|Alternative Title||Research on Active Cooling Technology of Infrared Windows for Hypersonic Vehicles|
|Place of Conferral||北京|
When the hypersonic vehicle cruises or re-enters at a high Mach speed in the atmosphere, aerodynamic heating causes the temperature of the infrared optical window to rise rapidly. The aerodynamic thermal radiation effect caused by high temperature gas and infrared window may decay the performance of the infrared seeker to recognize the probability and the tracking accuracy or even fail because of the detector saturation. In order to ensure the operation of the infrared window and improve its performance, it is necessary to adopt cooling measures. At present, the common cooling methods of infrared windows mainly include internal cooling and external cooling. In the 1980s, the United States conducted research on active cooling of infrared windows, and achieved certain results successfully applied to hypersonic vehicles. In the late 1990s, China also carried out some preliminary research on the active cooling of internal cooling scheme. But so far, there has been no comprehensive and systematic research report in the public literature.
In this paper, the infrared window cooling technology at home and abroad is comprehensively researched, and the infrared window intensity calculation and temperature influence analysis of hypersonic vehicle are carried out. The numerical simulation and experimental research of the infrared window internal cooling method are completed, which mainly includes four aspects:
Firstly, according to the characteristics of the atmospheric infrared window, we choose sapphire as the material of medium infrared window, after considering the mechanical properties, physical properties and optical properties of infrared optical materials in many aspects. In this paper, the strength analysis of circular and rectangular infrared windows is carried out by using the thin plate theory. The relationship between the minimum thickness of the infrared window and the load or feature length is obtained according to the structural strength design criteria. Typically, a circular infrared window with a diameter of 60 mm has a minimum thickness of 3.82 mm; a square infrared window with a side length of 60 mm has a minimum thickness of 4.46 mm.
Secondly, the aerodynamic thermal engineering method is used to analyze the stagnation heat flow and head heat flow distribution of hypersonic vehicles. The temperature range of the infrared window under typical flight conditions is calculated. When the heat flow on the surface of the infrared window is 100 kW/m2, the temperature of the sapphire with an initial temperature of 300K is about 600K when heated for 40s. In order to study the influence of infrared window temperature on the performance of the detection system, this paper analyzes the variation of infrared emissivity and transmittance of sapphire window with temperature and thickness, carries out the theoretical calculation and experimental study of window thermal radiation effect, and quantitatively obtains the influence of temperature and thickness on the detection performance of sapphire infrared window. The results show that the increase of sapphire window temperature will lead to an increase in emissivity and a decrease in transmittance. When the temperature of the 5 mm thick window increases from 293 K to 373 K and 473 K, the emissivity increases respectively by 13.7% and 32.9%, the transmittance decreases respectively by 1.7% and 3.2%. When the infrared window temperature is high, its radiation greatly reduces the effective dynamic range of the detection system. Studies have shown that when the integration time of infrared detector is 2ms, the temperature of sapphire infrared window should not exceed 453K.
Furthermore, this paper analyzes the internal cooling method of the infrared window of the hypersonic vehicle and gives the main influencing factors of the cooling effect. An infrared window with a diameter of 63mm and a thickness of 8mm and a 2mm×2mm annular internal cooling channel on the side was designed. The influence caused by the factor which includes coolant type, flow rate, initial temperature, and cross-sectional shape of the cooling channel on the cooling efficiency of the infrared window was studied by using finite element software. When the heat flow on the surface of the infrared window is 100kW/m2, the flow rate of the coolant water is 1 m/s, the initial temperature is 275 K, and the cooling channel is 2 mm × 2 mm, the infrared window temperature can be lowered by about 150K compared to no cooling. Similarly, when the heat flux is 25 kW/m2, the sapphire infrared window is cooled to about 70K.
Finally, the paper also carried out experimental research on active cooling of infrared window. The electric heat fan is used to simulate the heat flow, and the high temperature heating furnace is used to simulate the stable ambient working temperature. When the electric heating fan stably heats the sapphire infrared window with a heat flow of 25 kW/m2, the infrared window cooled by water with an initial temperature of 290K and a flow rate of 1 m/s, the maximum temperature of the infrared window has dropped by at least 65K. The experiment of sapphire infrared window active cooling research is basically consistent with the numerical simulation. When the temperature of the high temperature furnace is 473K and 523K, the infrared window cooled by water with an initial temperature of 290K and a flow rate of 1 m/s, the maximum temperature of the infrared window has dropped respectively by 140K and 170K.
The research results in this paper can provide reference for the design of infrared window for hypersonic vehicles.
|舒开才. 高超声速飞行器红外窗口主动冷却技术研究[D]. 北京. 中国科学院大学,2019.|
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