Secondary droplet breakup of impaction-pin nozzle: Comparison between experimental and CFD-DPM modelling

Spray systems play a crucial role in various industrial and environmental applications, where precise control over droplet size is critical for achieving efficiency. Despite extensive studies on primary breakup, which involves the disintegration of liquid jets or sheets into droplets, the dynamics of secondary breakup, where droplets fragment post-formation, remain less understood. In environmental applications, among various nozzles, impaction-pin nozzles have enabled the production of fine misting droplets at micron and submicron levels. One of the applications of these impaction-pin nozzles is to produce an artificial fog using high pressure seawater to shade corals, a technology under investigation within the Reef Restoration and Adaptation (RRAP) program. This study aims to model and characterise the secondary breakup dynamics in impaction-pin nozzles using a combined numerical and experimental approach. Simulations are performed using Discrete Phase Model (DPM) to model droplet dynamics and size distribution, leveraging its efficiency and accuracy for dispersed-phase tracking. The numerical model incorporated stochastic breakup, coalescence, and evaporation models within Euler-Lagrangian framework, alongside unsteady RANS modelling for gas-phase flow. Experimental validation was performed using a Scanning Electrical Mobility Sizer (SEMS) and an Aerodynamic Particle Sizer (APS), ensuring high-resolution particle size measurements particularly at micron and submicron levels. The impaction-pin nozzle (MeeFog IP-2115-08) used in this study atomised seawater droplets under controlled conditions. Both the experiment and simulations yielded similar log-normal distributions of dry particle sizes upon evaporation. The mean diameter for numerical CFD distribution was 322.4 nm with humidified distribution at 51 % of relative humidity had mean of 236.3 nm and initial dry particles at 675.1 nm, keeping the ranges within the experimental and numerical errors. The model also predicted the spatial distribution of droplets and spray characteristics with experimental visualisation, such as angle variation during spray development, which correlated well with experimental observations. This work provides valuable insights into secondary breakup dynamics and offers a validated framework for optimizing impaction-pin nozzle spray systems for applications requiring precise droplet size control.