同轴离心式喷嘴燃烧稳定性裕度评估

Combustion Stability Margin Assessment of Two Swirl Coaxial Injectors

关键词:  同轴离心式喷嘴  缩进长度  振荡燃烧  稳定性裕度  衰减系数

Key words:  Swirl coaxial injector  Recess length  Combustion oscillation  Stability margin  Damping coefficient

摘要:

为评估同轴离心式喷嘴燃烧稳定性裕度，开展了两种不同缩进长度喷嘴的稳定性试验研究。试验中燃料流量不变，逐步增加氧化剂流量，两种喷嘴在相同的混合比（2.28）发生振荡燃烧，均对应燃烧器第二阶纵向声模态频率。喷嘴A（缩进长度8.5mm）和喷嘴B（缩进长度12.5mm）最高振荡幅值对应的混合比分别为2.28和2.81。基于二阶非线性耦合振子模型，利用喷嘴稳定燃烧阶段的压力数据，分别从频域和时域识别系统衰减系数，发现两种方法得到的衰减系数在数量上一致，验证了两种分析方法的等价性。喷嘴A在不稳定发生前衰减系数突然增大，喷嘴B衰减系数呈递减趋势，不稳定发生时降到0.03以下，说明衰减系数可以评估喷嘴的稳定性裕度，但需要考虑噪声相干共振等非线性现象的影响。

Abstract:

An experimental investigation on determining safety margin to combustion instability of two laboratory-scale swirl coaxial injectors with different recess lengths was carried out. For a constant kerosene flow rate， with increasing mass flow rate of oxygen diluted with air， combustion oscillation corresponding to the second longitudinal mode of cylindrical combustor were excited at the same mixture ratio 2.28 for two injectors. The mixture ratios corresponding to the maximum oscillation amplitudes of nozzle A （recess length 8.5mm） and nozzle B （recess length 12.5mm） are 2.28 and 2.81， respectively. Based on the model structure of nonlinear second-order oscillators， the two approaches from time domain and frequency domain were used to quantify damping coefficient using the pressure data of steady combustion respectively. It is found that the damping coefficients obtained by the two approaches are consistent quantitatively， which verifies the equivalence between the two analysis methods. For injector A， damping coefficients rise up unexpectedly before the occurrence of combustion instability， while damping coefficients of injector B decreases to below 0.03 as the system moves from being strongly stable to the stability line. It is concluded that the damping coefficient is a sensitive index for predicting the safety margin to combustion instability while coherence resonance should be considered which is associated with the phenomenon of noise induced enhancement of deterministic dynamics in a nonlinear system.