Hydrocarbon Fueled Scramjet Engine Research Articles

Effects of fuel conversion ratio on cooling and drag reduction performance for supersonic film using gaseous hydrocarbon fuel

Film cooling especially combined with regenerative cooling is an effective solution to meet the heat protection and the drag reduction demands for hydrocarbon-fueled scramjet engines. Typically, fuel is the only available coolant to organize the active cooling for hypersonic vehicles, and the coolant used in the film cooling is extracted from the regenerative cooling channels; however, the fuel pre-cracking in the regenerative cooling process must be considered as it can change the initial composition and energy level of the coolant in the film cooling. Numerical simulations based on the RANS method are conducted to evaluate the effects of film pre-cracking on its cooling and drag reduction performance, by changing the fuel conversion ratio from 0 to 40 %. Results show that the influence of film inlet cracking on its drag reduction performance is limited, and the relative change of the wall friction under different conversion ratios is less than 5 %. However, the film inlet cracking has a notable negative influence on its thermal protection performance. Specifically, for every 10 % increase in the film conversion ratio, the average adiabatic wall temperature will rise by about 20 K. Further analysis reveals that although the total amount of the fuel chemical heat sink is comparable to its physical heat sink, only around 25 % of the film chemical heat sink can be effectively used to enhance the fuel film cooling performance. The loss of the effective chemical heat sink during the pre-cracking process is the main reason for the decrease of film cooling performance as the film inlet conversion ratio increases.

Effects of wall thermal state on the cooling and friction reduction characters for supersonic film using gaseous hydrocarbon fuel

• The region influenced by the wall thermal state is limited within 1 mm off the wall. • Increase of the wall cooling heat flux improves the hydrocarbon film cooling performance. • The cooling enhancement can be equivalent to an additional wall heat flux of −0.36 MW/m 2 . • Increase of the wall cooling heat flux is unfavorable for reducing skin friction. Film cooling using gaseous hydrocarbon fuel is an effective way to meet the thermal protection and friction reduction demands of hydrocarbon-fueled scramjet engines. The cooling and friction reduction characteristics of the hydrocarbon film can be influenced by the wall thermal state through the variation of endothermic and exothermic chemical reactions in the film. Numerical investigations based on the RANS method have been performed to evaluate the effects of the wall thermal state on the hydrocarbon slot film cooling performance. Four wall thermal states which represent different regenerative cooling levels are simulated. Results show that an increase of the wall cooling heat flux is beneficial to the film cooling performance while has unfavorable effects on the film friction reduction performance. Further analysis reveals that the net energy effects of the chemistry in the hydrocarbon film can be equivalent to an additional constant wall cooling heat flux of −0.36 MW/m 2 within a certain range. Besides, the proportion of this equivalent wall cooling heat flux in the total wall cooling heat flux decreases from 100% to 26% for different wall thermal states. Therefore, the cooling enhancement brought by the endothermic chemistry is comparable to the regenerative cooling. However, the wall shear stress is less sensitive to the wall thermal state. The maximum deviation of the total wall friction for different wall thermal states is less than 8% compared to the reference value.

Effects of combustion on supersonic film cooling using gaseous hydrocarbon fuel as coolant

For hydrocarbon fueled scramjet engine, gaseous hydrocarbon fuel has been promoted to conduct supersonic film cooling. During the continuous mixing in the cooling process, the possibility of the coolant film combustion must be seriously considered. In this paper, a numerical model of gaseous hydrocarbon fueled supersonic film cooling in terms of the coolant combustion has been developed and validated. The results indicate that the combustion of the coolant film occurs in the boundary layer. The combustion flame is formed apart from the wall and restricted near the boundary layer edge. The interesting thing is that the combustion first reduces and then increases the wall temperature rather than only increases the wall temperature significantly as expected, which brings double effects on film cooling. The combustion reduces the wall temperature due to low-temperature endothermic chemical reactions occur and dominate the near wall region over a long distance after the film injection. Moreover, the turbulent energy transport to the wall is suppressed due to the combustion heat release. Furthermore, after the combustion flame occurs, the skin friction is reduced. The skin friction reduction by combustion is attributed to the combined effects of Reynolds stress reduction and the viscosity stress reduction.

