Flag models have recently emerged as promising flexible vortex generators for enhancing heat transfer in laminar channel flows; however, systematic comparative studies of their thermohydraulic performance remain limited. In this work, a comprehensive numerical investigation is conducted on three representative flag configurations—the standard flag model (SFM), the inverted flag model (IFM), and the wall-mounted flag model (WFM)—to examine their heat transfer enhancement characteristics over a wide range of bending stiffnesses ($K_b^{*}$), channel Reynolds numbers (Re), and vertical placements ($h^{*}$). The comparative analysis reveals that the three models enhance heat transfer through distinct vortex-generation mechanisms. The SFM is dominated by wake-induced mixing, the IFM by leading-edge-generated coherent vortices with deep core penetration, and the WFM by persistently wall-attached vortical motions that strongly interact with the thermal boundary layer. The bending stiffness $K_b^{*}$ determines whether unsteady flapping dynamics can be activated, while Re and $h^{*}$ further regulate the coherence and transport effectiveness of the induced vortical structures once such dynamics are established. For all three configurations, the heat transfer efficiency increases with Re; however, the location of the high-efficiency regime in the $K_b^{*}\text{−}\mathrm{Re}$ parameter space is strongly configuration dependent. Among the three models, the IFM achieves the highest peak heat transfer efficiency with its high-efficiency region occurring at intermediate $K_b^{*}$ values and high Re. Nevertheless, this superior performance is confined to a relatively narrow operating window and exhibits pronounced sensitivity to $h^{*}$. In contrast, the WFM exhibits a broader and more robust high-efficiency regime at comparatively larger bending stiffness under high-Re conditions, whereas the SFM provides more modest enhancement with weaker sensitivity to parameter variations. Overall, these results clarify how flag-induced dynamics and vortex organization govern thermal transport in laminar channel flows.

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• Flow-structure interactions
• Mixing enhancement
• Thermal convection