Study On Diesel Spray Combustion Under Oxygen-enriched Conditions

Oxygen-enriched combustion has demonstrated superiorities in pollutant emission and thermal efficiency,and has been applied in several industrial fields.Researchers have carried out investigations on oxygen-enriched combustion in diesel engines,but there are few studies focusing on flame characteristics under oxygen-enriched conditions.In this paper,multiple optical diagnostics are applied to investigate diesel spray flame at different oxygen-enriched levels,and the tests are implemented in an optical constant volume vessel simulating the in-cylinder conditions of diesel engines,in order to investigate the appearance,development,stabilization and extinguish processes of diesel spray flame as well as the flame characteristics and typical species distribution.Firstly,high-speed imaging technology is applied to investigate the effects of various oxygen level(21-90%)and types of buffer gases(argon,nitrogen and carbon dioxide)on the combustion processes and characteristics of diesel spray flame.Experimental results show that with the rising of the oxygen level,the ignition delay shortens,the end of combustion advances,flame luminosity increases,the width and length of the flame reduce,the spherical soot head in the flame tip tends to shrink and fluctuate,and a large blue region emerges in the tip of the diesel spray flame.Then,three types of buffer gases,argon,nitrogen and carbon dioxide are tested.And experimental results show that with the same level of oxygen,flame luminosity of diesel spray flame buffered with argon is the highest while that buffered with carbon dioxide is the lowest.That is to say,oxygen level plays an important role in the combustion processes and characteristics of diesel spray combustion,and in the meanwhile,the type of buffer gas also affects flame luminosity.Based on this,two-color method and OH chemiluminescence imaging are applied to investigate the distributions of flame temperature and soot concentration,and the lift-off length of the diesel spray flame at different oxygen level of 21-60% buffered with argon,and then the flame natural emission spectra of diesel spray flame at oxygen level of 60% are tested.The results show that with the rising of oxygen level from 21% to 60%,the flame area shrinks,the temperature of flame goes up,the tendency of soot formation weakens,and the flame lift-off length first decreases and then increases.And at oxygen level of 60%,flame emission spectra from the points with the distance of 15-70 mm from the nozzle tip are dominated by soot radiation,while those from the points with the distance of 70-100 mm from the nozzletip are dominated by chemiluminescence which is from some combustion intermediate species such as OH and CO.And the chemiluminescence of CO oxidation is the main reason for the blue region in the tip of diesel spray flame at high oxygen level.The experimental results of high-speed imaging,two-color method,OH chemiluminescence,and flame emission spectra are discussed and compared with the existing conceptual model of diesel spray combustion,trying to give an explanation of the effects of oxygen-enriched level on diesel spray flame.This study provides some basic supports for further application of oxygen-enriched combustion in real diesel engine,and makes effort to develop new technology of engine combustion control.