实验流体力学

Progress in the basic application issues of the pulse detonation rocket engine

Fan Wei, Lu Wei, Wang Ke

2019, 33(1): 1-13. doi: 10.11729/syltlx20180105

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Abstract:
The pulse detonation rocket engine is a propulsion system which obtains thrust by repeatedly generating detonation waves. It has the potential advantages of high thermal efficiency, simple structure and wide applications. In order to realize the potential advantages, various theoretical and engineering problems need to be solved before it goes into practical application. Numerous efforts have been made to establish a systematic set of theories and guidelines that could be universally applied to detonation-based propulsion designs, thereby laying solid theoretical and technical foundations for the birth of novel aerospace propulsion engines. The progress in some basic application issues in the past few decades is summarized, including the two-phase de-tonation engine technology, short-distance and low-loss detonation initiation technology, perfor-mance optimization and prototype integration. For the two-phase detonation engine, researches on the velocity deficit in two-phase detonation and researches on the atomization and fuel-oxidizer mixing are introduced. Some new ideas are developed in recent years. To pursue a better propulsion performance, liquid fuel is pre-heated to improve the atomization performance. For the short-distance and low-loss detonation initiation, researches on the detonation initiation improvements using solid obstacles, fluidic obstacles, hot jet or shock focusing are involved. Employing solid obstacles is the most common technology used in the pulse detonation engine, whereas employing fluid obstacles can accelerate the detonation initiation with lower loss of propulsion performance. For the performance optimization, the partial filling effect, high efficient nozzles and high frequency operation technologies are discussed. Partial filling and proper nozzles are proved effective to enhance the propulsion performance, while more accurate theory is needed. Recent researches indicate that pulse detonation engines can reach a high operating frequency in the valveless mode. For the prototype integration, a few pulse detonation rocket engines launched by different institutions and the relevant experimental researches are introduced as well.

Progress on light-round ignition dynamics in annular combustor

Wang Gaofeng, Xia Yifan, Ye Chenran, Hu Keqi, Linghu Changhong

2019, 33(1): 14-28. doi: 10.11729/syltlx20180090

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Abstract:
Annular combustors are generally applied in aero-engines. Study on light-round ignition dynamics of annular combustors is important for ignition reliability. Laboratory-scale mo-dels have become a feasible way to investigate the ignition mechanism of annular combustors due to its low economic cost and high precision. Several typical laboratory-scale annular combustors and related experiments are reviewed, including the annular combustor MICCA from EM2C in France, the premixed or non-premixed annular combustor made by Cambridge University, the annular combustor model referred from an industrial gas turbine in Technical University Munich, and the annular combustor TurboCombo with the coupling of the combustor and the turbine interaction made by Zhejiang University. The ignition process can be generally divided into three phases:(1) the formation of a flame kernel in a flammable mixture around the igniter; (2) the kernel expands and grows to be a swirling flame, which is stabilized and anchored upon the adjacent burner; (3) the propagation of the flame (light-round), which successively igniting all the burners and then reaching to steady state. The factors influencing the light-round process are quite complicated. Previous experimental and numerical investigations focus on the equivalence ratio, ignition mode, thermal power, bulk velocity, spacing between burners and so on, which influence the characteristics of the ignition, flameout, flame propagation mode and light-round time in the annular combustor. Recently, the spray combustion is also studied in the similar models. Meanwhile, the application of advanced laser diagnostics with high resolution would promote the understanding of the light-round mechanism.

Progress on spray autoignition under the extreme conditions in aero-engines

Gao Wei, Zhang Chi, He Chunlong, Lin Yuzhen

2019, 33(1): 29-40. doi: 10.11729/syltlx20180120

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Abstract:
The lean premixed prevaporized (LPP) combustion is the most advanced low emission combustion technology of civil aero-engines. However, the LPP technology faces the risks of autoignition and flashback during the premixing process, which becomes the bottle-neck restricting the development of LPP technology. Under the extreme inlet conditions of high temperature (up to 1000K) and high pressure (up to 6MPa) in the aero-engine combustor, the autoignition in premixed and prevaporised section is a liquid fuel spray autoignition process in confinement. This paper reviews and analyzes the experimental research on the spray autoignition. Firstly, the spray autoignition process in the LPP combustor of civil aero-engine is described, the influencing factors and characteristics of spray autoignition are analyzed, and the key research directions are pointed out. Secondly, the study on the chemical autoignition is briefly reviewed and the effects of parameters on chemical autoignition are summarized. Finally, the experimental research status of spray autoigniton is analyzed, the research progress of autoigniton randomness under the extreme conditions in aero-engines is demonstrated, and the issues and subsequent developments are discussed.

