摘 要
本研究探討單流體微型噴嘴之霧化機制及其霧化特性，微型噴嘴孔口直徑為50 μm 、100 μm、150 μm及200 μm等，工作流體為純水。研究項目包括微液柱之暫態演變過程及其穩態破裂過程，並探討微液柱在高壓下，共振效應對其霧化特性之影響。所探討的參數包括噴嘴出口孔徑、共振腔高度、共振腔直徑、多孔噴頭孔數及孔距等。微液柱之暫態演變過程及其穩態破裂過程以IDT公司之高速攝影機拍攝，噴霧粒度及粒度分佈以Malvern公司雷射繞射式RT-Sizer粒徑分析儀量測。研究結果顯示，由於微流體表面張力之作用甚大，液體先在微噴嘴出口處形成半月型之液膜，此液膜隨時間成長，液體慣性力逐漸遞增，並與液體表面張力產生強烈交互作用，故液體從半月型之液膜演變為球狀體，再拉伸成為具錐體、頸部及頭部之複雜結構，最後產生破裂現象。在相同噴射壓力下，當微噴嘴之孔口直徑增大，液體之慣性力相對增加，液柱因為拉伸而變細，故不穩定拉伸變形及破裂過程之時間亦縮短。研究結果亦顯示，微液柱破裂長度及其破裂機制隨液體雷諾數而變。當液體出口雷諾數小於392時，微液柱之破裂機制主要為軸對稱不穩定模態，破裂長度隨雷諾數增加而遞增。當液體出口雷諾數392＜Re＜635時、微液柱之破裂機制從軸對稱不穩定模態轉變成為螺旋不穩定模態，此時液柱破裂長度隨雷諾數增加而遞減。另外，當液體出口雷諾數進ㄧ步增加到Re ≧ 635時，微液柱之破裂機制從螺旋不穩定模態再次轉變成液柱擺動模態。
研究結果亦顯示，微型噴嘴之出口孔徑大小為噴霧產生率及噴霧粒度之主要控制參數。噴霧產生量之增加率與噴嘴出口孔徑成二次曲線關係。噴霧粒度隨噴嘴出口孔徑增加而遞增。例如在液體壓力125 bar下，噴嘴孔徑為100 μm、150 μm及200 μm時，噴霧產生率分別為3.95 kg/hr、8.4 kg/hr及12.33 kg/hr，而其噴霧平均粒徑則分別為6.65 μm、9.42 μm及11.53 μm。另外，多孔噴嘴之孔數為噴霧產生率之控制參數。在液體操作壓力為125 bar下，微型噴嘴孔徑為100 μm時，若從單孔變為雙孔時，噴霧產生率從3.95 kg/hr增至6.91 kg/hr，但是噴霧粒徑並未隨孔數增加而有太大之變化，故知此微型噴嘴可以在保持相同之噴霧品質下，以增加噴孔數來控制噴霧產生率。此外，多孔噴頭之孔距亦為改變噴霧特性之重要參數，以雙孔配置之多孔噴頭，噴孔孔距與噴嘴孔徑比為10時，可獲得較微細之噴霧，顯示在多孔微噴頭結構下，微噴霧間之交互作用，具有改善霚化特性之效果。

Abstract
Atomization performance of pressure type micro-atomizers with resonant effects is investigated in this research program. The evolution of the micro-jet during the transient break-up processes was studied using IDT-high speed video camera. The particle size distribution was measured by Malvern RT-Sizer. Results showed that the liquid first formed a meniscus around the outlet of the orifice because of the higher surface tension effects associated the micro-injector. The meniscus was transformed to the spherical shape when the inertia force was increased with time. The spherical shape was then stretched to a column as the inertia force was further increased. Finally, the liquid jet became quite unstable and formed a conical shape near the orifice, a cylindrical column as the neck and a spherical head in the down stream. The above unsteady processes were preceded in a short time as the diameter of the nozzle was increased due to the increased inertia of the liquid column. Results also showed that the intact-length of the micro-jet increased with Reynolds number as Re＜ 392. However, the intact length reached a maximum and then began to decrease as Reynolds number was further increased. The instability modes of the liquid jet changed from the axi-symmetric mode to helical mode as Reynolds number reached 392. The helical instability was transformed to the sinusoidal motion as Reynolds number was further increased (i.e., Re＞635). It was also found that the diameter of the orifice was the control parameter of the spray production rate and the particle size. The spray production rate increased with the orifice diameter to the second power. The particle size of the spray also increased with the orifice diameter. For example, under the injection pressure of 125bar, the spray production rates were 3.95 kg/hr, 8.4 kg/hr, and 12.33 kg/hr and mean particle sizes were 6.65μm, 9.42μm, and 11.53μm for the cases of orifice diameters of 100μm, 150μm, and 200μm, respectively. The spray production rate was further increased with the multiple-orifice design. For example, under the liquid infection pressure of 125bar and the orifice diameter of 100μm, the spray production rate increased from 3.95kg/hr to 6.91 kg/hr as the number of orifice was doubled. However, the particle size of the spray was insensitive to the change of the number of orifice. Hence one may control the spray production rate by increasing the number of orifice without degrading the quality of the spray. The pitch of the multiple orifice design also affected the atomization performance. Ultra-fine spray could be obtained as the ratio of the pitch to orifice diameter was more than 10. It turned out that the interaction between the spray jets with the multi-injection design could be used to enhance the atomization processes of the micro-jet.

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