Stabilized detonation triggered by a single ignition kernel in hypersonic ramp flow

Detonation initiation by a hypersonic ramp holds requirements in both energy deposition and chemical kinetics, known as the energetic and kinetic limits, respectively. If the post-shock ignition delay time exceeds the flow residence time on the ramp (the kinetic limit being unsatisfied), detonation cannot be formed since autoignition does not occur behind the shock. This work aims to evaluate the feasibility and conditions under which an additional ignition kernel can enable the transition from a ramp-induced inert shock to a stabilized detonation despite the kinetic limit being unmet. To this end, numerical simulations incorporating detailed chemistry are performed for hypersonic ramp flows, with and without extra ignition kernels, across varying freestream Mach numbers and pressures. Baseline simulations without extra ignition kernels reveal three combustion regimes, i.e. , detonation, shock-induced combustion, and inert shock. These numerical results align well with the theoretical predictions by the energetic and kinetic limits. Upon introducing the ignition kernels, three distinctive combustion regimes are identified: failed initiation, stabilized detonation, and destabilized detonation. Results show that whether detonation can be triggered by the ignition kernel or not is jointly influenced by the freestream Mach number and pressure. The boundary separating the detonatable and undetonatable regions aligns well with the theoretical energetic limit, confirming the feasibility of using an extra ignition kernel to facilitate detonation initiation. Nevertheless, stabilization of the triggered detonation is found to be strongly correlated to the formation of a normal detonation wave behind the ramp-induced shock. A third criterion called the standing limit is hence proposed to characterize this nature by defining a nondimensional parameter V _S = V ₂/ D _CJ2 (where V ₂ and D _CJ2 denote the post-shock flow and Chapman–Jouguet velocity). Detonation stabilizes only if V _S is smaller than or approximates unity, with a prompt pattern for V _S < 1 and a delayed pattern for V _S ⪆ 1. Novelty and significance statement The problem of shock-induced autoignition and its transition to detonation in hypersonic ramp flows has been extensively studied previously. However, little attention was paid to detonation initiation in scenarios where autoignition cannot occur. In this work, we numerically investigate the feasibility of using an additional ignition kernel with low ignition energy to facilitate detonation initiation in hypersonic ramp flows. For the first time, we reveal the specific conditions under which a single ignition kernel can enable the transition from an inert ramp-induced shock to a stabilized detonation and propose quantitative detonation initiation and stabilization criteria for prediction. The significance of this paper lies in two aspects. First, the findings offer new insights into fundamental detonation initiation problems. Second, the ignition method and relevant theoretical criteria proposed have implications for future detonation engine design.