Dominant processes and characteristic timescales of intermodal coagulation for bimodal aerosols in decaying homogeneous isotropic turbulence

Aerosol coagulation is a key process governing particle size distributions (PSDs) in engineering and environmental flows. Most traditional models are developed for unimodal aerosols and therefore struggle to capture intermodal coagulation in bimodal PSDs. In this study, an aerosol-turbulence modeling framework was established by coupling a bimodal population balance equation (PBE) with a decaying homogeneous isotropic turbulence field simulated in Open Field Operation and Manipulation (OpenFOAM). The PBE for Brownian and turbulent coagulation was solved using the Taylor-expansion method of moments (TEMOM), which was compared with the quadrature method of moments, the direct quadrature method of moments (DQMOM), and the inverse Gaussian distribution method of moments. The consistency of the TEMOM closure and the reliability of the OpenFOAM flow field were validated. The temporal and spatial evolutions of the moment fields were analyzed to identify the relative contributions and characteristic timescales of Brownian and turbulent coagulation to intermodal coagulation. In addition, the sensitivities of coagulation dynamics to PSD parameters and flow-field conditions were examined. The results showed that TEMOM provided reliable predictions of the low-order moment evolution throughout the process. For the robustness tests, the maximum relative error of TEMOM with respect to the sectional method remained below 10%, confirming the robustness of the closure over the tested parameter range. The simulated decaying homogeneous isotropic turbulence field exhibited the expected spectral structure and decay characteristics. Intermodal coagulation in bimodal aerosols was initially dominated by turbulent coagulation, and gradually shifted to Brownian coagulation as the turbulence decayed. Moreover, the crossing of the Brownian and turbulent characteristic depletion times occurred earlier than the transition identified from intermodal mass transfer. The former only indicates comparable instantaneous particle-number depletion effects in the smaller mode, while the latter represents the change in the dominant mechanism governing intermodal mass transfer. This transition point was insensitive to the particle number concentration ratio, but was significantly delayed as the geometric mean diameter ratio and Reynolds number increased in the investigated parameter range. This study provides an efficient framework for predicting the evolution of bimodal aerosol systems in turbulent environments.