Research progress on active thermal protection for hypersonic vehicles
Introduction
Hypersonic vehicles (Ma>5) [1,2] are not only needed to achieve a very high speed flight speed, but they also represent one of the strategical development directions for both civil and military aerospace engineering. The most promising engine for a hypersonic vehicle is the air-breathing scramjet [1]. The outer surface of the vehicle as well as the inner surface of the main powerplant encounter a severe thermal environment with the increasing of flight Mach number and cruise time in hypersonic flight. Therefore, thermal protection technology becomes the key remedy for airbreathing hypersonic vehicles to tolerate the extreme thermal conditions [3,4]. The thermal protection technology for hypersonic vehicles mainly includes the protection of the outer surface and the protection of the engine inner surface, as both surfaces are exposed to severe thermal conditions.
Taking a wide view of the thermal protection technology for flight vehicles from low speed to high speed, the thermal protection can be divided into two categories, one is active thermal protection and another is passive thermal protection [3]. The heat insulation structures using a heavy and expensive thermal shield to protect the inner structures of the air breathing hypersonic vehicles was proposed in the 1960s [5,6]. Although the thermal shield can insulate heat for the vehicles well, the take-off weight added by the heavy shield and the flight performance impaired by the changes in aerodynamic shapes due to the wall ablation limit the application range of this method. In contrast with the heat insulation structures concept, the active cooling concept proposed by Becher et al. [7] is more attractive. In addition, the investigations from Helenbrook et al. [8] clearly indicate the obvious advantages of active cooling in terms of weight and costs over the heat shield.
The commonly used active cooling technologies include radiative cooling, film cooling, transpiration cooling and regenerative cooling. The radiative cooling utilizes the thermal radiation of the material in high temperature condition. The requirement of the material for radiative cooling is strict and the amount of the heat dissipated by radiation is relatively small. Therefore, radiative cooling is not suitable in the condition of high wall heat load, and the application in the hypersonic vehicles is also limited [7,8]. Film cooling can be divided into liquid film cooling and gas film cooling. Due to the extremely limited liquid fuels in air breathing hypersonic vehicles and the low cooling performance caused by the poor stabilization of the liquid film in contact with the high speed gas environment, the most common application of the film cooling in hypersonic vehicles is gas film cooling [3,4]. Based on the analysis, the active thermal protection technology of hypersonic vehicles covered in this paper is discussed only in terms of gas film cooling, transpiration cooling, regenerative cooling and some combined cooling developed on the base of the first three elementary cooling types.
Gas film cooling is a kind of heat insulation active cooling thermal protection method, and is very commonly adopted in air breathing flight vehicles with low or medium speed because of the superior cooling performance and simple structures. The cooling source of gas film cooling is generally the incoming airflow, however, under the hypersonic flight conditions, the total temperature of the inflow air is too high to be used as cooling gas directly. Consequently, the application of gas film cooling in hypersonic vehicles usually requires either a cooling device for the incoming airflow or another gas cooling source to replace the air. Transpiration cooling is also a kind of heat insulation active cooling thermal protection method with excellent cooling performance, and either the inflow air or the fuel can be used as coolant. However, the transpiration cooling has two main drawbacks in hypersonic flight circumstances. First, the fuel transpiration cooling will significantly impair the engine performance. Second, the structure for the transpiration cooling is rather sophisticated leading to a low structure reliability, besides, the fuel coking effects can also result in a cooling structure failure. The regenerative cooling is a kind of endothermic convective cooling method, which shows great potential and advantages in a wide speed range up to hypersonic flight. The regenerative cooling concept was initially used in liquid rocket engines in 1903 [9], and then migrated to the scramjet later. The regenerative cooling is also attractive from the energy point of view, because the heat absorbed from the hot side is not dissipated, but carried by the propellants and finally returned to the combustion chamber [10].
In addition, during the development of the hypersonic vehicles, several novel cooling methods were also aggressively explored to remedy the limitation of the single elementary cooling type and to cope with the challenges of increasingly high flight Mach number, such as the combined cooling method, the cooling ability increasing method based on the energy management, and the third fluid cooling method.
The particularity of active cooling for hypersonic vehicles lies in the great challenge of resolving the conflict between the huge aerodynamic heating effects and the extremely limited cooling source, brought by the hypersonic flight speed and the working characteristics of the air breathing engine, respectively.
This paper focuses on the important issue of the active thermal protection methods for hypersonic vehicles. Firstly, the challenges and difficulties of the active thermal protection methods for hypersonic vehicles are thoroughly discussed, followed by the conclusions on the key technical difficulties and the solving ideas of each active cooling method. Then, the investigations on the key technologies of each active thermal protection method covered in the previous discussion are summarized and analyzed. Finally, on this basis, the development trend of the related technologies is discussed in detail in order to provide some help for the design of a thermal protection system in hypersonic vehicles.
Section snippets
Severe shortage of coolant for active thermal protection
Take the regenerative cooling, which is the mostly widely studied method for the active thermal protection of hypersonic vehicles, as the example. The incoming air cannot be used as coolant due to the extremely high total temperature of air at high Mach number Ma>5. In this case, the fuel on board becomes the only cooling source for the hypersonic vehicles. As for the regenerative cooling using the fuel as coolant, the cooling capacity of the coolant can be explained as follows:
Working principles of regenerative cooling
The regenerative cooling using fuel as coolant is becoming one of the most promising cooling methods for hypersonic vehicles [1,2,4], which is mainly used for cooling the combustion chamber inner surface and auxiliary cooling of the outer wall of vehicles. The regenerative cooling was first proposed for liquid rocket engines, and then used for scramjet engines [17]. Fig. 2 gives the working principle of the regenerative cooling of a scramjet engine, which is the most common propulsion device
Working principle of supersonic film cooling
Film cooling is a kind of active cooling method, in which the relatively low-temperature gas is introduced from discrete holes or slots on the wall to effectively isolate the heat in the hot downstream region. The schematic of the film cooling is illustrated in Fig. 19. The functions of the film cooling are not only protecting the wall from overheating, but also reducing friction drag, blowing off the boundary layer and controlling the lateral thrust.
Since the 1970s, the film cooling has become
Working principle of transpiration cooling
Transpiration cooling, which is a bionic technique for regulating the temperature of objects by mimicking the perspiration of biological skins, can be divided into three categories: solid transpiration cooling, gas transpiration cooling and liquid transpiration cooling. Among them, the solid transpiration cooling and a few types of liquid transpiration cooling are mainly free transpiration cooling, while the gas transpiration cooling and most types of liquid transpiration cooling are forced
Combined cooling
In order to sufficiently use the cooling capacity of fuel and increase the cooling efficiency of the cooling system for hypersonic vehicles, researchers have proposed various combined thermal protection methods.
Conclusions and outlooks
For the long run and for reusable purpose, active thermal protection methods have become crucial techniques for hypersonic vehicles. Regenerative cooling, supersonic film cooling, transpiration cooling and their combinations are most promising for hypersonic vehicles application, depending on the purpose and the position that cooling methods are used for.
In this paper, the “relative heat load” is proposed to evaluate the challenges and difficulties of the thermal protection for hypersonic
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research work is supported by the National Natural Science Foundation of China (Grant No. 51806049), and is supported by the National Postdoctoral Program for Innovative Talents (No. BX201600043), Shenzhen Technology Projects (No. ZDSYS201707280904031), National Natural Science Foundation of China (No.51976046).
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