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文中对如图1所示的直升机模型展开研究,其长L=14.5 m,高H=5.0 m,宽W=5.1 m,直升机主旋翼平面与机身水平面夹角为5.3°。两台涡轴发动机被安装在机身两侧的机匣中,机腔轴心到直升机中心面的距离是0.97 m。主旋翼和尾桨都被简化为圆盘,如图2所示,主旋翼直径D=14.6 m,尾桨直径d=2.8 m。
发动机机匣模型和红外抑制器模型及其在直升机上的位置如图3(a)所示。涡轴发动机由粒子分离器、压气机、燃烧室、高压涡轮、低压涡轮组成,建模时仅考虑发动机各部件的机匣,如图3(b)所示。采用如图3(c)所示的多路分流引射式红外抑制器,涡轮后排气通过四个进口进入抑制器,其中左、右进口面积相等,并与上、下两进口面积之和相等,以保证三股主气流流量相同地进入三根平行的混合管,且混合管外有遮挡罩包裹。对于单个抑制器,其主流进口面积与次流进口面积、抑制器出口面积的比值为1∶1.23∶2.32。
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文中的计算域及其外廓尺寸如图4所示,计算域的边长为直升机主旋翼直径D的10倍,考虑到旋翼下洗气流和前飞来流方向分别为z轴负向和x轴正向,将直升机放在计算域左上区域以保证多股气流有足够大的发展空间。直升机机头正对的外场面为速度进口面,其他五个面为压力出口面。将直升机各壁面设置为无滑移壁面,由于机身内部无流场区域,机身蒙皮仅存在与外部环境的换热;排气系统壁面为金属材料,其导热系数较高,且壁面厚度一般在1~3 mm,壁面内外侧几乎没有温差;因此建模时将壁面设置为零厚度壁面[13-16]。所有壁面均视为漫灰表面,其发射率固定为0.85。
将简化的发动机机匣模型(图3(b))的各分段表面按等壁温边界处理,各分段表面温度设置如表1所示。假定发动机贫油燃烧,其涡轮后排气参数如表2所示。考虑直升机处于巡航状态,其飞行速度为270 km/h,大气组分为O2和N2,其质量百分数分别为0.244、0.756。
表 1 涡轴发动机机匣的壁面温度
Table 1. Temperature turboshaft engine casing wall
Serial number Temperature/K Serial number Temperature/K 1 338 6 673 2 423 7 450 3 478 8 600 4 673 9 773 5 573 - - 表 2 飞行时涡轮的排气参数
Table 2. Exhaust steam parameters in flight
Flow/kg·s−1 Temperature/K Percent mass of each component CO2 H2O O2 N2 3.4798 856 0.064 0.026 0.136 0.774 认为巡航状态下直升机高度1500 m,且位于北回归线上(23°26′N,120°47′E),当地气温在不同季节的最大值与最小值出现的时刻为14:00、4:00,其温度数值如表3所示,当日其余时刻的环境温度在最大与最小温度之间均匀变化,环境压力为84556 Pa。
表 3 环境温度
Table 3. Environment temperature
Season Temperature at 4:00/K Temperature at 14:00/K Vernal equinox 286.44 290.42 Summer solstice 290.44 294.42 Autumn equinox 284.44 288.42 Winter solstice 280.44 284.42 为了获得主旋翼与尾桨产生的旋转气流,将主旋翼与尾桨简化为以旋桨叶长为半径的圆盘,并给定圆盘面上的速度分布,以减少计算网格和仿真的复杂度。此简化方法的可行性在文献[16]中已详细阐述。使用多参考坐标系(MRF)方法对旋翼和尾浆进行仿真,提取并拟合出旋翼和尾浆盘面上的速度分布,如图5所示,旋翼径向长度为横坐标,盘面气流速度为纵坐标,编制UDF用于直升机整机的流场仿真。
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考虑到直升机整机中的部件尺度跨度大、且形状复杂,文中采用结构化与非结构化混合网格。如图6所示,对复杂的直升机壁面及红外抑制器区域采用非结构化网格以更好地贴合壁面,并降低近壁面网格尺寸以满足y+的要求,对排气喷流出口区域加密网格,以更精确捕捉排气喷流的流动特性;外场为结构化网格,其与非结构网格交界处通过金字塔型网格平滑过度,并在延申至远场边界过程中网格尺寸逐渐变大,覆盖足够的外场空间同时节约网格数量。
调整机身壁面和排气喷流区域网格尺寸设计4套网格以进行网格无关性验证,表4是右侧红外抑制器引射冷气流量,考虑计算精度与网格数量,最终选择的网格数为1590万。CFD计算时各项残差值均收敛至10−5以下。
表 4 不同网格数下右侧红外抑制器的引射冷气流量
Table 4. Ejector cool air flow rate of right infrared suppressor with different grid numbers
Grid number/
×106Ejector cool air flow rate of right
infrared suppressor/kg·s−119.7 1.854 15.9 1.852 13.3 1.843 11.5 1.821 -
根据计算模型进行三维计算流体力学仿真,控制方程包括质量守恒方程、动量守恒方程和能量守恒方程,以及各组分输运方程和辐射换热方程,这些方程在文献[19]已有详细描述。湍流模型选择SST k-ω,文献[16]通过实验与仿真的对比,证明该湍流模型能有效预测直升机机身后表面的温度场。
采用平均天气条件下的直接太阳辐照方程[20]来模拟太阳辐射:
$$ Edn=\frac{A}{{\exp}\left({{B}/{{{\rm{sin}}}\left(\beta \right)}} \right)}$$ (1) 式中:A和B分别为基于晴天的地球表面空气质量的表观太阳辐照和大气消光系数;$ \; \beta $为水平面以上的太阳高度(单位:(°))。
垂直表面上的漫射太阳辐照方程为:
$$ Ed=CYEdn $$ (2) 式中:$ C $为文献[20]中给出的一个常数;$ Y $为垂直表面上的天空漫射辐射与水平表面上的漫射辐射之比。
除垂直表面外,其他表面的漫射太阳辐照方程为:
$$ Ed=CEdr\frac{1+{{\rm{cos}}}\left(\varepsilon \right)}{2} $$ (3) 式中:$ \varepsilon $是表面与水平面的倾斜角(单位:(°))。
