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红外抑制器分流喷管进口定义为流量进口边界,质量流量为6.17 kg/s,总温为842 K,并假定排气热力参数在分流喷管均匀。为了模拟分流喷管引射式红外抑制器从外界引射环境冷气,且保证混合管排气充分发展,在红外抑制器外围构建了足够大的外流场区域,如图3所示,外流场边界设置为压力出口,压力为101 325 Pa,外流场环境冷气温度为288.15 K。
假定来自发动机动力涡轮后的排气是完全燃烧的燃气,主要为氮气、二氧化碳和水蒸气,其质量占比为0.706、0.209、0.085。引射的环境冷气组成成分为氮气和氧气,质量占比分别为0.756和0.244[15-16]。红外抑制器的固体壁面设置为无滑移的流固耦合面进行传热计算,同时考虑固体壁面之间的辐射换热,红外抑制器的固体壁面发射率取为0.8。
采用ICEM (Integrated Computer Engineering and Manufacturing)软件对计算模型进行网格划分。由于分流喷管引射式红外抑制器整体结构较为复杂,且外流场区域尺寸较大,综合考虑计算效率与精度,对红外抑制器结构使用适应能力较强的非结构化网格进行网格划分,并对分流喷管、混合管等壁面区域进行局部网格加密处理,对大的外部流场使用结构化网格进行网格划分。如图4所示,对比红外抑制器网格数量为580万、660万、750万、830万和920万下的引射系数和总压恢复系数,最终确定总体的网格数约为750万。
图 4 不同网格数下红外抑制器引射系数和总压恢复系数计算结果
Figure 4. Computed results of Infrared suppressor pumping coefficient and total pressure recovery coefficient under different grid numbers
参考大量红外抑制器的数值模拟文献[10-18],选用SST (Shear Stress Transfer) k-w湍流模型进行数值模拟,加入组分输运方程获得主流与次流掺混后的组分分布。流动传热与组分输运方程中的对流项和扩散项均采用二阶迎风差分格式离散,压力与速度耦合采用SIMPLEC (Semi-Implicit Method for Pressure-Linked Equations Consistent)算法。选取离散坐标辐射模型(DO模型)计算辐射换热。计算收敛判据设置为各项残差均小于10−5。
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在流场和温度场计算的基础上,红外辐射特性计算采用经过验证的正反射线追踪法[19-20]。文中计算和分析了分流喷管引射式红外抑制器3~5 μm波段和8~14 μm波段的红外辐射特性。考虑红外抑制器结构在铅锤方向上的对称性以及内侧被机身遮挡,因此将探测点设置在水平探测面−90°~0°和铅锤探测面0°~90°范围内,其中正对混合管排气出口的水平探测面−30°~0°和铅锤探测面0°~30°范围每5°一个探测点,水平探测面−90°~−30°和铅锤探测面30°~90°范围每10°一个探测点,探测距离为200 m,红外辐射特性计算时红外探测点位置分布如图5所示。
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引射系数Φ是衡量红外抑制器的分流喷管引射能力的重要参数,定义如下[21]:
$$ \varPhi = \sum\limits_{}^{} {\frac{{{m_b}}}{m}} $$ (1) 式中:mb为通过百叶窗入口进入的引射次流流量;m为主流流量。
主流与引射次流的掺混势必造成排气的流动损失,从而直接影响发动机的性能,因此引入总压恢复系数对此进行评价。总压恢复系数σ定义为[10]:
$$ \sigma = \frac{{{m_{{\rm{out}}}}{P_{{\rm{out}}}}}}{{mP + {m_b}{P_b}}} $$ (2) 式中:mout、m和mb分别为混合管出口流量、主流流量和引射次流流量;Pout、P和Pb分别为混合管出口、主流入口和百叶窗引射入口的总压。
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图6为实验系统示意图,主流从离心式压气机流出,经燃烧室燃烧后进入稳流段,最后进入实验段。主流通道设有流量计,可以监控实验的主流流量。根据数值计算模型,设计了基准模型和两种波瓣出口模型的1/2缩比实验,三种喷管出口实物如图7所示。抑制器分流喷管主流流量为0.3 kg/s,温度为773 K。
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图8为实验系统实物图,引射系统由四根吸风喇叭管和集气腔组成,在吸风喇叭管内布置了静压测量点,弯曲混合管的引射流量通过测量的静压换算而得,压力采集设备为PSI (Pressure Scanner Implement) 9116智能压力扫描阀,测量误差在0.05%以内。混合管的出口温度通过铁制热电偶耙测量,竖向共分布有7根热电偶,每根热电偶之间间隔约5 cm。内侧混合管壁面温度通过预留的测点测量,测点位置距离混合管出口平面10 cm,测量时将热电偶插入旋紧即可,竖向共分布有7根热电偶,每根热电偶之间间隔约5 cm,热电偶耙实物图和热电偶测量位置如图8所示,温度采集设备为多路温度测试仪(anbai-AT4764),测量误差在±0.5 K以内。测量红外辐射强度的设备为红外遥感光谱辐射计(VSR-3),测量误差在3%以内,探测点位于为实验模型的−90°方向(参考图5),距离实验段40 m。
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对实验工况下的实验模型进行数值模拟。实验模型引射系数的实验结果与仿真结果如表1所示。可以得到,相比Origin模型,Lobe_2模型引射系数的实验结果基本不变,Lobe_1模型引射系数的实验结果明显降低;缩比模型引射系数的数值结果与实验结果吻合度高,误差在4%以内。
表 1 实验模型引射系数
Table 1. Pumping coefficient of experimental model
Model Experiment Simulation Deviation Origin 0.988 0.975 1.32% Lobe_1 0.896 0.925 3.24% Lobe_2 0.981 1.018 3.77% 图9为实验模型内侧混合管壁面与混合管出口温度分布的实验与数值模拟结果对比。三组实验的内侧混合管壁面温度在1号热电偶位置的数值模拟结果均高于实验结果,并存在较大误差,最大误差达到10%,造成误差的原因在于1号热电偶未旋紧到与混合管壁面良好接触,其他位置的数值模拟结果与实验结果吻合度较高,误差在8%以内。数值模拟和实验测量得到的混合管排气出口温度分布趋势一致,误差基本在5%以内。
表2为实验模型的红外辐射强度。数值模拟和实验测量得到的红外辐射强度变化趋势一致,两者最大误差为6.42%,最小差值为2.98%,吻合程度较高。总体来看,数值模拟结果与实验测量结果吻合度较高,满足工程计算需求[22]。
