Visualization on spray and flame characteristics of wall-impinging spray under marine diesel engine conditions

Abstract

Due to the longer nozzle-to-wall distance, spray-wall interaction with only vapor fuel impinging on the wall occurs generally in large-bore marine diesel engines. The interaction between the vaporized spray and wall not only impacts the combustion process, heat transfer, but also may condense and dilute the oil film on the cylinder wall. However, different from the scenario involving both liquid and vapor impinging on the wall in direct injection vehicle engines, the unique characteristics of vaporized diesel spray impingement have not been addressed in detail. To address the behavior of vapor diesel impinging spray and its influence on subsequent combustion in marine diesel engines, a 0.3 mm diameter single-hole injector, commonly used in marine diesel engines with a bore diameter around 170 mm, was utilized. The inclination angle of the injector is 30° and a flat wall was positioned 40 mm below the nozzle tip to approximate the relative positioning of the piston to the injector when the spray impinges on the wall in the engine. The interactions between spray and wall were studied under both inert and reactive conditions by altering the ambient gas to N2 and air, respectively. Back-illumination imaging, schlieren, and natural luminosity imaging were applied to study the impacts of ambient pressure, ambient temperature, and injection pressure on the evolution of impinging spray and diesel combustion within an optically-accessible constant volume chamber. Specifically, to evaluate the impact of spray-wall interaction with only vapor diesel impinges on the wall, the impinging sprays were compared to free sprays under the same experimental conditions. It shows that the impinging spray has a similar development mechanism with the free jet, while due to the varying air–fuel mixing rates at ambient pressures of 1 MPa and 3 MPa, the fuel film height on the wall exhibits different trends along the wall. At an ambient pressure of 5 MPa, the ignition delay of vaporized impinging spray is 17.62 % longer than that of free jet, while exhibiting a notable reduction in the soot emissions. The average time-integrated natural luminosity for jet impingement is found to be 71.28 % lower compared to the free jet with the same conditions.