本文以數值方法在對稱式背向階梯燃燒室中，探討噴嘴之位置及收縮比對流場之影響。利用模擬軟體FDS( Fire Dynamics Simulator )及大渦漩模擬法（Large Eddy Simulation）搭配Smagorinsky次網格黏滯應力模式( SGS )對流場的穩態、暫態流況及定量上的性質變化做一模擬分析並與實驗文獻所得之結果作一比較。當數值使用之入口假設條件所獲得之結果與實驗流場使用之入口條件所得結果比對後，顯示數值與實驗在長短邊再接觸長度上有明顯差異。由於數值之入口流發展為層流狀態，實驗之入口流則已為紊流狀態，因此形成誤差之存在。此差異亦呈現出層流狀態之再接觸長度較紊流狀態之再接觸長度來得長。為消除此誤差可利用改變入口流狀態以獲得改善。 穩態分析中，未加噴嘴之突張流場在短邊迴流區下游之三維壁面出現第三迴流區。加裝噴嘴則會造成短邊迴流區與第三迴流區的結合形成較對稱之流場形態。水平平均速度、垂直平均速度與紊流強度則因入口流狀態與實驗之差異，在階梯後方產生不同之分佈趨勢。 暫態分析中，未加噴嘴之突張流場的壓力擾動頻率較為雜亂。加裝噴嘴則使的壓力擾動頻率有一明顯之的頻率出現。噴嘴之收縮比會使流場壓力擾動頻率有一明顯之主頻，噴嘴位置則會改變流場壓力擾動之頻率。利用數值可視化觀察發現渦流之配對與流逸發生在流場兩低壓區之間，壓力擾動之頻率則影響渦流之配對與流逸。透過三維流線與壓力之切面觀察得知角落渦漩之產生是來自第三維兩側壁面流體迴流至階梯角落所造成且其迴流與迴流區內之壓力變化有關。 This study presents the numerical simulation of a backward-facing step combustor with nozzle, results of influence of the contract ratio and position of nozzle in sudden expansion flow were shown in this thesis. Utilizing FDS ( Fire Dynamics Simulator ) and LES( Large Eddy Simulation )with subgrid scale (SGS) viscosity stress of Smagorinsky model to simulate the steady flow, the transient flow and to quantitatively analyze then compared with the results of the document of the experiment. When the numerical results using the condition of the inlet flow of experiment compared with the experiment results, the results show that the short and long reattachment length of the simulation have obviously difference with the experiment. The reason is that the inlet flow of the simulation is laminar flow but turbulent flow is in the experiment, so the error is produced by difference between conditions of the inlet flow. The difference between the simulation and the experiment also show that the reattachment length of laminar flow is more long than turbulent flow. By changing the condition of the inlet flow will reduce to the error and improve the result. The analysis of the steady, sudden expansion flow without nozzle appears to the third recirculation region at the downstream zone of short recirculation region in three-dimensional side. After installing the nozzle, the third recirculation region combine into the short recirculation region, leading to the flow which is more symmetrical . The distribution of horizontal average velocity , vertical average velocity and turbulent intensity behind step in the simulation are different from that in experiment, since the conditions in the inlet flow state of experiment and simulation are not the same . The analysis of the transient, the fluctuant frequency of pressure of sudden expansion flow without nozzle is disorderly . After installing the nozzle , the fluctuant frequency of thepressure appears to a obvious peak. The contract ratio of nozzle can make fluctuant frequency of pressure have a fixed value . The position of nozzle can change the fluctuant frequency. The combination and flowing of the vortex appears Between two low-pressure zone by numerical visualization. the fluctuant frequency of pressure influences combination and shdding of the vortex .