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发动机进气总温总压光纤复合探针结构如图1所示。光纤法珀(Fabry-Perot, F-P)总压传感器和光纤布拉格光栅( Fiber Bragg grating,FBG)总温传感器采取并排式安装的方式安装在滞止罩中。其中,F-P总压传感器进行压力测量,FBG总温传感器既可以进行温度测量,又可以用于FP压力数据的温度解耦。这种总压传感器和总温传感器并排式的设计使压力敏感膜片不易受到上游滞止压力损失的影响,具有良好的频率响应,可以获得较为准确的滞止压力值,同时有助于减小FBG上的热损失,实现对滞止温度的精确采集以及温度解耦,进而实现总温总压精确测量。
为了实现发动机进口气流总压总温的测量,必须使运动的气流绝能等熵的滞止到速度为零的状态,又称为气流滞止状态。在气流总温总压测量中,为了减小速度误差,总温总压探针应该有一个接近于1且稳定的恢复系数。恢复系数包括总温恢复系数Ct和总压恢复系数Cp,可以表示为:
式中:Tg为测量有效温度;T为静温;Tt为总温;PT,probe为探头测得的总压;PT为总压;PS为静压。
能够有效提高传感器恢复系数的方法是在传感器的外端设计合理的滞止罩。综合考虑传感器的安装以及对流场的影响,滞止罩直径设计为5 mm。利用有限元法对多组参数(进出口面积比,传感器位置)进行仿真,设计合理的滞止罩。文中引入压力基求解曲线坐标系下的守恒N-S方程,湍流模型选用通过重正规化群理论分析的(SST) k-omega模型,为了提高收敛速率及求解精度,求解方法选择SIMPLEC方案,流体材料选择ideal-gas[13]。不同进出口面积比下的恢复系数如表1所示,可以看出当进出口面积比为4∶1时,恢复特性最好。进出口面积比为4∶1时的仿真结果如图2所示,滞止罩及流场流速在0.49 Ma以内,总温传感器和总压传感器表面速度滞止为0。
Import/export
area ratioFlow rate/Ma Ct Cp 4.5 0.55 0.85 0.91 4 0.49 0.91 0.93 3.5 0.53 0.84 0.88 Table 1. Recovery coefficient of stagnation cover at different inlet and outlet area ratios
Figure 2. (a) Stagnation cover velocity cloud; (b) Total pressure cloud; (c) Total temperature cloud
经验表明[14],总温传感器应具有一定的换热功能,感温单元表面流速为0.1~0.2 Ma时,能实现更加精确的温度测量。由于FBG栅区置于出口气后方时对气流没有滞止效果,故只对栅区中心与出气口中心平行及位于出气口前方的情况下建立计算模型,进行流场模拟。FBG表面速度仿真结果如图3所示,在出气口中心处流速最大,当光栅中心位于出气口前方时,表面流速分布更加均匀,有助于减小由气流引起的机械扰动的误差。
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传感器结构如图4所示,由刻蚀的硅敏感膜片、高硼硅玻璃、打孔硅和打孔玻璃四层结构经过三次键合,最后与光纤集成[15]。当压力发生改变时,硅敏感膜片发生形变,F-P腔长发生改变,进而压力信号发生变化,通过干涉谱进行处理获得F-P腔腔长,即可获得传感器所在环境的压力值。
从图4可以看出,当光进入光纤后,首先部分反射,然后透射光在玻璃晶圆端面和硅晶圆的端面之间多次反射。光多次反射回玻璃晶片形成了多光束干涉,干涉光谱由下式确定:
式中:I1和I2分别为入射光和透射光的强度。
根据弹性力学理论,在施加均匀分布的压力下,圆形传感器膜片的中心偏移和灵敏度为:
式中:W为膜片中心偏移量;Y为膜片灵敏度;P为施加在硅敏感膜片上的压力;E为杨氏模量;μ为泊松比;r和h分别为膜片的有效半径和厚度。硅敏感膜片采用晶向N型<100>的单晶硅材料。
总温总压光纤复合探针要求硅总压传感器具有足够小的尺寸。因此,综合考虑传感器的制备工艺,硅膜片的直径设计在1 mm以内。对硅敏感膜片厚度与压力量程和灵敏度的关系进行参数模拟仿真,得到结果如图5所示。
基于小挠度理论,综合考虑工艺水平、解调腔长及灵敏度等,确定总压传感器结构参数如表2所示。
Figure 5. (a) Relationship between pressure range and diaphragm thickness; (b) Relationship between sensitivity and diaphragm thickness
Performance Symbol Value Diaphragm radius/mm r 0.45 Diaphragm thickness/μm h 18 Sensor sensitivity/μm·MPa–1 Y 9.20 Table 2. Structural parameters of the total pressure sensor
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当气流流入滞止罩后,在FBG表面滞止,动能转换为内能,引起温度的升高。其感受到温度变化,反射谱中心波长产生图6中的漂移,其原理如下式所示:
式中:
$ \Delta {\lambda _B}$ 为光栅中心波长的变化量;$\Delta {n_{{\rm{eff}}}}$ 为纤芯折射率的变化量;$ \Delta \varLambda$ 为光栅栅格周期的变化量。搭建的飞秒激光微加工平台包括飞秒激光器、高精密位移平台和光路系统。利用表3所示参数的飞秒激光脉冲在不剥除涂覆层的情况下,在聚酰亚胺光纤上直写光栅,制备机械性能良好、可靠性高的总温传感器。
Performance Symbol Value Wavelength/nm λ 1030 Frequency/kHz F 200 Pulse-width/fs S 250 Frequency multiplier - 2 Table 3. Femtosecond laser processing parameters
Design of a fiber-optic composite probe for engine intake total temperature and pressure (invited)
doi: 10.3788/IRLA20220685
- Received Date: 2022-09-22
- Rev Recd Date: 2022-10-09
- Available Online: 2022-11-02
- Publish Date: 2022-10-28
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Key words:
- total temperature and pressure /
- stagnation cover /
- Fabry-Perot /
- fiber Bragg grating /
- temperature decoupling
Abstract: A stagnation cover type total temperature and pressure fiber-optic composite probe is designed for the demand of total temperature and pressure measurement at the engine inlet. Firstly, the aerodynamic simulation of the probe structure was carried out by using the finite element method, and the influence of the probe structure parameters on the measurement results was analyzed. On this basis, the total temperature and pressure fiber-optic composite probe was fabricated and its working performance was tested by building a static test system. The experimental results show that the total temperature and pressure fiber-optic composite probe can work under the temperature and pressure composite environment of 150 ℃ and 0.25 MPa. The maximum nonlinearity of the total pressure sensor is 0.5% in the range of room temperature - 150 °C, and the maximum nonlinearity of the total temperature sensor is 4.04% in the ambient pressure environment. Finally, the temperature decoupling of the pressure parameters was completed, and the pressure measurement error in the full temperature range did not exceed 1.55%. The designed total temperature and pressure fiber optic composite probe diameter is 5 mm, which effectively reduces the interference of the probe to the flow field.