射流扩散火焰在日常生产生活和能源动力等领域的应用非常普遍，掌握射流扩散火焰特性对于实际燃烧设备设计以及相关的火灾安全工作具有重要指导意义。稳定性是扩散火焰中关注的重点问题之一，对层流和湍流气体射流扩散火焰已存在较多的研究工作，但由于涉及流动、化学反应、热质输运以及它们之间的相互耦合，过程和机理十分复杂，目前对很多问题的认识还不清楚。地面常重力环境中，浮力流动及其效应影响火焰的结构和稳定，其与射流流动、化学反应之间的非线性叠加使得问题更难处理。火焰向湍流状态转捩是扩散燃烧研究中的一个基本问题，同时，转捩将层流与湍流火焰联系起来，对转捩火焰和弱湍流火焰（低Reynolds数火焰）进行研究有助于认识和理解扩散燃烧的基本过程、火焰稳定特性以及浮力影响。本文以气体射流扩散火焰的转捩过程为对象，开展了系统的实验工作，对层流、转捩和湍流火焰的稳定特性以及浮力效应的影响进行研究。主要工作及结论如下：

通过实验和数值模拟方法对层流附着火焰的稳定机理进行研究，发现在层流附着火焰根部局部气流速度远小于燃烧速度，混合层内部分预混火焰的传播因局部熄灭而终止，提出了基于火焰传播和熄灭动态平衡的火焰稳定机理，揭示了火焰根部热损失的规律，并基于能量平衡对火焰根部建立简化的熄火模型，分析得到了混合层厚度与局部气流速度和层流燃烧速度之间的关系式，通过该式能够合理解释和预测火焰根部的稳定。此外，通过对比反向射流（燃料射流垂直向下喷射）火焰与常规的正向射流（燃料射流垂直向上喷射）火焰，考察了浮力效应对附着火焰结构和稳定性的影响，浮力作用形式的改变导致反向射流火焰高度增大、碳烟形成区更大、吹熄极限更高。

对附着火焰的转捩过程进行实验，并通过伴流考察了浮力效应对火焰转捩过程和稳定特性的影响。相比静止环境，较大的伴流速度能够减小浮力效应对火焰转捩过程的影响，使得转捩发生时的临界Reynolds数（Re_cr）增大，即推迟转捩，而较小的伴流速度下，Re_cr保持不变。伴流速度较小时，转捩过程中的射流火焰发生周期性振荡，振荡幅度随伴流速度的增大而减小，继续增大伴流速度将限制浮力对流对火焰不稳定性的作用，中心射流对火焰的控制占据主导地位，火焰振荡转而呈现随机性。不同状态火焰的燃烧产物特性也有明显的不同，特别体现在转捩阶段CO和NO的浓度急剧下降。

首次对推举火焰的转捩过程进行实验研究，并考察了伴流的影响。推举火焰向湍流的转捩主要由紊乱射流和火焰根部之间的相互作用引起，转捩完成之前，射流表现出间歇性破碎现象，此现象消失可作为火焰转捩阶段完成的标志之一。转捩过程中推举火焰发生周期性振荡，振荡频率与层流附着火焰一致。但与附着火焰相反，伴流导致推举火焰的转捩提前发生，这主要是因为相较于燃料射流与周围空气之间的剪切层速度梯度而言，推举高度对伴流更为敏感，而推举高度较大时火焰根部附近的紊乱射流可以更为充分地发展，促进火焰转捩。由推举火焰转捩机制的特点可推测，浮力效应对转捩临界条件的影响较小。层流、转捩和湍流火焰的燃烧产物有显著的差异，尤其是转捩阶段CO和NO₂的浓度大幅增加，而NO的浓度急剧减小。

