Numerical Investigation of Heat Transfer Augmentation in Axial and Parallel Rotating Channels with Inserts

Rotating cooling channels are an effective way to enhance convective heat transfer in high-speed, high-power-density electric machines. This study numerically investigates the flow and heat transfer in axial and parallel rotating channels across a range of axial Reynolds numbers (Rea = 5000–25,000) and rotation numbers (N = 1–3). The numerical study is conducted using three-dimensional RANS simulations with the realizable k-ϵ model, validated against established rotating pipe correlations. Parallel rotation consistently produces higher heat transfer enhancement than axial rotation under identical operating conditions, with the Nusselt number (Nu) increasing with the eccentricity ratio (H/D). The largest enhancement is observed for the parallel configuration at H/D = 8, where Nu increases by 88% compared to the axial case at Rea = 5000 and N = 3, resulting from amplified centrifugal forces that generate intense secondary vortices. The effect of a Kenics static mixer is also evaluated in both configurations. The insert enhances Nu by 17–38% in axial channels, where rotational effects alone are insufficient to promote strong secondary flow, but produces only marginal gains of 0–8% in parallel channels at H/D = 8, despite a significant increase in pressure drop. Thermal Performance Factor (TPF) analysis against the stationary pipe benchmark reveals that all empty pipe rotating configurations achieve TPF > 1, reaching a maximum of 1.47 for the parallel H/D = 8 configuration at Rea = 5000 and N = 3, while all Kenics-equipped configurations produce TPF < 1 throughout the study. The scientific contribution of this study is twofold. First, the Kenics mixer provides substantial heat-transfer enhancement that is strongest in axial channels and diminishes with increasing eccentricity (0–8% at H/D = 8). Second, despite this heat-transfer enhancement, the Kenics-equipped configurations yield TPF < 1 in every case examined, meaning Nu gains do not offset the associated pressure penalty on a performance-weighted basis. By contrast, the empty rotating configurations—particularly parallel rotation at high eccentricity—achieve TPF > 1 up to 1.47 (H/D = 8, Rea = 5000, N = 3). These results indicate that rotation alone is the preferred augmentation strategy when overall thermo-hydraulic performance is the design objective, while Kenics inserts should be reserved for applications where maximum heat-transfer enhancement is prioritized over the pressure penalty.