Experimental analysis on free spray and impinging spray of the swirl nozzle under low injection pressure

摘要

Diesel Particulate Filter (DPF) is a very effective after-treatment device to limit particulate emissions from diesel engines. As the collection amount of PM in the DPF is increased, pressure loss increases and purification rate decreases. Therefore it is necessary to regeneration the DPF regularly. Injecting fuels into the exhaust line upstream of the Diesel Oxidation Catalyst (DOC), hydrocarbons are oxidized on the DOC, which increases the exhaust gas temperature at the DPF inlet as one system to regenerate the DPF. In burning the particulate matter efficiently, it is necessary that the fuel is completely vaporized and uniformly mixed with exhaust gas. However the distance between injector and DOC is short, there is problem that evaporation and dispersion of the spray deteriorates. Therefore it is important to understand spray behavior in the tail pipe. Injection pressure of the spray in engine cylinder is between hundreds atmospheric pressure and thousands atmospheric pressure. However injection pressure of the spray in the tail pipe is tens atmospheric pressure. There are many research focused on the spray formation in engine cylinder but there is little the research focused on the spray under low injection pressure. This report focuses on free spray and wall impingingt spray to understand its physical phenomena using laser sheet scattered light method and Super High Spatial Resolution Photography (SHSRP) with swirl nozzle and n-dodecane. As a result, swirl spray injected in a state with swirling force, liquid film grows to radial direction by centrifugal force, become a hollow cone shape. When liquid sheet spreads, inside of the spray became negative pressure. And flow field from the outside to the inside of the spray is formed, follow up spray is formed. As injection pressure increases, this follow up spray becomes bigger. Also liquid and ligament region in the condition that injection pressure is 1 MPa is bigger than 4 MPa. Moreover the ligament and hole occurs irregularly in the liquid film, and the liquid film breakup of the swirl spray has occurred in the center of the liquid film as well as the tip. Therefore the breakup mechanism of swirl spray under low injection pressure is different from Fraser's theories and break up is irregular due to the instability of the liquid film. Also, in the wall superheating degree of non-boiling region and the nucleate boiling region, the wall impingement droplets splash upward, and the micro-droplets occurs. As a result, the vortex swirl up the droplets and spray height after spray impingement is maximum.