Study on the Effect of Air Throttling on Flame Stabilization o fan Ethylene Fueled Scramjet Combustor

When the flight Mach number is bigger than 6, the air flow remains completely supersonic throughout the scramjet combustor in a few milliseconds. Ignition and flame stabilization have been a serious concern in the design of scramjet engines due to the difficulties to anchor flames in a high-speed environment.

Air throttling was an effective method to achieve ignition and combustion establishment. A proper shock train is generated by air throttling, and then the velocity decreases and the pressure increases in the combustor section which can effectively enhance the ignition and flame stabilization. When the air throttling was removed after the flame establishment, the shock train was retained leading to sustained combustion if sufficient heat release was produced in the combustor. Conversely, insufficient heat release might result in an unstable shock train and causes flame blowout. Control and optimization of ignition transient and flame development in a scramjet engine using air throttling were studied, the locations and operation timing of air throttling and fuel injection on the engine start-up transient characteristics were examined systematically, and a dynamic optimization scheme based on the genetic algorithm (GA) was developed to maximize the performance of air throttling. The transient process of flow establishment and combustion process were studied for an ethylene fueled direct-connect dual mode scramjet combustor with air throttling.

Air throttling was confirmed to be effective means of the ethylene ignition and combustion establishment. A series of experiments were conducted to study the ignition characteristic of a flush wall, liquid kerosene fueled scramjet combustor. Experiment results showed that successful ignition could be achieved by the air throttling method. Flow development, fuel/air mixing, ignition transient, and flame development in an ethylene fueled scramjet engine with air throttling were also studied. The research showed that air throttling could generate a precombustion shock train in the isolator, the resultant decrease in the flow velocity and increases in the temperature and pressure in the combustor section to improve the ignition characteristics and the flame stabilization process. The flame stabilization of a Facility and Scramjet Configuration. The scramjet combustor was experimented in the direct pulse combustion wind tunnel (Figure 1).

A vitiated air heater was used to generate high enthalpy air flow supplied into the combustor. Oxygen was fed into the heater to obtain test gas with its mole fraction of oxygen being equal to that of standard air. Total temperature (𝑇 𝑡) of the test gas was 900 K and total pressure (𝑃 𝑡) was 0.8MPa. The test gas was accelerated by the nozzle to Mach 2.0; the running time of the facility was about 400ms.

The location of air throttling in the scramjet combustor was 875mm from the entrance of the combustor. The mass averaged velocity between the exit of the isolator and the front of the air throttling was about 400∼600m/s, which was much lower than that of the scramjet combustor without air throttling, which was about 800∼900m/s. The lower velocity of air flow in the scramjet combustor with air throttling resulted in a longer residence time for better fuel/air mixing. The mass averaged pressure of the scramjet combustor with air throttling was higher than that of the scramjet combustor without air throttling, which was helpful to ignition in the scramjet. The temperature of the scramjet combustor with air throttling was much higher than that of the scramjet without air throttling from the exit of the isolator. The temperature of the scramjet with air throttling was higher than 700K but that of the scramjet without air throttling was about 500K; the influencing zone of the shock train produced by air throttling in the scramjet combustor was from the exit of the isolator to the exit of the combustor. Thus it could be seen the thermodynamic parameters of the scramjet combustor flow field were influenced obviously by the shock train. Compared with the case without air throttling, the flow field with air throttling had a lower velocity and higher pressure, temperature, and vortices intensity, which were helpful to ignite in the scramjet combustor. Intense flow recirculation zones were formed downstream of the cavity due to the back pressure rise and separation of the boundary layer by air throttling. But that could not be seen from the scramjet combustor without

The effect of air throttling on flame stabilization of an ethylene fueled scramjet combustor was investigated by CFD and experiment. The results indicated that air throttling had a great effect on the thermodynamic parameters in the flow field. Compared with the combustor without air throttling, the flow field with air throttling had a lower velocity and higher pressure, temperature, and vortices intensity. Air throttling was an effective way to achieve flame stabilization, the combustion in the combustor without air throttling was nearly blowout, and only a small portion of the flame was anchored in the rearward step of the cavity and in the boundary layer of the up wall. With air throttling, an intensive combustion in the whole cavity and wall boundary layer can be achieved; the shock train could be seen clearly in the isolator because of the pressure increased in the combustor. It was important to choose the location and time sequence of air throttling for fuel ignition and flame stabilization.