Abstract

Analysis of dynamic combustor stability is essential for the development of practical devices that employ distributed reaction zones. The fluctuation of reaction zone (RZ) was studied to compare normal air combustion and distributed combustion at an equivalence ratio of 0.9 in a swirl-stabilized burner using methane fuel. Distributed combustion was fostered by diluting the inlet air stream with carbon dioxide that resulted in a gradual drop in the adiabatic flame temperature (T_ad). High-speed broadband chemiluminescence at 3 kHz was used to study the global combustion dynamics. Vortex shedding observed along the shear layers influenced the oscillation of flame base in normal air combustion. However, relatively stable bases were observed in distributed combustion. Fast Fourier Transformation performed on RZ base fluctuation data supported the observations from chemiluminescence signatures. Proper orthogonal decomposition (POD) analysis indicated various vortex-shedding patterns as the dominant structures in conventional swirl flames. The power spectral density (PSD) of POD modes showed the rotational effect of RZ and weak von Karman vortex shedding to influence fluctuation in distributed combustion. The self-sustained flame oscillations in normal swirl flames were investigated from the acoustic and heat release fluctuation signals. Both acoustic and heat release fluctuations gradually reduced when approaching the distributed combustion. The PSD of these signals demonstrated a common peak at ~ 174 Hz (in normal swirl flames) with possible thermo-acoustic coupling. This peak was consistent with the peaks obtained from the base fluctuation and POD spectra. The peak gradually diminished when approaching the state of distributed combustion. A similar observation of gradually decreasing acoustic and heat release fluctuation was made with a constant T_ad. However, the drop in chemiluminescence signal intensity with CO₂ dilution was relatively low in case of constant T_ad compared to the variable T_ad case. The local and global Rayleigh index obtained for the variable T_ad cases at different dilution levels showed positive values at O₂ = 21 and 20%, which signified self-sustained instability. However, the distributed combustion possessed a negative Rayleigh index indicating dampening of instability. These results support the gradual decline of thermo-acoustic instability when approaching the distributed combustion condition.

Graphic abstract

中文翻译：

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分布式燃烧降低火焰波动的实验研究

摘要

动态燃烧室稳定性的分析对于开发采用分布式反应区的实用设备至关重要。研究了反应区（RZ）的波动，以比较在使用甲烷燃料的旋流稳定式燃烧器中，当量比为0.9时的正常空气燃烧和分布式燃烧。通过用二氧化碳稀释进口空气流来促进分布式燃烧，从而导致绝热火焰温度逐渐降低（T _ad）。3 kHz的高速宽带化学发光用于研究整体燃烧动力学。沿剪切层观察到的涡旋脱落影响了正常空气燃烧中火焰基体的振荡。但是，在分布式燃烧中观察到相对稳定的碱。对RZ基波起伏数据执行的快速傅里叶变换支持了化学发光标记的观察结果。正确的正交分解（POD）分析表明，各种旋涡脱落模式是常规旋流火焰中的主要结构。POD模式的功率谱密度（PSD）显示了RZ的旋转效应和弱的von Karman涡旋脱落，从而影响了分布式燃烧的波动。从声学和热释放波动信号中研究了正常旋流火焰中的自持火焰振荡。接近分布式燃烧时，声音和热量释放的波动都逐渐减小。这些信号的PSD在可能发生热声耦合的情况下，在〜174 Hz（在正常旋流火焰中）显示出一个共同的峰值。该峰与从基础涨落和POD光谱获得的峰一致。接近分布式燃烧状态时，峰值逐渐减小。在常数不变的情况下，也观察到逐渐减小的声波和热释放波动的类似现象。这些信号的PSD在可能发生热声耦合的情况下，在〜174 Hz（在正常旋流火焰中）显示出一个共同的峰值。该峰与从基础涨落和POD光谱获得的峰一致。接近分布式燃烧状态时，峰值逐渐减小。在常数不变的情况下，也观察到逐渐减小的声波和热释放波动的类似现象。这些信号的PSD在可能发生热声耦合的情况下，在〜174 Hz（在正常旋流火焰中）显示出一个共同的峰值。该峰与从基础涨落和POD光谱获得的峰一致。接近分布式燃烧状态时，峰值逐渐减小。在常数不变的情况下，也观察到逐渐减小的声波和热释放波动的类似现象。牛逼_的广告。然而，与可变T _ad情况相比，在恒定T _ad的情况下，用CO ₂稀释的化学发光信号强度的下降相对较低。在不同稀释水平下，可变T _ad病例获得的局部和全局瑞利指数在O ₂  = 21和20％时显示正值，这表明自我维持的不稳定性。但是，分布式燃烧的瑞利指数为负，表明不稳定性得到了抑制。这些结果支持当接近分布式燃烧条件时热声不稳定性逐渐下降。

图形摘要