Flow regime in the ISO standard 22197–1:2016 photocatalytic Reactor: PIV-Visualisation, Modelling, and Application

• Flow in the ISO reactor shows stable vortices despite low Reynolds numbers. • A wide-angle diffuser induces coherent structures, enhancing mixing. • Vortices boost NO x transport beyond molecular diffusion limits. • Owing to increased mass transport, the photocatalytic performance is enhanced. • Results challenge laminar assumptions in photocatalytic reactor models. Photocatalysis is a promising technology for air purification, but accurately predicting its performance under real-world conditions requires detailed knowledge of airflow characteristics in standardised testing reactors. Accurate assessment of photocatalytic reactor performance requires a comprehensive understanding of internal flow structures, which are often oversimplified in models that rely solely on macroscopic flow parameters. In this study, we experimentally determine the flow regime within a reactor conforming to ISO Standard 22197–1:2016 using time-resolved Particle Image Velocimetry (PIV). Despite operating at low Reynolds numbers (Re ≈ 150–200), which nominally indicate laminar conditions, the flow field exhibits complex, coherent structures arising from boundary layer separation induced by the wide-angle inlet diffuser (≈60°). The resulting quasi-periodic laminar vortices dominate convective mass transport, greatly exceeding molecular diffusion in the spanwise and wall-normal directions. These vortical structures are stable, large-scale, and advected with the mean flow, leading to enhanced mixing and reactant flux to the photocatalytic surface. This non-trivial flow topology explains the high photocatalytic NO x degradation rates observed under conditions where purely molecular transport would be rate-limiting. The findings underscore the inadequacy of traditional laminar flow assumptions in such systems and provide critical data for refining kinetic models and scaling methodologies for gas-phase photocatalytic reactors.