Knudsen minimum for surface reactions in microscale tube flow

Knudsen minimum for surface reactions within microscale tube flow is explored. A direct simulation Monte Carlo (DSMC) method is employed to analyze these phenomena across a broad Knudsen numbers and tube aspect ratios. While appropriate normalization is a recognized prerequisite for observing the Knudsen minimum in mass flux, a comparable method for analyzing surface reactions has been lacking. To overcome this, we introduce a generalized normalization framework. This framework enables consistent comparison of surface reaction rates and reveals a clear minimum that depends on the tube aspect ratio. Similar to the Knudsen minimum for mass flux, the depth of this minimum increases with increasing tube aspect ratio. However, a key distinction emerges: unlike mass flux, the observed minimum for surface reactions shifts toward the continuum regime as the tube aspect ratio increases. This unique shift is attributed to variations in the average reactant mole fraction, governed by the balance between reactant transport and surface reactions. This aspect ratio-dependent shift offers a novel benchmark for validating surface reaction models that account for non-equilibrium effects, thereby enhancing our comprehension of microflows involving surface reactions.