Experimental Study Of Droplet Impact Dynamics On Liquid Pool

As a substitute for halon fire extinguishing agents,water mist fire extinguishing technology is widely used in many important fire-fighting places,such as industrial plants,laboratories,buildings and forests,since it has the clean,efficient and non-polluting characteristics.However,most of the current research on water mist fire extinguishing technology is focused on the fire extinguishing efficiency at the macro level.Only a limited number of scholars have studied the microscopic mechanism,and however the microscopic mechanism has an important influence on the further development of water mist fire extinguishing technology.Therefore,in order to explore the microscopic phenomena and mechanisms of water mist fire extinguishing,a series of small-scale experiments were carried out on the interaction of single droplets with high and low boiling oil pools.This will help people to scientifically understand the microscopic mechanism of water mist fire extinguishing.In order to further investigate the dynamic process of a droplet impacting on high and low boiling oil pools,two experimental facilities were built to observe the dynamic process of a single water droplet impinging on the oil pool and the flame expansion process induced by a water droplet impacting on the burning cooking oil.All experiments were carried out at room temperature(20?).In the study,high-speed camera and digital camera were used to record the microscopic process of water droplets colliding with the liquid surface and the instantaneous process of flame expansion.Moreover,the impact characteristics,including droplet impact velocity,crater size,jet height,bubble duration,droplet absorption heat,flame height and flame volume,were further measured and calculated.By changing the ignition conditions,impact height,liquid depth,liquid temperature,and droplet diameter,the characteristic impact parameters and micro-impact phenomena of single water droplets impacting on high and low boiling oil pools under different environmental conditions are further revealed.The details are as follows:The research revealed the effects of impact height and liquid depth on the kinetics of single water droplet impacting on an unburned heptane pool.Firstly,the dynamic behavior of single droplet impacting on the unburned shallow/deep pool was recorded with a 2000 fps high-speed camera,and three kinds of impact phenomena were defined: penetration-second penetration/floating,crater-jet/crater-jet-secondary jet,and bubble.Secondly,the distribution diagram of impact phenomena determined by Weber number We and liquid depth d* was established,and the critical Weber number of various phenomena was found with the help of a large number of repeated experiments.Finally,the variation trends of impact characteristic parameters such as crater size,jet height,bubble duration,and loss rate of impact velocity were analyzed quantitatively,and the detailed mechanism explanation of the evolution of impact characteristic parameters was further discussed.The research revealed the effects of Weber number and liquid depth on the kinetics of single water droplet impacting on a burning heptane pool.The entire experiment was carried out while heptane was in quasi-steady state combustion.Firstly,according to the different number of jets,the regimes of burning heptane pool at low Weber number,including shallow pool,transition region,and deep pool,were founded,and the different dynamic behaviors of jet and bubble formation in shallow and deep pools were compared and their detailed formation mechanism was emphatically explained.The variation trends of liquid depth and Weber number on the impact behaviors were analyzed qualitatively.Secondly,by comparing the difference between the maximum crater size and jet height under different liquid depths and Weber numbers,the influence factors of crater and jet parameters in the shallow/deep pool were especially explained,and their evolution rules under different conditions were further revealed.Finally,according to the physical model of the droplet in the falling process,the damping effect of fire plume on droplet impact velocity was explored and the theoretical correlation between droplet impact velocity and impact height was proposed.The research revealed the dynamics of single water droplet impinging on a high-temperature and burning peanut oil pool.Firstly,the dynamic process and distribution law of impact behaviors of a single water droplet impacting on high-temperature peanut oil surface were showed in detail.The critical conditions of vapor explosion and the way of explosion contact were emphatically analyzed.Based on the way of explosion contact,three evolution stages of water vapor were further divided.Secondly,the heat absorbed by a single water droplet was deduced.The relationship between the total absorption heat of a single water droplet and the droplet diameter was established.Based on the volume calculation model of the water vapor bubble,the heat absorbed by the maximum daughter droplet was also explored,and the ratio of the absorption heat of the maximum daughter droplet to that of a single droplet was revealed.Finally,considering the effect of the droplet diameter on the flame expansion,the instantaneous process of flame expansion was described in detail and the variation rules of the maximum flame expansion height under different droplet diameters were emphatically studied.The influence law of maximum flame expansion height and volume multiple with the change of droplet diameter was further revealed by using the gray threshold method.