Abstract:Microreactors, known for their superior mixing performance and efficient mass transfer characteristics, are widely used in many fields, such as chemical production, drug synthesis, and energy conversion. This study focuses on the design of microchannel structures and explores the underlying mechanisms of mass transfer enhancement by the passive microreactors. Four types of microreactor designs are investigated, including butterfly microchannel, Tesla microchannel, snake microchannel, and herringbone microchannel; and their mixing performance is systematically evaluated. Both numerical simulations and experimental tests are employed, and key parameters, such as mixing efficiency (η) and segregation index (Xs), are used to evaluate the mixing performance. The effects of Reynolds number (Re) and fluid viscosity on mixing efficiency and pressure drop are analyzed in detail. Numerical simulations suggest that different microchannel designs enhance mass transfer through different underlying mechanisms. The butterfly microchannel achieves efficient fluid dispersion and recombination by introducing obstacles; the Tesla microchannel facilitates fluid collision and flow interlacing through counterflow; the snake microchannel induces velocity differences and secondary flows via curved flow paths; the herringbone microchannel generates transverse secondary convections by incorporating grooves on the inner walls. The four microchannels all significantly improve the mixing performance of the microreactors, with butterfly and Tesla microchannels showing better mixing performance at low Re and snake and herringbone microchannels exhibiting lower pressure drops at high Re. Numerical simulations also demonstrate that the dominant mode of mass transfer shifts from molecular diffusion to convective transfer as Re increases. The mixing performance is poor when Re is low, as evidenced by the large segregation index and low mixing efficiency, and mass transfer mainly relies on molecular diffusion at low Re. The mixing performance gradually improves as Re increases, as indicated by the decreased segregation index and increased mixing efficiency, suggesting that the dominant mode of mass transfer shifts from molecular diffusion to convective transfer at high Re. In addition, the impact of fluid viscosity on mixing performance predicted by numerical simulations suggests that the mixing efficiency is weakened as fluid viscosity increases. The simulation results are further tested by experiments. Experimental results confirm that microchannel structures have a significant impact on the mixing performance at low Re, and the butterfly microchannel shows better mixing performance than the snake microchannel. In conclusion, experimental tests and numerical simulations show consistent results on the dependence of mixing performance on Re, the influence of microchannel structure on mixing performance, and the underlying mechanism of mass transfer enhancement. This study provides theoretical guidance and experimental support for the design and optimization of efficient microreactors, offering valuable insights for practical applications. Close-