| The flame extension behaviors beneath the ceiling can be divided into the extension for an impinging flame or a directly extended flame beneath the ceiling.If there is a certain distance from the fire source to the ceiling and the heat release rate is large enough(for example,the fire caused by the vehicles in the tunnel),the upward flame will impinge the ceiling and extend a certain distance along the ceiling due to the effect of buoyancy and the blocking of the ceiling.This is called flame extension behaviors beneath the ceiling for an impinging flame.If the fire source is close to the lower surface of the ceiling(for example,a fire caused by a leakage of a gaseous fuel pipe at the ceiling or the ceiling fire caused by the flammable materials underneath the ceiling;i.e.a ceiling fire),the flame will directly extend underneath the ceiling driven by the buoyancy of itself.This is called flame extension behaviors for a directly extended flame beneath the ceiling.Previous works about the flame extension behaviors resulted from an impinging flame have been primarily concerned with un-confined ceilings and no forced air flow conditions.Only a few researchers investigated the flame extension behaviors beneath a confined ceiling for an impinging flame with the air flow.However,there usually exist air flow in the actual tunnels or corridors fire due to the existence of the exhaust fans in tunnel or windows at both ends of the corridor.Besides the impinging flame will touch the side walls of the ceiling if the heat release rate is large enough.There are still very limited studies on the flame extension behaviors resulted from a natural ceiling fire.Only a few works studied the scaling law for this ceiling fire,however they just focus on the stand-off distance of the extension flame,There is a lack of systematic research on the horizontal dimension of the ceiling fire.Besides,the previous researchers only theoretically analyzed the forming mechanism for the observed cellular flame,but the evolution of an inner cell structure(i.e.,the cell size)along with the spreading distance from the source has never been quantified before.Therefore,this paper is expected to filling in gapes of previous studies on these two kinds of flame extension behaviors through experimental research,theoretical analysis and mathematical statistics.In this paper,the experiments about the flame extension behaviors resulted from an impinging flame or a natural ceiling fire was respectively carried out.For the experiment about the impinging flame beneath the ceiling,Square porous gas burners of different sizes(15cm,20cm)were employed as the fire source,using propane as fuel with various heat release rates(11 kinds sceneries from 51-153.1kW),source-ceiling heights(35cm,40cm,45cm)and air flow(0m/s,0.6m/s,1.2m/s,1.8m/s).The flame extension lengths beneath the ceiling were measured by video processing methods.Results show that the flame extension lengths beneath the ceiling increases with heat release rate,but decreases with source-ceiling height,channel width,burner size or longitudinal air flow speed.Non-dimensional correlations are proposed for the flame extension lengths(upstream,downstream and total length),based on the unburnt fuel distribution upstream and downstream,as well as considering air entrainment of the ceiling flow,which further consumes the unburnt fuel along the ceiling.These correlations are shown to fit the data well.For the experiment about the flame extension behavior resulted from a natural ceiling fire,experiments were carried out employing both circular and line-source gaseous burners with various dimensions(with diameter of 40mm,60mm,80mm for circular burner and dimensions of 2(W)×142.5(L)mm、2(W)×217(L)mm for the linear burner)and heat release rates(9 kinds sceneries from 6.12-12.24kW).A cellular flame structure was formed beneath the ceiling.This special flame structure is due to the inherent "Rayleigh-Taylor" instability:a hot diffusion flame burning beneath the cold fuel gas.issuing from the ceiling above.Thus,there will develop an accelerated upward flow beneath the ceiling,resulting in an unstable situation in terms of the Rayleigh-Taylor instability.The horizontal flame extension lengths both for the circular and linear burner were measured.The correlations are proposed for the horizontal extension lengths of the cellular flame both for the circular and linear burner based on the scaling analysis.Meanwhile,the flame characteristic cell size at the edge of the cellular flame for the circular burner was statistically calculated.And a correlation relating the cell size at the edge of the cellular flame and the distance of the edge cell from the source origin based on the "Rayleigh-Taylor" instability scaling analysis.Besides,the relationships between the total cell area of the cellular flame(for the circular and linear burner)and the heat release rate were also investigated. |
