本研究以實驗方法探討二氧化碳雪花噴霧結構及其暫態演變過程，並且探討不同二氧化碳雪花形成腔體對二氧化碳雪花特性之影響。實驗結果顯示，二氧化碳雪花噴流在形成的過程，會經歷霧化、蒸發、凝結等暫態演變過程，所形成之二氧化碳雪花噴流具有雙層結構，其內層結構是由低溫二氧化碳氣相與固相所組成之雪花流體，而外層結構則為環境中的水氣凝結之水霧結構。因為高壓二氧化碳流體首先在雪花形成腔體內產生降壓及相變化，形成氣、固兩相混合之二氧化碳雪花流體。當此氣、固相混合流體由雪花形成腔體噴出後，復由於雪花粒子在噴流中之昇華現象以及雪花粒子間之碰撞結合，會改變噴流中二氧化碳雪花之粒徑大小、粒子濃度及其空間分佈。實驗結果發現，當二氧化碳雪花形成腔體長度增加，二氧化碳雪花粒子濃度有上升的趨勢，這是因為在較長的雪花形成腔體中，二氧化碳流體有較長的時間完成相變化及雪花成核過程。實驗結果亦顯示雪花形成腔體長度並不影響大顆粒雪花之產生，但會影響小顆粒雪花之數量。當雪花形成腔體長度增加，二氧化碳雪花之小顆粒數量隨之遞增，可以運用於電子微製程之清洗應用上。另外，雪花形成腔體直徑大小對雪花特性亦有相當大之影響，當雪花形成腔體直徑增加，二氧化碳雪花粒子濃度隨之遞增，且其噴射過程穩定，這是因為當雪花形成腔體管徑變大，流體速度降低，因此二氧化碳在腔體內的滯留時間變長，使得高壓液態二氧化碳流體有較長的時間完成相變化及雪花成核過程。但若雪花形成腔體直徑太小，則在雪花形成腔體內易產生氣阻現象，導致雪花呈現間歇性噴流。二氧化碳供應壓力亦會影響二氧化碳雪花粒徑之分佈。若噴射壓力較高時，所形成之二氧化碳雪花粒徑分佈較均勻，且操作穩定性較佳，故壓力亦為控制二氧化碳雪花粒徑分佈之重要參數。

This paper investigates the formation processes and the effects of the design parameters on the characteristics of the CO2-snow jet. Liquid-CO2 was first supplied to a CO2-snow formation chamber through an orifice. The pressure and phase changes in the CO2-snow formation chamber resulted in the production of a mixture of gaseous CO2 and solid particles of dry ice. The solid particles were called CO2-snow. The mixture was then injected from the CO2-snow formation chamber to form a CO2-snow jet. Result showed that the CO2-snow jet had the double layer structure. The inner layer was formed by the cryogenic CO2 snow. The outer layer was formed by the water drops which were condensed due to the heat transfer to the cryogenic CO2 layer. The sublimation and the collisions between the CO2 particles in the jet flow resulted in the change of the particle size, concentration and distribution of the CO2-snow. Result showed the concentration of the CO2-snow particles was increased as the length and diameter of CO2-snow formation chamber were increased because of the increase in residence time of the CO2-flow in the formation chamber. It indicated that the CO2-flow needs enough time to complete the phase change and the nucleation processes of the CO2-snow particles. It was also found that the change of the length of the CO2-snow formation chamber affected the production of the small particles. But the large particles were less affected. As a result, the amount of the smaller particles was increased as the length of the CO2-snow formation chamber became longer which is useful in the cleaning processes of the electrical production processes. Result also showed that the change of the diameter of the CO2-snow formation chamber affected the stability of the CO2-snow jet. It turns out that the operation of the CO2-snow jet was more stable as the diameter of the snow formation was increased. The phenomenon of chocking in the CO2-snow formation chamber could be avoided as the diameter of the chamber was increased. It was also found that the pressure of the liquid-CO2 would affect the particle distribution of the CO2-snow. The particle size became more uniform and operation was more stable under high pressure injection. Hence the adjustment of injection pressure of the CO2 liquid is important to control the particle size of the CO2-snow jet.

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