Entrainment in the core regions of weak axisymmetric round fountains in homogeneous fluids with coexistent temperature and concentration effects

• Enhanced Entrainment at Interfaces: The study reveals significant entrainment enhancement at the interfaces between upflow and downflow,and downflow and ambient fluid in weak axisymmetric round fountains,driven by temperature and concentration gradients.This finding highlights the critical role of these interfaces in mixing dynamics. • Impact of Froude Number and Buoyancy Ratio: Increasing the Froude number(Fr)significantly intensifies momentum-driven mixing and entrainment with the ambient fluid,while the buoyancy ratio(N)exerts a pronounced inhibitory effect on entrainment.These results provide insights into the complex interplay between momentum and buoyancy forces in thermal-concentration fountains. • Dominance of Concentration-Induced Buoyancy: At low N,concentration-induced buoyancy dominates the mixing and entrainment processes,influencing the fountain's behavior and interaction with the surrounding fluid.This highlights the importance of considering both temperature and concentration effects in the design and optimization of fountain systems. The mixing and entrainment characteristics of a weak axisymmetric round fountain in a homogeneous fluid with coexistent temperature and concentration effects was investigated numerically with 0.25 ≤ Fr ≤ 1.75, − 0.75 ≤ N ≤ 5.0, all at Re=100, where Fr is the Froude number, N is the buoyancy ratio reflecting relative contribution of concentration to density compared with temperature, and Re is the Reynolds number. The results demonstrated that the upflow within the core region showed minimal sensitivity to entrainment of temperature and concentration. However, notable entrainment enhancement was observed at the interfaces between (i) upflow and downflow, and (ii) downflow and ambient fluid. A quantitative analysis revealed that increasing the Fr led to significantly intensified momentum-driven mixing and entrainment with the ambient fluid, resulting in a progressive reduction in the vertical profiles of velocity, temperature and, concentration in the core region. Furthermore, the N had significant influence on the entrainment process, with increasing N values showing a pronounced inhibitory effect on entrainment. Notably, the concentration-induced buoyancy dominated the mixing and entrainment dynamics at low N. This study provides references for complex interplay of momentum and buoyancy forces in thermal concentration fountains, with significant implications for applications in environmental and industrial fluid systems.