Investigation On Spark Ignition In Kerosene Scramjet Engine At Low March Flight Condition

In the thesis,we investigate the spark ignition of kerosene in the scramjet engine at low flight Mach condition.Both experiments and numerical simulation are used to investigate the ignition and flame propagation process.The effects of single and combined upstream injection on ignition are analyzed.The mechanism of these effects are discussed.We summarize the combustion modes during the spark ignition process,and come up with the concept of local flame,downstream cavity flame and global flame.The characteristics,forming process and stabilization of each combustion modes have been analyzed.The results shown that,the final combustion mode is decided by flame propagation process,which makes the combusiton modes in spark ignition different from the modes in flame-holding research.The stabilization of local flame is affected by local equivalence ratio inside cavity.The downstream flame is formed by the downstream propagation of wake flame,and the wake flame can enhance the downstream flame.These are three ways to establish the global flame,and the three ways are associate with the intensity of local flame,which makes the local flame the key factor.Although the global flame is intense,if the flame inside cavity is blow out,the global flame will also been blow out.The effects of injection pressure,injection distance and orifice size of upstream injection on spark ignition have been invesitaged.The results shown that,when the injection pressure increase,the local equivalence ratio inside cavity first increase and then decrease.When the cavity is fuel lean,the local equivalence ratio increase along with time,which makes the spark ignition success after the spark begin a period of time.When the cavity is fuel rich,the initial flame can formed,but cannot develop to stable local flame,which make the cavity ignition and blowout over and over again.The injection with long injection distance tend to makes the cavity fuel rich.The injection with bigger orifice size has a higher penetration,which tend to makes the cavity fuel lean.The expand wall of combustor will also affect the penetration.The straight wall has the bigger penetration than expand wall.As the spark ignition of single upstream injection has the problem of local flame unreliable and few gloal flame states,we raise the combined injection scheme.The results indicated that,combined injection can improve the local flame unreliable caused by fuel lean.By adjusting combined injeciton,the fuel lean inside cavity has been imporved.However,the improvements on global flame is different in schemes.After the assistant injection shutdown,the pre-injection pressure has a transient pressure down process,which can be used to enhance the local flame.If the downstream region of ignition cavity has some flame stabilization ability,the wake flame can establish thermal choke and develop to global flame by flame inverse spreading.On the contrary,if the thermal choke is hard to form,the combined injection has little help on global flame.The downstream cavity has a important role on the forming of global flame.We also conduct experiments on full size scramjet combustor.With the flight Mach condition of 4.5 and 4.0,the effect of cavity length on spark ignition has been verified.In flight Mach 4.5 condition,the short cavity can form local flame with upstream injection and direct injection.But the local flame is too weak to ignite the fuel in main flow.By opening and closing the upstream injection again and again,the local flame can be induced to global flame.The local flame with long cavity is enough intense to spread to global flame.The cavity length is an important factor on the expanding process of local flame.In flight Mach 4.0 condition,the intensity of local flame decrease.The local flame is easy to been extinguished by direct injection.Compared with the short cavity,the intensity of local flame inside long cavity is still larger.