Science and Technology on Scramjet Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11572346).
Received Date:06 May 2019
Accepted Date:09 June 2019
Available Online:01 September 2019
Published Online:05 September 2019
Abstract:Mixing enhancement for supersonic mixing layer is of great importance for developing scramjet engine. The growth rate of supersonic mixing layer is smaller than that of subsonic mixing layer. As the compressibility increases, the mixing enhancement becomes more difficult. Plasma synthetic jet is regarded as a promising flow control technology. The plasma synthetic jet generator can produce high energy jet. This generator has no moving parts and does not need additional gas source. It is the first time that plasma synthetic jet has been used to enhance the mixing in supersonic mixing layers. The influence of plasma synthetic jet on the supersonic mixing layer is investigated experimentally and numerically. The experiments are conducted in the low noise supersonic mixing layer wind tunnel. The Mach number of upper stream and lower stream are 1.37 and 2.39 respectively. The convective Mach number of this wind tunnel is 0.32. The plasma synthetic jet actuators are installed in the splitter plate. The distance between the jet hole and the splitter plate end is 15 mm. The nanoparticle-based planar laser scattering (NPLS), particle image velocimetry (PIV) and schlieren are used to obtain the response of the supersonic mixing layer to single pulse plasma synthetic jet perturbation. The NPLS successfully captures the large-scaled vortex structures induced by the plasma synthetic jet in the supersonic mixing layers. The effect of plasma synthetic jet is remarkable. The schlieren images show the process of the perturbation. An oblique shock wave is generated when the jet is ejected. The PIV is employed to obtain the influence of plasma synthetic jet on the velocity field. The y-velocity standard deviation increases due to the perturbation. The actuators’ mixing enhancement effects and actuators’ performances at three locations are investigated by two-dimensional numerical simulation. The three actuators are located on the upper, bottom and end surface of splitter plate respectively. The numerical simulation results show that the mixing layer thickness is increased by the plasma synthetic jet perturbation. There are two mechanisms of perturbations while actuators are located at different positions. The actuators installed on the upper and bottom surface of splitter plate influence the mixing layer through perturbing the upper and lower stream respectively. The actuator installed at the end of splitter plate affects the mixing layer directly. The response time of supersonic mixing layers to the perturbation of the actuator installed at the end of splitter plate is shorter than those of the others. The performance of each actuator is sensitive to the location. Keywords:mixing enhancement/ plasma synthetic jet/ flow control/ supersonic mixing layers
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2.1.实验风洞
实验在低噪声超声速混合层风洞中进行. 混合层风洞如图1所示, 实验段长度为350 mm, 高度为60 mm, 宽度为200 mm. 为消除流向压力梯度, 风洞的上下壁面有1°的张角. 厚度为10 mm的隔板从风洞入口到喷管出口将风洞从中间分为两部分. 风洞实验段实物图如图2所示. 风洞上侧喷管马赫数为1.37, 风洞下侧喷管马赫数为2.39, 根据对流马赫数(Mc)计算公式[23,24] 图 1 超声速混合层风洞示意图 Figure1. Schematic of the supersonic mixing layer wind tunnel.
图 2 超声速混合层风洞实物图 Figure2. The test section of supersonic mixing layer wind tunnel.
表1压力匹配情况下校测流场参数 Table1.Flow parameters of supersonic mixing layer.
22.2.等离子体合成射流激励器 -->
2.2.等离子体合成射流激励器
等离子体合成射流的详细原理在文献[25-27]中有较多的介绍. 图3是安装有等离子体激励器阵列隔板的示意图. X, Y, Z分别代表流向、横向和展向的方向. 激励器安装在距离隔板尾端约15 mm处, 实现对上侧气流的扰动. 5个激励器采用串联放电方式工作. 每个激励器由圆柱形放电腔体和一对电极组成. 采用抗放电烧蚀能力强的钨针作为电极, 电极直径为1 mm, 腔体采用的是树脂材料. 放电电极之间的间距为1 mm. 圆柱形放电腔体的直径为12 mm、高度为6 mm、体积为678.24 mm3. 有一个直径为2.5 mm的射流孔, 如图4所示. 电源采用KD-1高压脉冲电源[16], 最大输出电压为10 kV, 脉冲频率为1—50 Hz, 单次脉冲最大输出能量为20 J. 本次实验使用的放电电容为640 nF. 图 3 安装有等离子体合成射流激励器隔板在风洞中的位置 (a) 隔板在风洞中位置; (b) 激励器在隔板上的位置 Figure3. Schematic of the wind tunnel and the actuator mounted inside a plate: (a) Splitter plate in the wind tunnel; (b) actuator in the splitter plate.
