同軸式噴注器多用於大推力之液態火箭引擎，主要功能為霧化燃燒所需之液態氧化劑與氣態燃料。本研究採用光學技術探討影響同軸噴流破碎及霧化混合之相關參數，如速度比(gas to liquid velocity ratio, varied from 2.9 to 60)、氣層厚度(gas layer thickness, varied from 1.2 mm to 2.4 mm)及液體的物理性質(表面張力及黏滯係數)。在實驗分析方面，本研究採用霧化角、平均粒徑、核心粒徑等指標並配合高速攝影觀察噴流之破碎機制、過程等，藉此分析相關參數對同軸噴流於不同實驗條件設定狀態下之影響。
實驗結果顯示，外圍高速氣流是影響此同軸流場的重要因素，氣層除限制同軸噴流的空間分布外，主要為增強液柱本身的不穩定性使其進入螺旋式運動(spiral-type motion)，液柱於此運動中拉伸並形成帶狀液膜(ligament)而破碎。對於中低速度比之噴流來說(即VR<15)，當液注破碎後進入次霧化區時(secondary breakup region)，此時外圍氣流與液滴的速度差已不足以使液滴再次破碎，氣體對霧化後的流場本身無顯著的影響。在這樣的機制控制下之同軸噴流，其霧化角會隨著氣液間速度比的提升而增加，並隨氣層厚度增加而下降。破碎霧化後的同軸噴流之平均粒徑呈現均勻、中心較集中且穩定的分布。
於使用低表面張力液體之同軸噴流中可觀察到大量呈纖維狀分支之液膜。高速氣流之剪力作用於纖維狀液膜，可使更多細小的粒滴破碎並脫離液柱表面，此機制使噴流的平均粒徑下降。增加工作流體的黏滯係數對於同軸噴流有強烈之影響，高黏滯係數液體本身之聚合力可在螺旋運動狀態下之帶狀液膜破碎前，使其向外伸展而穿透氣液交界，液柱可藉由此機制增加高速氣流與液柱表面間之作用面積，進而更有效的利用高速氣流協助其霧化。故在高黏滯係數液體的同軸噴流中可觀察到較大的霧化角及較小的平均粒徑及核心粒徑。

Coaxial injector is mainly used in large-scale liquid rocket engines. By utilizing optical methods, this research studies the effects of gas to liquid velocity ratio (2.9 to 60), gas layer thickness (1.2 mm to 2.4 mm), and liquid physical properties (surface tension and viscosity) on the breakup and atomization of coaxial liquid jet spray. The spray angles, droplet size (SMD, SMD0.1) distributions as well as the breakup processes at different experimental conditions are observed and analyzed.
The results reveal that the significance of the breakup of coaxial jet spray is the surrounding high speed air confines the spray spatially, and forces the inherent unstable liquid column into spiral-type turning, then elongates into ligament before instantaneous breakup. Once the liquid jet has broken up, the outer high speed air appears to have no further effect on the spray. Due to this controlling breakup process, the spray angle appears to be increased with increasing velocity ratio but decreased with increasing gas layer thickness, and the droplet size distribution of coaxial jet spray appears to be uniform, concentrated, and invariant after jet has broken up.
Coaxial jet with a lower liquid surface tension apparently shows more branching fiber growth from the liquid column surface that induces smaller droplets stripping from main wavy jet. Increasing liquid viscosity in this study shows an intense effect on coaxial jet spray. With higher viscosity, the liquid ligament is stretched wider to penetrate the liquid and gas boundary before disintegration. Hence, the liquid ligament could utilize more energy of surrounding high speed air for liquid atomization, thus to produce larger spray angle and smaller droplet sizes as observed in the experiments.

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