Studies On Behaviors And Characteristic Parameter Models Of Fire Plumes And Ceiling Jets Driven By Rectangular Fire Sources

In reality,the fire sources have different shapes changing from axisymmetric-type(aspect ratio equals to 1)to line-type(aspect ratio is very large),which can be simplified as rectangular fire sources with different aspect ratios.Many research results on the behaviors and characteristic parameters of rectangular fire plumes have been reported in literatures.However,some special but common rectangular fire plume behaviors still have not been studied:(a)Fire plumes induced by horizontally oriented rectangular burners.Reaserches on a horizontally jet fire,for example a fire issued from a leakage on the side of a gas tube,are focused on roud burners.However,the leakage can be a rectangular one or a line-type one.(b)Ceiling jet driven by a line-type fire.Many studies have been conducted for free rectangular fire plumes,and the ceiling jet flows are focused on the axisymmetric-type fire sources.There are few studies reported on the ceiling jet flows driven by line-type fire sources.(c)Varying wall-fire separation distances for a rectangular fire plume.Since wall has an important effect on the air entrainment of a fire plume,which can further affect the flame characteristics,many studies have been conducted on wall attached fires.However,in practice,many fire plumes are affected by the wall with varying separation distances,which have different evolution behaviors,for example,a chair or a sofa may be attached the wall or near the wall with different wall-fire separation distances which can cause a near wall fire.(d)Inclined ceiling jet flow driven by wall attached fire.People study the wall attached fires and the ceiling jet flows separately.More studies on the ceiling jet flows(especially with an inclined ceiling)driven by a wall attached fire need to be done.It happens in enclosure fires when the fire source is attached the wall.Therefore,for the above fire scenarios,we need to study the behaviors and characteristic parameters of fire plumes and ceiling jets driven by rectangular fire sources.Experimental researches and theoretical analysis have been carried out to study the behaviors and the characteristic parameters of fire plume and ceiling jet driven by rectangular fire sources.A series of rectangular burners with different aspect ratios were used as the fire source.Firstly,we study the fire plume behaviors and the flame lengths of horizontally oriented rectangular fires.Secondly,we research the temperature profiles and the flame extents of ceiling jet flows induced by a line-type fire.Thirdly,we study the flame touching wall probability and the evolution behaviors of the flame characteristic parameters of near wall fire plumes with varying wall-fire separation distances by using rectangular burners.At last,we investigate the temperature distribution and the flame extents induced by wall-attached fire(rectangular burner)impinging upon an inclined ceiling.The major findings include:The effects of burner aspect ratio on the flame length(in the horizontal direction)of a horizontally oriented buoyant turbulent rectangular jet fires were revealed and prediction model was proposed for the flame length.In this work,experiments were conducted by using 8 horizontally oriented rectangular burners with aspect ratio,n(nozzle length to nozzle width: n=L/W),varied from 1:1 to 71:1.Results showed that the flame length increased with the heat release rate growth.At the same time,the flame length decreased with the increase of the aspect ratio n for a given nozzle exit area.A non-dimensional function was then derived for the flame length,in which a characteristic length scale was found in relation to the nozzle length and width,based on the balance of the momentum flux to the buoyancy flux of the projected flame.A new non-dimensional heat release rate was defined based on the proposed characteristic length scale.The data were shown to be well correlated by the derived function in two regimes:(1)for relative small non-dimensional heat release rates,the flame length has a 2/3 power dependency on the heat release rate as for a 2-D trend fire;(2)for relative large non-dimensional heat release rates,the flame length has a 2/5 power dependency on the heat release rate as for a 3-D trend fire.Prediction models on the temperature profiles and the flame extent evolution behaviors of ceiling jet flows induced by a line-type fire source were proposed.It is found that the temperature profile induced by a line fire source(aspect ratio 18:1)is two-dimensional and decays faster in the x-direction than that in the y-direction(x-direction: parallel to the longer side of the line source;y-direction: perpendicular to the longer side of the line source).The characteristic length scales for characterizing the temperature profiles in the two directions are then