Rarefied transport in planar Y-junctions: Effects of geometric asymmetry, surface scattering, and upstream velocity memory

We investigate nonequilibrium kinetic transport and trajectory-resolved septum energy transfer in reservoir-fed symmetric and asymmetric planar Y-junctions at a collisional Knudsen number of 10. A two-dimensional, two-velocity-component event-driven kinetic Monte Carlo model quantifies conditional septum energy exchange, upstream velocity-memory effects, and sensitivity to outer-transition morphology, while a complementary Direct Simulation Monte Carlo framework defines the population-based estimators required for entropy-production analysis. Increasing the total included daughter angle from 2 ° to 90 ° raises the septum-interception probability from approximately 0.31 to 0.39. Surface scattering sets the statistical structure of conditional energy exchange: at a septum diffuse-reflection probability of 0.96, the complete conditional variance ranges from 15.5 to 17.5 and the zero-transfer probability from 0.7 % to 1.3 %; at a probability of 0.05, the variance drops to 0.73–0.82 while the zero-transfer probability mass increases to 78.6 %–81.0 %. Architectural asymmetry elevates the condition-mean variance by 1.1 %–5.3 % relative to matched symmetric junctions. Upstream conditioning and outer-transition morphology act primarily through terminal passage: wide–short and narrow–long conditioning channels increase transmission through the asymmetric 90 ° junction by 7.15 % and 18.05 %, respectively, relative to reservoir injection, whereas replacing sharp outer backsteps with a 16-panel cubic-Hermite transition approximation reduces transmission by up to 9.35 %. These results identify distinct kinetic controls, included angle governs septum exposure, surface scattering governs localized energy-exchange statistics, and inlet history and outer-transition morphology govern terminal transmission.