Plasma-Assisted Fuel Atomization and Multipoint Ignition for Scramjet Engines

Estimates of energy and power requirements for shortening the ignition delay time in hydrocarbon-fueled scramjet engines using nonequilibrium plasma generation of radicals show that the uniform volumetric plasma ignition would be associated with extremely high power budget and substantial losses of total pressure. A multipoint plasma ignition scheme based on electron beams is proposed that would emulate the volumetric ignition while reducing the power budget by several orders of magnitude. The paper also explores an approach where the fuel is injected into the core flow as a liquid jet, and low-power electron beams are used to electrostatically charge the droplets and thus to control droplet breakup, atomization, mixing, and ignition. With a subcritical microwave field applied to the injection region, local enhancement of electric field strength at the surface of droplets, together with seed electrons and ions produced by the electron beam, would create subcritical microwave discharges and thus initiate combustion in multiple spots. The multispot ignition would help in spreading the flame across the combustor. Additionally, the initiation of combustion at the droplet surface, where local equivalence ratio is high, could help ignite lean (in average) mixtures.

Analysis of the maximum flight Mach number of hydrocarbon-fueled scramjet engines under the flight cruising constraint and the combustor cooling requirement

Scramjet is the optimal propulsion system for hypersonic vehicles, and it is important to study its maximum flight Mach number. Upper limit and its influence factors of the flight Mach number of dual-mode scramjet used in hypersonic vehicles are investigated in this paper. Scramjet performance analysis model with the constant-area heating process, cooling requirement analysis model of the combustor and cruise model of the scramjet-powered hypersonic vehicle are established. Specific impulse of dual-mode scramjet engine with hydrocarbon fuel is similar to that of liquid oxygen kerosene rocket engine when the freestream Mach number is above 10. The ratio of hypersonic vehicle reference area to scramjet inlet capture area is introduced to establish the size relationship between the scramjet engine and the hypersonic vehicle. Considering the balance of thrust and drag, smaller ratio of hypersonic vehicle reference area to scramjet inlet capture area leads to higher flight Mach number that hypersonic vehicle can cruise. When the angle of attack is 10 degrees, the ratio of hypersonic vehicle reference area to scramjet capture area should be less than 5.5 to ensure that the vehicle can work at Mach 8. The maximum length to diameter ratio of the combustor is used to indicate active cooling requirements, and it is less than 5 under the cooling requirement when flight Mach number is above 9. Reducing the heat flux at the combustor wall and increasing the fuel heat sink is helpful to increase the allowable combustion chamber length, so as to increase the maximum flight Mach number of the scramjet engine on the premise of meeting thermal protection requirement.

Modeling of incomplete combustion in a scramjet engine

In the present study, an empirical theoretical split chemistry model was developed to describe the phenomenon of incomplete combustion for scramjet engines. The model was developed by decoupling flow into two distinct regions, namely, unburned and burned, as in real scramjet flows. The conservation equations for the combustor and the rate equations for the supersonic nozzle were calculated independently for these two regions using the split ratio of the volume occupied by the fuel–air mixture to the overall volume. The split chemistry model was implemented in a one-dimensional flow solver by assuming that the combustion efficiency is known. The effect of incomplete combustion on the performance of a hydrocarbon-fueled scramjet engine was investigated by performing a parametric study along the entire flow path through the scramjet engine, including the inlet, isolator, combustor, and supersonic nozzle. The results showed that, for a combustion efficiency of 0.5 with a global equivalence ratio of 0.5, the overall temperature and the thrust performance along the flow path through the combustor and the nozzle significantly decrease owing to incomplete combustion. It was also observed that the chemical composition of the fuel-only region varies, regardless of the change in combustion efficiency, because efficiency is a function of the extent of the combustion reaction and the split ratio. It was found that the present split chemistry model is useful to describe incomplete combustion, and it can be effectively utilized for the preliminary design and analysis of scramjet engines.

Performance evaluation of regenerative cooling/film cooling for hydrocarbon fueled scramjet engine

Thermal protection is considered to be a significant challenge for scramjet engine. In this paper, a combined cooling method which combines film cooling (F.C.) with regenerative cooling (R.C.) instead of single regenerative cooling for hydrocarbon fueled scramjet engine by using gaseous hydrocarbon fuel coming out of the cooling channel exit as film coolant has been proposed to increase the engine's flight Mach number without bringing extra fuel on board. One-dimensional (1-D) model of the combined cooling in terms of supersonic combustion in combustor and cracking reaction in regenerative cooling channels has been built and validated in order to evaluate its performance. The calculation results indicate that the engine wall temperature can be reduced significantly with R.C./F.C. and the flight Mach number of engine can be increased by nearly 8% under the stoichiometric fuel flow rate when material limit of the engine wall is set to be 1300 K. It is found that the effect of film inlet temperature on the cooling performance can be ignored because of the high total temperature of main flow, while the flow direction inside the cooing channel will have significant effects on the cooling performance of R.C./F.C.. The heat flux imposed on the engine wall is less uniform with “parallel flow” inside the cooling channel and the cooling performance is better and more sensitive to the change of film injection position under this condition. In addition, the cooling performance of R.C./F.C. present a very slight change when the number of the slots stays at a very low value and 2–3 film slots may be more practical and useful for the hydrocarbon fueled scramjet engine.

