Study on Cavitation Flow in a Nozzle Hole and Its Effect on Spray Characteristics

通过实验研究了喷嘴孔内空化气泡的行为对液体射流破碎过程的影响。研究了喷注液过热对喷嘴孔内空化气泡产生的影响。通过孔径为0.5尼米的喷嘴将液体连续注入大气中。在喷嘴上安装超声换能器，检测空化气泡产生、发展和破裂的声强。在喷嘴孔壁上安装一对细电极，测量液体沿喷嘴孔横截面的阻抗。用电阻法测量了液体射流的破裂长度。对液体射流的破碎过程进行了显微观察。主要研究结果总结如下。(1)超声换能器检测空化气泡的发生、发展和液压翻转等行为。(2)液体在喷嘴孔内的阻抗检测液体流面与喷嘴孔壁的附着和脱离。(3)当喷射压力略低于液压翻转压力时，液体射流上的波结构非常复杂。(4)水力翻转发生时，观察到平行于喷嘴孔轴的波浪。据推测，这些波与喷嘴孔中液体流动的再附着线有关。(5)注入过热液体使喷雾分散性很大，破碎长度很小。(6)过热度对喷雾特性的影响在长孔喷嘴中比在分选孔喷嘴中更为明显。

An experimental study was made to clarify the effects of cavitation bubbles' behavior in a nozzle hole on the breakup process of a liquid jet. Effects of superheat of the injection liquid, which promoted the onset of the cavitation bubbles in the nozzle hole, were also investigated. The liquid was injected continuously into the atmospheric air by the nozzle whose hole diameter was 0.5 nun. An ultrasonic transducer was attached to the nozzle to detect the acoustic intensity due to the onset, develop and collapse of the cavitation bubbles. Impedance of the liquid along the cross section of the nozzle hole was measured by a pair of thin electrodes which were installed in the nozzle hole wall from the outside. Breakup length of the liquid jet was measured by an electric resistance method. Microscopic observation was made of the breakup process of the liquid jet. Main results are summarized as follows.(1) The ultrasonic transducer detected the behaviors of the cavitation bubbles, such as the onset, develop and hydraulic flip.(2) The impedance of the liquid in the nozzle hole detected the attachment and the detachment of the liquid flow surface to the nozzle hole wall.(3) Waves on the liquid jet shows a very complicated structure when the injection pressure was a little lower than the hydraulic flip pressure.(4) Waves parallel to the nozzle hole axis were observed when the hydraulic flip occurred. These waves were supposedly related to the reattachment lines of the liquid flow in the nozzle hole.(5) Injection of the superheated liquid made the dispersion of the spray very large and the breakup length very small.(6) The effect of superheat on the spray characteristics is more dominant for the longer hole nozzle than the sorter hole one.