将太阳光视为一束平行光束,在Fluent中设置太阳辐射热流的强度与方向,以计算太阳辐射热流对直升机蒙皮的作用[14,18]。考虑太阳辐射在一日内不同时刻、不同季节、直升机不同飞行方向,设置如图7和图8所示不同的太阳辐射加载工况,图7为不同时刻下太阳光辐照角度示意图,其中A1~A5分别代表8:00、10:00、12:00、14:00、16:00时刻,另设置4:00时刻无太阳辐射状态为A0。图8为不同季节下太阳辐照角度示意图,时刻为当日中午,其中B1~B4分别代表一年中春分、夏至、秋分、冬至。图9为直升机不同飞行方向下太阳光入射方向示意图,假定时间为夏至10:00时刻,C1~C4表示直升机飞行方向分别为正东、东南、正南、西南。
在完成直升机整机模型三维流场和温度场的稳态计算后,采用正反射线追踪法[13]对直升机的红外辐射强度开展计算,算法的详细介绍与有效性验证可参考文献[16]。图10所示为文中探测水平面(xoy平面)、横切面(xoz平面)与纵切面(yoz平面)的红外辐射强度空间分布,坐标原点设在喷管出口面的中间位置,探测距离即探测点至坐标原点的距离为1 km,在各探测面上每隔10°设置一个探测点。
Numerical investigation of solar radiation effects on helicopter infrared radiation characteristics
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摘要: 太阳辐射对飞行中的直升机局部蒙皮有加热作用,从而改变整机红外辐射的分布特征。构建了包含直升机机身蒙皮、主旋翼、发动机机匣以及排气系统的物理模型,综合考虑发动机机匣、排气系统与发动机舱蒙皮的换热,耦合直升机前飞来流、旋翼下洗气流、尾桨气流,以时刻、季节、直升机朝向为变量,计算分析太阳辐射对直升机8~14 μm波段红外辐射特性的作用规律。计算结果表明:夏季正午太阳直射可使机身向阳面整体升温20 K以上,局部最高可达25 K。直升机向阳面机身蒙皮8~14 μm波段红外辐射强度在全天变化趋势呈山峰状,其峰值出现在12点前后。越靠近机身顶部向阳面,太阳辐射对8~14 μm波段红外辐射强度增强作用越显著,最高可达25%。以冬季为基准,秋分、春分、夏至时的整机红外辐射分别增加7%、11%、21%左右。除夏季外,其他季节的机身两侧8~14 μm波段红外辐射强度分布都呈现不对称性,春、秋两季两侧相差在5%左右,冬季在6.5%左右。整体上,夏季上午10点的太阳辐射对不同飞行方向的直升机8~14 μm波段红外辐射强度分布影响较小。Abstract:
Objective The development of long-wave infrared radiation imaging technology is more and more threatening to helicopter. The sources of helicopter long-wave infrared radiation include the infrared radiation of the helicopter's own power system and its energy transfer with the engine compartment as well as the local heating of the fuselage by solar radiation. A large number of experiments and numerical analyses have been carried out on the highly efficient ejection blending system and infrared suppressor, which can effectively reduce the infrared radiation of the helicopter itself. Solar radiation has a heating effect on the local skin of the helicopter in flight, thus changing the infrared radiation distribution characteristics of the whole aircraft, but it is often ignored in numerical simulation calculation, and there are relatively few studies on the characteristics of solar radiation and infrared radiation of the whole aircraft considering various factors. Therefore, it is necessary to carry out the research on the effects of solar radiation on the fuselage infrared radiation characteristic. Methods A physical model including helicopter fuselage skin, main rotor, engine casing and exhaust system was constructed to establish a structured and unstructured hybrid grid (Fig.6). The heat transfer of engine casing (Tab.1), exhaust system (Tab.2) and engine compartment skin is comprehensively considered, coupled with the helicopter forward incoming flow, rotor downwash flow and tail rotor flow (Fig.5). The solar radiation is simulated by the equation of normal direct irradiation applying the fair weather conditions method. A forward-backward ray-tracing method is used to calculate the helicopter infrared radiation. Results and Discussions The whole helicopter model including the engine casing and the exhaust infrared suppressor is simulated and calculated (Fig.3). In the calculation of the flow field, the mixed flow