Review of studies on oblique detonation waves in supersonic flows

Miao Shikun, Zhou Jin, Liu Yu, Liu Shijie, Lin Zhiyong

2019, 33(1): 41-53. doi: 10.11729/syltlx20180078

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Abstract:
The research progress and development status of studies on oblique detonation waves in supersonic flows are summarized. Experimental investigations on the initiation and stability of oblique detonation waves are firstly provided, especially on the initiation criterion and the formation condition of different transition structures. Then studies on the transition structure and the cellular structure are reviewed, where the effects of the inflow condition and the wedge angle on the wave structures are introduced. Besides, studies on the interaction between the oblique detonation wave and the turbulence boundary layer are reviewed. Some challenges in current studies are analyzed and some suggestions on future studies on oblique detonation waves are given.

Mechanism of in-cylinder turbulence on the distribution of fuel activity in hybrid combustion

Feng Yifang, Xie Hui, Chen Tao, Zhao Hua

2019, 33(1): 54-61,71. doi: 10.11729/syltlx20180096

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Abstract:
The interaction between fuel and turbulence under the in-cylinder limited conditions is the key issue for hybrid combustion controlled by fuel activity stratification. Dimethyl ether (DME) is injected to the cylinder to produce high activity fuel stratification. Particle image velocimetry, laser Rayleigh scattering, Mie scattering and high speed imaging combined with heat release analysis on the optical engine experiment platform are used to observe the flow field and combustion process of hybrid combustion in the limited space of cylinder. 3D Computational Fluid Dynamics (CFD) simulation are used to explain the experimental phenomena. Result shows that there is a large range of counter-clockwise vortex field in the cylinder, and the diffusion and evaporation process of DME is influenced by the flow. Under the flow field, the combustion process in the cylinder shows characteristics of DME auto-ignition in the distribution area, flame propagation, multi-point auto-ignition.

Flow field visualization for ethylene/air auto-ignition at different pressures and temperatures in a rectangular shock tube

Liu Erwei, Xu Shengli

2019, 33(1): 62-71. doi: 10.11729/syltlx20180051

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Abstract:
Flow field visualization for the auto-ignition of the ethylene/air mixture is key important in understanding properties and characteristics of the flame generation and propagation. High-speed photography was used in experiments and ICCD camera was triggered by the flame signal located at the test section. The series of images for the auto-ignition flow field were obtained at different temperatures (T₅) and pressures (p₅) in a rectangular shock tube. In the case of p₅ being 106kPa, auto-ignition occurs and the initial flame is close to the tube end of shock reflection when T₅ is 1210K. Then, the flame propagates upstream (right side) and evolves as a planar one. The vortex-lets are obviously embedded in the flame surface. The flame front gradually evolves to be vertical to the tube axis as it propagates upstream. In contrast to decreasing T₅, the initial flame becomes thicker and moves away from the tube end of shock reflection. More-over, the flame emission becomes weaker corresponding to the lower reaction rate. Also, the flame decays from a planar front into several irregular zones. When T₅ is 1077K, the auto-ignition occurs far away from the tube end of shock reflection, and then propagates upstream and downstream. With increasing p₅, the flames emit strongly and the vortex-lets almost disappear on flame surfaces. When p₅ are 265 and 419kPa, respectively, the local explosions take place frequently. These local explosions merge into an approximately planar front and propagate upstream with time going by.

Experimental study on flow structure and flame development in a hydrogen-fueled supersonic combustor

Tian Ye, Le Jialing, Yang Shunhua, Zhong Fuyu

2019, 33(1): 72-78. doi: 10.11729/syltlx20180027

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Abstract:
The flow structure and flame development were studied experimentally in a hydrogen-fueled scramjet combustor. Wall pressure measurement, schlieren, differential interferometry, high-speed framing of flame luminosity and OH-PLIF (planar laser-induced fluorescence) were introduced to characterize the combustion flow. The synchronous measurement path diagram of schlieren, differential interferometry and PLIF was developed in this study, and the coupling results of flow structure and flame were obtained. The results show that:in the five studied cases, the combustion flow structure is unstable when the equivalent ratio of hydrogen is greater than 0.17. The fragmentation flame spreads in the flow between the top and bottom wall. When the equivalent ratio of hydrogen is less than 0.17, the continuum flame is stable and located in the cavity shear layer.

