An Experimental Study On The Interaction Between Flames And Superfine Powderbased Agents

As one of the most useful fire suppression agents,traditional dry powder has been applied in portable fire extinguishers,fire suppression equipment and commercial fire suppression system in the past fifty years.When the production of Class I ozone depleting substances(ODS),such as Halon 1301,began to stop after adoption of the Montreal Protocol,powderbased fire suppression agents are considered as good alternatives because of their environmental compatibility,fast fire extinguishment and low cost.Among the proposed Halon replacements and alternatives,more attention should be devoted to optimizating the agents formulation, selecting and designing of the distribution ways.In order to develop the powderbased fire suppression technology,a method for powder screening,compounding and superfining was proposed in this paper.Seven types of powderbased agents were prepared.The microcosmic structure of the powders was observed by scanning electron microscope(SEM).Thermal Gravity Analysis(TGA),Fourier transform infrared(FTIR)spectra of KBr wafers and X-ray diffraction spectra(XRD)techniques were used to analyze the thermal behavior and spectra characters of all kinds of powder.By analyzing the results,it can be concluded that XRD pattern of K-powder confirmed the formation of organic mineral complex during sample preparation.The FTIR spectra confirm that surface treatment agents and organic groups were present in the K-powder,which was beneficial to fluxion and surface property of fire extinguishing powders.The microstructure and surface properties were very distinct between the powder after superfiing and normal BC powders.And then,we built up a laboratory-scale immovable fire suppression system.In this system,seven kinds of powder agents including normal ammonium phosphate dry powder,sodium bicarbonate dry powder and superfine and surface treatment powderbased agents were test.The relationship between the characters of the powders and their effectiveness on fire suppression were studied.The fire extinguishing time and temperature changes were recorded when changing powder types,particle sizes, working pressures and spraying direction.Results showed that there was a significant impact on fire suppression effectiveness when the particle size is decreased.The working pressure also influences the fire suppression effectiveness.By analyzing the results of the research,it is concluded that the pressure,the particle size of the powder and microcosmic structure of powder surface mainly determine the capability of the powder to extinguish the fire.In the following part,the interaction between a small-scale diffusion flame and powder agents was studied.In this part,simple flame apparatuses widely used in the combustion research were firstly reviewed.The cup burner apparatus was finally selected considering its flame structure comparability.Experiments were conducted to explore the CH₄/air flame characteristics in the Cup Burner,which provided the base for further experiments.Interaction of CH₄/air diffusion flames with powder agents were then investigated in details.We observed the dynamic process of the flame instability,color changing and extinguishing when the powder loading mass was increased.The diffusion Cup Burner flame structure was considered as the "worst case" for fire suppression.As the powder concentration was increased to the suppression limit,the flame base became weaker and could not come back from the innermost position,thereby drifting upward to blow off.The mass-based concentrations of powders required to extinguish flame were measured in the cup burner apparatus and compared with data in other references.A comparison of the agents showed that on average,the superfine BC powder and K-powder had reached or exceeded the effectiveness of normal BC powder on a mass basis.The flame atomic emission spectrometry changes before and after powder loading confirmed that the free radicals in methane combustion chain reactions were scavenged,the specific spectras of potassium and sodium radicals peaks appeared.The Damkohler number was introduced to discuss the fire suppression.The different means to extinguish the fire can be unified as the influences to the critical extinguishing Damkohler number.The mechanisms of fire suppressing powders were considered in two parts:thermal mechanism and chemical mechanism(including both homogeneous(gas phase)and heterogeneous(surface)chemistry).The particle size effects and movements of powders in the flame were discussed based on the "grid thermal death" and flame quenching theory.Mass powder particles can act as a wall or grid effects to extinguish fire.Powder cloud is taken as a three-dimensional grid "put" on a flame by researchers.The mechanism of flame suppression consists of fast thermal destruction(or recombination)of FFRs due to their collision with a surface of particle grid elements.The three-dimensional powder particles become an "inert wall" which absorbs part of FFRs energy.Then,it is obvious that what is called "grid" size or powder particles distance between each other becomes the key factor to fire extinguishing.At last,suggestions were given for applications of powders in supertall buildings. Generating non-toxicity and clean superfine powder which are high fire extinguishing efficacy will extend the application of these fire suppressing agents.More theoretical and experimental investigations should be carried out.