Experimental and Numerical Evaluation of the Cavitation Performances of Self-Excited Oscillating Jets

Self-excited oscillating jets(SOJ)are used in several practical applications.Their performances are significantly affected by structural parameters and the target distance.In this study,a geometric model of the SOJ nozzle accounting for multiple structural parameters is introduced,then the related cavitation performances and the optimal target distance are investigated using a Large-Eddy Simulation(LES)approach.Results are also provided about an experiment,which was conducted to validate the simulation results.By analyzing the evolution of the vapor volume fraction at the nozzle outlet,a discussion is presented about the effect of the aforementioned structural parameters on the cavitation performances and the target distance.It is shown that the distribution of cavitation clouds at the outlet of the SOJ nozzle displays a non-monotonic trend(first increasing,then decreasing).Under working conditions with an inlet pressure of 4 MPa,a SOJ nozzle outlet/inlet diameter ratio(D_(1)/D_(2))of 1.2,and a chamber diameter ratio(D/L)close to 1.8,the nozzle outlet cavitation performance attains a maximum.The optimal structural parameters correspond to the optimal target distance,which is near 50 mm.The experiments have revealed that the SOJ nozzle with the above parameters displays a good cavitation erosion effect at the target distance of 50 mm,in satisfactory agreement with the numerical simulation results.

Impact frequency variation of self-excited oscillation pulsed supercritical carbon dioxide jets

In order to obtain the impact frequency of resonant coal breaking by self-excited oscillation pulsed supercritical carbon dioxide(SC-CO_(2))jet,large eddy simulation was used to analyze the formation and development process of self-excited oscillation pulsed SC-CO_(2)jet,the variation of jet impact frequency in the nozzle and the free flow field,and the variation of jet impact frequency at different positions in the jet axis and under different cavity lengths.The test device of jet impact frequency was developed,and experiments were performed to verify the conclusions of the numerical simulations.The results show that the frequency of the self-excited oscillation pulsed SC-CO_(2)jet is different in the nozzle and the free flow field.In the nozzle,the frequency generated by the fluid disturbance is the same,and the jet frequency at the exit of the nozzle is consistent with that inside the nozzle.In the free flow field,due to the compressibility of CO_(2),the pressure,velocity and other parameters of SC-CO_(2)jets have obvious fluctuation patterns.This feature causes the impact frequency of the self-excited oscillation pulsed SC-CO_(2)jet to decrease gradually in the axis.Changing the cavity length allows the adjustment of the jet impact frequency in the free flow field by affecting the disturbance frequency of the self-excited oscillation pulsed SC-CO_(2)jet inside the nozzle.

THEORY AND EXPERIMENTAL STUDY OF THE SELF-EXCITED OSCILLATION PULSED JET NOZZLE

Comparing with usual continuous jet nozzle, the self-excited oscillationpulsed jet nozzle SEOPJN) can make jet generate a higher peak of pressure and larger scouringvolume. And it can make jet increase the effective standoff distance, too. The basic theories of theSEOPJN are introduced. Some experimental results are shown. According to the results, using tricornbits assembled the SEOPJN to drill oil well, the ROP increases by 8 percent approx 77 percent, andthe rates of the footage for tricorn bit increases by 6.7 percent approx 44.0 percent.

Mechanism of the self-excited oscillation pipeline flocculator

This paper presents the research on the dynamic mechanism of flocculation based on the characteristcs of turbulent flow. The shearing force and the centrifugal force transferred by the vortex are the main forces to cause collision of flocculated grains in water and the shearing force is the primary one. Based on this mechanism, a new type of self-excited oscillation pipeline flocculator is designed.

