Experimental Study On Inhibition Of Fuel Evaporation And Flame Extinguishing Of Oil Tank Fires By Compressed Gas Foam

The oil storage tanks pose a big threat in the chemical industry area due to their high concentration of energy,which are prone to fire and explosion accidents.Among the candidate extinguishing agents,compressed gas foam has been proven to be effective and efficient.The available research mainly focused on the performance characteristics and engineering parameters of the foam,where the theoretical model was relatively simple,without taking the spreading characteristics of foam and the process of heat and mass transfer into comprehensive consideration.Therefore,aimed at unveiling the underlying mechanism of the compressed gas foam extinguishing tank fires,this paper will address the key scientific issues of the non-steady-state heat transfer,flow,and combustion in the mutual activities between the foam,flame,and fuel.Among them,the emphasis is put on the inhibition of the fuel evaporation and the extinguishment of the flame,which are the leading function effects of the compressed gas foam.Firstly,an experimental system for producing compressed gas foam was designed,and the foam was characterized by measuring the foam expansion rate and drainage rate.Fire-extinguishing efficiency of the foam is primarily affected by the spreading rate of foam on the liquid surface.Therefore,a power law model of spreading length of foam was given based on the experimental results.Later,a theoretical model was established to predict the reduced burning rate after the discharge of the foam by introducing a mass transfer B number.It was found that,owing to the oleophobic effect of the fluorocarbon surfactants,the greater thickness of the foam layer and lower diffusion coefficient had coupling chemical and physical effects,which hindered fuel transportation,diminished oxygen mixing and blocked flame radiation feedback,thus the burning rate was reduced eventually.Further,the foam mass loss rate and fuel evaporation rate under a stable radiation source were experimentally studied.It was found that,the drainage rate of the foam increased with the increase of the initial foam thickness,but it was not affected by the radiation level,primarily dependent upon the unique parameter of the expansion rate of the foam.In addition,due to the better stability of compressed nitrogen foam,its drainage rate was slower.Meanwhile,the foam evaporation rate increased with the increase of radiative heat flux,but was independent of the initial foam thickness.Additionally,the inhibition fuel evaporation rate due to the foam was represented in the form of the pilot ignition time.The fuel ignition time increased with the increase of the initial foam thickness or the decrease of the radiation value,indicating less fuel evaporation rate.Compared with compressed air foam,with the coverage of the compressed nitrogen foam,the ignition time was slightly longer,which was resulted from that the nitrogen foam with better stability provided a stronger barrier to the mass transfer of oxygen and fuel vapor,further the nitrogen acted as an inert gas with an additional effect preventing combustion more effectively.Additionally,the effect of fuel thickness and foaming gas on the flame extinguishing under actual fire conditions was studied.The results showed that the extinguishing time decreased with the increase of foam flow rate,but the rate of decrease became slower at the higher foam flow rate.The variation of foam amount with the foam flow rate showed a U-shaped curve,with the existence of an optimal point,where the foam amount was the lowest and the fire-extinguishing time was shorter.It was found that,at lower fuel thickness,with the additional heat feedback from the hotter solid tank wall,foam evaporated more easily accompanied with the reduction in the foam layer thickness,therefore,the reduction of the rate of burning was slowed,which increased the time needed for the flame to extinguish.Lastly,after the compressed nitrogen/air foam was discharged,the variation trends of the flame temperature,area,frequency and burning rate with time were observed to be similar with an obvious instantaneous increase.Three monotonous regions associated with two time delays were found in the whole fire extinguishing duration.For these two suppressing agents,underlying reasons for the two time delays were similar,which were the movement of flame sheet,the R-T instability,air entrainment and intensified combustion.However,the important difference was reflected in the larger shift of flame sheet for the compressed nitrogen foam.The burning rate under compressed nitrogen foam decreased faster corresponding to a shorter extinguishing time.The main advantages of the compressed nitrogen foam were:the faster increase in boundary layer thickness,lower oxygen mass fraction,less heat flux by radiation and foam drainage fraction.The disadvantage for compressed nitrogen foam was attributed to the lower cooling effect of the flame,caused by the less foam evaporation under less flame heat feedback.Therefore,for compressed nitrogen foam,it had relatively better inhibition ability to extinguish fire,which was reflected in the theoretical burning rate model and a smaller Da number.