Youhai Zong

Dynamic Characteristics of Combustion Mode Transitions in a Strut-Based Scramjet Combustor Model

T HE supersonic combustion ramjet (scramjet) engine is expected to be the most efficient propulsion system in the hypersonic flight regime [1]. Given the broad range of aerothermodynamic conditions experienced during hypersonic flight, the scramjet would operate under different combustionmodes [2], andmode transition is a critical phenomenon in designing such engines because the thrust and specific impulse of the fuel in each mode varies considerably. In much of the previous work, researchers experimentally achievedmode transition and investigated the static characteristics of different combustion modes. In the open literature, Billig [3] first demonstrated mode transition in ground tests. Heiser and Pratt [4] used a one-dimensional (1-D) analysis approach to comprehend the complex aerothermodynamics of a dual-mode combustion system. The flowfield can be illustrated for threemodes: scramjet with shockfree isolator and oblique shock train, and ramjet with normal shock train. Takahashi et al. [5] and Kouchi et al. [6] observed four different combustion modes with respect to the fuel flow rate, namely, blowout,weak combustion, strong combustion, and thermal choking. As the mode transition occurred, thrust and heat-flux distribution [7] varies considerably. Sullins [8] experimentally achieved the mode transition from a scramjet with a precombustion shock system having a high pressure ratio to a scramjet with no precombustion shock system by increasing the total temperature of airflow to simulate a real acceleration process. Micka and Driscoll [9] reported two distinct reaction zones in a combustor with wall injection and a cavity flameholder corresponding to jet wake stabilization and cavity stabilization. The reaction zonewas found to only appear in the cavity stabilized mode in the scramjet mode, even for conditions where the ramjet modewas jet-wake stabilized. Also, the spreading of scramjet mode combustion is significantly less than that of the ramjet mode. Masumoto et al. [10] investigated the effect of combustor length and total temperature on combustion modes and suggested the minimum combustor length to attain supersonic or dual-mode combustion. However, there have been few studies on the dynamic characteristics of combustion mode transition, and the open literature did not fully investigate the combustor performance changes with the fuel flow rate small changes (∼1 g∕s) near the critical conditions. One interesting phenomenon, rather different from the results available in the open literature, is that the wall pressure and thrust show obvious catastrophe near the critical point of combustion mode transition. The combustion mode transition depends on the path taken (i.e. the fuel flow rate is increasedordecreased).With the sameexternal parameters, the scramjet engine may be a different combustion mode, known as hysteresis effect according to the nonlinear dynamics theory. During hypersonic flight, itmay bringgreat difficulties to the precise control of the vehicle, have a great impact on the flight safety, and even cause a flight accident [11]. Therefore, the successful development of a scramjet engine depends on further understanding and control of the nonlinear mode transition process. In this research, particular attention was focused on the dynamic characteristics of combustion mode through ground tests, especially the nonlinear catastrophic and hysteresis phenomena. As known, the transition between ramjet and scramjet mode is determined from the magnitude of ΔT0∕T0;air (either by decreasing or increasing the amount of heat release). In this paper, we linearly changed the fuel mass flow rate along two adverse paths; that is, increased and decreased the fuel equivalence ratiowhile the rate of changewas held approximately constant. In particular, to obtain performance of the model combustor around the critical conditions in detail, the heat release was changed little by little every time (corresponding to an increase in fuel equivalence ratio of 0.0125). Compared to strut injection in the center of the combustor, the transverse wall injection disturbs the boundary layer significantly. The wall injection plume forms a barrel shock, and induces a bow shock that leads to separation and the formation of a recirculation region in front of the wall injection location. These unnecessary disturbances make it difficult to determine the exact mode transition mechanism [12]. Therefore, a central strut injector has been employed, which also improved fuel mixing in the supersonic core stream and combustion performance in supersonic combustors. Because the liquid hydrocarbon fuel has greater fuel densities and endothermic cooling capabilities than hydrogen, particularly for hypersonic vehicles limited to Mach 8 flight, kerosene was used as the fuel in this research.

