Numerical investigation of macroscopic and microscopic characteristics of an air-assisted spray from a multi-hole nozzle

To facilitate the transition of marine energy systems toward low-carbon and environmentally sustainable solutions, this study numerically investigates the medium-assisted atomization characteristics of a marine methanol tri-fuel boiler (methanol, diesel, and heavy fuel oil) under realistic operating conditions. A CFD-based VOF-DPM framework coupled with adaptive mesh refinement (AMR) is employed, in which the VOF method captures the evolution and breakup of the continuous liquid phase, while the DPM tracks the dispersed droplets and their size characteristics, with AMR applied to both phases to ensure adequate spatial resolution. Key parameters analyzed include the evolution of co-current atomization for different fuels, velocity-field distribution, droplet-size distribution, and atomization angle. The results demonstrate that methanol, owing to its low viscosity and surface tension, generates fine and uniformly distributed droplets predominantly within the 0–10 μm range, with a maximum atomization angle of 25°–27° and a Sauter mean diameter (SMD) consistently below 100 μm. Diesel produces droplets concentrated mainly between 40 and 50 μm, accompanied by noticeable fluctuations in atomization uniformity, whereas heavy fuel oil exhibits pronounced primary fragmentation after saturated-steam-assisted atomization, yet its velocity decays rapidly. Distinct differences are observed among the three fuels in terms of spray-penetration distance and atomization angle: methanol shows the strongest lateral dispersion but the weakest axial penetration, heavy fuel oil displays the opposite trend, and diesel lies between the two. This study elucidates the effects of fuel properties and atomizing media on the spray performance of marine methanol-fired triple-fuel boilers, offering theoretical insights for nozzle optimization and the practical application of low-carbon fuels in marine energy systems.