Flame Spread Behavior Characteristics Over Typical Charring Solid Surfaces

Flame spread over solid surfaces is a basic issue in fire safety. The development of flame spread influences the initial growth of fire and the final fire scale. It is a very complicated process including the physical heat and mass transfer coupling with the chemical reaction both in solid and gas phases. Moreover, the multi-parameter coupling effects make the flame spread behaviors over solid surfaces more complex, and have not been entirely understood yet. It is benefical to investigate the multi-parameter effects on flame spread characteristics and to build a scientific and rational flame spread model for predicting the fire development accurately and preventing fire. In addition, more and more researchers begin to pay their attentions to the plateau fire safety. Investigating the flame spread behaviors over solid surfaces in the plateau is helpful to the plateau fire prevention and controlling.The whitewood is chosen as the typical charring combustible solid and the comparative flame spread experiments over which are carried out in the plateau (the Lhasa city) and the plain (the Hefei city) respectively. In these experiments, some typical features (flame shape, flame temperature, and flame spread rate, etc.) of flame spread over solid surfaces are measured, and the multi-parameter effects (the ambient pressure, the sample width, the sample orientation, and the external radiation intensity, etc.) on the flame spread over solid surfaces are investigated. Combined with the theories of heat transfer and combustion, the controlling mechanisms of flame spread over solid surfaces are investigated, and the prediction models for flame spread behaviors are built.Firstly, the jointly influences of the ambient pressure and the sample width on flame spread are investigated, and a flame spread model coupling with the pressure and sample width effects is built based on the heat transfer theory. This model indicates that the width effects on flame spread is controlled by two different heat transfer regimes:the convective regime and the radiative regime. These two competitive regimes cause that the flame spread rate rises firstly with the increasing sample width and then drops.In addition, the coupling effects of the ambient pressure, the sample orientation and the external radiation intensity on the flame spread behaviors are investigated. The investigation proposes that the coupling effects of the sample orientation and the radiation intensity changes the flame spread state, the temperature rise profile in solid phase, and the change profile of flame area as well. These joint increments would reduce the influence of chemical reaction (the pressure effects).Finally, the occurrence mechanisms of the mutations in flame spread over solid surfaces (the extinction, the deceleration and the acceleration, etc.) are discussed in detail. The deceleration of flame spread is firstly found and well explained in this paper by the energy and mass transfer balance theory between the solid phase pyro lysis and the gas combustion. The deceleration is caused actually by the unbalance between the ignition energy and the required heat of flame spreading. Furthermore, the occurrence mechanisms of extinction are discussed and a theoretical model is built to predict the critical extinction angle. In addition, the flame shapes and the heat transfer processes are analysis on the flame spread in the plain and plateau. The occurrence mechanisms of the acceleration in flame spread are discussed based on these analyses, and a theoretical model is built to predict the acceleration of flame spread, viz. Πc=1. This model takes the coupling effects of the ambient pressure, the ambient oxygen concentration and the sample orientation into account on flame acceleration.