In this study, the influence of injector diameter on the combustion of diesel sprays in an optically accessible combustion chamber of marine engine dimensions and conditions has been investigated experimentally as well as numerically. Five different orifice diameters ranging between 0.2 and 1.2 mm have been considered at two different ambient temperatures: a “cold” case with 800 K and a “warm” case with 900 K, resulting in a total of ten different test conditions.
In the experiment, the reactive spray flames were characterized by means of high-speed OH* chemiluminescence imaging. The measurements revealed a weak impact of the injector diameter on ignition delay (ID) time and flame lift-off length (LOL) whereas the influence of ambient temperature was found to be more pronounced, consistent with former studies in the literature for smaller orifice diameters.
Computational fluid dynamics (CFD) simulations were performed using the Conditional Moment Closure (CMC) combustion model and the numerical results have been validated with the experimental data by means of ignition delay time, flame lift-off length and temporal evolution of flame region. The simulation was capable to reproduce ID very well while the calculated LOL were generally underestimated. The early flame development after ignition was reasonably well captured. Following this validation, the influence of the orifice diameter on the ignition behavior and quasi-steady spray characteristics has been further explored numerically. Overall, CMC was found to offer a promising framework also for the simulation of sprays from nozzle orifices in the millimeter range, relevant for large two-stroke marine diesel engines.