Performance Investigation of a Double Split and Recombine Multifunctional Heat Exchanger Reactor

Abstract

Mixing and heat transfer at small scales remain major challenges in process intensification. In laminar flows, diffusion is often too slow, and heat transfer depends on how effectively the geometry renews fluid near the walls. Passive split and recombine (SAR) reactors overcome these limitations by repeatedly dividing, redirecting, and recombining the flow, enhancing mixing and heat transfer without moving parts or external energy input. This thesis presents a detailed investigation of a circular double split and recombine (DSAR) multifunctional heat-exchanger reactor. The work is organized into three parts: a detailed CFD and experimental characterization of the circular DSAR; a cross-sectional shape study comparing it against the square DSAR under two matching criteria, equal hydraulic diameter and equal cross-sectional area; and a CFD benchmark against the classical Chen SAR and Gray SAR configurations. All geometries are matched by internal active volume to ensure a fair comparison. Performance was quantified through pressure drop, friction factor, mixing index, residence time distribution, average Nusselt number, mixing energy cost, and performance evaluation criterion (PEC), combined into an equal weighted normalized ranking. Experimental pressure-drop measurements agreed with CFD within acceptable uncertainty. Among the cross-section variants, the circular DSAR achieved the highest first-element mixing index (MI=0.94) and the highest thermal performance (PEC=3.75), outperforming the square baseline. Against the classical benchmarks, the circular DSAR reached near-complete mixing within a single element, compared to 2-3 elements for Chen and up to 12 for Gray. At Re=300, this translated to a mixing energy cost of 456 mW, approximately half that of either benchmark (~930 mW). Chen and Gray showed higher local Nusselt numbers, a consequence of their shorter elements, but the mixing and energy penalties dominated the aggregate score. The circular DSAR was identified as the best overall SAR topology with an aggregate score of 0.887, providing the most favorable trade-off between mixing, heat transfer, and energy efficiency for passive flow reactor applications.