| Low Temperature Combustion (LTC) engines were widely concerned as they showed great potential in aspect of clean and efficient combustion in terms of the internal combustion engine. At the same time, researches on diversity alternative fuels were vigorous. Renewable, low emission and nontoxic, bio-ethanol attracted much of the attention. Meanwhile, the physical and chemical properties of ethanol were quiet suitable for low-temperature combustion engines. However, most of the current researches mainly concerned the engine performance that relied on the engine bench test. Therefore, experimental researches on ethanol+diesel fuel spray combustion at low temperature combustion environment are necessary. It can not only provide experimental data for ethanol+diesel spray, but also promote the theory of LTC. The comparative study on atomization, evaporation and combustion processes was implemented to unite the fragmented knowledge that discovered by former researchers, the results provided an important reference for the spray combustion model improvement.A Constant Volume Combustion Bomb (CVCB) was constructed to simulate the spray combustion environment applying the premixed combustion heating method. This CVCB can simulate a wide range of the spray combustion environment which included the typical environmental parameters of low temperature combustion engine cylinder. The fuel injection pressure ranged from50MPa to200MPa, the ambient gas temperature ranged from300K to1200K, the ambient gas pressure ranged from0.1MPa to10MPa, the ambient gas oxygen concentration ranged from0%to21%, and the ambient gas density was set as15kg/m. Image and pressure signals were obtained simultaneously. This CVCB can be used not only to implement spray atomization research, but also spray evaporation, spray combustion and premixed combustion experiments. Spray image processing programs based on Matlab were developed according to the image characteristics. It distinguished the spray vapor boundary from the complex schlieren background.Experimental researches on E20ethanol diesel spray atomization, evaporation and combustion were conducted. The influence of three factors, fuel injection pressure, ambient temperature and ambient oxygen concentration were investigated. The injection pressure varied from50MPa to200MPa, the ambient temperature was set at383K for atomization spray and ranged from800K to1200K for evaporation and combustion spray, the environmental oxygen concentration was0%for atomization and evaporation spray, and varied from15%to21%for combustion spray.0#diesel was also researched as the reference fuel. Nozzle flow characters, spray atomization characteristics and mixing characteristics were studied in the spray atomization experiments. The results showed that the higher the injection pressure, the better the mixing quality. E20atomization characteristics were similar to that of diesel, but mixing quality was better than diesel as E20contains oxygen itself. Elevated injection pressure and usage of oxygenated fuel were the practical ways for intensified mixing.The characteristic parameters of combustion spray based on image, such as light based ignition delay, flame lift-off length, and pressure based parameters such as combustion pressure, pressure based ignition delay were analyzed. The results showed that the lower the ambient temperature and oxygen concentration, the longer the ignition delay, combustion duration, flame lift-off length and combustion pressure, but the smaller the flame area and integrated natural flame luminosity. Based on the flame lift-off length at the quasi-steady phase of the spray flame, the fuel-air equivalence ratio was calculated. The results showed the that the impact on the chemical reaction was larger than the physical mixing when the oxygen concentration decreased. When the oxygen concentration was0.15and the ambient gas temperature was800K, the fuel-air equivalence ratio at the flame lift-off length was slightly higher than2, the flame almost reached the smokeless combustion state.Comparative study on atomization, evaporation and combustion spray showed lots of undiscovered structures about the low temperature combustion spray. After the end of the injection, some fuel droplet remained at the spray region, and the shape of the conical spray became twisted; Ignition take place at the middle of the spray beam, and the ignition of spray tip was mainly due to auto ignition as there was no obvious flames propagation. Comparison on microscopic images and combustion images at nozzle orifice illustrated that, after the end of injection, there existed small amount of low-speed large sized droplets near the orifice. These droplets would burn and emit bright light when the ambient temperature exceeded1000K, but became an important source of unburned hydrocarbons at lower ambient temperature. |
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