|Alternative Title||Dominant Influence and Flow Mechanics of Hypersonic Inlet Restart Capability|
|Thesis Advisor||张新宇 ; 岳连捷|
|Place of Conferral||北京|
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.
|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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