| The autoignition and combustion characteristics, as well as their influence factors and boundary conditions, are important for diesel HCCI combustion. Therefore, this paper proposed a research method for autoignition and combustion stabilization of fuel jet in Controllable Active Thermo-Atmosphere (CATA) through analyzing the base of the lifted triple flame. A novel CATA combution system was developd to provide homogeneous temperature field for the central jet flame. The diesel spray was issued into the CATA and the autoignition and combution characteristics were studied. The different stabilization mechanisms in different temperatures were concluded and the Critical Temperature (TC) was found to be the key factor which influences the combustion stabilization. The effects of fuel properties, spray quality, injection parameters and coflow additives on the TC were also discussed.The CATA combustor was developed from the prototype of the Dibble burner. Based on this combustor, the CATA combustion system was built. The operating characteristics of this system show that this system can provide a controllable active thermo-atmosphere between 700K and 1500K safely. There exists a nearly constant temperature cylinder covering the axial distance from the nozzle to 150mm downstream and the radial distance up to 40mm, which enables the research of jet flame at different coflow temperatures.Since it is hard to get homogeneous mixture with liquid fuels due to their weak vaporization, a novel research method was proposed. The fuel vapor in a lifted flame is mutually diffused with the surrounding oxidizer around the flame base where the equivalent ratio spans a wide range and there exists a stoichiometric mixture area where autoignition occurs originally. Therefore the autoignition phenomenon of diesel fuel can be investigated through analyzing the flame base of lifted flame without preparing homogeneous mixture. Based on this method, autoignition and combustion characteristics of diesel spray flame were studied and, furthermore, could be used to validate the feasibility of the proposed method. It can be concluded that autoignition position approaches to nozzle tip as the coflow temperature increases. Autoignition delay changes sensitively under low coflow temperature and when coflow temperature increases 26K, the autoignition delays more than 4ms. When the coflow temperature is high enough, the autoignition delay decreases slowly. The liftoff height of diesel spray flame decreases with the increasing of coflow temperature. The lower the temperature, the more evident increase of liftoff height. Especially, when temperature is below 1022K(±3K), stable lifted flame is hard to realize.High-pressure injected diesel spray was issued into thermo-atmosphere and the multipoint autoignition phenomenon was observed. The effects of injection parameters on autoignition and combustion characteristics were experimentally studied. The test results show that the autoignition delay depends strongly on the coflow temperature and has nonlinear relationship with the temperature. The injection parameters, in terms of nozzle hole diameter, injection pressure and pump speed, have some effects on autoignition delay while these effects depend on the coflow temperature. The significant effect lies in low coflow temperature. The injection parameters influence the autoignition delay because it can shorten the physical delay by improving the spray quality and enlarging the spray angle or penetration rate to improve mixture quality.The effects of CO2 and H2O2 concentrations in the coflow on the autoignition characteristics and soot emission of central jet flames were investigated. The results show that with the increasing of the CO2 concentration in the coflow, the autoignition of the central jet delays and the liftoff height of the lifted flame increases. The soot emission decreases with the CO2 concentration increasing or coflow temperature decreasing. With H2O in the coflow, the autoignition delays and soot emission decreases. When the H2O2 concentration in the coflow increases, autoignition delay shortens and soot emission increases firstly and then decreases. The test results also show that soot emission has a close relationship with the liftoff height and soot emission increases sharply with lowering flame liftoff height.The fluctuation of the lifted flame base of diesel fuel in the vitiated coflow was analyzed and the different stabilization mechanisms in various temperatures were found: flames is stabilized by mixture autoigniting at low temperature while by flame propagating at high temperature.TC was found to be the key factor influencing the autoignition and combustion stabilization. The relationship between flame liftoff height and autoignition delay validates the equivalence for the two criterias of TC. The TC was further researched by changing the fuels, spray quality, injection parameter, concentration of coflow additives. The TC for low pressure injection diesel fuel is 1074K. The results show that the TC for diesel fuel is lower than that for biodiesel, and TCn-heptane lower than TCiso-octane. As the spray quality improved due to increasing injection pressure, the TC decreases from 1074K to 1048K. Improving spray quality by enlarging nozzle diameter, increasing injection pressure or pump speed will decrease the TC for diesel fuel. CO2 has the potential for increasing the TC while H2O2 will decrease the TC.A transient HC detector was used to measure the HC concentration distribution in the central flames. The effect of coflow temperature on HC distribution was analyzed. The emission data indicates that the HC concentration in spray axes decreases downwards. The peak value of HC concentration distributes axial symmetrically. In the pre-selected experiment range, the HC emissions exist within a radial distance of 30mm. All these experimental data could build a database for consequent simulation work.
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