火灾是现代社会最普遍的威胁公众安全和社会发展的主要灾害之一，传统的灭火技术在扑救室内高位火源时难度较大，对常规水雾进行定向活化和感应荷电并借助外加磁场对活化荷电细水雾进行引导，使荷电细水雾在洛伦兹力作用下发生偏转使其喷向高位火源或传统雾化喷头的喷射死角，可有效避免火势的蔓延。而在火场条件下，外加磁场的施加必然会对燃烧过程产生影响。由于火灾导致的扩散射流燃烧十分普遍，扩散射流火焰成为了火灾科学研究的重点。因此本文开展外加磁场作用下射流火焰响应机制研究，能为揭示荷电活化细水雾在磁场导流作用下破坏火焰结构的微观作用机制奠定基础。

本文在自由射流燃烧试验台的基础上增设一对电磁铁，研究了外加磁场作用下射流火焰的响应机制。利用高斯计对射流出口上方不同励磁电压的磁场分布规律进行了测量；利用高速摄像机对不同磁场条件下的火焰形态变化规律展开了研究，并结合MATLAB软件对火焰形态特征参数进行提取分析；采用接触式测温方式测量了磁场作用下火焰平均温度和射流出口温度；通过自制离子探针得到了射流火焰离子浓度分布及磁场作用下离子的迁移规律，定性揭示了外加磁场对火焰带电粒子的影响。

研究表明，射流出口上方负梯度磁场占主导地位，火焰轴线上存在磁感强度线性衰减的区域，衰减量在最大磁感强度的60%以上；相同喷嘴下自由射流火焰尺寸、火焰平均温度均随射流流量增加而不同程度地增加。相同流量下火焰高度、火焰平均温度随喷嘴尺寸的增加而减小。射流出口处的温度均随着喷嘴流量的增加呈现线性减小趋势。在火焰轴线上，火焰根部的离子电流最大，但最大不会超过1.0μA。

在负梯度磁场中，火焰形态被拉长，火焰的不稳定行为消失，表现为概率云图中高概率面积增大及火焰平均温度上升。射流出口处的离子电流值锐减，带电粒子迅速在靠近火焰尖端和火焰面的区域聚集。火焰流场变化是施加磁场后火焰燃烧特性响应的直接原因，根本原因是顺磁性的氧气在梯度磁场中受到磁场力导致其向磁场中心扩散造成火焰燃氧比的变化。火焰在磁场中的响应机制为磁场引导活化荷电细水雾灭火技术奠定了基础。

Fire is one of the main disasters that most commonly threaten public safety and social development in modern society. Traditional fire extinguishing technologies are more difficult to extinguish indoor high level fire sources. Directional activation and induction charging of conventional water mist are carried out, and the activated charged water mist is guided by an external magnetic field, causing the charged water mist to deflect under the Lorentz force, causing it to spray towards the high level fire source or the injection dead angle of traditional atomization nozzles. It can effectively avoid the spread of fire. In fire conditions, the application of an external magnetic field inevitably affects the combustion process. Due to the widespread of diffusion jet combustion caused by fires, diffusion jet flames have become a focus of fire science. Therefore, this thesis conducts a study on the response mechanism of jet flames in an external magnetic field, which can lay the foundation for revealing the microscopic mechanism of the damage to the flame structure by charged activated water mist under the guidance of a magnetic field.

In this thesis, a pair of electromagnets were added to the free jet combustion test rig to study the response mechanism of jet flame in external magnetic field. The magnetic field distribution above the jet outlet with different excitation voltages was measured using a Gauss meter; The change of flame shape in different magnetic field was studied by using high-speed cameras, and the characteristic parameters of flame shape were extracted and analyzed by combining MATLAB software; The contact temperature measurement method was used to measure the average flame temperature and jet outlet temperature in the magnetic field; The ion concentration distribution in the jet flame and the migration rule of ions in the magnetic field were obtained by using a self-made ion probe. The effects of external magnetic field on charged particles in the flame were revealed qualitatively.

The research shows that the inverse gradient magnetic field above the jet outlet dominates, and there is a linear attenuation region of the magnetic induction intensity along the flame axis, with the attenuation amount exceeding 60% of the maximum magnetic induction intensity; The size and average temperature of the free jet flame under the same nozzle increase to varying degrees with the increase of jet flow rate. The flame height and average flame temperature decrease with the increase of nozzle size under the same flow rate. The temperature at the jet outlet shows a linear decreasing trend with the increase of nozzle flow rate. On the flame axis, the ion current at the root of the flame is the highest, but the maximum will not exceed 1.0μA。

In the inverse gradient magnetic field, the flame shape is elongated. The unstable behavior of the flame disappears, manifested as an increase of the high probability area in the probability cloud map and an increase of the flame average temperature. The ionic current value at the jet outlet decreases sharply, and charged particles quickly gather in the region near the flame tip and flame surface. The change of flame flow field is the direct reason for the response of flame combustion characteristics in the magnetic field. The fundamental reason is that paramagnetic oxygen is subjected to a magnetic field force in the gradient magnetic field, resulting in its diffusion towards the center of the magnetic field, resulting in a change in the flame fuel oxygen ratio. The response mechanism of the flame in the magnetic field lays the foundation for the fire extinguishing technology of activation charged water mist guided by magnetic field.

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