The Theory and Experimental Study of the Self-Excited Oscillation Pulsed Jet Nozzle (Pipeline Pulsed Flow Generator)

In this paper, the basic theories of the Self-excited Oscillation Pulsed Jet Nozzle (SEOPJN) invented by the authors are introduced. Then, some experimental results are shown. According to the results, using tricorn bits assembled the SEOPJN to drill oil well, the rate of penetration (ROP) increases by 8% - 77%, and the rate of the footage for tricorn bit increases by 6.7% - 44%. Although the test was conducted in the water, good result was got in nature gas transportation. The volume of gas transportation could be increased by the Self-excited Oscillation Pulsed generator while the gas pressure drop could be decreased, since it significantly reduced the pressure loss during gas transportation.

Side force control on slender body by self-excited oscillation flag

Strong asymmetrical vortices appear on the leeward of slender body at high angles of attack, which has very unfavorable effect on the stability and control of the aircraft. A method is developed to control the side force of slender body at high angles of attack, and is verified in wind tunnel. A thin-film triangular self-excited oscillation flag is fixed at the tip of the slender body model whose semi-apex angle is 10°. Side force is approximately linearly proportional to roll-setting angle of self-excited oscillation flag at high angles of attack, and the slop of fitting straight line obtained by the least square method is -0.158. The linear relationship between side force and roU-setting angle provides convenience for developing side force control law of slender body at high angles of attack. Experimental data shows that the side force coefficients vary linearly with roll-setting angles when a specific plastic self-excited oscillation flag is used as the control flag. The range of side force coefficient and roll-setting angle are, respectively, -3.2 to 3.0 and -20° to 20°. The device is simple, effective, and is of great potential in engineering application.

Complex dynamics of an archetypal self-excited SD oscillator driven by moving belt friction

We propose an archetypal self-excited system driven by moving belt friction, which is constructed with the smooth and discontinuous （SD） oscillator proposed by the Cao et al. and the classical moving belt. The moving belt friction is modeled as the Coulomb friction to formulate the mathematical model of the proposed self-excited SD oscillator. The equilibrium states of the unperturbed system are obtained to show the complex equilibrium bifurcations. Phase portraits are depicted to present the hyperbolic structure transition, the multiple stick regions, and the friction-induced asymmetry phenomena. The numerical simulations are carried out to demonstrate the friction-induced vibration of multiple stick-slip phenomena and the stick-slip chaos in the perturbed self-excited system. The results presented here provide an opportunity for us to get insight into the mechanism of the complex friction-induced nonlinear dynamics in mechanical engineering and geography.

HARMONIC, SUBHARMONIC, SUPERHARMONIC, SIMULTANEOUS SUB/SUPER HARMONIC AND COMBINATION RESONANCES OF SELF-EXCITED TWO COUPLED SECOND ORDER SYSTEMS TO MULTI-FREQUENCY EXCITATION

Harmonic, subharmonic, superharmonic, simultaneous sub/super harmonic, and combination resonances of the additive type of self-excited two coupled-second order systems to multi-frequency excitation are investigated. The theoretical results are obtained by the multiple-scales method. The steady state amplitudes for each resonance are plotted, showing the influence of the different parameters. Analysis for each figure is given. Approximate solution corresponding to each type of resonance is determined. Stability analyses are carried out for each case.

Characteristics of Oscillation in Cavity of Helmholtz Nozzle Generating Self‑excited Pulsed Waterjet

Cavity flow oscillations in the axisymmetric cavity are critical to the operating efficiency of self-excited pulsed waterjets,which are widely employed in many practical applications.In this study,the behaviors of a turbulent flow in axisymmetric cavities causing cavity flow oscillations are investigated based on wall pressure characteristics.Experiments are performed using four Helmholtz nozzles with varying length-to-radius ratios at flow velocities of 20–80 m/s.Three orders of hydrodynamic modes in axisymmetric cavity are obtained through the spectral analysis of wall pressure.Based on the experimental results,the empirical coefficient of Rossiter’s formula is modified,and the values of the parameter phase lag and the ratio of convection velocity to free stream velocity are obtained as 0.061 and 0.511,respectively.In addition,the spectral peak with a relatively constant frequency shows that the flow-acoustic resonance is excited significantly.A modified model is introduced based on the fluidic networks to predict the lockon frequency.The results obtained can provide a basis for the structural optimization of the nozzle to improve the performance of self-excited pulsed waterjets.

