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

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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.