Studies Of Temperature Non-uniformity And Its Effects On Auto-ignition In A Rapid Compression Machine

The development and application of engine downsizing technology has been recognized as an effective way to solve the deteriorative energy and environmental problems and meet the stricter limitations of carbon dioxide emission.However,with the increase of compression and booster rate in engine downsizing,knocks will take place,which will restrain the further improvement of thermal efficiency and cause damage in engines.In order to solve this paradox,it is necessary to focus on the research of knock mechanisms and control strategies.Auto-ignition is the key to study the knock mechanisms.Therefore,in this paper,temperature non-uniformity and its effects on auto-ignition are studied in a rapid compression machine,which is widely used in the research about engines.At first,in order to study the influences of piston configurations on the non-uniformity of temperature in a rapid compression machine,the temperature field were simulated using non-reaction Fluent model.The results show that the vortex induced by the motion of the flat piston leads to the non-uniformity of the temperature field.The piston with certain volume of crevices can restrain the disturbance of the vortex and homogenize the temperature in the core region.However,the temperature field is not sensitive to the change of crevice shapes and the channel lengths.The piston with raised part can make the non-uniformity of temperature severer.Furthermore,the effects of temperature non-uniformity on auto-ignition time were studied.CHEMKIN reactors,Livengood-Wu integral and Fluent with a detailed chemical kinetic mechanism for hydrogen oxidation were applied respectively to compute the auto-ignition times in rapid compression machines with varying piston configurations.The results demonstrate that the Livengood-Wu integral,a relatively low computational-cost method,achieve a high accuracy in predicting auto-ignition times in a rapid compression machine,especially for the gases affected by vortex.The gradients of temperature field generate the gradients of auto-ignition times,the auto-ignition will take place in regions with high temperature first and an obvious flame propagation process can be observed.Finally,the influences of temperature non-uniformity on auto-ignition modes were investigated using the A-SURF code.The one-dimensional,compressible,multi-component auto-ignition wave propagation was simulated based on the temperature gradient in the rapid compression machine.The results show that the reflection of pressure waves induces a climb of temperature and pressure of end gas.the different initial temperature gradients lead to different auto-ignition modes.Threeauto-ignition modes are identified in this paper: deflagration,detonation and deflagration-to-detonation transition.