飞行器在高速飞行过程中形成的高热流、高阻力环境严重影响其安全、稳定的飞行。
针对飞行器内部的电子器件、燃烧室等高热流区域，采用平行流道结构的对流冷却是较为成熟的冷却技术，分形结构是自然进化的结果，将其应用于冷却结构的设计可以实现低耗高效的传热特性；针对飞行器外壳等直接与空气作用的高热流、高阻力区域，逆向射流对比于凹腔、减阻杆等方式是较为可控且稳定性高的降温减阻技术。CO2是无毒、不易燃的常见工质，超临界CO2粘度较低，用于循环系统可以大大减少功耗，CO2比空气、N2更易凝结，相变吸热有利于降低流场温度，且高压CO2无需泵推送，可大大简化系统，用于射流降温存在一定优势。因此以CO2为工质的飞行器热管理系统冷却部件的设计及其降温减阻方式的研究亟需展开。
在飞行器内部的冷却部件之中，以发动机进气口的预冷器、内部循环所采用的循环换热器为代表的双流体冷却结构和以燃烧室壁面再生冷却结构、电子设备热散热器为代表的单流体冷却结构等冷却部件的设计以及飞行器外部的射流降温减阻方式的研究对飞行器的稳定的运行至关重要。
本文构建了以CO2为工质的单流体仿生分形冷却结构的模型，采用控制变量法对单流体仿生分形换热结构的传热特性进行分析并对其结构进行优化。结果表明，当分支数增加时，分形散热结构的功耗不断增加，换热量不断增加；当管长增加时，分形散热结构的功耗不断增加，换热量不断增加，性能指标出现先增加后减小的趋势；当管径增加时，分形散热结构的功耗不断减小，换热量不断增加，性能指标出现先增加后减小的趋势；当入口流量增加时，分形散热结构的功耗不断增加；性能指标出现先减小后增加的趋势。采用遗传算法对分形散热结构的几何结构参数进行了多目标优化设计，优化结果显示：功耗量降低了44.0%，换热量减小了23.0%，但是性能指标提高了21.6%。
本文构建了以CO2为工质的双流体仿生分形冷却结构的模型，采用迭代壁温法实现了双流体仿生分形换热结构模型的计算，通过控制变量的方法对双流体换热结构的传热特性进行分析并对其结构进行优化。结果表明，在相同的设计参数下，逆流式分形换热结构的功耗、换热性能指标始终优于顺流式分形换热结构；随着分支数的增加，分形结构的换热量不断增加，功耗量先减小后增加存在最优值，性能指标先增加后减小存在最优值；随着管长的增加，功耗量和换热量不断增加，性能指标先增加后减小存在最优值；随着管径的增加，功耗量和换热量不断减小，性能指标先增加后减小存在最优值。采用遗传算法对分形换热结构的几何结构参数进行了以换热量和功耗量的线性组合的多目标优化设计，优化结果显示：功耗量降低了15.0%，换热量增加了2.4%，性能指标提高了16.0%，综合性能明显优于优化前的换热结构。
本文建立了飞行器前缘钝体的物理模型，对以CO2为工质的射流防护的降温减阻特性进行数值模拟研究。结果如下，逆向射流对钝头体降温减阻存在较为积极的作用，钝体总阻力下降可达20.5%，钝体前缘斯坦顿数最高下降93.7%；在其他条件不变的情况下，CO2射流的降温性能最优，N2射流降温能力次之，Air射流降温能力相对较差；减阻能力刚好相反；CO2射流因综合减阻能力略低于相同控制参数下的N2和Air；CO2射流降温能力高于相同控制参数下的N2和Air。当射流总温较低时，对钝头体前缘部分的降温效果明显，随着射流总温的增加，射流推离激波能力增加较为明显，总阻力明显增加；随着射流总压的提高，流场结构出现了长射流模式和短射流模式。钝体前缘的压强随着总压呈现先减小后升高再减小的趋势。随着总压的提高，钝头壁面的斯坦顿数不断下降，降温效果不断提升。随着射流角度的不断增加，射流防护减阻效果不断减弱，当射流角度大于30°时，热环境比无防护时更恶劣，当射流角度大于60°时，射流丧失了其减阻效果；随着来流攻角的不断增加，飞行器钝体迎风区域和背风区域的热环境和阻力环境不对称性不断增加，射流对迎风区域的降温减阻效果较小；当对飞行器外壳进行冷却调控时，随着射流总压的提高，减阻和降温效果不断提高，沿来流流动方向减阻效率和冷却效率呈现先快速下降后上升然后稳定不变的趋势；随着射流总温的升高，冷却效率不断降低；当来流攻角较低时，沿射流口轴线方向的冷却效率先减小后趋于稳定；随着来流攻角的不断增加，来流在射流孔附近形成激波，会使射流孔附近的热环境比无防护更为恶劣。

The high heat flux and high resistance environment formed by aircraft during high-speed flight seriously affect its safe and stable flight.
The use of a parallel channel structure for convective cooling in high heat flux areas, such as electronic devices and combustion chambers inside aircrafts, is a well-established cooling technology. The fractal structure, as a result of natural evolution, can be applied to the design of cooling components, resulting in low consumption and efficient heat transfer characteristics. Compared to concave cavities and drag reduction rods, reverse jet technology provides a more controllable and stable cooling and drag reduction technology for high heat flow and high resistance areas where the outer shell of the aircraft directly interacts with the air. CO2, a common working fluid, is non-toxic and non-flammable. Supercritical CO2 has low viscosity, which can greatly reduce power consumption when used in circulation systems. Moreover, CO2 has a higher freezing point compared to other common working fluids, such as air and N2, which makes it advantageous for jet cooling and drag reduction as it possesses a large latent heat energy. Therefore, it is crucial to conduct research on the design of cooling components and cooling and drag reduction methods for aircraft thermal management systems, using CO2 as the working fluid.
