雙基液體火箭推進中，液體-液體同軸渦旋噴注器具有混合效率高、空間利用佳與製程成本較低之優點，為液氧/煤油推進系統之經典構型。本研究參考蘇俄聯盟號 （Soyuz） 運載火箭第三級 RD-0110 引擎之同軸雙渦旋噴注器，以退縮距離 （Recess length, L） 為核心設計變數，設定了 L = 1.5 mm、L = 2.5 mm 與 L = 5.0 mm三種混合模式，探討其對噴注流場、霧化與混合特性之影響。
本研究自主設計並加工該噴注器，分別以陰影法、垂直平面雷射誘導螢光 （PLIF）與水平 PLIF，於常溫流體（水）及超低溫流體（液態氮與乙醇）兩種工作流體、以及單開內、單開外與內外同開等開閥方式下，觀測並量化噴霧之破碎長度、霧化角與混合效率。
於常溫流體實驗中，退縮距離決定內、外層液膜之撞擊位置與混合模式。且內、外層耦合時外層剪切會縮短內層破碎長度，並使內、外層噴霧趨於合流。混合效率隨退縮距離由 50.65 % 提升至 84.71 %，其主要提升發生於外部混合轉為尖端混合之過程；由尖端混合至內部混合仍有提升，但增幅較小。
於超低溫實驗中，本研究以液態氮 （LN_2）作為液態氧之模擬流體、外層燃料側採 95 % 乙醇，於約 90 psi、−165 °C 之穩定噴注條件下進行觀測。結果顯示，液態氮一離開噴注器後即破碎，未見常溫水所具有之連續液膜；其霧化角整體偏大，且於單開內時隨退縮距離增加而增大（並於尖端混合達最大），恰與常溫水相反（常溫水之內層霧化角於L= 5.0 mm 因退縮區之徑向約束而驟降至 15.5°），推測為液態氮極低之表面張力與黏度，噴出後之快速蒸發與閃蒸膨脹凌駕了退縮區之幾何約束。至於內外同開時，外層乙醇之連續環狀液膜被內層高速噴出之液態氮剪切、破碎為離散液滴而不再維持原始之連續液膜，整體霧化角由外層離心擴張與內層中心低壓回流之動量平衡所決定；由於三種退縮構型之混合比固定不變，該平衡不隨退縮改變，故霧化角穩定維持於約 98°~103° 而對退縮距離不敏感。綜上所述，退縮距離對噴霧之影響會隨工作流體之物性而改變，常溫冷流之結果（尤其霧化角）不宜直接外推至超低溫或真實推進劑條件，亦凸顯本研究進行超低溫噴注觀測之必要性與價值。

In bipropellant liquid rocket propulsion, the liquid–liquid coaxial swirl injector offers high mixing efficiency, compact use of space, and relatively low manufacturing cost, making it a classic configuration for LOX/kerosene propulsion systems. Referencing the coaxial bi-swirl injector of the RD-0110 engine—the third stage of the Soviet Soyuz launch vehicle—this study adopts the recess length (L) as the core design variable, setting three mixing modes at L = 1.5, 2.5, and 5.0 mm to investigate their effects on the injection flow field, atomization, and mixing characteristics.
The injector was designed and fabricated in-house. Using shadowgraphy, vertical planar laser-induced fluorescence (PLIF), and horizontal PLIF, the breakup length, spray angle, and mixing efficiency of the spray were observed and quantified under two working fluids—room-temperature fluid (water) and cryogenic fluids (liquid nitrogen and ethanol)—and under inner-only, outer-only, and combined injection modes..
In the room-temperature experiments, the recess length determined the impingement location of the inner and outer liquid films and thus the mixing mode. When the inner and outer flows were coupled, the shear from the outer flow shortened the inner breakup length and drove the inner and outer sprays toward merging. The mixing efficiency increased from 50.65% to 84.71% with increasing recess length; this improvement occurred mainly during the transition from external mixing to tip mixing, while from tip mixing to internal mixing, the efficiency still increased but with a smaller increment.
In the cryogenic experiments, liquid nitrogen (LN_2) was used as a surrogate for liquid oxygen, with 95% ethanol on the outer fuel side, and observations were carried out under stable injection conditions of approximately 90 psi and −165 °C. The results showed that, upon leaving the injector, LN_2 broke up immediately, without the continuous liquid film seen for room-temperature water. Its spray angle was generally broad and, under the inner-only condition, increased with increasing recess length (reaching its maximum at tip mixing), opposite to the trend for room-temperature water (whose inner spray angle drops sharply to 15.5° at L = 5.0 mm due to the radial confinement of the recess region). This is attributed to the extremely low surface tension and viscosity of LN_2, whose rapid evaporation and flash expansion after injection override the geometric confinement of the recess region. Under the combined injection condition, the continuous annular film of the outer ethanol was sheared and broken into discrete droplets by the high-speed inner LN_2 jet, no longer maintaining its original continuous film; the overall spray angle was instead governed by the momentum balance between the outer centrifugal expansion and the inner central low-pressure recirculation. Because the mixture ratio was held constant across the three recess configurations, this balance did not vary with recess, so the spray angle remained stable at about 98°–103° and was insensitive to the recess length. In summary, the effect of recess length on the spray varies with the physical properties of the working fluid; ambient cold-flow results—particularly the spray angle—cannot be directly extrapolated to cryogenic or real-propellant conditions, which further underscores the necessity and value of the cryogenic injection observations conducted in this study.

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