field including the forward incoming flow, the main rotor downwash flow, the exhaust jet flow and the tail rotor flow are considered (Fig.4). With time, season and helicopter flight direction as variables, different solar radiation loading conditions are set (Fig.8-10). The detection points are evenly arranged on the horizontal, transverse and longitudinal detection planes (Fig.7), to calculate and analyze the effect of solar radiation on the infrared radiation characteristics of helicopter in 8-14 μm band. Conclusions The calculation results show that the direct sunlight at noon in summer can increase the overall temperature of the fuselage to the sun side by more than 20 K, and the local maximum temperature can be increased by 25 K. The infrared radiation intensity of 8-14 μm band on the sun side of the helicopter fuselage showed a peak-like trend throughout the day, and its peak appeared around 12 o 'clock. The closer to the top side of the fuselage is, the more significant the enhancement effect of solar radiation on the infrared radiation intensity of 8-14 μm band is, up to 25%. Taking winter as the benchmark, the infrared radiation of the whole aircraft at the autumn equinox, spring equinox and summer solstice increases by about 7%, 11% and 21% respectively. Except summer, the infrared radiation intensity distribution of 8-14 μm band on both sides of the fuselage in other seasons presents asymmetry, and the difference between the two sides is about 5%. On the whole, the solar radiation at 10 am in summer has little effect on the infrared radiation intensity distribution of 8-14 μm band of helicopters in different flight directions. -
Key words:
- infrared radiation characteristics /
- solar radiation /
- numerical calculation /
- helicopter /
- season
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图 15 (a)特定探测点全天8~14 μm波段的红外辐射强度分布图;(b)特定探测点全天无量纲的红外辐射强度分布图
Figure 15. Distribution diagram of infrared radiation intensity in the 8-14 μm band throughout the day at specific detection points; (b) Distribution diagram of dimensionless infrared radiation intensity throughout the day at specific detection points
表 1 涡轴发动机机匣的壁面温度
Table 1. Temperature turboshaft engine casing wall
Serial number Temperature/K Serial number Temperature/K 1 338 6 673 2 423 7 450 3 478 8 600 4 673 9 773 5 573 - - 表 2 飞行时涡轮的排气参数
Table 2. Exhaust steam parameters in flight
Flow/kg·s−1 Temperature/K Percent mass of each component CO2 H2O O2 N2 3.4798 856 0.064 0.026 0.136 0.774 表 3 环境温度
Table 3. Environment temperature
Season Temperature at 4:00/K Temperature at 14:00/K Vernal equinox 286.44 290.42 Summer solstice 290.44 294.42 Autumn equinox 284.44 288.42 Winter solstice 280.44 284.42 表 4 不同网格数下右侧红外抑制器的引射冷气流量
Table 4. Ejector cool air flow rate of right infrared suppressor with different grid numbers
Grid number/
×106Ejector cool air flow rate of right
infrared suppressor/kg·s−119.7 1.854 15.9 1.852 13.3 1.843 11.5 1.821 -
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