旋转流动中浮力池火和火旋风的不稳定性机制与规律基础研究

The complex coupling between rotating flow and buoyant pool fire could result in many special types of combustion behavior including the fire whirl,which is a great threat to forest and urban fire safety. The research works mainly focused on the combustion dynamics of steady fire whirl until now, while quantitative experimental studies were few on the behavior before the formation of fire whirl and the instability in the transition process. Therefore, the coupling mechanisms and rules between external imposed circulation and buoyant fire plume are not clear at present. This work will build a rotating screen facility for both buoyant pool fire and fire whirl and experimental measurement systems. By controlling the external imposed circulation and fire burning rate precisely, this work will make systemic studies on the variation of combustion characteristics and velocity distributions under different fire source sizes, imposed external circulations and burning rates. Then we will reveal the dynamical mechanisms and rules of various instabilities and combustion behaviors that induced under different rotating conditions, and develop complete dynamical models for buoyant pool fire and fire whirl behaviors in rotating flow field. This work can enrich the scientific understandings on the mechanisms of instabilities induced by coupling effects between rotating flow and buoyant combustion and provide scientific basis for the forecasting and early warning of forest and urban fire safety. Therefore, this study has important scientific significance and application value.

旋转流动与浮力池火的复杂耦合作用会诱发包括火旋风在内的多种特殊燃烧行为，这对森林与城市消防安全构成了重要威胁。迄今为止，研究工作集中于稳态火旋风的燃烧动力学行为，缺乏对火旋风诱发前行为和转捩等不稳定性的定量实验研究，因而还不了解外加环量和浮力火羽流耦合作用引发的不稳定性机制和规律。本项目将建立旋转纱幕式浮力池火与火旋风模拟装置和实验测量系统。通过精确控制外加环量和火源燃烧速率，系统研究不同火源尺寸，外加环量和燃烧速率等关键参数条件下浮力池火和火旋风的燃烧特性和速度分布等的变化规律，从理论上揭示不同旋转条件下浮力池火和火旋风中出现的多种不稳定性及燃烧行为的动力学机制与规律，建立旋转流场中浮力池火和火旋风行为的完整动力学模型。本项目可以丰富对旋转流动-浮力燃烧耦合作用中不稳定性机制的科学认识，并为森林和城市消防预报、预警提供科学基础，因此具有重要的科学意义和应用价值。

火旋风是由普通浮力火焰与外加旋转流场耦合作用形成的典型极端火行为，它能大大增强火场强度并加快火蔓延速度。以往研究基本只关注准稳态常规火旋风的燃烧特性，对于很大外加环量范围内火旋风的燃烧行为及其临界形成等不稳定性不够了解。本项目新建了新式旋转纱幕式火旋风实验装置，实现了热释放速率和外加环量这两个关键参量的完全解耦和精细控制。此外，项目还改造了原有的中尺度固定边界式实验平台，实现了大功率气体燃料准稳态火旋风实验。通过开展系统精细实验和理论分析，1）发现了在特定的外加环量和热释放速率条件下会产生自由浮力火焰、倾斜火焰、弱火旋风、锥形火旋风、柱形火旋风、过渡态火旋风、弯曲柱状火旋风、火焰熄灭和不规则火焰等九种燃烧模式。火焰高度和直径随外加环量的变化与热释放速率和所处燃烧模式有很大关系。2）当外加环量小于临界值时，自由浮力火焰会转变为无旋倾斜火焰，且整个倾斜火焰绕着装置中心转动，其转动角速度和装置纱幕角速度完全相等。火焰倾斜角度的正切值大致正比于纱幕旋转速度。3）倾斜火焰与火旋风在不同的临界环量下能够相互转捩。对层流和湍流火旋风，临界无量纲外加环量Re分别与无量纲浮力流率B^1/4和B^1/3成比例。4）与普通浮力羽流相比，火旋风平均火焰区的空气卷吸系数减小了约1个数量级。新的火旋风火焰高度关系式将火焰高度与两个关键参量热释放速率和旋转强度耦合起来，并与不同类型燃料实验数据符合较好。5）火旋风火焰进动频率在固定边界式装置中接近于边界附近的流体转动速度，在旋转纱幕装置中等于纱幕转动频率。本研究完成了预定目标与任务，加深了对旋转流动和浮力燃烧非线性耦合机理与规律的科学认识，成果可为森林与城市火灾防控与应急管理提供科学依据，具有重要的科学意义和应用价值。