Detonation initiation by shock focusing is a promising approach for detonation engines; however, the role of viscous effects remains insufficiently understood because most previous studies relied on inviscid models. In this work, two-dimensional viscous simulations with detailed hydrogen–air chemistry are performed to investigate how reflected shock–boundary layer interactions govern ignition and detonation initiation in shock-focusing systems across a wide range of incident shock Mach numbers and reflector angles. Five distinct initiation modes are identified: ignition failure, nondetonative autoignition on the reflector wall, detonation initiated by autoignition on the reflector wall, detonation initiated by autoignition on the side wall, and shock-focusing-induced detonation. For the latter three modes, reflected shock–boundary layer interaction plays a decisive role by inducing boundary-layer separation, where intense viscous dissipation and prolonged fluid residence time generate localized heating that triggers mixture autoignition and subsequent detonation development. Quantitative assessment of boundary layer effects on the critical Mach number is conducted through comparison with inviscid simulations. Results demonstrate that neglecting boundary layer effects leads to qualitatively incorrect predictions of both ignition mode and critical Mach number. A striking nonmonotonic dependence of critical Mach number on boundary layer effects is observed as the reflector angle varies: boundary layer effects reduce the critical Mach number for θ = 100°, 110°, and 150° by facilitating detonation initiation while producing negligible effects for θ = 90°, 120°, and 125°. This nonmonotonic behavior arises from two competing mechanisms. Increasing reflector angle elevates the critical Mach number and postshock temperature, thereby favoring detonation development; simultaneously, it reduces boundary layer thickness and weakens reflected shock–boundary layer interaction above the reflector wall, suppressing autoignition on the reflector wall. This interplay causes a transition of detonation initiation from the reflector wall to the side wall across reflector angles, resulting in the nonmonotonic variation of boundary layer influence on the critical Mach number. These findings underscore the essential role of shock–boundary layer interactions in governing detonation initiation behavior in shock-focusing systems.

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• Boundary layers
• Detonation
• Shock waves