超音速燃燒衝壓引擎(Supersonic Combustion Ramjet, Scramjet)為一種高效率推進系統，本身僅需攜帶燃料不用額外的助燃劑，重量大幅減輕，是未來航空科技之重要研究課題。Scramjet周圍流場為高稀薄度之流場，在此區域連續體的假設將不成立，須以分子觀點來討論，其中直接模擬蒙地卡羅(Direct simulation Monte Carlo, DSMC)法為目前被認為是較精確且使用最廣泛的方法。 本文利用DSMC法模擬Scramjet之極音速稀薄氣體流場。飛行器飛行於低密度高空流場時，進氣道入口分子會受到外緣震波作用造成性質改變，進而影響引擎內流場之結構。而當流場稀薄度增加時，變化亦可能由下游延伸至上游。探討不同稀薄度之自由流在極音速流場下所呈現的物理現象。針對氣流流經不同突張結構之內流場所產生之特性(流場內震波與邊界層互相作用、階梯結構後方產生的膨脹波等)加以討論與分析。最後模擬三維引擎流場，並比較二維與三維結果差異。 Supersonic combustion ramjet (Scramjet) is a high-efficiency propulsion system. Scramjet only needs to carry fuel not to bring booster, the weight of vehicle reduces greatly. It is an important topic of aerospace sciences in the future. Because the flow around the scramjet is high-rarefaction, the assumption of continence isn’t valid. Hence, we have to discuss flow fluid by molecular model. In molecular gas dynamics, the direct simulation Monte Carlo (DSMC) method is a popular and accuracy simulation technique for low density flows. Hypersonic rarefied gas flows around a Scramjet inlet model have been analyzed using DSMC method. In the flows around vehicles operating at high altitudes, the molecules at the entrance and the flows properties inside the engine are affected by the shock wave. As the rarefaction of the flows field becomes increasingly, the effect may extend from downstream to upstream. This article discusses the physical phenomenon of different rarefied hypersonic flow field. We analyze the properties, including the shock-boundary interaction and expansion effect, of the gas flow in different sudden expansion structure. Finally, the results of three-dimensional scramjet flow are compared with those of two-dimensional scramjet flow.
