在超音速燃燒衝壓引擎之設計中，凹槽被視為重要之穩焰機構，故了解其對液態燃料噴注於超音速空氣流場之現象，是重要之工程設計基礎。本研究利用反射式震波風洞產生2馬赫之超音速自由流，並將液態燃料噴注於流場中不同相對凹槽位置。
本研究改變噴注速度及凹槽長深比(L/D)，藉由視流紋影法(schlieren photography)進行噴霧實驗觀察。實驗結果顯示，當於凹槽上游噴注燃料時，因L/D=3(開放型式，open-type)之凹槽較深，其剪切層發展無法深入凹槽(位置較高)，在凹槽內產生較大的迴流區，且其震波壓縮氣化區侷限於近後壁面；L/D=4.5(封閉型式，closed-type)之凹槽則因剪切層發展較深入凹槽內，迴流區略被分割成兩區域，但其震波壓縮氣化區較大，從凹槽底部延伸至後壁面；與平板實驗結果比對，顯示凹槽機構可提升噴霧之平均穿透高度，使噴霧更能深入流場。
為了瞭解震波交互作用對於噴霧之影響，本研究亦進行凹槽下游噴注實驗，利用封閉型凹槽內部產生之斜震波與噴霧所產生的弓形震波交互作用，觀察其對噴霧混合之影響。實驗結果顯示，震波交互作用會造成震波震盪以及流場不穩定性，進而幫助下游噴霧與超音速氣流混合；此外，改變噴注速度與凹槽底部角度 (δ=5°) 能使得震波交互作用位置較低，震波之間交互作用增強，進而加速燃料噴霧與氣流之混合，減短噴霧消散距離。

關鍵字：凹槽、剪切層、穿透高度、消散距離

SUMMARY

In scramjet engine combustor design, cavity is regarded as an important mechanism of flame-holding. This experimental research is conducted in a reflected shock tunnel providing Mach 2 supersonic air flow with cavity. Jp-4 fuel is injected laterally at different positions to justify the effects of cavity to fuel sprays.
Test conditions of various injected fuel velocity (V) and cavity L/D (length to depth ratio) are studied. Schlieren photography is used for visualization flow field and spray in cavity. Results indicate that for cavity with L/D=3 (open-type), the developed shear layer positions at a high position in cavity. A large recirculation region thus generates inside the cavity. However, a high temperature zone caused by shock wave compression is confined to the end wall of cavity that may allow the spray to vaporize. For cavity with L/D=4.5 (closed-type), the shear layer penetrates the cavity deeply and the recirculation region segmented into two regions; the spray gasification zone which caused by shock wave compression extends from cavity floor to rear wall. These results are then compared to the results with no cavity, and shows that cavity provides the spray with better average penetration height to let the fuel spray penetrates into core flow deeply.
For the purpose to study the effect of shock wave interaction on spray mixing, closed-type cavity downstream-injection experiment has also been conducted. Test conditions for injected fuel velocity and the angle of cavity floor are studied. In closed typed cavity, oblique shock waves are formed near the reattached point of shear layer, and separation shocks wave are also formed due to the flow separation at cavity rear wall. Those waves may interact with the bow shock induced by the fuel spray. The result of shock interaction strongly causes the bow shock to oscillate, the pressure field fluctuation, and enhances the instability of fuel spray. This phenomenon is beneficial to spray mixing. Moreover, changing the base angle of the cavity brings to a lower position of shock-shock intersection, which enhances the interaction among shock waves and shorten the dissipation distance of spray in the supersonic air flow.

Key words: cavity、shear layer、penetration height、dissipation distance

INTRODUCTION

Scramjet (supersonic combustion ramjet) provides vehicles a method of achieving hypersonic flight. However, the supersonic air flow passes through the engine combustor in a very short time(essentially milliseconds), that is, the mixing between air and fuel as well as flame-holding become significant issues in scramjet combustor design. The objective of this research is to understand the flame holding effect of cavity mechanism on laterally injected fuel spray in supersonic air flow, whereas the penetration height and the dissipation distance of the fuel spray are the index of comparison in analysis.

EXPERIMENTAL METHODS

The experimental study is conducted in a reflected shock tunnel which could provide Mach 2 supersonic air flow; the static pressure and temperature of air flow conditions are 1.1bar and 1200k, respectively. The cavity model with upstream JP-4 fuel injection and downstream injection are shown in Fig. 12 and Fig. 15, respectively. In addition, cavities (Fig. 13 and Fig. 14) with different L/D (length to depth ratio) are tested in upstream injection experiment, whereas models with (Fig. 16 and Fig. 17) two different base angle of cavity are tested in downstream injection experiment. Schlieren photography is adopted to provide the visualization of the flow field and fuel spray. The formation of shock waves induced by cavity and the fuel injection are observed.

RESULTS AND DISCUSSION

In order to understand the effect of cavity on flame-holding, the study started with upstream-injection experiment. Figure 29 shows the average penetration height distribution of L/D=3 cavity and flat plate at V=80m/s. It shows that cavity with L/D=3 possesses higher average penetration height than that with flat plate. The interaction between shocks induced by inner cavity supersonic flow and outer cavity supersonic flow causes fuel spray to oscillate, and increases the average penetration height. In figure 32, the fuel spray’s average penetration heights with L/D=3 cavity and flat plate at V=95m/s are given. It is clearly seen that penetration heights of L/D=3 cavity (open-type) are higher than that of simple injection.
Figure 35 and 38 show the average penetration heights of spray with L/D=4.5 cavity and flat plate at V=80m/s and 95m/s, respectively. It is seen that cavity with L/D=4.5 also produces higher average penetration height than that of flat plate. In addition, the difference is growing in evidence after the cavity section. This indicates the interaction between shocks produced by either closed-type or open-type cavities brings about higher penetration heights of fuel spray.
In order to study the effect of shock-shock interaction on mixing, close-type cavity downstream injection experiment has also been performed. Figure 44 shows the flow field images of the first derivative of intensity of flat plate injection spray and with cavity of δ=0° (flat floor) at V=80m/s. It indicates that the spray dissipation distance of simple injection and with cavity are 72mm and 62mm, respectively. As the supersonic flow over closed-type cavity, the shear-layer reaches to the cavity floor to induce oblique shocks. Separation shock waves are also formed due to flow separation near the cavity rear wall. The interaction among the oblique shock, separation shock, and the bow shock from fuel injection causes the bow shock to oscillate and induces downstream pressure field fluctuation. This enhances spray instability to have a better mixing with air flow and shorter its dissipation distance than flat plate. In figure 46, it can be seen that the dissipation distances of simple fuel injection at V=95m/s and with cavity of δ=0° are 87mm and 59mm, respectively.
The comparison with flat floor cavity and angled floor cavity at V=95m/s is shown in figure 53. The results reveal that cavity with δ=5° produces shorter dissipation distance than cavity with δ=0° (flat floor) because that the angled floor cavity induces a lower position of shock-shock intersection which enhances the pressure oscillation as well as the mixing between fuel and air at the downstream.

CONCLUSION

From the above discussions, it concludes that
(1)For the upstream-injection experiments, open-type cavity and closed-type cavity cause fuel spray oscillation; both of them possess higher average penetration height than that without cavity.
(2)For the downstream-injection experiments, the shock wave interaction induced by closed-type cavity benefits fuel spray mixing.
(3)The angled floor cavity provides shorter dissipation distance than the flat floor cavity owing to the lower position of shock-shock intersection which enhances the pressure fluctuation downstream.

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