Purpose

The purpose of this paper is to investigate the development process of the fire whirl in the fixed-frame facility and focus on the impacts of the fire whirl’s vortex core on the formation and flame structure of the fire whirl.

Design/methodology/approach

The complex turbulent reacting flame surface is captured by the large eddy simulation turbulence closure coupled with two sub-grid scale (SGS) kinetic schemes (i.e. the chemistry equilibrium and steady diffusion flamelet). Numerical predictions are validated thoroughly against the measurements by Lei et al. (2015) with excellent agreements. A double maximum tangential velocity refinement approach is proposed to quantify the vortex cores’ instantaneous location and region, addressing the missing definition in other studies.

Findings

The numerical results show that the transition process of the fire whirl is dominated by the vortex core movement, which is related to the centripetal force. The unsteadiness of the fully developed fire whirl was found depending on the instantaneous fluctuation of heat release rate. The steady diffusion flamelet scheme is essential to capture the instantaneous fluctuation. Furthermore, the axial velocity inside the vortex core is the key to determining the state of fire whirl.

Practical implications

Due to intensive interactions between buoyant fires and ambient rotating flow, the on-set and formation of fire whirl still remain largely elusive. This paper focused on the transition process of fire whirl between different development stages. This paper provides insights into the transition process from the inclined flame to the fire whirls based on the centripetal force.

Originality/value

This paper presented and compared two SGS kinetic schemes to resolve the fire whirl development process and the unsteadiness of its vortical structures. The modelling framework addresses the shortcoming of previous numerical studies where RANS turbulence closure and simplified combustion kinetics was adopted. Numerical results also revealed the fire whirl transition process and its relationship to centripetal force.

中文翻译：

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使用两种燃烧动力学考虑捕捉火焰结构和火焰旋涡的过渡过程

目的

本文旨在研究固定框架设施中火旋涡的发展过程，重点研究火旋涡的涡核对火旋涡的形成和火焰结构的影响。

设计/方法/方法

复杂的湍流反应火焰表面被大涡模拟湍流闭合结合两个亚网格尺度（SGS）动力学方案（即化学平衡和稳定扩散小火焰）捕获。Lei等人的测量结果彻底验证了数值预测。（2015 年）达成了良好的协议。提出了一种双最大切向速度细化方法来量化涡核的瞬时位置和区域，解决了其他研究中缺失的定义。

发现

数值结果表明，火旋涡的过渡过程以涡核运动为主，与向心力有关。充分发展的火旋涡的不稳定取决于热释放率的瞬时波动。稳定扩散小火焰方案对于捕捉瞬时波动至关重要。此外，涡核内部的轴向速度是确定火旋涡状态的关键。

实际影响

由于浮力火和环境旋转流之间的强烈相互作用，火旋风的发生和形成仍然很大程度上难以捉摸。本文重点研究了火旋风在不同发展阶段之间的过渡过程。本文提供了基于向心力的从倾斜火焰到火焰漩涡的转变过程的见解。

原创性/价值

本文介绍并比较了两种SGS动力学方案，以解决火旋发展过程及其涡旋结构的不稳定性。该建模框架解决了以前采用 RANS 湍流闭合和简化燃烧动力学的数值研究的缺点。数值结果还揭示了火旋流转变过程及其与向心力的关系。