通过PIV流场测量和统计分析，获得了推举火焰根部附近流场的脉动速度和积分尺度进一步计算得到了转捩和湍流火焰根部的燃烧速度，并与局部气流速度进行比较，发现层流和转捩初期二者基本平衡，那么可以通过燃烧速度与气流速度之间的平衡来解释火焰的稳定，但随着射流Reynolds数增加，二者之间的差距有扩大趋势，表明火焰存在新的稳定特性。此外，考察了火焰与失稳流动（湍流）之间的相互作用，发现当射流破碎以后火焰能够很快响应表现出湍流特征，而当湍流射流变得相对平静时，火焰并不能很快响应，其在一定时间内仍保持湍流状态，火焰与流动之间发生失耦。转捩阶段，由于火焰上游射流边缘产生了较为规则的大尺度结构，速度场出现较为低频的流场脉动。涡结构的运动会引起火焰向上收缩或向下拓展，涡结构内的气体可以由不同成分组成，当主要包含燃料/空气的混合气体时接近火焰引起其下移，而当主要包含空气时则引起火焰的上移，涡结构对火焰的稳定有不可忽视的作用。

通过落塔实验对微重力下推举火焰的转捩过程进行研究，并与常重力结果进行比较，进一步认识了浮力对转捩过程和火焰稳定特性的影响。相比常重力下的转捩过程，微重力环境中由于浮力对流基本消失，转捩结束时的Reynolds数增大300左右，但转捩发生时的临界Reynolds数一致。转捩过程中，微重力火焰高度为常重力结果的2倍左右，推举高度却始终低于常重力结果，该差异随射流速度的增大逐渐减小。两种环境下由于射流的间歇性破碎火焰均发生严重的分裂现象，继续增加射流速度，该现象逐渐变弱。利用火焰Froude数和无量纲火焰高度关联了两种环境中的转捩火焰和弱湍流火焰高度，浮力控制区无量纲火焰高度随Froude数的增大而增大，而当Froude数较大时，其增长速率明显变缓，表明微重力下的转捩和弱湍流火焰基本处于动量控制区，但仍受到残余重力的影响。根据湍流预混理论公式预测的湍流火焰推举高度与本文实验结果存在一定的差距，从另外一方面表明弱湍流射流推举火焰的稳定机理还需要考虑其他影响因素的作用，对弱湍流火焰开展研究将为认识扩散火焰的基本特性提供一个值得关注的途径。

Jet diffusion flames are widly employed in areas of dialy life, industry, energy and power and so on. Investigating characteristics of jet diffusion flame is of important significance to the design of pratical combustion equipments, and the related fire safety issues. Flame stability is one of the important problems in diffusion flames. There are many investigations about the characteristic of stabilization for laminar and turbulent gas jet diffusion flames. However, due to involving in flow, chemical reaction, heat and mass transport, and the coupling of these elements, making the process and mechanism very complex, our understandings about many problems are not clear. In the normal gravity environment, buoyant flow has effects on the flame structure and stability. Furthermore, the nonlinear coupling with jet flow and chemical reaction makes it more difficult. Flame transition is a basic problem in combustion. Moreover, the transitional stage connects the laminar- to turbulent-stage. Therefor, investigating the transition from laminar to turbulent flame has important roles in understanding the basic combustion process, the flame stabilization, and buoyancy effects. This work investigated the flame stabilization characteristics and buoyancy effects during the transitional process for the attached and lifted gas jet diffusion flames by conducting system experiments. The main work and conclusions are as follows:

Experiments and simulations on the stabilization mechanism of the burner-attached flames in laminar non-premixed jets were conducted. For the burner-attached flames, the stabilization mechanism is contrary to the balance between the local flow velocity and the burning speed, indicating that the propagation of partially premixed flame within the mixing layer at flame base is inhibited due to flame attachment. The flame stabilization mechanism based on the dynamic balance between the extinguishment and the propagation was put forward. And the effects of heat losses on the flame stabilization were revealed. In addition, a simplified analysis is provided based on the balance between the heat released by chemical reaction and the heat lost by conduction and convection, deriving a correlation for the clear dependence of the thickness of the fuel/air mixing layer on the local jet velocity and the laminar flame speed. From such relation, the flame base stability can be reasonably explained and predicted. On the other hand, buoyant laminar jet diffusion flames were studied experimentally in an inverted configuration, where gaseous fuel-stream jets vertically downward into air. By comparing with the conventional jet flames, which are established when the fuel jets upward, the different relative directions of buoyant flows and jet streams result in flow deceleration within downward buoyant flames, thus slowing mixing process between fuel and air. Therefor, downward flame yields larger flame height, and contains more soot and the soot formation region is wider. Because of increased characteristic flame residence time, downward flames have higher blowoff limits.

