Effects of swirler position on flame response and combustion instabilities

This study is concerned with the experimental and theoretical investigation of the combustion instabilities in a premixed swirl combustor. It is focused on the effects of the swirl mixing distance on the intrinsic thermoacoustic mode. The swirler as an origin of the swirling flow is also the source of the flow disturbance, which has effects on the flame response. The location of the swirler is varied in the experiment to study the effect on combustion instabilities and flame transfer functions. ...     

Experimental study on flow excursion instability of supercritical hydrocarbon fuel in scramjet regenerative cooling parallel channels

Flow instability of supercritical hydrocarbon fuel is a crucial issue in scramjet regenerative cooling structure. In this study, flow excursion instability and flow distribution in parallel tubes were experimentally studied for supercritical fluids. Two types of flow excursion occur in a single tube. Type I and Type II excursions, and they are corresponding to decreasing and increasing flow rate respectively. They can trigger flow maldistribution between parallel tubes and the hysteresis phenomenon of flow distribution. The effects of system parameters, including inlet temperature, system pressure, and heat flux, on flow distribution were analyzed. In addition, the relationship between flow excursion and the pseudo-critical interval proposed in the literature was established according to the heated tube outlet temperature at the onset of flow instability. Finally, the flow excursion instability boundary was obtained using two dimensionless parameters. These experimental results can provide helpful insight on the mechanism of Scramjet regenerative cooling.     

Similarity phenomena of lean swirling flames at different bulk velocities with acoustic disturbances

In this study, flame responses to acoustic disturbances with different frequencies and amplitudes were experimentally investigated in a lean premixed swirl-stabilized combustor operating at different bulk velocities. The total heat release rate fluctuations and spatial CH* chemiluminescence distributions were captured using a photomultiplier tube and high-speed camera, respectively. The results indicate that the heat release rate exhibits a relatively drastic oscillation and high-order harmonics for low-frequency disturbances. When the bulk velocity and forcing frequency were doubled simultaneously, similar flame structures were observed in the CH* chemiluminescence distributions. As the bulk velocity increases, the gain of the Flame Describing Function (FDF) extends toward the higher frequencies, and the delay time of the flame response decreases. The similarity among FDFs at different bulk velocities was effectively captured by introducing a non-dimensional parameter, defined as the ratio of the flame response delay to the forcing time scale, to replace the dimensional forcing frequency. Furthermore, the availability of the newly defined non-dimensional parameter was verified for flames with different swirl numbers, as this played an important role in determining the flame structures and associated unsteady heat release rate.     

Effect of baffle injectors on the first-order tangential acoustic mode in a cylindrical combustor

Combustion instability is a very important issue in the development of the propulsion systems used in aerospace. It is very important to associate the high frequency combustion instabilities with the acoustic characteristics of the combustion chamber. In this paper, the effects of various baffle injectors which were installed on the injector faceplate on the first-order tangential acoustic mode were investigated theoretically and experimentally. The effects of the gap between adjacent injectors on the first-order tangential acoustic mode in a cylindrical chamber were considered. The acoustic admittance of the injectors was derived. The results showed that the amplitude and frequency of the first-order tangential acoustic mode increase with the increase in the gap between adjacent injectors, but decrease with the increase in the number and height of the baffles. The baffle injectors have a greater influence on the amplitude and frequency of the first-order tangential acoustic mode than the baffle blades.     

低中等雷诺数超声速轴对称射流气动声场

采用线性抛物化稳定性方程(LPSE)方法对低、中等雷诺数条件下的超声速轴对称射流进行了研究。结果表明:射流中不稳定波的增长特性决定了声辐射的特性。在低雷诺数条件下，不稳定波的频率范围较窄，且由于不稳定波的增长区间较长，因此声辐射的范围较宽，声源不集中，声辐射的方向角不明确。在中雷诺数条件下，不稳定波的频率范围较宽，且由于不稳定波的增长区间较短，因此声辐射的范围较窄，声源集中，声辐射的方向角较明确；且随着斯特劳哈尔数增加，声辐射的方向角增加。在低雷诺数条件下，射流基本流的法向速度对射流气动声场的影响较弱，而在中等雷诺数条件下，其对射流的气动声场影响较强。      

静不稳定制导火箭弹控制系统设计

选取鸭式布局结构制导火箭弹作为研究对象,从改善制导火箭弹的飞行性能的要求出发,对静不稳定火箭弹自动驾驶仪进行了设计,对弹道上不同飞行高度和不同飞行速度条件下的自动驾驶仪响应进行仿真研究,结果表明,所设计的自动驾驶仪能够保证控制系统的性能稳定。     

基于纳米多孔薄膜的蒸发特性

实验研究了基于纳米多孔薄膜的蒸发特性，以氟化液FC-72为液态工质，对比分析了开口向上和向下时的相变特性。结果发现：两种工况下，其热流密度-过热度曲线具有相似的变化趋势，且均出现热流密度增加而温度保持不变的“薄液膜蒸发”区间。在低热流密度下，温度均较为稳定，但开口向下时的传热性能始终优于开口向上时，分析原因为重力对供液的影响和液膜厚度对热阻的改变。随着热流密度的增加，表面温度波动也越来越剧烈，甚至出现了温度的峰值，最终开口向下和向上达到的临界热流密度值分别为59 W/cm2和47W/cm2；同时，对蒸发过程进行理论建模，得出其蒸发系数分别为0.043 1和0.021 9。     

On a plastic instability criterion for ultra-large radial-axial ring rolling process with four guide rolls

A dynamic mechanical model is proposed to describe the complexing actions of all the rolls on the ring during the ultra-large radial-axial ring rolling(RARR) process with four guide rolls. Based on the model, the calculation models for bending moment and normal stress at any section of the ring are deduced by force method. If the maximum section bending normal stress exceeds the yield stress of the ring materials, the ring will be distorted thus leading to the instability of the RARR process. Ac...     

Numerical simulations of a continuously injected relativistic electron beam relaxation into a plasma with large-scale density gradients

In this paper the influence of large-scale decreasing and increasing gradients of the density of magnetized plasma on the relaxation process of a continuously injected relativistic electron beam with an energy of 660 keV ($v_b=0.9c$) and a pitch-angle distribution is studied using particle-in-cell numerical simulations. It is found that for the selected parameters in the case of a smoothly decreasing gradient and in a homogeneous plasma the formation of spatially limited plasma oscillations of large amplitude occurs. In such cases, modulation instability develops and a long-wave longitudinal modulation of the ion density is formed. In addition, the large amplitude of plasma waves accelerates plasma electrons to energies on the order of the beam energy. In the case of increasing and sharply decreasing gradients, a significant decrease in the amplitude of plasma oscillations and the formation of a turbulent ion density spectrum are observed. The possibility of acceleration of beam electrons to energies more than 2 times higher than the initial energy of the beam particles is also demonstrated. This process takes place not only during beam propagation in growing plasma density, but also in homogeneous plasma due to interaction of beam particles with plasma oscillations of large amplitude.     

A Review of the Attachment Line Instability for Hybrid Laminar Flow Control

This paper focuses on the investigation of the attachment line instability for Hybrid Laminar Flow Control （HLFC）, one of the most promising drag reduction technologies for modern transport aircraft respect to high Reynolds numbers and large sweep angles. The attachment line instability also plays an important role during laminar turbulent transition control and HLFC design on a swept wing. The overview of historical research is presented and knowledge gaps are pointed out as the conclusion.