마하 5.0 노즐을 장착한 스크램제트 엔진 시험설비의 시동 특성 연구

A Mach 5 nozzle and a diffuser of the Scramjet Engine Test Facility (SETF) were made for a hydrocarbon-fueled scramjet engine. SETF, attached with a diffuser guide, started with a model of 60% blockage, though the model engine could not start by over expansion of the facility nozzle. The model was moved into the nozzle to escape the shock generated from the nozzle exit, both SETF and the engine could start. The pitot rake experiments (blockage of 2.3%) were done for measuring the core flow in the test section. From the pitot experiments, the core flow was expanded by an under expansion. It means that the core flow in the test section was related with a model blockage. SETF and the engine with a blockage of 33% work normally. From a series of experiments, SETF started with a normal shock efficiency of 58%, regardless of a blockage ratio.

Research status of active cooling of endothermic hydrocarbon fueled scramjet engine

The active cooling with endothermic hydrocarbon fuel is proved to be a very effective approach for scramjet thermal protection. This article has summarized the research status of active cooling of endothermic hydrocarbon fueled scramjet engine from the following five aspects, cooling capacity and heat sink measurement, thermal and catalytic cracking, coking suppression, heat transfer characteristics, and injection, mixing, ignition, and combustion performance, and suggestions on the further study of active cooling of endothermic hydrocarbon fueled scramjets are put forward. The total heat sink of endothermic hydrocarbon fuel is found to be sufficient for scramjet cooling as the additional chemical heat sink is generated from cracking reactions, and catalytic cracking is proved to be better, because of its low cracking starting temperature, high conversion percentage, and good selectivity. As for coking mitigation, approaches are usually made from eliminating the oxygen dissolved in the fuel, reducing the amount of coking-foregoing substances, and doing some special treatments on the metal surface. The heat transfer of endothermic hydrocarbon fuels presents two enhancements. The first one is related to the variation of parameters near the critical point while the second one is a result of cracking reactions. The cracked products are proved to have better performance of injection, mixing, ignition, and combustion, and this is especially attractive for scramjets as effective mixing and stable ignition and combustion has always been difficult.

Experimental study on the performance of recooling cycle of hydrocarbon fueled scramjet engine

Recooling cycle (RCC) has been newly proposed as a new cooling method to increase fuel heat sink (cooling capacity). A testing system is established and an experimental study is conducted on the operating characteristics and performance of RCC with different flow, heat transfer and cracking conditions. Experimental results indicate that the utilization of fuel heat sink is restricted by the allowable material temperature of heated tube; RCC provides another chance for the continued use of fuel chemical heat sink and the repeated use of fuel physical heat sink in the secondary cooling by reducing the fuel temperature by doing work to turbine, and the total heat sink of fuel can be increased to 0.7MJ/kg when n-Decane is used as the fuel. RCC can also improve the “olefin/paraffin ratio” of cracking reaction, which facilitates the improvement of the combustion performance of fuel.

Comparison of Hydrogen and Hydrocarbon-Fueled Scramjet Engines for Orbital Insertion

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Development of Hydrocarbon-Fueled Scramjet Engines: The Hypersonic Technology (HyTech) Program

The Hypersonic Technology (HyTech) program was initiated in 1995 to maintain a core competency in hypersonictechnologiesafterthecancellation of theNationalAerospacePlaneprogram.HyTech isfocusedon expanding the technology base for liquid-hydrocarbon-fueled scramjet propulsion systems and is complementary in many ways to similar hydrogen-fueled hypersonic programs, such as Hyper-X. The overall effort consists of government sponsored industry efforts and an in-house technology base program. The technical challenges of these efforts associated with successful scramjet operation are addressed, which include activity in inlet/isolator operation, combustor operation and stability, nozzle operation, material advancement, fuel system development and integration and operability. The program has positively demonstrated the technologies that are critical to successful scramjet operation. The current status of work in each of these areas is discussed, followed by a discussion of upcoming activities for the program.