|Alternative Title||Mixing Enhancement and Flow Mechanism of Highly Underexpanded Jet Excited by Vortex Disturbance|
|Place of Conferral||北京|
|Keyword||高度欠膨胀射流 涡激励 混合增强 大涡模拟 超燃冲压发动机|
本文向高度欠膨胀射流施加的涡扰动由低压抽吸带来的速度剪切作用所诱导生成，而此涡扰动的强度随着抽吸压比（SPR）的减小而增大，其对流场的激励程度也随着SPR的减小而增大。研究中通过对自由和三组不同SPR（0.8、0.5和0.2）激励下高度欠膨胀射流的大涡模拟发现:（1）施加涡扰动会增加流场中的大小尺度涡结构并加快射流失稳和湍流转捩过程，从而使射流与空气的混合效率明显提高，并且涡扰动越强，混合增强效果越明显，但同时射流的核心区也明显变短;（2）通过对射流波系结构的瞬态演化分析，涡扰动会显著改变波系结构的建立过程，并破坏激波振荡周期，使得马赫盘从直线形变为弯曲形，同时使其宽度明显变大，马赫盘的强度减低，初始膨胀区变大，马赫盘后的激波胞格结构消失，且伴随涡扰动的增强，这种效应也越明显;（3）涡扰动会明显改变涡量的生成和消失规律，SPR = 0.5的激励射流中涡扰动使所有涡量生成项大小在扰动施加区域增大了至少一个量级，并改变了流场中涡量的空间分布;（4）通过分析射流声场发现，涡扰动改变了声波传播的角度和声源位置，涡扰动越强的激励射流其声源位置越靠近扰动施加位置；（5）通过螺旋度和压力脉动频谱给出了射流剪切层的主不稳定模态，发现涡扰动将自由射流剪切层的“1+1”单螺旋模态改变为顺逆时针螺旋交替出现的“n+n”多螺旋分支模态，同时在定量上也改变了激波啸叫频率，激发了整个压力脉动频域，增大涡扰动会使剪切层脉动振幅越大，特征频率越多，表现出更为复杂的主不稳定模态。
The mixing problem of supersonic fuel jet with air is considered as the key technology in scramjets, and has attracted many researchers around the world. The mixing efficiency of fuel and oxidant directly affects the later combustion performance, since improving the mixing efficiency can contribute to a higher combustion efficiency and combustion stability, and can also broaden the combustion limit. The highly underexpanded jet, as a typical flow pattern for parallel injection in the combustion chamber of supersonic engines, is the main research object for studying the mixing problem of fuel/air. It has been shown that imposing a certain degree of external excitation to the jet body can achieve the fuel/air mixing enhancement by regulating the flow characteristics of the jet. On account of the complex structures and short flow characteristic time of the underexpanded jet, large eddy simulation (LES) is regarded as one of the preferred methods to accurately predict the characteristic structures and turbulent fluctuations in the flow fields. Therefore, the present study takes the supersonic highly underexpanded parallel jet as the research object, and applies the LES method with a high spatial-temporal resolution to model free highly underexpanded jet and excited jet with vortex excitation. The results can deepen the theoretical knowledge of the mixing mechanism of the compressible highly underexpanded jets, and also contribute to the optimization of fuel injection schemes and mixing enhancement methods in engineering applications. The main research contents and conclusions are summarized as follows:
The vortex excitation applied to the underexpanded jet in this study is induced by the shear stress caused by the low-pressure suction. The intensity of the vortex excitation increases as the suction pressure ratio (SPR) decreases, so the jet flow fields tend to be more excited as the SPR decreases. The phenomena revealed by LES of free jet and three sets of excited jets (SPR = 0.8, 0.5 and 0.2) are concluded as follows: (1) the successful imposing of vortex excitation increases the number of small-scale vortices and promotes the development of the large-scale vortices, which is beneficial to accelerate the jet instability and turbulence transition, and then significantly enhance the mixing of fuel/air. The stronger the vortex excitation, the more efficient the mixing enhancement. However, the jet potential core tends to be shorter when the jet is obviously excited; (2) the analysis of wave structures revealed that the establishment process of the wave structures is remarkably changed by the vortex excitation, for example, by changing the Mach disk from a straight shape to a curved one and making its width significantly larger, which is related to a larger initial expansion area, a reduced strength of the Mach disk and vanishment of the shock cells behind the Mach disk. Moreover, the vortex excitation destroys the shock oscillation periods of the free underexpanded jet. The above changes tend to be more obvious with the enhancement of the vortex excitation; (3) the vortex excitation distinctly changes the distribution of vorticity generation and disappearance. The vortex excitation in excited jet with SPR = 0.5 amplifies all vorticity generation terms by at least one order of magnitude in the region where the excitation is imposed, and changes the distribution of the vorticity generation terms in the whole flow fields; (4) by analyzing the sound fields of free and excited jets, it is found that the vortex excitation alters the propagation angle of the sound waves and the location of the sound source. The stronger the vortex excitation, the closer the sound source location is to the region where the excitation is placed; (5) the dominant instability modes of the jet shear layer are shown by the helicity and the frequency spectrum of pressure fluctuations. It is found that the vortex disturbance converts the “1+1” single helical mode in the free jet shear layer to the “n+n” multi-branch helical mode with alternating clockwise and counterclockwise helices. Quantitatively, the vortex disturbance changes the shock screech tone, excites the whole frequency domain. Moreover, enhancing the vortex excitation can lead to larger amplitudes of shear layer pulsation, more characteristic frequencies, and also more complicated dominant instability modes.
|汪也. 涡扰动激励下高度欠膨胀射流的混合增强特性及流动机理[D]. 北京. 中国科学院大学,2019.|
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