proposed based on the linear increase assumption of the up-rising plume dimension.By using the temperature data in the literature and the large scale experimental data of this work,as well as combining the energy conservation analysis of the turbulent buoyant plume under the ceiling with the proposed characteristic length scales,an equation is derived to describe the two-dimensional temperature profile.For the flame extents of ceiling jet flows,experiments are conducted in this work employing a line-source burner(aspect ratio 71:1).Experimental results show that the flame length in x-direction is shorter than that in y-direction.Their difference decreases with increasing of ceiling height,meanwhile increases with increasing of the fuel supply rate.A new global model is derived to describe the flame lengths underneath the horizontal ceiling based on the analysis of un-symmetrically distributed un-burnt fuel mass flows in the two directions reflected by their characteristic lengths.The flame touch wall behavior and the flame characteristics of rectangular source fires(aspect ratio 1:1-71:1)near a wall with different separation distances were studied,and related prediction models were proposed.For the wall attached fires,experiments were conducted to investigate the flame height,maximum mean flame thickness(normal to the wall),maximum mean flame width(parallel to the wall)and flame fluctuation.These data are also compared with those of free fires.The experiments showed that the flame heights for the wall fires are nearly the same as those for the free fires when appropriately normalized using the mirror image approach for the wall fires.And wall fires experience a two-dimensional to three-dimensional transition as free fires do.The flame height fluctuation and the normalized flame thickness of wall attached fires are smaller than those of free fires,and both decrease with the increase of the fire source aspect ratio.The normalized flame width of the wall attached fire is also smaller than that of the free fire.The observed behaviors for the flame geometries are explained by the opposite effects of the flame fluctuation and air entrainment,both of which decrease for wall attached fires compared to free fires.The flame transition behavior from completely-attaching(wall fire),to partly-touching and finally non-touching upon the wall(free fire)with increasing wall-fire separation distance was investigated.With increasing of separation distance,the flame lowest touching-wall point height increased,meanwhile the total flame height decreased.The physical mechanisms of flame touching wall behavior and the evolution behavior of the flame characteristics with different wall-fire separation distances were revealed.Based on theoretical analysis,prediction models of the flame touch wall probabilities and the flame characteristic parameters were proposed.Research reveals the temperature profiles and the flame extent evolution behaviors induced by wall-attached fire impinging upon an inclined ceiling.Firstly,a square burner attached to the wall with an inclined ceiling above was used to study the temperature profiles of ceiling jet flows.The temperature in the direction normal to the intersection line of the wall and the ceiling crossing through the nozzle center was measured.Results show that the inclination angle of the ceiling has no effect on the maximum temperature rise under the ceiling.The plume temperature near the plume impinging point increases with increasing of the ceiling inclination angle.And the plume temperature of the positive inclination angles in the far field from the plume impinging point are close to each other,however,that of the negative inclination angles decreases to the ambient temperature sharply.Improved prediction models of temperature profiles for different ceiling inclination angles were established based on the previous studies on horizontal ceiling jet flows in the literatures.Four rectangular burners of various aspect ratios(1:1-71:1)were used as fire sources to study the flame extent induced by wall-attached fire impinging upon an inclined ceiling.Five angles of the inclined ceiling were considered,namely-20°,-10°,0°(horizontal ceiling),10° and 20°.The flame extent in the x direction(normal to the intersection line of the wall and the ceiling)increased,whereas that in the y direction(parallel to the intersection line of the wall and the ceiling)decreased with increasing of the ceiling inclination angle(from-20° to 20°).The inclination angle has little effect on the flame area expanded under the ceiling.The measured flame extent data were then analyzed regarding the buoyancy effect on the change of the impinged flow velocity field distribution over the ceiling.Based on the mass and momentum conservation of the fire plume right impinging on the ceiling,new models were finally obtained to correlate the flame extents in the two directions.