图 9 实验模型测点温度的实验与数值模拟结果对比
Figure 9. Comparison of experimental and numerical simulation results of experimental model temperature distribution
表 2 实验模型红外辐射强度
Table 2. Infrared radiation intensity of experimental model
Model 3-5 μm infrared radiation intensity/W·sr–1 8-14 μm infrared radiation intensity/W·sr–1 Experiment Simulation Deviation Experiment Simulation Deviation Origin 0.407 0.429 5.41% 3.285 3.187 2.98% Lobe_1 0.420 0.436 3.81% 2.958 3.073 3.89% Lobe_2 0.358 0.381 6.42% 2.464 2.594 5.28%
Numerical and experimental research on the effect of outlet structural parameters of diverter nozzle on infrared suppressor performance
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摘要: 在模型实验验证的基础上,采用数值模拟的方法,对比分析了分流喷管出口构型对直升机红外抑制器气动性能、温度场和红外辐射强度的影响。研究结果表明:相比基准分流喷管模型(Origin),分流喷管出口带一定外扩张角的波瓣出口结构(Lobe_1)的引射系数略微降低、总压恢复系数降低,中间混合管出口排气温度峰值却降低了65.1 K,同时降低了混合管上下方区域的壁面温度,但造成混合管中后段外侧壁面局部区域温度升高;外扩张角为0的波瓣出口结构(Lobe_2)增加引射系数3.8%,总压恢复系数与Lobe_1结构基本相当,中间混合管出口排气温度峰值也降低了62.8 K,尤其是其降低混合管壁面温度的效果最佳;分流喷管出口突片结构(Tab)增加引射系数10.6%,但总压恢复系数降低0.7%,同时,内侧混合管出口排气平均温度降低19.3 K,混合管壁面降温效果相对较差。总体来看,波瓣和突片结构都起到增强引射、强化混合的作用,尤其是波瓣出口结构(Lobe_2)对降低抑制器总体红外辐射效果最好,在3~5 μm波段的红外辐射强度最大可降低21%;在8~14 μm波段,其红外辐射强度最大可降低15%。Abstract:
Objective With the rapid development of advanced infrared detection technology and infrared tracking and striking technology, armed helicopters are increasingly threatened by infrared guided missiles from ground and air in the modern high-tech battlefield. In order to improve the battlefield survivability and combat assault capability of armed helicopters, advanced infrared stealth technology must be developed. The research shows that the use of shielding technology and the improvement of the ejector capacity of the suppressor have a significant effect on reducing the infrared radiation intensity of the exhaust system, but the specific technical means should depend on the structure of the infrared suppressor. For the diverter nozzle ejector infrared suppressor, limited to the size and shape of the helicopter, it is difficult to improve the ejector capacity of the diverter nozzle and reduce the exhaust and wall temperature in a limited space. Therefore, it is necessary to discuss the modification scheme of the diverter nozzle outlet to reduce the infrared radiation intensity of the diverter nozzle ejector infrared suppressor. Methods A physical model was established including diverter nozzle, gas-collecting chamber, ejected gas inlet, curved mixing tube, covering shelter, and outer cover ( Fig.1 ). The structured and unstructured hybrid grids were established, and the infrared radiation of the infrared suppressor was calculated by the forward-backward ray-tracing method. The calculation method is verified by experimental data (Tab.1 -2 ,Fig.9 ). By comparing the pumping coefficient, total pressure recovery coefficient, outlet and wall temperature distribution of the mixing tube and infrared radiation intensity of the diverter nozzle ejector infrared suppressor (Fig.10 -14 ), the effect of outlet structural parameters of diverter nozzle on infrared