Modelling of temporal and spatial evolution of sulphur oxides and sulphuric acid under large, two-stroke marine engine-like conditions using integrated CFD-chemical kinetics

In this work, three-dimensional computational fluid dynamics (CFD) studies of sulphur oxides (SO_x) andsulphuric acid (H₂SO₄) formation processes in a large, low speed two-stroke marine diesel engine are carriedout. The current numerical study aims to investigate the conversion of sulphuric dioxide (SO₂) to sulphurictrioxide (SO₃) and the possibility of H₂SO₄ condensation which are the prerequisites to betterunderstand the corrosion-induced wear phenomenon. This is achieved with the aid of the implementationof a multicomponent surrogate model, which comprises a skeletal n-heptane mechanism and areduced sulphur subset mechanism. In the present work, performance of the coupled CFD-chemicalkinetic model is evaluated using both qualitative and quantitative methods. The modelling results showthat the temporal and spatial evolutions of SO_x predicted by the skeletal model are similar to those by thebase mechanism. Predictions of the variations of SO_x and the associated SO₂ to SO₃ conversion inresponse to the change of fuel sulphur content, swirl velocity, start of injection, scavenge pressure andhumidity qualitatively agree with numerical and experimental results from the literature. The model isfurther evaluated using the measured SO₂ to SO₃ conversion levels in a low load, low scavenge pressurecase and a low load, high scavenge pressure case. The absolute values of simulated and measured conversionlevels are close, although the former appear to be higher. The current results show that the flameimpinges at the cylinder liner near top dead centre. The gas is cooled rapidly by the wall temperatureand H₂SO₄ is produced in the region where the local temperature is less than 600 K. Based on the fluegas correlation, the acid dew point temperature is higher than the wall temperature, suggesting that acidcondensation may begin early at the top part of the cylinder liner. The predicted distribution correspondswell with the distribution of corroded parts observed in service engines. The model is expected to serve asan important tool to simulate the rates of SO₂ absorption into lubricating oil film and H₂SO₄ condensationin this combustion system.