A review of flow channel optimization methods for reducing flow resistance in local ventilation components

Local ventilation components in ventilation and air-conditioning systems are major sources of energy loss due to flow resistance. Although various flow channel optimization methods have been proposed, systematic comparisons among them remain limited, hindering clear guidance for method selection. This review summarizes optimization approaches for local flow channels, together with the underlying resistance-reduction mechanisms and methods for evaluating local resistance. The results show that suppressing vortex structures induced by boundary layer separation and secondary flow is the dominant mechanism for reducing resistance. For resistance evaluation, the resistance coefficient and equivalent length methods provide fast estimates for fully developed flows, while CFD offers high-accuracy predictions. The truncation method is applicable to non-fully developed flows but is restricted to Reynolds numbers above 5 × 10 5 . Among optimization strategies, adjoint-based inverse design methods, particularly those parameterized by cell porosities or coordinates, provide high computational efficiency and wide applicability. However, the resulting geometries are often highly complex and difficult to manufacture. Future studies should incorporate fabrication constraints into the optimization frameworks to ensure both manufacturability and low-resistance performance. Overall, this review establishes a systematic framework for resistance reduction in local ventilation components and highlights adjoint-based inverse design as a promising approach toward energy-efficient ventilation systems.