Effect of annulus ratio on the residence time distribution and Péclet number in micro/milli‐scale reactors

Micro/milli‐scale annular reactor with straight and helical forms has excellent heat and mass transfer performance due to the short molecular diffusion distance and dual‐wall surface transport. The annular gap spacing is scalable by adjusting the inner and outer tube diameter. The influence of diffusion and convection effects on axial dispersion as expanding the flow scale requires further elucidation with the help of residence time distribution (RTD) curves and Péclet (Pe) numbers. The correlation of RTD characteristics with annulus ratio γ = Dh/D (ratio of annulus characteristic size to outer diameter) is investigated using computational fluid dynamics. Results show that with enlarging the straight annular gap from micro‐scale to milli‐scale, RTD characteristics exhibit opposing patterns. This can be attributed to the transition from diffusion‐dominated to convection‐dominated on momentum transfer, and the transition interval is 0.167 < γ < 0.250. Correlation equations of Pe number with Reynolds (Re) number and γ are established under diffusion‐dominated and convection‐dominated states. The symmetrically distributed secondary flow in the helical annular gap effectively elevates the Pe (Pemax > 100). Correlation equations of Pe with Re and γ are established in helical annular gaps with 0.083 < γ < 0.208 and 0.167 < γ < 0.500. The above results contribute to understanding the annular flow RTD characteristics for better applications of tube‐in‐tube reactors.