Experimental And Numerical Simulation For The Pulsating Process Of Laser Cavitation In Micro Holes

Laser underwater drilling has the advantage of high punching efficiency.However,the study on the interaction of the laser,the small hole and the water during the punching process which causes the water migration to promote the large amount of slag discharge process,is still very limited.In this paper,the mechanism of laser cavitation in microholes with radius of several hundred micrometers and depth of millimeters is studied in order to provide a theoretical reference for underwater laser drilling.First of all,we observed the transient process of nanosecond laser acting on water medium,including shock wave,phase ablation,bubble pulsation and water splashing.In the bubble expansion stage,the free surface of the water forms the phenomenon of the water graves.In the stage of collapse,the jet of the bubble collapse had strong impacts on the bottom of the hole.Collapse formation and collapse time of double bubble pulsation process was different from the single bubble pulsation process.The bubble pulsation was confirmed as the main mechanism of water migration and bubble pulsation is divided into three stages,including expansion in the hole,attraction on the surface of the water and collapse.Furthermore,according to the experimental phenomena and based on the theory of fluid mechanics,the pulsation process of the bubbles in the hole with full water was simulated using the finite element method.The experiment verified the rationality of the numerical model.Furthermore,the numerical calculations reveal the mechanism of the impact on the bubble pulsation of the initial bubble pressure and the location of the bubble generation.As the initial bubble pressure increases,the speed and pressure of the single bubble jet increase.However,when the initial bubble pressure reaches 35 MPa,the leakage phenomenon occurs,and the velocity and pressure of the jet decrease.The greater the initial bubble pressure of bubble near the surface of the water,the stronger the inhibitory effect on the bubble at the bottom of the hole.The greater the initial bubble pressure of bubble at the bottom of the hole,the greater the jet velocity and pressure ultimately generated.The closer to the bottom of the hole the bubble is generated,the stronger the migration of the bubble pulsation to the water.The closer the bubble is to the orifice,the earlier the jet is formed and the greater the jet velocity and pressure.Finally,the effects of hole depth,aperture and hole shape on the cavitation process of the bubble were investigated.The results showed that the maximum radius and the expansion velocity of the bubble in the 2mm deep hole were greater than in the 1mm and 3mm deep holes.The larger the hole depth,the more foam energy the cavitation bubble required to consume,and the smaller the hole depth was after the excessive expansion of bubble out of the hole.Moderate hole depth can enhance the migration of water in the holes.The smaller the small hole radius,the larger the bubble wall radius and the expansion velocity.The bubble can be concentrated to the outside of the hole due to the constraint of the hole wall.The reduction of the hole diameter can strengthen the migration of water in the small hole.The shape of the hole has a great influence on the maximum bubble radius and the instantaneous expansion rate.Because hole wall constrains the expansion of the bubble,the conical hole weakens the migration of water in the hole by the bubble,and the inverted conical hole strengthens the migration of water in the hole by the bubble.