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高超声速进气道再起动特性影响规律研究
英文题名Dominant Influence and Flow Mechanics of Hypersonic Inlet Restart Capability
贾轶楠
导师张新宇 ; 岳连捷
2018-05-25
学位授予单位中国科学院大学
学位授予地点北京
学位类别博士
学位专业流体力学
关键词高超声速进气道、再起动、激波-边界层相互作用
摘要

高超声速进气道再起动性能是评估超燃冲压发动机的重要参数之一。早期Kantrowitz基于理想气体条件、进气道内收缩段入口处存在正激波、进气道内流动为一维等熵流动、喉道处为声速的假设,提出对进气道再起动性能进行预判的Kantrowitz极限。但由于边界层的存在,实际进气道再起动过程的与Kantrowitz极限的假设条件存在很大差异。本文通过激波风洞实验,着眼进气道内收缩段激波边界层干扰相关要素,发现进气道内波系结构、唇口截面气动参数、壁面温度以及压缩方式对进气道的再起动性能存在显著影响,并将不同影响因子解耦,获得其对进气道再起动性能的影响规律,进一步通过数值仿真揭示再起动过程的流动机理,为高超声速进气道的工程设计提供理论依据。

论文实验观察并定义了进气道再起动过程稳定后存在的三种流态:起动流态,不起动流态以及临界流态。在临界流态下,进气道唇口处无溢流,但大尺度分离区及其诱导的斜激波无法通过喉道,而是稳定在进气道内压缩段,进气道的流量捕获与起动流态的流量捕获相同但总压恢复能力大幅度下降。

二元高超声速进气道内的波系结构对再起动性能存在主导影响,随唇罩激波强度的增加,进气道再起动极限收缩比减小,性能曲线存在突降现象。针对这种特性,论文设计了唇罩分级压缩改善进气道再起动能力,并进行了验证实验。唇口截面马赫数的增加除有效提升进气道的再起动性能外,还使激波强度-再起动性能曲线的突降向大角度方向移动。同时,唇口马赫数的增加使进气道再起动性能的突降发生在更宽的压缩角度范围内。进气道肩点膨胀波对再起动能力存在促进作用。对于大唇口气流偏转角度的工况,分离区同侧的膨胀波可以加快当地气流速度,降低局部压强,促使大尺度分离区向下游的移动,明显提升进气道的再起动性能。

对于弱唇罩压缩激波工况,进气道再起动性能不再由唇罩激波强度单一主导,唇口截面边界层相对厚度的作用开始显现,且唇口截面边界层相对厚度达到特定值后,边界层厚度的增加使进气道再起动性能出现突降。

论文通过数值模拟考察了壁面温度对进气道再起动性能的影响。发现对于唇罩压缩激波较强的进气道,冷却壁面无法改善进气道的再起动能力。但对于唇罩压缩激波相对较弱的进气道模型,壁温总温比Tw/Tt的减小会诱导进气道再起动性能发生突变。且随着唇罩角度的减小,进气道再起动性能突变向高无量纲壁面温度Tw/Tt的方向移动。随唇口截面边界层厚度的减小,进气道再起动性能突变向高壁温方向移动。

对于唇罩激波强度、唇口截面边界层厚度、壁温总温比对进气道再起动性能的突变现象,论文通过进一步数值计算发现,进气道再起动性能突降前,进气道可以通过的最大流量由几何喉道约束。但对于突降后性能较差的进气道,其再起动过程中内压缩段的大尺度分离区形成气动喉道,代替几何喉道限制了进气道可以通过的最大流量。几何喉道约束-气动喉道约束的转换造成了进气道再起动性能的突降。同时,研究发现突变区域往往伴随大范围的临界流态的出现。

唇罩压缩方式通过改变压缩激波与底板边界层的相互作用模式影响进气道的再起动能力。由于侧压后掠激波与底板边界层干扰中三维效应的影响,进气道再起动性能规律与顶压式进气道存在明显差异。对于由几何喉道决定能都实现再起动的进气道,压缩方式对进气道再起动性能的影响不明显。对于由压缩激波-边界层干扰诱导分离区形成的气动喉道决定能否再起动的进气道,唇罩压缩变为侧板压缩能有效的提高进气道的再起动性能。

