实验流体力学

A review of optical diagnostic platforms and techniques applied in internal combustion engines

HE Xu, WU Yue, MA Xiao, LI Yanfei, QI Yunliang, LIU Zechang, XU Yifan, ZHOU Yang, LI Xiongwei, LIU Cong, FENG Haitao, LIU Fushui

2020, 34(3): 1-52. doi: 10.11729/syltlx20200003

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Abstract:
The Internal Combustion engine (IC engine) is one of the most widely applied power machines in modern industry. Investigating the mechanisms of and developing control strategies for IC engines are of practical importance and give rise to interesting scientific issues, as the fuel penetration, evaporation and ignition inside the engine can tremendously affect the structure reliability, power efficiency and pollutant generation. In recent years, lots of efforts have been performed to achieve deeper understanding of the working processes of IC engines by applying experimental optical diagnostic techniques in engine-like laboratory platforms. This review starts with introducing the engine-like platforms (e.g. Constant Volume Combustion Bomb(CVCB), Rapid Compression Machine(RCM), optical engine, etc.) developed to experimentally simulate the practical working processes of practical IC engines. Moreover, multiple advanced optical diagnostic techniques are discussed, including their basic principles and particular applications for the study of detailed processes in IC engines. Specifically, two categories of optical diagnostic techniques are respectively discussed, including the traditional diagnostic techniques based on conventional optics (e.g. schlieren, Two Color Method, etc), and the laser-based diagnostic techniques (e.g. Particle Image Velocimetry, Laser Induced Fluorescence, etc). These techniques offer advantages to examine the spraying, evaporation and combustion processes of the IC engines by measuring the temperature, concentrations, droplet sizes and other valuable characteristics with multi-scale resolution. Furthermore, the diagnostic techniques enable deeper insights into the nature of the flow/combustion under high ambient pressure and temperature, which benefits us from understanding the physical and chemical mechanisms of engine processes in both macro and micro scales. This brief review is intended to be beneficial for both researchers and engineers to analyze the current shortcomings and limitations of the IC engines, and to design the state-of-the-art IC engines with better power performance, energy efficiency and pollutant reduction. Besides, the review paper is also intended to provide a guideline for researchers to conduct further fundamental experiments in IC engines to investigate the flow and combustion mechanisms.

Experimental measurement of the flow field of a swirling flame under large amplitude acoustic forcing

LIU Xunchen, WANG Guoqing

2020, 34(3): 53-60. doi: 10.11729/syltlx20190165

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Abstract:
The dynamic of a swirling flame under high acoustic perturbation characterizes its non-linear acoustic response. Here, it shows that the time-dependent flow field of a swirling flame under large amplitude acoustic forcing, measured by high-repetition rate pulse burst laser Particle Image Velocimetry (PIV) technique, is highly non-linear. The periodic vortex structures formed in the inner and outer shear layers of the swirling flame can interact with the flame front and flow field though distinct manners:the vortex ring formed in the outer shear layer folds the flame front and alters the flame heat release rate; the vortex ring formed in the inner shear layer mainly affects the velocity distribution in the recirculation zone. We quantitatively analyzed the effects of the outer/inner vortex rings, including the trajectory, vorticity, circulation and size. We found that exit velocity and acceleration are the main factors that determine the formation and release of the out shear layer vortex ring.

Experimental study on ignition and combustion of pulverized coal particles clouds under laminar and turbulent conditions

