Experimental study on flow resistance of mini-channels fabricated via in-situ low-resistance selective laser melting

Additive manufacturing (AM) enables the fabrication of mini-channel heat exchangers for high-power-density systems but introduces surface roughness and geometric deviations that affect flow resistance. To address these issues, a low-resistance selective laser melting (L-SLM) process is developed, employing a small laser spot diameter, a thin powder layer, and fine powder feedstock. This configuration reduces internal surface roughness by nearly one order of magnitude compared with conventional SLM while improving cross-sectional dimensional accuracy. A deep-learning-based characterization framework is established to extract geometric parameters from metallographic images. This framework enables batch measurements of dimensional deviations with micrometer-scale resolution and provides inputs for hydraulic analysis. Flow resistance tests are performed on circular, square, rounded-square, and rectangular channels under laminar, transitional, and turbulent conditions (Re ≤ 10,000). The results indicate that flat-contour geometries suppress fusion defects and yield lower friction factors. Unlike smooth tubes, the critical interplay between the decaying viscous sublayer and emerging roughness elements leads to an early transition and a roughness-dominated regime, which is systematically verified across the non-circular geometries. Comparative analysis substantiates the maximum profile height (Rz) as the critical descriptor, showing that friction factor predictions based on Rz achieve significantly higher accuracy (deviation ∼1.5%) compared to averaging parameters (Ra, Rq). The L-SLM process achieves an order-of-magnitude reduction in roughness (Ra = 1.36-2.53 μm) and a 20-40% reduction in flow resistance compared to conventional DMLS channels reported in the literature, effectively mitigating the roughness penalty. The proposed roughness-geometry-flow coupling framework links these critical descriptors to friction-factor behavior, supporting roughness-aware design of additively manufactured mini-channel heat exchangers.