A New Methodology for Early Detection of Thermoacoustic Combustion Oscillations Based on Permutation Entropy

We carry out a series of experimental investigations in a model combustor to detect a precursor of thermoacoustic combustion oscillations based on permutation entropy,which can amplify the subtle changes effected in the time sequence to identify the anomaly.By changing the flame’s location or the fuel flow to a value,an abrupt switch from aperiodic small-amplitude oscillations to periodic large-amplitude oscillations would occur in pressure fluctuations.The characteristic frequency of combustion oscillation is obtained by spectral analysis,with which a modified algorithm of the permutation entropy is proposed.The impact evaluation on key parameters such as moving step sizes and window sizes reveals that the moving data permutation entropy has strong robustness,and can accurately detect the onset of thermoacoustic oscillations.Further nonlinear analysis exhibits peculiar dynamics of the combustion system,which result in specific patterns in the time series and provide a theoretical basis for anomaly detection.Our results suggest that the permutation entropy has a certain potential in early warning and detection of combustion oscillations.

Internal Vibration and Synchronization of Four Coupled Self-Excited Elastic Beams

The vibration behavior and the synchronization between some internal points of four coupled self-excited beams are numerically studied. Coupling through the root of the beams is considered. The transverse displacements of the internal points and the beam tips are monitored, and the power spectra of the resulting time series are employed to determine the oscillation frequencies. The synchronization between beams is analyzed using phase portraits and correlation coefficients. Numerical results show multiple frequencies in the vibration pattern, and complex patterns of synchronization between pairs of beams.

Characterization of vortex structures with self-excited oscillations based on Liutex-Omega vortex identification method

The self-excited oscillation effect produces a continuous periodic pulsation without an external excitation source.It is widely used in fluid heat and mass transfer,cavitation and resistance reduction,and other related fields.The self-excited oscillation effect is significantly influenced by the vortex structure created by the jet passing through the specially designed cavity.The flow field in a self-excited oscillation cavity is simulated in this paper using the large eddy simulation(LES)method.The Liutex-Omega([Math Processing Error])method is used to analyze the vortex structure’s evolution inside the cavity and is contrasted with the Q-criterion,the λ_(2)-criterion,and the Omega(Ω)method.The studies indicate that the[Math Processing Error]method is less sensitive to threshold selection compared with other methods,while it is more capable of identifying weak vortices.The change in cavity vortex structure can be devided into the four stages of vortex ring priming,growth and development,wall touch separation,and fragmentation.The turbulent energy generated by shear effect can promote the growth and development of the vortex ring structure and has an important influence on the formation of the vortex ring structure.The vortex strength reveals the interaction mechanism between the shear effect and vortex rings.The vortex core area illustrates that the small-scale vortices are mainly distributed inside the collision walls of the cavity and the downstream flow channel.The Liutex-omega method has unique advantages in analyzing the cavity flow field and revealing the mechanism of self-excited oscillations.

