Research On Dynamic Characteristics Of Laser-induced Bubbles In Venturi

The bubble is a complex fluid dynamic phenomenon. The collapse of bubble will produce high temperature and pressure, and result in a strong shock wave and micro-jet in a very short period of time. The bubble can be used to drive microfluidic device, accelerate the rate of chemical reaction and remove harmful cells, which indicates it can be widely used in microfluidic device technology, biomedical and chemical engineering. In this paper, the dynamic characteristics of laser-induced bubble in venturi have been studied by experiments and simulation. In addition, the effect of a single bubble on the flow field in venture is also studied. The aim is to realize the generation and collapse of bubble in fixed location and provide technique supports for the active control and effective use for bubble. The content of the research is as follow:The laser-induced bubble generates when the YAG laser bombards the metal target. The results show that the size of bubble is related to the laser pulse width and current intensity, however, there is no completely proportional relationship between them. The laser-induced bubble rises to the water surface after its generation, and it stays bellow the water surface for some time and then collapses. The collapse of bubble produces a water jet.The system to test the laser-induced bubble generation and movement in venturi is set up. The move process and collapse condition of bubble in venturi is explored in detail. The results show that the generation and collapse of bubble in a fixed location can be realized by using YAG laser and venturi. The collapse of bubble depends on flow condition. Bubble will collapse in the state of turbulent flow, but not in laminar flow. The more severe of turbulence, the more obvious of collapse phenomenon.The bubble dynamics in venturi is simulated by the method of computational fluid dynamics. The results show that the bubble will not collapse when the inlet pressure is less than25kPa. When the inlet pressure is larger than30kPa, the bubble will collapse. The intense of bubble collapse increases if increasing of inlet pressure. Theses results are consistent with the experiments.Based on the Rayleigh-Plesset-Keller-Kolodner equation, the dynamics of single bubble is calculated in the consideration of liquid viscosity and surface tension. The flow field in venturi is simulated by fluid dynamics. The results show that, for the whole flow field, with the increase of λ, the velocity of Venturi throat and the maximum of the cavitation volume fraction increase. The distribution areas in which the cavitation volume fraction is not zero become wider. The throat pressure decreases at first and then tends to be stable. For a single bubble, with λ increasing, the maximum growth rate and inner cavitation bubble pressure pulse significantly increases, and the number of bubble growth and collapsing also increases. The effect of a single bubble dynamics in the Venturi on the cavitation flow field depends on turbulence intensity. The more severe the turbulence in venturi, the less the effect of single bubble on the whole flow field.