旋流数对小限制率旋流喷雾及火焰特性的影响

Effects of Swirl Number on Swirled Spray and Flame Characteristics at Small Confinement Ratio

关键词:  贫油直喷  小限制率  旋流数  回流区  喷雾  火焰结构

Key words:  Lean direct injection  Small confinement rate  Swirl number  Recirculation zone  Spray  Flame structure

摘要:

分布式贫油直喷（Distributed Lean Direct Injection，DLDI）燃烧室是国外多点贫油直喷（MLDI）燃烧室的实用发展形式。本文对DLDI燃烧室的主燃级单元LDI开展研究，主要关注外旋流器旋流数（Sn）从0.65降低到0.33对流场、喷雾和火焰结构的影响。利用FLUENT软件、采用雷诺平均方程（RANS）对时均流场进行求解；利用离散相模型（Discrete Phase Model）对喷雾散布进行模拟；利用Mie散射和激光粒度测量仪对喷雾散布和SMD（Sauter Mean Diameter）进行测量，并对仿真结果进行验证；利用高速摄像机拍摄火焰结构。研究结果显示Sn变化直接改变流场结构：随着Sn的减小，外旋流射流对内旋流射流的压制逐渐变强，内旋流射流的张角和中心回流区尺寸都逐渐缩小；尤其在Sn为0.33时，角涡回流区演化成壁面回流区。流场变化影响液雾散布：Sn在0.38~0.65区间时，喷雾核心主要由内旋流射流输运，喷雾张角由内旋流射特性决定；当Sn为0.33时，喷雾核心会在自身惯性下射入外旋流射流，在外旋流射流以及壁面回流区的作用下均匀散布。喷雾散布结果表明喷雾核心射入外旋流射流时更有利于液雾的散布。火焰结构的研究结果显示在小限制率条件下，Sn为0.33时，中心回流区回流量不足、无法稳定火焰，此时形成的壁面回流区创造了新的稳火点来帮助稳定火焰。

Abstract:

Distributed lean direct injection （DLDI） combustor is a development form of multi-point lean direct injection （MLDI） combustor. The elemental main-stage LDI injector of the DLDI combustor was studied in this paper. Effects of the swirl number （Sn） of the outer swirler varying from 0.65 to 0.33 on the flow field， the spray dispersion and flame characteristics have been studied. The commercial software FLUENT was used to simulate the flow field by using the RANS method. The DPM （Discrete Phase Model） was used to simulate the spray dispersion. Mie scattering and Laser-particle-sizer were used to measure the spray dispersion and SMD， respectively. The accuracy of simulation calculation can be checked by measured results. The high-speed camera was used to capture the flame structure. Results show that the flow field distribution is directly affected by the change of the swirl number. With the decrease of swirl number， the outer swirling air jet makes a stronger suppressing effect upon the inner swirling air， which leads to the both decrease of the opening angle of the inner swirling air and the size of the center recirculation zone. Especially when Sn=0.33， the corner recirculation zone gradually evolves into the wall recirculation zone. Changes of the flow field affect the spray distribution. When Sn varies from 0.65 to 0.38， the spray core is mainly dispersed under the transportation of the inner swirling air jet， due to which the opening angle is determined by characteristics of the inner swirling air jet. When Sn=0.33， the spray can inject into the outer swirling air jet under the inertia effect， and then distributed by the simultaneous action of the outer swirling air jet and the wall recirculation zone. It can be concluded that it is benefit for even spray distribution when the spray core can inject into the outer swirling air jet. Flame structure shows that at small confinement ratio， when Sn=0.33， the small recirculated air flow rate reflects the poor recirculation capability of the center recirculation zone， which cannot hold the flame. At this time， the wall recirculation zone can help to hold the flame.