Study On Ignition Characteristics Of Diesel Surrogate Fuel In High Concentration Of Carbon Dioxide Atmosphere

CO₂ emissions from diesel engines are not normally considered a pollutant in air,whereas conventional diesel engines have the problem of competing with workers for oxygen and elevated CO₂ concentrations in a closed environment,thus threatening their lives.In order to solve the CO₂ emissions problem in a closed environment,closed-cycle diesel engines that auto-ignition and combustion in CO₂/O₂ atmospheres are created and developed.However,the diesel engine auto-ignition in the CO₂/O₂ atmosphere becomes difficult to predict,and even occurs misfire phenomenon when the CO₂ concentration is high.The diesel fuel auto-ignition characteristics problem in CO₂/O₂ atmosphere needs to be solved.In order to provide theoretical reserve and technical support for the design and development of future liquid oxygen solid carbon closed-cycle diesel engines,it is necessary to achieve the ignition delay time accurate prediction and the analysis of the whole auto-ignition and combustion process in CO₂/O₂atmosphere.Therefore,studying the diesel fuel auto-ignition characteristics problem in CO₂/O₂atmosphere is of great importance at both the theoretical and practical level.The diesel fuel physical and chemical properties,such as auto-ignition,reactivity at high temperatures,molecular structure and hydrocarbon ratio,are considered and 70%n-heptane/30%toluene blends are used as a diesel surrogate fuel and study for auto-ignition characteristics in CO₂/O₂atmosphere.Firstly,an ignition delay time（CDE）model considering the high concentration CO₂ effect on auto-ignition and flame retardance is established,and the sub-models of turbulence,spray and combustion are coupled.The geometric model of the constant volume combustion chamber（CVCC）is constructed by advanced cutting Cartesian cell grid division,and the detailed n-heptane/toluene coupling mechanism is established and simplified to realize the auto-ignition and combustion processes simulation at the top dead center（TDC）.Secondly,the test platform for the CVCC visualization is built and the test conditions（air,60%CO₂/40%O₂,50%CO₂/50%O₂ and 40%CO₂/60%O₂）are set.The 70%n-heptane/30%toluene auto-ignition and combustion processes in different working conditions are visualized by high-speed camera system and the auto-ignition and combustion test data are obtained.The composition and function of the test platform six main systems are described,and the formulation of the test scheme is completed and the auto-ignition data processing method is briefly described.Finally,based on the experiment and simulation results,the auto-ignition process,ignition delay time,flame temperature cloud maps,flame natural luminosity and other auto-ignition and combustion characteristics are discussed and analyzed,and the initial temperature,pressure and the OH radical rate of production（ROP）that have a significant impact on the auto-ignition are simulated.The results show that the CDE model can well predict the ignition delay time in CO₂/O₂atmosphere,and the maximum error is 8.18%in 40%CO₂/60%O₂ atmosphere.From the flame high temperature duration and flame cumulative natural luminosity analysis,it is concluded that70%n-heptane/30%toluene is most suitable for auto-ignition and combustion in 50%CO₂/50%O₂ atmosphere.The high concentration CO₂ effect on auto-ignition is mainly reflected in its physical effect,and the chemical effect is not obvious,while the third body effect is enhanced when the CO₂volume fraction is higher than 60%.When the volume fraction of CO₂ increases from 40%to 60%,the R4（O+H₂O→2OH）OH radical ROP peak decreases significantly and decreases by 180%.