Scenario-Based Multi-Objective Optimization of HVAC Supply Parameters for Respiratory Particle Control in a Subway Carriage

Respiratory infectious diseases pose substantial transmission risks in enclosed public-transport environments. Under a defined fully seated reference scenario, this study examines whether the same HVAC supply parameter governs two respiratory-particle control objectives in a subway carriage: whole-carriage particle removal and local seated breathing-zone exposure. Computational fluid dynamics (CFD) simulations, single-factor analyses, an L16 orthogonal design, analysis of variance (ANOVA), and grey relational analysis (GRA) were used to evaluate supply velocity, temperature, and angle. The results reveal an objective-dependent control hierarchy. Supply velocity is the leading factor for whole-carriage removal (contribution rate, 40.14%; p = 0.047), whereas supply angle contributes most to the seated breathing-zone particle proportion (59.30%), although the corresponding ANOVA result does not reach statistical significance at the 0.05 level (p = 0.056). The two single-objective optima share the same velocity and angle but differ in temperature, indicating a partial rather than complete mismatch. GRA identifies 2.0 m/s, 24 °C, and 0° as the best compromise among the investigated cases. This combination is specific to the modeled carriage, passenger arrangement, source location, and operating conditions. The more generalizable finding is that global clearance and local breathing-zone exposure are governed by different control variables.