Experimental study of the flow fields of the impinging jet flames using Laser Doppler Velocimetry (LDV)

Fang Yuanqi, Li Lin, Zhong Liang, Yu Yu, Cai Guohan, Chen Kaixi, Chen Jiao, Wang Gaofeng

2019, 33(1): 79-88. doi: 10.11729/syltlx20160161

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Abstract:
A Laser Doppler Velocimetry system containing a particle generator, a particle collector and a motorized precision translation stage is built to diagnose the fluid fields of a vertical flow burner. The free jet flame and impinging jet flame are investigated, both for a single nozzle (200W power) and a coaxial dual-nozzle (1200W power). An adaptable signal to noise ratio (SNR) threshold is analyzed and employed for post-processing. The experimental data shows high repeatability and accuracy in multiple measurements. For impinging jet cases, the Reynolds numbers (Re) of low power and high power flame are 1200 and 7200, respectively. The mean velocity vectors and contours are sketched from the measurements at different axial and radial positions, displaying the main characteristics of the impinging jet flame. Meanwhile, a peak of the horizontal velocity occurs roughly at one-nozzle-diameter distance departed from the nozzle axis in the near-wall region. This feature possibly provides an explanation for the mechanism of the secondary peak of the heat transfer captured in previous literatures. For the cases of coaxial jet, a mixing region exists between the outer annular jet and the core jet:the mixing zone is gradually damped with the development of the free jet flame, whereas radially expanding in the impinging flame driven by the high-pressure stagnation region.

Investigation on flow field structure of rotating detonation using the method of characteristics

Gong Jishuang, Zhou Lin, Zhang Yining, Teng Honghui

2019, 33(1): 89-96. doi: 10.11729/syltlx20180072

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Abstract:
The characteristics theory and the corresponding calculation methods are classical methods of gas dynamics, which have high computational efficiency in the rotating detonation flow field analysis. The analysis of the rotating detonation flow field is simplified in the wave-fixed reference frame, and the steady-state flow field calculation method is established by using the method of characteristics and the unit processes. The effect of the equivalence ratio and the plenum stagnation conditions on the rotating detonation flow field structure of premixed hydrogen/air, methane/air and octane/air is studied. The calculated results show that the detonation wave height and inclination angle are significantly affected by the equivalence ratio and the stagnation temperature, and decrease as the fuel changing from the small molecule hydrogen to the macromolecular hydrocarbon. The equivalence ratio and the stagnation temperature mainly affect the macroscopic flow field structure by affecting the propagation velocity, height and inclination angle of the detonation wave.

Study of characterization methods of supersonic combustion flame based on fractal geometry

Cheng Liuwei, Zhong Fengquan, Du Mengmeng, Gu Hongbin, Zhang Xinyu

2019, 33(1): 97-102. doi: 10.11729/syltlx20180084

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Abstract:
Fractal geometry is a new subject of graphics. By means of fractal geometry, ir-regular graphics can be studied, the self-similarity characteristics of graphs can be revealed, and quantitative data of graph self-similarity can be given. In this paper, the fractal geometry is used to analyze the flame morphology in the supersonic airflow, and the variation law of the flame fractal dimension under the condition of different equivalent ratios and fuel component molar ratios is quantitatively analyzed. The relationship between the velocity of the turbulent flame propagation and the fractal dimension of the flame boundary is studied. In this paper, the flame CH* self-luminescent transient image obtained by the high-speed photography is used to record the flame morphology of different fuels in the Mach number 2.5 supersonic airflow. The experimental results show that the fractal dimension of the frontal boundary of the supersonic combustion turbulent flame increases approximately linearly with the increase of the equivalent ratio, and increases with the increase of the hydrogen component in the fuel.

Numerical study on propagation characteristics of back-pressure in a pulse detonation engine

Li Qing'an, Wang Ke, Sun Tianyu, Fan Minghua, Fan Wei

2019, 33(1): 103-110. doi: 10.11729/syltlx20180093

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Abstract:
An engine flow path of a base geometry consists of the elaborately designed isolator and a detonation combustor with a length to diameter ratio of 20, along with a comparison group of four different geometries, were studied numerically by means of single detonation to investigate the propagation characteristics of back-pressure in an air-breathing pulse detonation engine. Parameters of the back-pressure, including propagation speed, pressure peak, and its decay rate as well as the total pressure recovery coefficient of the base model were considered and discussed. The results demonstrate that the designed isolator is able to reduce the back-propagation speed and the back-pressure peak effectively. The detonation combustor with a larger length to diameter ratio contains more fuel and oxidant, which needs more efforts to prevent the back-pressure. The decay rate of the pressure peak is mainly affected by the geometry of the isolator at the early stage of the back-propagation process, and afterwards it depends on the distance of the back-propagation. When the inlet pressure is given, the detonation combustor with a smaller length to diameter ratio has a more rapid exhaust process, and therefore, a slighter back-pressure propagation phenomenon. Large total pressure losses are not found in the designed isolator when the Mach number of the incoming flow is between 0.15~0.80 under sea level conditions.

2019 Vol. 33, No. 1