QI Sheng, LIU Siyu, XIN Shirong, HE Yong, LIU Yingzu, WANG Zhihua

2020, 34(3): 61-69. doi: 10.11729/syltlx20200033

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Abstract:
The ignition and combustion behaviors of pulverized coal particles clouds in a jet with different levels of turbulence and primary O₂ mole fractions were evaluated in a lab-scale optical entrained flow reactor. Bituminous particles were injected into the hot flue gas environment produced by a Hencken burner, with environment temperatures varying from 1200 K to 1700 K and secondary O₂ mole fractions from 0.1 to 0.3. Digital photography and OH-Planar Laser-Induced Fluorescence (OH-PLIF) techniques were employed to record the coal jet flame behaviors and capture the transient structure of the flame. The coal jet flame was narrow and smooth under laminar conditions, and become wider as the turbulent intensity of the primary flow increased. The OH-PLIF data reveals that upstream of the turbulent coal jet flame, reactions occur only at the periphery of the clusters of the pulverized coal particles where the high-temperature environment provides sufficient heat and oxygen. Downstream of the turbulent coal jet flame, reactions could occur also within the clusters of the pulverized coal particles, because of the continuous coal devolatilization process along the streams, and the mixing process between the volatiles and the oxygen entrained by the secondary flow. The ignition delay time is reduced and the combustion intensity is enhanced with the increase of the environment temperature and O₂ mole fraction in the primary or secondary flow. The frontal OH edge expands inward to the centerline of the clusters of the pulverized coal particles with the increase of the primary O₂ mole fraction. In addition, the effects of the environmental temperature and O₂ mole fraction on the ignition of pulverized coal particles clouds exhibit a threshold phenomenon. Once the environment temperature or the O₂ mole fraction exceeds a certain threshold, its influence on the ignition delay of the coal jet becomes weaker and the leading factors controlling the ignition behavior of the pulverized coal particle clouds change accordingly.

Visualization of flame structure in supersonic combustion by Planar Laser Induced Fluorescence technique

WU Ge, LI Yun, WAN Minggang, ZHU Jiajian, YANG Yixin, SUN Mingbo

2020, 34(3): 70-77. doi: 10.11729/syltlx20190168

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Abstract:
Planar Laser Induced Fluorescence (PLIF) can be used to visualize the flame structure with high temporal and spatial resolution and investigate the mechanism of supersonic combustion. In this paper, OH-PLIF and CH-PLIF techniques were used to study the flame structure in supersonic combustion. The cavity-stabilized reaction zone structure of the three streamwise sections and two spanwise sections in a supersonic combustor was obtained by using the OH-PLIF technique. The experimental results show that the combustion occurs in the inner cavity and the OH radicals are distributed symmetrically along the central axis at a low global equivalence ratio. The OH radicals are primarily distributed at two-side-wall of the combustor and the location of the flame is higher than that of the cavity at a high equivalence ratio. The heat-release structure of the cavity-stabilized flame was observed by the CH-PLIF technique. It is found that the heat-release zone is highly wrinkled and distorted in the supersonic combustion and it is distributed in a narrower region than the reaction zone.

Research progress on ground-to-flight correlation of aerodynamic force and heating data from hypersonic wind tunnels

LUO Changtong, HU Zongmin, LIU Yunfeng, JIANG Zonglin

2020, 34(3): 78-89. doi: 10.11729/syltlx20200006

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Abstract:
Hypersonic flow characteristics such as high temperature real gas effect, viscous interference effect, Mach number effect and scale effect, do not follow the similarity simulation criterion of experimental gas dynamics, which makes the hypersonic flow phenomenon beyond the range that can be accurately predicted by the classical gas dynamics theory. How to use the ground experimental data to predict the flight state, that is, the problem of ground-to-flight (G2F) correlation, is the key scientific problem that restricts the development of new aerospace vehicles. This paper summarizes the latest research progress of G2F correlation, and focuses on the multi-space correlation theory and correlation method of intelligent functional optimization. According to the theory, from a high-dimensional point of view, the experimental results of different wind tunnels are intrinsically related, and the flight test can be regarded as an ideal wind tunnel experiment. Based on the experimental data of wind tunnel groups (different types of wind tunnels which can simulate different parameter sections), using a specialized intelligent learning algorithm in functional space, the correlation method is performed by starting from the high-dimensional full parameter space. And then by carrying out a series of dimension reduction and adaptive space transformation, the invariant law is automatically deduced that different wind tunnels abide by together, so as to get a formula for G2F correlation. The results of verification examples and preliminary applications show that the multi-space correlation theory and functional intelligent optimization correlation method are effective, which would be a new trend in the research of G2F correlation for hypersonic aerodynamic force and heating.