燃气轮机燃烧场热声振荡的测试研究进展

氢燃料燃气轮机是目前最具有前景的研究方向,氢燃料特别是绿氢能源能够减少温室气体排放、提高能源利用率,但由于氢能燃烧化学反应速率较快、燃烧速度高,容易引发热声振荡现象。热声振荡是声波与热释放在相位差小于90°时耦合作用的结果,该现象会对燃气轮机造成疲劳损伤,甚至损坏部件,热声振荡的诱因包括火焰热释放波动、声压脉动、燃烧室内流场波动等,对这些参量的耦合研究可加深对热声振荡的认识,为预测热声振荡提供理论基础。热声振荡研究涉及到的测试参量包括动态压力、火焰热释放、温度、当量比、速度,对应的测试手段应具备高频测量的能力。动态压力的测试手段主要包括压力传感器和麦克风,由于压力的时域响应快,动态特征明显,是热声耦合作用的关键参量,研究最为广泛。火焰热释放主要使用OH*化学发光或者荧光信号来进行表征,测试手段包括图像增强型高速相机(ICCD)、光电倍增管(PMT)、平面激光诱导荧光(PLIF)等。温度测量除了常用的热电偶,光学诊断技术有可调谐二极管激光吸收光谱(TDLAS)、PLIF、拉曼光谱(RS)等。当量比作为对燃烧有直接影响的参量,烟气分析仪等传统手段难以进行动态测量,早期使用当量比波动方程来进行评估,后来发展的TDLAS、PLIF、激光诱导击穿光谱(LIBS)等光学测试技术都具备获得当量比动态波动的能力。速度脉动是直接作用于热声振荡的参量,测试技术包括一维的双麦克风测速、热线风速仪、激光多普勒测速(LDV),以及多维的粒子图像测速(PIV)、纹影等。至今为止,关于热声振荡测量的大部分技术发展较为成熟,列举了这些燃烧诊断技术的原理以及在热声振荡或不稳定燃烧现象中的应用,并且总结了热声振荡测试的发展与展望。

惰性气体射流协同控制火焰振荡和NO_(x)排放性能研究

预混火焰燃烧通常会出现热声振荡等问题,严重制约了燃气轮机或油气锅炉的安全运行和洁净排放。为此,提出通过惰性气体射流来调控预混燃烧过程,从而实现火焰振荡和NO_(x)排放的协同控制。惰性气体射流将导致预混火焰振荡模态迁移,既可降低火焰振荡幅值,还可使火焰振荡频率发生改变。相对原子质量大的氩气比相对原子质量小的氦气的控制效果好。惰性气体射流能够显著改变火焰结构,使预混火焰长度变短。

Experimental study and analysis of shock train self-excited oscillation in an isolator with background waves

A study of shock train self-excited oscillation in an isolator with background waves was implemented through a wind tunnel experiment.Dynamic pressure data were captured by high-frequency pressure measurements and the flow field was recorded by the high-speed Schlieren technique.The shock train structure was mostly asymmetrical during self-excited oscillation,regardless of its oscillation mode.We found that the pressure discontinuity caused by background waves was responsible for the asymmetry.On the wall where the pressure at the leading edge of the shock train was lower,a large separation region formed and the shock train deflected toward to the other wall.The oscillation mode of the shock train was related to the change of wall pressure in the oscillation range of its leading edge.The oscillation range and oscillation intensity of the shock train leading edge were affected by the wall pressure gradient induced by background waves.When located in a negative pressure gradient region,the oscillation of the leading edge strengthened;when located in a positive pressure gradient region,the oscillation weakened.To find out the cause of self-excited oscillation,correlation and phase analyses were performed.The results indicated that the instability of the separation region induced by the leading shock was the source of perturbation that caused self-excited oscillation,regardless of the oscillation mode of the shock train.

Self-excited oscillation of spinning solar sails utilizing solar radiation pressure

The present paper proposes a control method to excite spinning solar sail membranes for three-dimensional use.Using optical property switching,the input is given as the change in magnitude of the solar radiation pressure.The resonance point of this system varies with the vibration state due to its nonlinearity and the change in equilibrium state.To deal with this,a state feedback control law that automatically tracks the resonance point is developed in the present study.The proposed method enables decentralized control of the actuators on the sail,each of which determines the control input independently using only the information of vibration state.The proposed method is validated using numerical simulations.The results show that the nonlinear system behaves differently from the linear system,and the vibration grows using the decentralized control regardless of resonance point variation.

