Research On The Ignition Process Of Cavity-based Flameholder In Ramjet Combustor

Numerical and experimental investigation of flameholder based on cavity is carried out, and the ignition characteristics, ignition process and the structure of flame is examined carefully.With the pressure of sea level, the ignition characteristics of cavity are examined. The ignition characteristics are influenced by the structure of cavity and the injection mode of the fuel. With the same cavity structure and ignition mode, successful ignition and steady combustion is achieved with the fuel injecting transversely from upstream, while igniton failed with current-wise injection upstream. The decrease of cavity depth raises the mixing ratio, and the increase of cavity length is in favor of successful ignition. But there is an optimal value for the cavity length. Compared with Vee gutter flameholder, the ignition limit for cavity is wider for cavity-based flameholder, and the combustion efficiency is nearly the same for these two kinds of flameholder while the pressure loss of cavity-based flameholder is smaller.The ignition characteristics of cavity are examined with the pressure of 15KPa in ramjet combustor with cavityb-based flameholder. with current-wise injection upstream, the increase of cavity depth or the adaption of double-cavity will increase the resident time of the fuel and then in favor of ignition. The recirculation of cavity in the center of the ramjet is unsteady and the flame goes out when the torch is shut down. The ignition characteristics of configuration of wall mount cavity combined with cavity in the center is in favor of the successful ignition. The results suggest that the ignition characteristics are strongly influnced by the decrease of pressure. The variety of pressure changes the penetration hight of the fuel injected transversely from the wall, the quality of the injection decreases and the ignition energy increases with the decrease of pressure. Also the increase of total temperature and fraction of oxygen will make the ignition easier to achieve.Successful ignition is achieved with the pressure of 60KPa under the circumstance of clean air compared with vitiated air heated by combustion process. Three stages of the ignition is observed, which are the discharge of igniter, the formation of the flame kernel, and the spread of the flame kernel. Injection mode of the fuel is a decisive factor for ignition process. So that fuel is injected directly into the cavity with tranverse mode. Both the formation and spread of the flame kernel is highly influened by the location of the igniter. The fuel distribution and flow velosity of adjusent place of the igniter should be proper for the birth and survive of a flame kernel. The structure of the cavity is another important factor for the ignition process. With bigger length-depth-ratio, the cavity becomes unsteady and ignition becomes difficult to achieve. Also lower aft wall height results in a negetive influence for the ignition process.Also the working process and the distribution of flame is analyzed with the pressure of 60KPa under the circumstance of clean air.It is found that spread of flame from preceding cavity to rear cavity for double-cavity flameholder is easy to achieved , but only when the fuel is injected into the mainstream and ignited, the flame spreads from rear cavity to preceding cavity. The flameout process is also investigated. At last the distribution of the flame is analyzed. The results suggest that the recirculation is more stable when the length-depth ratio is small.