The transition to turbulence of coflow burner-attached jet diffusion flames was investigated experimentally with varying air coflow velocities, and the buoyancy-suppressing effect of coflow was analyzed to reveal the buoyancy effects on the transition and stability of diffusion flames. Compared to jet flames in quiescent air, a coflow with relatively high velocity is shown to suppress the influence of buoyancy on transitional flames, resulting in a larger critical nozzle Reynolds number of transition to turbulent flame, i.e., the transition process is delayed by coflow; when the coflow velocity is small, however, the critical Reynolds number stays almost the same. In the transitional regime, diffusion flames are characterized by periodic oscillation, and the oscillation amplitude decreases with increasing coflow velocity. As coflow velocity increases further, it suppresses effects of buoyant flow on the developing of the instability occurred in flame boundary. Thus random oscillation is observed. Besides, the combustion products of laminar, transitional and turbulent flames have clear difference, especially for the transitional stage in which concentrations of CO and NO all occur a sharp decrease.

Studied the transition to turbulence of lifted jet diffusion flames experimentally at various air coflow velocities, effects of coflow air on the transition and stability were analyzed. The transition to turbulence for the lifted flame is mainly caused by the interaction between turbulent jets and the flame base. Before the end of transitional regime, the breakup of jet exhibits intermittency. However, as the jet velocity increases, this intermittency disappears in the transition end point. Therefore, this phenomenon can be as one of the symbols for determining the transition end point. During the transitional process, the lifted flames are also characterized by periodic oscillation, and the frequency range is similar to results of the laminar buoyant flames. In addition, it is shown that the combustion product gases during the transition process are significant different, particularly to the transitional regime where concentrations of CO and NO₂ increase rapidly, and the NO concentration occurs a sharp decrease.

By the PIV measurement results and statistical analysis, the intergral scale and the turbulence intensity near the lifted flame base are obtained, especially revealing characteristics of the flame transition and the combustion fields for the weak turbulent flames. Further calculating the turbulent flame speed, and compared to the local flow velocity, it is found that the flame speed is nearly equal to the local flow velocity at laminar- and the original transitional-stage, indicating that the flame stabilization can be explained based on their balance, while the tendency of difference between the two velocities increases with the Reynolds number, indicating the new stabilization characteristics. Besides, the flow velocity exhibites relatively regular oscillation because the jet boundary produces the relatively regular large scale structures. The flame becomes turbulent immediately after the jet breakup, while as the turbulent jet gradually becomes laminar, it still keeps turbulent within some time, occurring decoupling between the flame and the flow. The large scale structures will make the flame base move up or down. When the vortex mainly cantain fuel/air mixing gases, the flame will move up. But when the air is the main gas, the flame will move down as it approaches to the base. Effects of votices on the flame stabilization can not be neglected.

The transition to turbulence of lifted jet diffusion flames was investigated by microgravity experiments. Results show that the lifted flame in microgravity yields a larger Reynolds number at the end of the transitional stage (ng: Re = 2650; μg: Re = 2940), while the critical Reynolds numbers of transition to turbulent flame in different gravity levels are the same. During the transitional process, the flame heights in microgravity are approximately twice the normal gravity results. The liftoff heights in microgravity are always lower than results of normal gravity, and this difference gradually becomes less with the jet velocity increasing. Besides, as the jet velocity is increased to a certain number, the transitional flames in normal- and micro-gravity all occur severe separation. As the jet velocity is increased, this phenomenon is gradually weak. In addition, connecting the flame heights in normal- and micro-gravity through the flame Froude number, it is found that the dimensionless flame heights in the buoyancy-controlled region increases with the flame Froude number. While its growth rate decreases clearly when the Froude number is relatively large. It indicates that the transitional flame and the weak turbulent flame in microgravity are mainly controlled by the momentum. But they are still influenced by buoyancy due to the remnant gravity. Lastly, according to the turbulent premixed flame theory, the turbulent flame liftoff heights are predicted. There are some differences between the predicted and the present data, indicating that the stabilization mechanism of the weak turbulent flames is influenced by other elements. So it provides a good approach to understand the basic characteristics of diffusion flames by investigating the weak turbulent flames.