Studies On Facade Flame Entrainment And Flame Height From A Compartment Under Various Opening And Side Wall Constraint Boundary Conditions

In recent years, three dimensional flame spread upon the building facade has been observed in many high-rise building fire accidents around the world, which can lead to catastrophic loss of casualties and properties. Most of the building fires begin with compartment fires inside. For a fully developed under-ventilated compartment fire, the flames can be observed to eject from the opening of the compartment with hot gases and smokes, then spread to upper floors. Thus, the special phenomenon of flame ejecting behavior has been formed. As the first stage of three dimensional flame spread, extensive investigations have been addressed on such flame ejecting behavior, on which the characteristic parameters and regulations are focused, including temperature profile, heat radiation intensity, heat flux profile as well as flame shape and dimensions. On the other hand, various boundary conditions (e. g. vertical facing wall and slopping facing wall) and atmospheric pressures have been also taken into consideration to the flame ejecting behavior studies, with some prediction model proposed. However, most of the current works are based on a very simple model, that the flame is ejected from a compartment with only one opening, which is also differed from the actual building constructions. Moreover, since little work has been reported on the process of flame ejection, the opening related factors (e.g. vertical opening position and opening amount) and effects of exterior side walls, some more works are still needed here to clarify these points.This paper presents an experimental investigation and theoretical analysis on flame ejecting behavior upon building facade from a fire compartment. Based on similarity principle, reduced scale model of1:4and1:8were established as fire simulation test facility. We set up a series of experiments for different opening dimensions, amounts and vertical positions, as well as various heat release rates to study the flame ejecting behavior, with measurement system collects the temperature, flame height and heat flux signals. Two side walls were also employed in our experiments to look into the phenomenon and investigate the side wall effects on flame ejecting behavior, where the separation distances and lengths of side walls varied. Combined with the knowledge of classical fire dynamics of building fires and the analysis of flame entrainment model for different conditions, dimensionless characteristic parameter theoretical prediction models were brought up. The specific work includes the following three aspects:(1) For free boundary condition, the transitional regime of flame intermittent ejected from the opening is observed through experiments. We define the probability of flame intermittent ejection to describe such flame intermittent ejecting behavior. The lower and upper critical condition, as well as the range of the transitional regime is obtained. From energy conservation equation, we discuss the temperature profile inside the fire compartment and analysis on the opening-factor-coupled heat generation and heat loss competition. The correlation of flame intermittent ejection probability with the heat release rate and opening factor is proposed. On the other hand, the vertical opening position is changed in the experiments to reveal the difference in critical condition of flame ejection and flame base height. The revolution and mechanism of facade flame height under various vertical opening position are discussed and the explicit solution model of flame height is proposed.(2) Taking the multiple opening condition in real building rooms into account, we carry out the study on spill plume from a fire compartment with double parallel openings. The flame merging behavior is observed in the experiments, after they spilled out from the opening, and the flame merging probability is defined to describe such phenomenon. The critical condition of flame merging is obtained incorporating some relevant factors. The correlation of flame merging probability is proposed. Based on image process method, the flame merging distance from neutral plane is acquired with dimensionless exponential model proposed. Considering the equivalent model of a rectangular fire, the comparison has been made between the entrainment of ejected flame from single opening and double parallel opening conditions, indicating an interaction mechanism between flames. An additional entrainment area is introduced in facade flame height discussions. The facade flame height model including the opening dimensions, opening separation distances and heat release rates is then built up.(3) Considering the side wall structure in real buildings, we conduct the investigation on flame ejecting behavior and measure the characteristic parameters under side wall constraint boundary conditions. The effect of side wall separation distance on flame entrainment within different flame ejecting stages (i.e."wall fire" and "half axisymmetric fire") has been presented, and the entrainment model with side wall constraints proposed. Furthermore, we also discuss the side wall length effect. Two different flame ejecting regimes are found while the side wall length is increased, then the entrainment model is modified. Compared the facade flame heights under side wall constraints with those of free boundary condition, the correction factor incorporating the side wall separation distance, side wall length, characteristic length scales as well as heat release rates is introduced here to establish the model of facade flame height and vertical heat flux profile under various side wall constraint boundary conditions.