Investigation On Initiation Of Ignition And Mechanism For Enhancement In Supersonic Flowfield

Aiming at the initiation process of ignition transient in scramjet combustor, the initial flame kernel generation, propagation and enhancement mechanism for self-ignition and forced ignition in supersonic flow-field were generally investigated, with high-speed camera, PIV, schlieren photography system and other flow-field measurement technologies, combing with numerical simulation method.Firstly, the experimental research on the process of self-ignition flame kernel generation was conducted. The effects of fuel types, total temperature, intensity of incident shock wave, injection pressure, injection distance, ratio of length to depth of cavity, flow-field choking, degree of premixness and other factors on the ignition process were compared and analyzed. Results revealed that the event of ignition was depended on the competition between ignition delay time and resident time. Increasing the total temperature and injection pressure, introduction of incident shock wave and flow-field choking and other flow accommodation method, which was to shorten the ignition delay time or extend the resident time, were all beneficial for the establishment of self-ignition flame kernel. The dynamic of flame evolvement demonstrated that the emergence of the first flame kernel was controlled by the self-ignition mechanism, but could not sustain under the condition of just floor equipped without the help of cavity, which was due to the oscillation characteristic of incoming flow. Circulation zone within cavity which was used for upstream flame propagation at the flame initial phase and acted as ignition source of flame holding during stable combustion phase was necessary for the flame establishment. After the establishment of flame, the flame-holding process was mainly dominated by the flame propagation and downstream diffusive combustion.Secondly, according to the measured and simulated results before ignition, the ignition of the gaseous ethylene and liquid kerosene fuel by local high energy spark was observed and examined. It was found that the circulation zone trapped in the fore corner of cavity was essential for the creation of initial flame, in which the ignition energy accumulated and the flame expanded and transported downstream until forming a cavity trapped flame. The feedback process of ignition finally established until the pre-combustion shock wave train found in the combustor caused by heat release of convective combustion. For spray ignition, the several ignition failure modes were found a relation with the fuel distribution around cavity, which was decided by the injection pressure. Compared with gaseous fuel, the ignition boundary of spray was relatively narrow and more depended on the enhanced mixing and evaporation of pre-combustion shock wave train. The spreading flame was prone to be extinguished by the decalescence of unevaporated droplets. The ignition performance can be improved by optimizing flame spreading routine with the help of increasing flame temperature and downstream convective combustion.Then, the effectiveness and mechanism of local oxygen addition within cavity for spray ignition enhancement was verified and explored experimentally and numerically. It demonstrated that the characteristic of flame distribution and intensity of combustion in cavity were affected by the scheme of oxygen addition. Results confirmed that the performance of cold-start in combustor can be improved with oxygen addition, which can intensify the combustion within cavity due to modulation of the fuel concentration distribution. The flame propagation was maintained with the help of increased adiabatic flame temperature, which promoted the performance of spray evaporation. Besides, the calculation supported that the requirement for minimum ignition energy was lower with more oxygen content.Lastly, the spray ignition piloted by pre-burned flame such as ethylene flame and hot jet were studied. Effect of shock wave train and structure of pre-combustion flame and its interaction between spray trace on the ignition process were focused. The structure and intensity of ethylene flame had important effect on ignition event. For the strong flame case, the enhanced mixing and evaporation process by the pre-combustion shock wave train was another mechanism for piloted ignition, in addition to high temperature heating of ethylene flame. For ignition induced by hot jet, the enhanced ignition mechanism was mainly characterized by the far-field diffusive burning between spray and hot jet. The trapped flame within cavity was created due to the flame propagation upstream, which was the dominant mechanism of successful flame establishment.