Study On Gas-liquid Synergistic Mechanism And Application Of A Co-flow Water Mist System

With the acceleration of urbanization,the accident risks of urban fire and gas pipeline leakage enhance.Clean and efficient fire fighting and leakage gas disposal methods are in urgent need to protect people’s lives and property,and reduce environmental contamination.Fire extinguishing and leakage gas disposal techniques using water mist have attracted more and more attention due to their high efficiency and environment-friendly.Currently,increasing spray pressure could induce spray penetration distance,which leads to larger energy consumption,and higher construction and maintenance costs.In order to reduce the working pressure of the water mist system and enhance the penetration of water mist,it is meaningful to investigate the generation and control of water mist under lower pressure.Based on the fire-extinguishing mechanism of flame cooling,the internal relations of different factors affecting fire-extinguishing performance of water mist were well clarified.At present,a novel water mist system namely a co-flow water mist system was proposed,which could significantly improve the penetration and vaporization of water mist.Meanwhile,the mechanisms of gas-liquid synergism in a co-flow water mist system and application designs of this co-flow water mist system for different scenarios were also further studied.The main contents and conclusions are listed as following:（1）The interactions among the various factors affecting the fire extinguishing performance of water mist are well clarified based on the fire-extinguishing mechanism of flame cooling.Those direct-acting factors were integrated into the single droplet motion,heat,and mass transfer models.The influence of droplet initial characteristic parameters and fire characteristic parameters on the penetration and vaporization of water mist was studied detailly.The effectiveness of existing methods in improving the utilization efficiency of water mist was also evaluated.A method to enhance the utilization efficiency of water mist named co-flow water mist system was then proposed.Those direct-acting factors ranked according to their influence degree from high to low are initial diameter of droplets d_p0,heat transfer temperature difference?T,initial velocity of droplets u_p0,flame plume velocity u_f.The penetration and vaporization of water mist can be simultaneously improved by increasing u_p0.Compared with the existing methods,the penetration and vaporization of water mist are significantly improved in a co-flow water mist system.The penetration distance of a droplet with u_p0=10 m/s and d_p0=100μm in the reverse airflow with?T=600 K and u_f=?4 m/s is increased approximately 5 times,and the vaporization rate is increased by 37.5%.（2）A 2D computational model of a high Reynolds number jet was formed due tothe good symmetry and stability of the flow field,where the compressibility of gas also was considered.The effects of nozzle structures and working pressure on the dynamic characteristics of the co-flow were studied detailly.The generation method of the co-flow that meets the demands of the flow field dynamics characteristics was determined.The working pressure of the Laval is the highest,but that is the lowest for the pipe with the same outlet momentum.The jets issuing from these two nozzles have a potential core that increases with the working pressure.In the near field,the centerline axial velocity of the jets issuing from the Laval and the pipe decay slower,which is negatively correlated with the working pressure.There is a parabolic dependence of the mass flow of jet m on the outlet momentum of jet M,the density of the surrounding fluidρ_∞,the axial position x,in the form of m=B₂+A₁x（Mρ_∞）^1/2+A₂[x（Mρ_∞）^1/2]².In the far field,the centerline axial velocity of jets is nearly independent of the nozzle structures,but the asymptotic values increase with the working pressure.There is a linear relationship between m and x（Mρ_∞）^1/2in the form of m=K₁x（Mρ_∞）^1/2+B.In the applications where the required Mach number of the outlet is no more than 1,the jet issuing from the pipe has a better performance.In addition,the Laval is better as the required Mach number is greater than 1.The outlet should not be under-expanded.（3）An interactive model of the co-flow and water mist was established based on the Eulerian-Lagrangian method,where the breakup,collision,and coalescence of droplets were considered.The effects of water mist on velocity,turbulence intensity,diffusion of the co-flow,as well as the effects of the co-flow on velocity,particle size,concentration distribution,momentum of water mist were analyzed.Mechanisms of gas-liquid synergism in a co-flow water mist system were discovered.A method to select optimization structure and operating parameters for a co-flow water mist system was proposed.It was found that the decay of the centerline axial velocity,the diffusion,the entrainment,and the turbulence intensity of the gas phase is reduced owing to the injection of droplets,which result in a larger penetration distance of the two-phase flow.The motion of droplets is mainly affected by their inertia and average velocity of the co-flow,so the concentration field of droplets is similar to the velocity field of the co-flow.The droplets with a diameter between 50 and 150μm have better acceleration and velocity retention performance.The action-angle of droplets is related to the initial momentum of the co-flow.The control of Sauter mean diameter（SMD）and momentum decay rate of droplets can be implemented by adjusting the spray pressure difference,initial spray angle,gas-liquid mass flow ratio（GLR）,spacing from the co-flow nozzle to the atomizer.（4）The interactions of the co-flow water mist system and leakage jets/flame plumes were studied.The instantaneous trajectories,concentration distribution,and penetration of droplets were visualized by numerical simulation.The coupling relationships between the co-flow water mist system and leakage jets/flame plumes were discovered.A method to determine the minimum spray intensity of a co-flow water mist system was established based on the fire-extinguishing mechanism of flame cooling.A design method for the co-flow water mist system which could be used to prevent and control the leakage of hazardous gas or extinguish a fire was proposed.Instantaneous trajectories of the droplets depend on the initial momentum ratio of the reverse flow and the co-flowΦ’.Φ’≤1 is necessary to effectively control the leakage jets,but 0.5≤Φ’≤1 is better for the suppression of the high-temperature flame.A natural exponential function can be used to fit the dimensionless interaction boundary height z_b*and the characteristic axial momentum ratioΦfor the scenarios of a co-flow water mist system interacting with leakage jets,which is z_b*=1.694-1.570e^-0.753Φin this dissertation.