Theoretical Analysis Of Atomization And Fire Extinguishing Experimental Research On Swirling Water Mist Nozzles With Straight

With the limited use and faced elimination of halone fire extinguishing chemical,fire suppression technology employing water mist demonstrates its wide promising potential due to its environmental benign,high efficiency,lower water consumption,less bveakage to the shielded objects and so on.It has become one of the hot topics in fire safety community.In domestic,the currently-used water mist fire suppression systems are featured with complicated design structure and high installation and operation costs mostly due to the high operating pressure.These are significantly detrimental to the generalization of the water mist fire suppression technology.The water mist nozzle is the core part of various water mist fire suppression systems.The performance of the water mist nozzle directly determines whether or not the designed water mist fire suppression system can be effectively applied to various fire scenarios.In China,the most developed high pressure water mist nozzles have not only complex design structure but also too many hard-controlled operating parameters.Therefore,it is significantly important to develop a kind of single-phase water mist nozzle with small size,light weight,simple deign structure, ease-to-manufacture-and-install,and good atomization performance within low-to-medium operating pressure.In domestic fire safety community,there are no literatures combining the nozzle structure,atomization mechanism,spray characteristics and fire suppression effectiveness with the whole systematical engineering for studies.The present work experimentally studied the relationship among the nozzle structure,atomization mechanism,spray characteristics and fire suppression effectiveness. The main works and progresses done in the current study are as follows.1.The present work developed a novel low-to-medium swirl water mist nozzle.The structure parameters of nozzle were optimized through theoretical analysis,numerical simulation and experimental verification.Following the discussion on the relationship between the nozzle structure and spray characteristics,the corresponding theoretical model was developed.The effects of the nozzle's structure parameters on the spray characteristics had been investigated using the FLUENT code,through which the internal and the external flow fields' of the nozzle were calculated.The numerical results provided theoretical foundation to parameters optimization of the subsequent design of the water mist nozzle.2.The movement equations of water droplet were established by balancing various forces imposed on the droplet appeared in air.The numerical calculation was performed on the droplets with typical diameter and various exit velocities of the droplet.The basic movement law of the water droplet was then obtained.Dependences of the droplet's terminal velocity on the droplet diameter between spuared-low and lineear-low was determined,the corresponding critical droplet diameter equal to 80μm.3.On the basis of numericai simulation,the cold field tests on the spray performance of the water mist nozzle were carried out.The experimental results were compared with the numerical calculations.The results showed that the optimized single-fluid and low-to-medium swirl nozzle presented excellent spray characteristics.For most fire scenarios,the current nozzle can replace the high pressure nozzles or two-phase flow nozzles.It not only decreases dramatically the initial investment of water mist fire suppression system but also enhances the safety and reliability of operating the system.4.The fire suppression effectiveness of water mist subjected to external wind disturbance was both experimentally and numerically numerically investigated.The fire suppression model of water mist subjected to external wind flow was established and without wind or with validated.The results showed that water mist can extinguish the pool fire the external disturbed wind flow.However,the fire can not be extinguished at a critical wind velocity.5.The experimental setup was established.The kerosene pool fire suppression tests using water mist in a confined space were performed. The effects of ventilation condition,the preignition time and the operating pressure on fire-extinguishing time were examined.It was concluded that pool fire in confined space showed a core reaction region, a ventilation-controlled region and a fuel-controlled region.Following the experimental results and qualitative analysis,it was found that the principal mechanisms of the pool fire suppression by water mist in confined space were gas phase cooling and flame cooling.Moreover,the basic assumptions of extinguishment model of the chamber fire,the fire extinguishment theory and the model of calculating the critical water flow rate for fire suppression were experimentally verified.