Flow Instability of Double-Diffusive Natural Convection Under Wall Radiation Effects

The present study numerically investigates the effects of wall radiation on the flow instability and heat and mass transfer of double-diffusive natural convection in a rectangular cavity under opposing thermal–solutal buoyancy conditions. The governing equations are solved with the finite volume method, and dynamic mode decomposition is employed to uncover the underlying instability mechanisms. Two representative regimes are examined, namely a nearly balanced buoyancy regime and a thermal-buoyancy-dominated regime. In the nearly balanced regime, wall radiation suppresses flow instability by reconstructing the thermal boundary conditions and redistributing perturbation energy, whereas the intrinsic oscillatory double-diffusive instability originating from buoyancy balance remains unchanged. By contrast, in the thermal-buoyancy-dominated regime, wall radiation promotes flow instability by inducing Rayleigh–Bénard-type thermal stratification near the horizontal walls, which triggers secondary vortices that couple nonlinearly with the global circulation and accelerate its inertial instability. Moreover, increasing wall emissivity substantially enhances wall heat transfer, which becomes dominated by the radiative component, while the convective Nusselt number remains nearly unchanged, and the Sherwood number decreases once wall radiation is activated. These findings offer theoretical insights into flow stability control and heat and mass transfer regulation, with potential implications for high-temperature thermal systems.