suppressor performance is analyzed from multiple perspectives.Results and Discussions The experimental data are used to verify the calculation method. The pumping coefficient and total pressure recovery coefficient of the infrared suppressor under different diverter nozzle outlet structures are compared and analyzed ( Fig.10 ). The exhaust temperature distribution of the mixing tube outlet plane of the infrared suppressor under different diverter nozzle outlet structures is shown (Fig.11 ). The temperature distribution of the outer mixing tube wall surface of the infrared suppressor under different diverter nozzle outlet structures is shown (Fig.12 ). And the infrared radiation intensity distribution of the infrared suppressor with different diverter nozzle outlet configurations on the horizontal and lead hammer detection surfaces in the 3-5 μm band (Fig.13 ) and 8-14 μm band is shown (Fig.14 ).Conclusions Compared with the original model, the pumping coefficient of the Lobe_1 with a certain expansion angle is slightly reduced, the total pressure recovery coefficient of the Lobe_1 is reduced, and the peak exhaust temperature at the outlet of the intermediate mixing tube is reduced by 65.1 K. For Lobe_1, the wall temperature in the upper and lower areas of the mixing tube is reduced, but the temperature in the local area of the outer wall of the middle and rear sections of the mixing tube is increased. The lobed outlet structural (Lobe_2) with an outer expansion angle of 0 increases the pumping coefficient by 3.8%. The total pressure recovery coefficient is basically the same as that of the Lobe_1 model, and the peak exhaust temperature of the intermediate mixing tube is also reduced by 62.8 K, especially the effect of reducing the wall surface temperature of the mixing tube is the best. The outlet of the diverter nozzle with tab structure increases the pumping coefficient by 10.6%, but the total pressure recovery coefficient decreases 0.7%, and the average exhaust temperature of the inner mixing tube decreases by 19.3 K. Tab model has a poor effect on cooling the wall temperature of the mixing tube. In general, both lobe and tab structures play a role in ejection and mixing. In particular, the lobed outlet structure (Lobe_2) has the best effect on reducing the overall infrared radiation of the suppressor, and the infrared radiation intensity in the 3-5 μm band can be reduced by up to 21%, in the 8-14 μm band, the infrared radiation intensity can be reduced by 15%. -
表 1 实验模型引射系数
Table 1. Pumping coefficient of experimental model
Model Experiment Simulation Deviation Origin 0.988 0.975 1.32% Lobe_1 0.896 0.925 3.24% Lobe_2 0.981 1.018 3.77% 表 2 实验模型红外辐射强度
Table 2. Infrared radiation intensity of experimental model
Model 3-5 μm infrared radiation intensity/W·sr–1 8-14 μm infrared radiation intensity/W·sr–1 Experiment Simulation Deviation Experiment Simulation Deviation Origin 0.407 0.429 5.41% 3.285 3.187 2.98% Lobe_1 0.420 0.436 3.81% 2.958 3.073 3.89% Lobe_2 0.358 0.381 6.42% 2.464 2.594 5.28% -
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