英文摘要

The restart capability of hypersonic inlet is a crucial factor of scramjet. In the early studies, Kantrowitz proposed a theoretical model for supersonic inlet restart based on the assumptions that a normal shock wave stands at the cowl lip station and the quasi-steady, one-dimensional, isentropic internal flow has a sonic condition at the inlet throat. However, owing to the existence of boundary layer, the practical flow pattern in the hypersonic inlet differs a lot from Kantrowitz’s assumption. In this dissertation, wind tunnel experiments and numerical simulations were done to further understand the unsteady flow pattern during the inlet restart process. It was found that the wave pattern in the inlet, the flow conditions at cowl lip station, wall temperature and how the captured air compressed were key factors of inlet restart capability. These key factors were decoupled and the influence rule was obtained. Further analysis was done and the design to improve the inlet restart capability was put forward. 

Three flow patterns were observed after the inlet restart process, namely, restart flow pattern, un-restart flow pattern and transitional flow pattern. When the inlet restart process ends up in transitional flow pattern, the separation-induced shock does not pass through the throat. However, the shock finally stands in the internal contraction section and impinges on the cowl, implying that the inlet mass capture may be the same as the started inlet. Owing to the loss in the separation bubble and the separation shock in the transitional mode, the inlet cannot take in sufficient air with high total pressure to work properly in the combustor.

The wave pattern in the inlet shows strong effect on the inlet restart performance. With the increase of cowl angle, the restart Maximum ICR decreases with a sudden change domain. Based on the results of the cowl shock effect, a design concept of multiple noncoalesced cowl shock waves was proposed for a large cowl turning angle to improve the inlet restart by reducing the strength of cowl shocks. The increase of Mach number at cowl lip station can not only enhance the inlet restart capability, but also shifts the sudden change domain of cowl angle to larger cowl angle direction. The sudden change of Maximum ICR occurs in broader horizontal axis as Mach number increases. When the air flow deflection angle is relatively large, the expansion wave originated from the shoulder accelerates the local flow velocity and decreases the static pressure, which promotes the separation to move downstream. Thus, the inlet restart capability can be enhanced. 

When the cowl shock strength is relatively weak, the boundary layer thickness at cowl lip station starts to emerge. The experiment shows that when the relative boundary layer thickness reaches a certain value, the inlet restart capability drops suddenly with the increase of boundary layer thickness.

Numerical simulations were performed to study the effect of wall temperature on  inlet restart capability. For the inlet with strong cowl shock, cooling wall has little influence on the inlet restart performance. However, for the inlet with relatively weak cowl shock, the inlet restart Maximum ICR increases significantly as Tw/Tt decreases. With the decrease of inlet cowl angle, the sudden change domain shifts to higher Tw/Tt. As relative boundary layer thickness rises, the sudden change domain of Maximum ICR shifts to higher Tw/Tt.

It is noted that the sudden change domain exists where the Maximum ICR drops sharply as cowl angle, the relative boundary layer thickness at cowl lip station and Tw/Tt alters. Further analysis reveals that, for the inlet with bad restart performance, the separation bubble inside the contraction section is so large that an aerodynamic throat forms and prevents the separation bubble from being swallowed. While the inlet with Maximum ICR before the capability’s sudden drop can restart as long as the inlet geometric throat can get through all the captured flow. Accordingly, the reason of the inlet restart capability’s sudden change is the transformation from inlet aerodynamic throat choke to inlet geometric throat choke. Transitional phenomenon was observed in the sudden change domain.

The way that the captured air compressed changes the way compression shock-boundary layer interacts. Thus, the capability of inlet restart is influenced. Due to the obvious three-dimensional structure of the separation induced by the swept shock, the inlet restart performance of sidewall-compression inlet differs from that of cowl-compression inlet. For the un-restart inlet caused by geometric throat choke, the method of sidewall-compression shows little effect on inlet restart capability. For the un-restart inlet caused by aerodynamic throat choke, sidewall-compression can enhance the inlet restart capability effectively compared to cowl-compression.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73160
专题高温气体动力学国家重点实验室
作者单位中国科学院力学研究所
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贾轶楠. 高超声速进气道再起动特性影响规律研究[D]. 北京. 中国科学院大学,2018.
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