Surface tension coefficient and thickness measurements in planar soap-film flows

ZHOU Zeyou, WAN Dongmei, XU Haitao

2020, 34(3): 90-96. doi: 10.11729/syltlx20190128

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Abstract:
Steady planar soap-film flows have been used widely in experimental investigations of fluid mechanics, e.g., as models for two-dimensional flows or supersonic flows. Despite the significant advances in the measurement techniques for soap films over the years, there is still room for improvement, especially for flowing soap films. Here we present two developments in measuring the surface tension coefficient and the film thickness of flowing soap films. For a vertically flowing soap film between two elastic wires, based on the force balance of the boundary wires, we derived an exact solution for the curve shape of the boundary wires. We verified our solution by direct measurement of the wire shape in our flowing soap film. Our exact solution thus provides support to a recently proposed semi-empirical method. Moreover, based on our solution, we proposed a new, easy-to-implement method for measuring the surface tension coefficient of flowing soap films. For film thickness measurement, we proposed a new method based on the interference of the incident light with a single-wavelength. When coupled with a conventional vertical velocity profile measurement, such as particle image velocimetry (PIV), the interference method can provide the film thickness profile using a single monochrome camera.

A correction method of icing testing scaling law with dynamic effects

LI Weihao, LI Weibin, YI Xian, WANG Yingyu

2020, 34(3): 97-103. doi: 10.11729/syltlx20190166

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Abstract:
The icing scaling law is an important theoretical method for converting flight conditions into test conditions. But existing icing scaling laws are mostly based on small droplets, which can cause large errors when applied to scaling transformations of large droplets. In response to this situation, research on icing scaling law considering the dynamics of droplets was carried out. Firstly, based on the ONERA scaling laws, the scaling parameters of the droplet deformation/breakup and splash were incorporated, and the modified icing scaling law was proposed. Secondly, based on these two scaling laws, the numerical simulation method was used to calculate the collection coefficient before and after the correction. The validity of the correction method was verified. Finally, the variation of the test parameters obtained by the two correction methods with the size reduction ratio was analyzed. The application of the scaling law in the icing test was given. The results show that the proposed method improves the coincidence of the local collection coefficient and reduces the average error of the collection coefficient and the impact limit. In addition, the scaling test parameters are within the design range of the icing wind tunnel. These correction methods can provide guidance for scaling transformations in supercooled large droplets ice wind tunnel test.

Mach number sensitivity analysis of cavity noise

LIU Jun, CAI Jinsheng, ZHOU Fangqi

2020, 34(3): 104-110. doi: 10.11729/syltlx20190079

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Abstract:
Sensitivity analysis plays an important role in assessing the importance of parameters and calculating the uncertainty. Mach number sensitivity analysis of cavity noise at subsonic, transonic and supersonic speed has been investigated by wind tunnel experiments in this study. At subsonic speed, the Mach number is changed by adjusting the opening of the valve, and the nominal increment of the Mach number is 0.01.At supersonic speed, the continuous change of the Mach number is achieved by changing the angle of attack of the model, and the increment of angle of attack is 1 degree. The experimental results show that the fluctuating pressure coefficient at the rear of the cavity increases with the increase of the Mach number at the subsonic speed, while decreases with the increase of the Mach number at the supersonic speed. At transonic speed, the fluctuating pressure coefficient has the largest sensitivity derivative to the Mach number. In the case of modal switching, the derivative of St to Mach number for the dominant acoustic mode is negative. The peak value of the dominant acoustic mode's power spectral density increases with the increase of Mach number at subsonic speed, while decreases at supersonic speed. The obtained sensitivity results can be used not only for the uncertainty assessment of the aerodynamic noise load in the embedded weapon bay, but also for a better understanding of the cavity noise characteristics.

Docking test verification of high temperature gas ejector system

ZHAO Fang, REN Zebin, WANG Haifeng, LI Xianfeng, SHI Yu, GUO Longde

2020, 34(3): 111-116. doi: 10.11729/syltlx20190068

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Abstract:
The ejector system is the key component of the chemical laser exhaust system. In order to improve the ejection efficiency of the exhaust system and meet requirements of miniaturization and economy, an ejection gas source scheme based on the gas generator to generate high temperature gas is proposed. A test stand of the ejection system composed of a gas generator based on structure of aero-engine mono-injector combustor and the two-stage supersonic ejector is established. An independent thermal test of the gas generator has been carried out and a docking test with the ejector has been successfully realized. Results show that ignition of the gas generator is reliable and the operation is stable; the ejector starts quickly and works smoothly under the action of high temperature gas injection, and performance indexes are better than design points:the ejector-gas temperature variation within a certain range does not affect the ejector performance. Research results expand the application of the gas generator, and at the same time, provide a powerful reference for the design and selection of ejector system solutions in this field.

2020 Vol. 34, No. 3