Combustion characteristic using O2-pilot strut in a liquid-kerosene-fueled strut-based dual-mode scramjet

A set of exploratory experiments are conducted to test a newly designed strut for fuel injection and flame holding in a liquid-kerosene-fueled dual-mode scramjet combustor. The thickness of the strut is 8 mm and the front blockage is about 8%. To organize stable combustion in a Mach number equal to 2.6 air flow under this thin strut using room-temperature liquid kerosene in a flash wall combustor without any cavity and other flame holders, some oxygen is injected through a set of orifices at the back of the strut, based on which a stable center local flame is generated at the back of the strut and the main flow combustion can be organized around this local flame. Experimental results show that stable combustion can be achieved at the center of the combustor with a wide range of equivalence ratio from 0.19 to 1 based on this center flame strut strategy. Through the analysis of the pressure distribution along the combustor, different combustor modes appear with different equivalence ratio. The article also gives some discussions about different influence of the oxygen to the combustion process under different equivalence ratio.

Combustion characteristics of a dual-mode scramjet injecting liquid kerosene by multiple struts

In order to optimize fuel allocation between multiple injection struts in scramjet combustor, experimental investigation has been carried out to study the combustion characteristics of a dual-mode scramjet, in which liquid kerosene was injected from three staggered arranged struts, and the effect of fuel allocation between the three struts on the combustion characteristics has been analyzed. The results show strong influence of the quantity and streamwise position of struts on the wall pressure distribution and both the equivalence ratio limits of inlet unstarting and lean blowout. To obtain a wide stable operation range, just one strut is needed for injecting most of the fuel in the combustors with configuration and size as in this work, and a streamwise position closer to the downstream divergent segment is a better injection location. Lower lean blowout limit can be obtained by injecting the overall much lean fuel from just one strut with a relatively closer distance to the downstream flameholder.

Effects of upstream strut on the combustion of liquid kerosene in a model cavity scramjet

A supersonic combustion organizer that consists of both cavity flameholder and strut injector was applied in a liquid-kerosene-fueled model scramjet. The experimental results indicated that the strut injection can improve the combustion performance. When the strut was mounted near the cavity, transverse injection from the strut gave the best performance. However, the excessive long distance between the upstream strut and the cavity led to upstream spreading of combustion to the isolator and pressure rise at the isolator entrance. Besides that, parallel injection was found difficult to establish effective combustion due to the poor spreading performance, except in the condition that the strut was mounted close to the cavity and wall injection was used simultaneously.

Experimental study on combustion mode transition effects in a strut-based scramjet combustor

A strut-based scramjet combustor was operated in three different combustion modes under Mach 2.5 inflow conditions, scramjet mode, weak ramjet mode, and strong ramjet mode. The combustion mode transition effects are investigated through ground experiments. The scramjet mode occurred below the overall fuel equivalence ratio of around 0.44. Transition from the scramjet mode to the weak ramjet mode occurred at ER ∼ 0.44, accompanied by a sudden increase in thrust. By contrast, transition from the weak ramjet mode to strong ramjet mode occurred at ER ∼ 0.6125, accompanied by a sudden decrease in thrust. In particularly, combustion mode transitions significantly change the dominant characteristics of the flow field.

Effect of Fuel Injection Allocation on the Combustion Characteristics of a Cavity-Strut Model Scramjet

The cavity-strut supersonic combustion organization method has been found to improve the combustion performance of ethylene in recent studies. In this work, liquid kerosene combustion experiments have been carried out in a model scramjet combustor with a cavity and small strut upstream of the cavity. The effect of fuel injection allocation between the transverse injectors at the strut leading surfaces and duct wall upstream of the cavity on the combustion characteristics has been investigated. In the experiments, a larger bench thrust increment, a higher specific impulse, and an improvement in isolating the interference from the downstream region have been obtained as more of the fuel was injected from the strut injectors. Increasing the strut injection proportion makes the combustion zone translate to the divergent segment downstream of the cavity, and leads to the dominant flow changing from subsonic to supersonic in the minor fuel-rich condition.