Among the cooling components inside the aircraft, the design of double-fluid cooling structures, such as the precooler at the engine inlet and the circulating heat exchanger used for internal circulation, and the single fluid cooling structure, represented by the regenerative cooling structure on the combustion chamber wall and the electronic equipment heat sink, are critical for the stable operation of the aircraft. Additionally, research on jet cooling and drag reduction methods outside the aircraft is also essential.
In this study, an analytical model of a single fluid cooling structure using CO2 as the working fluid is developed. The Control Variates method is utilized to analyze the heat transfer characteristics of the single fluid heat transfer structure and optimize its structure. The results indicate that as the number of branches increases, the impact of local resistance dominates, and the power consumption of the fractal heat dissipation structure continues to rise. Furthermore, the heat exchange area and maximum temperature increase proportionally. As the pipe length increases, the influence of frictional resistance becomes dominant, leading to an increase in the power consumption of fractal heat dissipation structures. In contrast, the heat exchange area, heat exchange rate, and maximum temperature continue to rise, and the performance indicators show an increasing and then decreasing trend. As the pipe diameter increases, the flow velocity inside the pipe decreases steadily, resulting in a corresponding decrease in the power consumption of the fractal heat dissipation structure. The heat exchange area, heat exchange rate, and maximum temperature still increase, and the performance indicators exhibit an increasing and then decreasing trend. As the inlet flow rate increases, the flow velocity inside the pipe continuously rises, and the power consumption of the fractal heat dissipation structure also increases. Meanwhile, the maximum temperature decreases continuously, and the performance indicators show a decreasing and then increasing trend. This implies that the split heat dissipation structure cannot satisfy the demands of both low power consumption and high heat exchange. High heat transfer performance will cause significant pressure loss, and vice versa. A multi-objective optimization design is conducted on the geometric structure parameters of the fractal heat dissipation structure using a genetic algorithm. The optimization results indicate that the power consumption is reduced by 44.0%, the heat transfer is decreased by 23.0%, while the performance indicators are improved by 21.6%.
This article presents an analytical model of a double fluid biomimetic fractal cooling structure, which employs CO2 as the working fluid. The iterative wall temperature method is utilized to calculate the biomimetic fractal heat transfer structure model. The heat transfer characteristics of the double fluid heat transfer structure are analyzed, and its structure is optimized by controlling variables. The results indicate that the power consumption and heat transfer performance indicators of the counter flow fractal heat transfer structure are consistently superior to those of the parallel flow fractal heat transfer structure under the same design parameters. The greater heat transfer achieved by fractal heat transfer structures under countercurrent flow is due to a larger average temperature difference in heat transfer. When other structural parameters remain constant, the number of branches' increase leads to increased heat transfer of the fractal structure. The power consumption initially decreases and then increases, and there is an optimal value. The performance index initially increases and then decreases, and there is an optimal value. As the length of the pipe increases, the power consumption and heat exchange continue to increase, and there is an optimal value for performance indicators that initially increase and then decrease. As the pipe diameter increases, the power consumption and heat exchange continue to decrease, and there is an optimal value for performance indicators that initially increase and then decrease. This indicates that the split heat transfer structure cannot simultaneously meet the requirements of low power consumption and high heat transfer. Good heat transfer performance leads to significant pressure loss, and vice versa. To optimize the geometric parameters of fractal heat transfer structures based on a linear combination of heat transfer and power consumption, a multi-objective optimization design using genetic algorithm is performed. The optimization results show a 15.0% decrease in power consumption, a 2.4% increase in heat transfer, and a 16.0% improvement in performance indicators. Overall, the performance is significantly better than that of the optimized heat transfer structure.
This article presents a geometric model of the bluff body at the leading edge of an aircraft, and a numerical simulation of the cooling and drag reduction mechanism using CO2 as the working fluid jet protection. The results indicate that the reverse jet has a positive effect on the cooling and drag reduction of the blunt body. The jet is able to push the shock wave away from the leading edge, forming detached and reattached shock waves. This results in a total resistance reduction of 20.5% and a maximum Stanton number reduction of 93.7% at the leading edge of the blunt body. Under unchanged conditions, CO2 jet provides the best heat protection performance, followed by N2 jet, and Air jet has relatively poor heat protection performance. However, the drag reduction ability is exactly the opposite. CO2 has a larger latent heat of condensation, which allows it to carry away more heat than N2 and Air, resulting in a higher cooling ability under the same control parameters. When the total temperature of the jet is low, the proportion of CO2 condensation is larger, and the cooling effect on the leading edge of the blunt body is significant. As the total temperature of the jet increases, the ability of the jet to push away shock waves increases significantly, resulting in a significant increase in total resistance. Under the combined action of different jet modes, the pressure at the leading edge of the blunt body first decreases, then increases, and then decreases with the total pressure. As the total pressure increases, the Stanton number of the blunt wall surface continuously decreases, and the thermal protection effect continuously improves. As the jet angle increases, the drag reduction effect of jet protection weakens. When the jet angle is greater than 30 degrees, the thermal environment is worse than when there is no protection. When the jet angle is greater than 60 degrees, the jet loses its drag reduction effect. As the angle of attack of the incoming flow increases, the asymmetry of the thermal and drag environments in the upwind and enclosed areas of the blunt body of the aircraft increases, and the cooling and drag reduction effect of the jet on the upwind area is relatively small. When regulating the cooling of the aircraft shell, as the total pressure of the jet increases, the drag reduction and heat prevention effects continue to improve. The drag reduction efficiency and cooling efficiency along the incoming flow direction show a trend of first rapidly decreasing, then increasing, and then stabilizing.