Experimental And Theoretical Studies On Water Mist Flashover Restraining In Confined Space

As a critical indication for the full development of confined compartment fires, flashover would always lead to heavy casualties and huge property loss.Compartment flashover is affected by the room dimensions,fire source location,opening and fuel load.After fire suppression agent discharging,flashover could be restrained by reducing heat release rate,depressing smoke temperature and obstructing thermal radiation.As the leading Halon alternatives,water mist technology is emphasized by researchers with its advantages:clean,high efficiency,low cost and little damage to protected objects.But prevenient investigation mostly focused on fire restricting or suppression effect,little attention was paid to the flashover restraining efficiency. With obstructions,obstructed fire and deep fire would always be difficult to be suppressed.Then,water mist flashover restraining shows decisive influence on evacuation and rescue.Thus,with theoretical and experimental study,characteristic mechanism and influencing regularity of water mist flashover restraining will be investigated here.Consulting foregoing flashover experiments,two different confined flashover compartments were designed first,with regular water mist system,ultra-fine water mist systems based on ultrasonic atomization and vapor condensation for flashover restraining.Flashover phenomena was reappeared with gasoline,diesel oil,alcohol, polyethylene and wood crib fires.Results showed that the minimum fuel area needed for flashover is proportional to evaporation heat,and inversely proportional to combustion heat.Time to flashover decreases following the increasing of fuel area.Considering that the existing nonlinear flashover model only suits chamber with single opening,this paper presented a modified model which would be more widely applicable after revising the dimensionless parameters,based on analyzing the calculation methods for smoke neutral plane and outflow mass rate from compartment with multiply openings.A relationship formula between dimensionless temperature and a parameter coupled with combustion characteristics and fuel area was presented. Following the forecasting model,analytical results anastomose well with experiments, indicating the model deduced here predicts flashover effectively.With experiments on water mist pool fire suppression and analysis on main mechanism during extinguishing,it was found that fire could not be suppressed with the nozzle pressure under 2MPa,which means flashover may occur then.In confined space,main mechanism of fire suppression would be oxygen displacement.While it comes to gasoline pool fire,cooling effect on fuel surface could be ignored comparing to the cooling of flame area.Thus,in experiments of water mist flashover restraining, consideration only the mist decalescence in smoke and flame would be enough and doable.Nonlinear dynamics was chosen here for investigating water mist flashover restraining.Since mist mainly alters the convection endotherm dimensionless factor in smoke energy balance equation,obtaining the relationship between convection factor and water mist flow rate would lead to critical condition for flashover restraining. Based on different mist evaporation processes,mist movements in smoke and flame with none aerification,partial aerification and total aerification were studied,leading to mist endotherm rate,prediction model for critical mist flow rate and the criterion between defferent aerification processes.During experiments,flashover occurs under low nozzle pressure.Flashover would be postponed with nozzle pressure increasing, and be restrained finally.It was found that model derived here could predict mist restraining effect conveniently after comparison to experiment results.Ultra-fine mist will evaporate completely in confined space with diameter less than 50μm.Critical mist volume flux was obtained after analyzing mass rate into smoke and flame by self-entraining and forced ventilation.Interactions between flashover and ultra-fine mist were simulated in two scale enclosures with mist designed.The result came out that restraining efficiency depends on mist flux rate, discharge time and discharge manner.Also,experiments and predictions came to an accord.In summary,this paper investigated regular and ultra-fine water mist flashover restraining effect in confined compartment with theoretical analysis,experiments and models.Prediction model and critical condition were obtained,offering a useful forecasting approach for flashover restraining under water mist.