| In reality,the scenes of fire accident are often complicated,for example,fires in compartment,corridors,tunnels,etc.are often accompanied by the constraints effects of wall and ceiling.The existence of these constraints factors will seriously affect the air entrainment and flow behavior of fire plume,as well as the distribution of characteristic parameters such as heat flux and temperature,thus directly changing the development and hazard of fire.Therefore,fire plumes behavior under the constraint space as a fundamental scientific problem in fire safety should be concerned.In this paper,the process of burning behavior in the building was considered as the fire combusting under constraint,then impinging the ceiling and further ignite the ceiling combustible,as well as the effect of environmental wind.The fire plume and flame diffusion behavior of wall and ceiling constraints are studied in the whole fire development process.Through the combined research method of experimental research,theoretical analysis and numerical simulation,the evolution of the flame behavior under different constraints was concerned.The flame morphological characteristic parameters were quantified.The distribution of temperature and total heat flux were analyzed.The ceiling cellular flame structure and ceiling pulsation behavior were focused and the flame flow field structure was analyzed by the computational fluid dynamics software.A series of theoretical prediction models are proposed.Relevant conclusions can provide important reference for fire risk alarm,fire protection standards,fire facilities design,and numerical simulation research.The major findings include:(1)The flame morphology evolution law under the coupling effect of side-wall constraint and cross wind was revealed,and the model of flame morphological parameters was proposed.Most of the previous studies mainly focused on the side wall flame in no wind condition,while coupling between side wall constraint and cross wind usually exists in fire scenarios.In this paper,the morphological characteristics of diffused flame with or without sidewall constraints and cross wind are compared and analyzed.The evolution behavior of sidewall flame plume under cross wind is demonstrated by the wall temperature profile.Through the physical analysis,the characteristic length scales were proposed to characterize the side wall entrainment limitation,the competition between cross wind momentum and flame buoyancy,and the influence of burner size on mixed flow.By dimensional analysis,a new dimensionless characteristic parameter is derived,and the dimensionless models of flame morphological parameter were proposed by coupling the effect of side wall and cross wind.(2)The flame extension behavior of flame plume impinging the ceiling under corner constraint was revealed,and the model of impinging flame lengths and ceiling temperature and heat flux profile were proposed.Previous studies have carried out research on the fire plume impinging the ceiling under the free or near wall condition,but in reality,the buildings have corner wall with different angles.In this paper,the effects of different corner angles on the impinging ceiling flame structure was analyzed.It is found that the impinging flame under the ceiling presented a V shape structure under the corner constraint.Based on the effect of corner constraints on air entrainment and fuel mixing before-and after flame impinging ceiling,a dimensionless formula was proposed for impingement flame length of different corner constraints.Meanwhile,the temperature and heat flux profiles of various corner angles were analyzed,and then the normalized formulas of the temperature and heat flux were proposed by using the total flame length as the characteristic scale.(3)The flame diffusion combustion behavior under ceiling constraint was studied,the evolution of ceiling cellular flame structure,temperature and heat flux distribution and flame pulsation were revealed.The ceiling fire with burning of combustible ceiling itself is completely different from that of a flame impinging the ceiling,due to there is no accelerated rise stage of the fire plume.In this paper,the special cellular flame structure under ceiling constraints was studied.Through the scaling analysis,the characteristic length scale of ceiling flame evolution behavior were proposed,and the models of ceiling flame extension length,temperature and heat flux profile was proposed.The temperature gradients of the ceiling flame in the vertical direction was found.Based on Rayleigh-Taylor instability theory,the formation mechanism of cellular flame was revealed,and the size and spatial distribution of flame cells were analyzed.Meanwhile,the special pulsation behavior of the ceiling flame was found,the physics of ceiling flame pulsation modes was revealed combined with numerical simulation,which is different from that of free flame.The flame lengths fluctuation scale and kinematic features of vortical structures were analyzed.Based on the air entrainment characteristics of ceiling flame,a dimensionless model of ceiling flame pulsation frequency was proposed.(4)The flame extension behavior of a long confined ceiling fire with cross wind was revealed,and the model of flame morphological parameters were proposed.The burning of long confined ceiling combustible is usually coupled with the influence of cross wind.In this paper,the evolution of confined ceiling flame morphology with cross wind speed was revealed.The flame macroscopic characteristics were analyzed by ceiling temperature profile.It is found that the long confined ceiling flame presented an ellipse structure in no wind condition,due to the asymmetric entrainment limitation of the two side walls.Meanwhile,the flame gradually shrinks downstream into a straight line flame with the increase of cross wind speed.Based on the special air entrainment of long confined ceiling fire,a dimensionless heat release rate was proposed to couple the effect of ceiling flame buoyancy and cross wind,and then the dimensionless models of flame morphological parameters were deduced. |
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