4.数值仿真由于实验获取数据较少, 对电源的要求较高, 因而采用仿真手段进行研究. 研究高频激励器布置在不同位置对超声速混合层的影响效果. 仿真对象的射流出口的为大长宽比的窄缝, 当出口的长宽比大于1∶4的时候可以看作是二维, 进而可以使用二维仿真进行研究[29]. 分别对无扰动(unperturbation)、激励器在隔板上表面(up)、激励器在隔板尾端(end)、激励器在隔板下表面(bottom)四种工况进行仿真, 物理模型见图10. 模拟频率为5 kHz, 单次释放能量为150 mJ. 假设每次释放的热量相同, 假设布置在不同位置激励器释放的热量也相同, 使用Fluent 15.0的大涡模拟, 时间精度采用的是二阶隐式, 对流通量使用三阶AUSM离散, 空间项使用三阶MUSCL离散. 图 10 仿真物理模型 (a) 无扰动; (b) 激励器在隔板上表面; (c) 激励器在隔板尾端; (d) 激励器在隔板下表面 Figure10. Physical model: (a) Unperturbation; (b) the actuator at the upper surface of the splitter plate; (c) the actuator at the end surface of splitter plate; (d) the actuator at bottom surface of splitter plate.
图 14 (T0 + 180 μs)时刻NPLS结果与数值仿真密度场对比 (a) NPLS结果; (b) 数值仿真密度场 Figure14.T0 + 180 μs, contour of density and NPLS result: (a) NPLS result; (b) contour of density.
24.2.激励器位置不同对混合层的影响 -->
4.2.激励器位置不同对混合层的影响
图15所示为(T0 + 555 μs)时刻瞬时数值仿真密度场, 与未受扰动的工况对比, 可以看出这三个工况涡结构都有明显的增大, 扰动已经影响到了整个流场, 诱导出连续大尺度涡结构. 图 15 (T0 + 555 μs)时刻密度场 (a) 未受扰动; (b) 激励器在隔板上表面; (c) 激励器在隔板尾端; (d) 激励器在隔板下表面 Figure15. Contours of density at T0 + 555 μs: (a) Unperturbation; (b) the actuator at the upper surface of the splitter plate; (c) the actuator at the end surface of the splitter plate; (d) the actuator at the bottom surface of the splitter plate.
图16为(T0 + 75 μs)瞬时温度云图和流线仿真结果. 图16(a)中激励器在隔板上表面布置, 可以看出热气流喷出后, 形成一个虚拟型面将来流抬高, 周期性的射流喷出可以实现气流的上下摆动, 使得y方向速度脉动量增加, 有助于气流掺混均匀. 图16(b)中激励器在隔板的尾端布置, 可以看出等离子体合成射流喷出后直接作用在混合层的再附点上, 从而加快混合层失稳, 达到增强混合的效果. 并且由图16(a)和图16(b)这两个图可以推知, 由于在隔板尾端布置的激励器可以直接作用在混合层上, 因而混合层对在隔板尾端布置的激励器扰动响应最快. 图 16 (T0 + 75 μs)时刻温度云图和流线 (a) 激励器在隔板上表面; (b) 激励器在隔板尾端 Figure16. Simulation of the temperature and flow: (a) The actuator at the upper surface of the splitter plate; (b) the actuator at the end surface of the splitter plate
图18为激励器出口参数. 由于在隔板尾端外部压力较小, 喷出射流获得了较大的速度, 因此气体膨胀做功转化的动能较多, 因而出口的动量是这三个工况中最大的. 而在隔板上下表面布置的激励器, 射流与来流相互作用, 气体膨胀做功转化为动能较少. 但是由于上下两股气流的引射造成隔板尾端布置的激励器腔体内气体密度较小, 因而喷出的射流质量流量最小. 同时可以看出在隔板尾端布置激励器出口压力也小于激励器布置在隔板上下表面的工况. 图 18 激励器出口参数 (a) 激励器出口质量流量; (b) 激励器出口速度; (c) 激励器出口动量率; (d) 激励器出口压力 Figure18. The parameters of actuator outlet: (a) The mass flow rate of actuator outlet; (b) the velocity of actuator outlet; (c) the momentum rate of actuator outlet; (d) the pressure of actuator outlet.
图19为激励器腔体内参数. 图19(a)为激励器腔体内气体密度曲线图, 从图中可以发现, 腔体内密度随着放电次数的增加而逐渐降低. 这是由于算例设置的激励器腔体是绝热壁面, 做功过后腔体内温度难以降低, 腔体内维持一定压力造成外部气体内难以回吸. 这样就导致随放电次数增多, 激励器做功能力下降. 对于高频的激励器来说, 应该采用六方氮化硼陶瓷等导热能力强的材料作为激励器腔体, 或者采用冲压式激励器[32]. 图19(b)为激励器腔体温度曲线图, 在热源释放热量相同, 腔体体积相同的情况下, 腔体内气体温度的变化与密度成反比. 由于激励器布置在不同位置造成腔体内密度不相同, 因此温度变化也不相同. 在隔板尾端布置的激励器由于气体密度最小, 所以温度升高也最高. 图19(c)为激励器腔体内压力曲线图, 可见激励器布置在隔板上下表面的工况腔体内的最大压力是相同, 而在隔板尾端布置的激励器腔体内的最大压力小于其他两个工况. 这可能是由于上下两股气流的引射, 造成腔体内密度较小, 因而最大压力要小于其他两个工况的最大压力. 同时可以看出这三个工况达到峰值压力的时间差别不大. 图 19 激励器腔体内参数 (a) 激励器腔体密度; (b) 激励器腔体内温度; (c) 激励器腔体内压力 Figure19. Parameters of actuator cavity: (a) Density of the gas in the actuator chamber; (b) temperature of the gas in the actuator chamber; (c) pressure of the gas in the actuator chamber.