combustor relies on cavity recirculation-dominated propagation, leading to longer ignition times. Applying 2-fold ignition energy partially compensates for scale-induced delay of ignition time, achieving a temporal ratio comparable to theoretical predictions (1:2). The larger combustor, benefiting from enhanced fuel mixing efficiency and relatively thinner boundary layers, enable reliable ignition across multiple positions. It offers essential insights that are crucial for optimizing ignition strategies in large-scale scramjet combustors.