Studies on Fire Plumes and Flame Diffusion Combustion Characteristics under the Constraints Effects of Wall and Ceiling

墙壁與頂棚限制火羽流與火焰擴散燃燒特性研究

Student thesis: Doctoral Thesis

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Award date11 Jul 2022

Abstract

In reality, the scenes of fire accidents are often complicated. For example, fires in a compartment, corridors, and tunnels are often accompanied by the constrained effects of walls and ceilings. These constraints will seriously affect the air entrainment and flow behaviour of fire plumes, hence the distribution of characteristic parameters, such as heat flux and temperature, thus directly changing the development and hazard of fire. Therefore, fire plume behaviour under the constraint space is a fundamental scientific problem in fire safety and should be a concern. This work considered the fire combusting under wall constraint to follow the process of burning behaviour in the building, then impinging the ceiling and further igniting the ceiling combustible, as well as taking into the effect of ambient wind. The fire plume and flame diffusion behaviour of wall and ceiling constraints are studied throughout the fire development process. Through the combined research method of experimental research, theoretical analysis, and numerical simulation, the evolution of the flame behaviour under different constraints was pursued. The flame morphological characteristic parameters were quantified. Moreover, the distribution of temperature and total heat flux were analysed. The ceiling cellular flame structure and ceiling pulsation behaviour were focused on. Many theoretical prediction models are proposed. Relevant conclusions can provide an essential reference for fire risk alarms, fire protection standards, fire facilities design, and numerical simulation research. The major findings include:

(1) The morphological characteristics of diffused flame with or without sidewall constraints and crosswind was compared and analysed. Most previous studies mainly concentrated on the sidewall flame in no wind condition, while coupling between sidewall constraint and crosswind usually existed in fire scenarios. The flame morphology evolution law under the coupling effect of sidewall constraint and crosswind was revealed. The wall temperature profile demonstrates the evolution behaviour of sidewall flame plume under crosswind. By dimensional analysis, the dimensionless models of flame morphological parameters were proposed by coupling the effect of sidewall and crosswind.

(2) The flame extension behaviour of flame plume impinging the ceiling under corner constraint was disclosed. Previous studies have researched the fire plume impinging the ceiling under the free or near wall condition. However, the buildings have corner walls with different angles in reality. This work analysed the effects of different corner angles on impinging the ceiling flame structure. 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 the ceiling, the model of impinging flame lengths and ceiling temperature, and heat flux profile were proposed.

(3) The particular cellular flame structure under ceiling constraints was studied. This ceiling fire with the burning of the combustible ceiling is completely different from that of a flame impinging the ceiling as there is no accelerated rise stage of the fire plume. The scaling analysis proposed the characteristic length of ceiling flame evolution behaviour, and the models of ceiling flame extension length, temperature, and heat flux profile was suggested. The temperature gradients of the ceiling flame in the vertical direction were found. Based on the Rayleigh-Taylor instability theory, the formation mechanism of cellular flame was revealed, and the size and spatial distribution of flame cells were analysed. Meanwhile, the special pulsation behaviour of the ceiling flame was found, and the physics of ceiling flame pulsation modes was disclosed. The flame lengths fluctuation scale and kinematic features of vortical structures were analysed. A dimensionless model of ceiling flame pulsation frequency was proposed based on the air entrainment characteristics of ceiling flame.

(4) The burning of long confined ceiling combustible is usually coupled with the influence of crosswind. This work revealed the evolution of confined ceiling flame morphology with crosswind speed. The macroscopic flame characteristics were analysed by ceiling temperature profile. The long confined ceiling flame presented an ellipse structure in no wind condition due to the asymmetric entrainment limitation of the two sidewalls. Meanwhile, the flame gradually shrinks downstream into a straight line flame with the increase of crosswind speed. A dimensionless heat release rate was proposed to couple the effect of ceiling flame buoyancy and crosswind based on the special air entrainment of long confined ceiling fire. Then, the dimensionless models of flame morphological parameters were deduced.

    Research areas

  • Sidewall constraint, Corner constraint, Ceiling constraint, Long confined ceiling, Flame morphological characteristics, Temperature and heat flux distribution, Cellular flame, Flame pulsation