Lumped parameter model for resonant frequency estimation of a thermoacoustic engine with gas-liquid coupling oscillation

Gas-liquid coupling oscillation is a novel approach to reducing the resonant frequency and to elevating the pressure amplitude of a thermoacoustic engine.If a thermoacoustic engine is used to drive low-frequency pulse tube refrigerators,the frequency matching between the thermoacoustic engine and the refrigerator plays an important role.Based on an acoustic-electric analogy,a lumped parameter model is proposed to estimate the resonant frequency of a standing-wave thermoacoustic engine with gas-liquid coupling oscillation.Furthermore,a simplified lumped parameter model is also developed to reduce the computation complexity.The resonant frequency dependence on the mean pressure,the gas space volume,and the water column length is computed and analyzed.The impact of different working gases on the resonant frequency is also discussed.The effectiveness of the models is validated by comparing the computed results with the experimental data of the gas-liquid coupling oscillation system.An increase in the mean working pressure can lead to a rise in the resonant frequency,and a lower resonant frequency can be achieved by elongating the liquid column.In comparison with nitrogen and argon,carbon dioxide can realize a lower frequency due to a smaller specific heat ratio.

热声发动机中的交变流动换热器特性研究

为阐明热声发动机交变流动换热器内基本换热与阻力特性,以10 kW直线热管耦合自由活塞斯特林发电机的发动机核心为例,对三维模型进行等效,分别利用Sage一维和CFD二维模型两种方法,对交变流动换热单元在额定工况下的沿程基本状态参数、时均能量的分布特征进行计算并比较。结果表明:Sage一维模型中的换热分布特征及换热总量均与CFD二维模型计算结果相近,即Sage一维模型可以较好地捕捉换热器内的瞬态与时均特征;同时,Sage一维模型对换热器界面的突变截面损失则与CFD二维模型计算差异较大,可利用CFD二维模型的阻力计算结果,提出采用有效压降与有效速度获得突变截面处的局部阻力系数修正,并添加修正因子后代入Sage一维模型,可使Sage一维模型的阻力特性与CFD二维模型的阻力特性基本吻合。所得热声热机中换热器特性规律及分析方法同样可用于指导其他热声热机的相关研究与工程设计,具有一定的普适性。

径向分级燃烧室热声特性的试验研究

为了探究径向分级燃烧室燃料分配比例对燃烧室热声特性的影响,对工业级单筒4 MW功率等级径向分级燃烧室进行试验研究.试验中通过改变径向上值班喷嘴和主燃喷嘴的燃料分配比例,分析燃料分配比例对动态压力特征频率和幅值的影响.采用相空间重构方法和递归分析方法,探究燃料分配比例变化过程中燃烧室内热声状态的变化特征.结果表明,燃烧室内整体存在低频(70~90 Hz)和高频(200~230 Hz)2个主导频率.在燃料分配比例调节过程中,出现低频向高频跃迁现象.相空间重构图和递归图显示,当燃料分配比例增大时,燃烧室内热声状态存在切换过程:由准周期状态过渡为混沌状态,最终切换至极限环状态.

结合系统辨识的甲烷掺氢预混火焰热声稳定性研究

为了研究甲烷掺氢燃料对燃气轮机燃烧室热声振荡特性的影响,利用计算流体动力学结合系统辨识(CFD-SI)的方法对某旋流预混燃烧室进行了数值研究。在非定常CFD模拟中提取上游速度与火焰整体热释放率波动的时间序列,并通过相关性分析进行系统辨识,获取了不同甲烷掺氢比下的火焰传递函数(FTF),并结合低阶网络模型对燃烧室热声不稳定性进行了分析。结果表明,随着体积掺氢比的提高,火焰逐渐由“V”形变为“M”形,燃烧器出口处的涡结构先形成后消失,这导致了燃烧室热声振荡特性的改变。在体积掺氢比低于30%时,火焰响应不发生较大变化,燃烧室振荡频率小幅提高;当掺氢比在30%~60%时,火焰对低频的响应逐渐增强,对高频响应先增强后减弱,燃烧室在200Hz附近热声模态的振荡消失,火焰稳定性增强,但高频纯声学模态被激发;而当掺氢比达到60%时,燃烧室内重新出现热声模态的振荡。对于以天然气作为燃料的燃气轮机,适当的掺氢比能改善燃烧室内的热声不稳定性,但氢气加入会更容易激发燃烧室的高频纯声学模态,使其在高频范围内出现振荡。