实验流体力学

Application of femtosecond-laser spectrum technology in combustion field

Zhang Dayuan, Li Bo, Gao Qiang, Li Zhongshan

2018, 32(1): 1-10. doi: 10.11729/syltlx20170141

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Abstract:
Based on the femtosecond laser combustion diagnostic technology, the on-line measurements of temperature, velocity and component concentration in combustion flowfield can be realized. Femtosecond laser diagnostic technology is an effective diagnostic tool and has wide applications in combustion field. It plays an important role in improving combustion efficiency and reducing combustion emissions. In this paper, the authors reviewed several applications of femtosecond laser diagnostic technology in combustion field, such as femtosecond multi-photon laser-induced fluorescence (fs-MPLIF), filament-induced nonlinear spectroscopy (FINS) and femtosecond laser electronic excitation tagging velocimetry (FLEET). Then the authors summarized the development of femtosecond laser combustion diagnostic technology. On this basis, the prospect of the femtosecond laser combustion diagnostic technology in the future was analyzed and discussed, which would provide reference for the relevant researchers.

Experimental investigation of turbulent flame-shock wave interactions based on abnormal combustion in internal combustion engine

Wei Haiqiao, Zhao Jianfu, Zhou Lei

2018, 32(1): 11-18. doi: 10.11729/syltlx20170144

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Abstract:
Abnormal combustion phenomena like knock or super-knock are inherent constraint limiting the performance and efficiency of downsized spark ignition (SI) engines.Essentially, engine knock or super-knock is always accompanied by the interactions of turbulent flames and shock waves, as well as rapid chemical energy release.Thus, it is of great significance to investigate the interactions of turbulent flame and shock waves which are the key to reveal the mechanism of knock and super-knock.The major objective of the present work is to experimentally investigate the process of flame acceleration, shock wave formation and interactions of turbulent flame and shock wave in a newly designed constant volume combustion bomb (CVCB) mounted with a perforated plate.In the CVCB, the perforated plate is used to achieve flame acceleration and produce turbulent flame and shock wave.High-speed Schlieren photography was employed to capture the interactions of turbulent flame and shock wave.Hydrogen-air mixture was chosen as the test fuel due to its fast flame propagation velocity and easiness to form obvious shock wave ahead of the flame front.Interactions of turbulent flame and shock wave at different levels could be obtained by changing the initial thermodynamic conditions (including initial pressure and equivalence ratio) and parameters of the perforated plate (including hole size and porosity).Flame acceleration, formation of shock wave and flame-shock wave interactions are discussed in this paper.Depending on the interactions of turbulent flame and shock wave, five combustion modes are obtained by experiments, such as normal combustion, periodically decelerating combustion, oscillating combustion, flame-front autoiginiton and end-gas autoiginiton.The maximum amplitude of the pressure oscillation at combustion models with autoiginiton exceeded 4.5MPa, 4~40 times greater than those without ignition. Therefore, autoiginiton caused by the interactions of turbulent flame and shock wave is the root cause of the intense pressure oscillation in the combustion chamber.

Network topology analysis on wrinkled structure of turbulent premixed Bunsen flame

Wang Jinhua, Nie Yaohui, Chang Min, Zhang Meng, Huang Zuohua

2018, 32(1): 19-25, 63. doi: 10.11729/syltlx20170147

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Abstract:
Turbulent flame structure represents the species, velocity and temperature field in the turbulent combustion, which reflects the interaction between the turbulence and the combustion. It is also important for combustion model validation. The conventional PDF of curvature method can not accurately reflect the folded regions in the turbulent flame, while the network topology analysis can demonstrate these regions as it can mark the key nodes or structure in a system. In this paper, the network structure of the turbulent premixed Bunsen flame is constructed to trace the folded regions in turbulent flames. Results show that the folded regions can be traced by network structure. These regions are mainly caused by DL instability in weak turbulence, while they are mainly affected by turbulence vortex wrinkling as turbulence intensity increases. The influence of DL instability on turbulent premixed Bunsen flames is constrained by flame development. At the bottom of Bunsen flame, the DL instability does not wrinkle the flame. As the flame propagates to the downstream, the flame becomes more wrinkled due to DL instability.

Simultaneous multi-species PLIF diagnostic on CH₄-Air inverse diffusion jet flame

Chen Shuang, Lukasz Jan Kapusta, Weng Wubin, Su Tie, Tu Xiaobo, Zhongshan Li

2018, 32(1): 26-32. doi: 10.11729/syltlx20170138

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Abstract:
The simultaneous multi-species Planar Laser Induced Fluorescence technique plays an important role in studying the flame structure and the two-dimensional distribution of intermediate species in combustion.The experimental system of OH/CH₂O/Acetone-PLIF was built in order to study the CH₄-Air inverse diffusion jet (IDJ) flame.The system consists of two sets of lasers, two intensifier-CCD cameras, a temporal controller and several lenses.The strategy of fluorescence excitation, the method of synchronous timing control and image calibration procedures are discussed.The IDJ flame was studied using the simultaneous multi-species PLIF technique, and the reaction zone, pre-heating zone and fuel zone of IDJ flame were determined.Experimental results suggest that the IDJ flame is different from either the normal diffusion flame or the premixed jet flame.The behavior of this type of flame reveals similarity to the partially premixed flame.Compared to OH chemilumiscence images, simultaneous multi-species PLIF can provide more detail and information about the flame structure and it has huge potential in fundamental combustion studies and industrial burner experiments.

Development of laser combustion diagnostic techniques for ground aero-engine testing

Hu Zhiyun, Ye Jingfeng, Zhang Zhenrong, Wang Sheng, Li Guohua, Shao Jun, Tao Bo, Zhao Xinyan, Fang Bolang

2018, 32(1): 33-42. doi: 10.11729/syltlx20170135

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Abstract:
Laser-based diagnostic techniques have the characteristics of non-intrusiveness, high temporal-spatial resolution and abundant measurement information, and have been demonstrated as powerful and indispensable tools for the turbulent combustion research of various engine devices.This paper reviews the basic principle, research status and development trend of several laser spectroscopic techniques, which have been used successfully in the measurements of engine combustion.Since there is no single laser diagnostic technique that can be applied under all circumstances, the particular application must be carefully considered according to the type of combustion flow and measurement requirements.For the temperature measurements, Coherent Anti-stokes Raman Scattering (CARS), as a point-wise technique, yields the best accuracy of less than 5% uncertainty for single-shot measurements.And the two color Planar Laser Induced Fluorescence (PLIF) technique may be employed to study temperature gradients or temperature fields but may lead to reduced absolute accuracy.For the velocity measurements, the Particle Image Velocimetry (PIV) technique is suitable for fine measurements of low-speed flow field, while the Hydroxyl Tagging Velocimetry (HTV) technique has been applied in high-temperature supersonic and even hypersonic flow fields with uncertainty less than 4%.In species concentration measurements, the Spontaneous Raman Scattering (SRS) and PLIF are used to measure the main components and the distribution of intermediate products, respectively.In terms of the high-temporal-spatial-resolution measurements of temperature, velocity and species concentrations in the aero-engine combustor, this paper reviews the basic principle, research status and development trend of relevant laser combustion diagnostic techniques.

Progress on tunable diode laser absorption tomography technique for combustion diagnostics

Hong Yanji, Song Junling, Rao Wei, Wang Guangyu

2018, 32(1): 43-54. doi: 10.11729/syltlx20160177

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Abstract:
Tunable Diode Laser Absorption Tomography(TDLAT) technique is a new technique for 2D flow field diagnostics, which combines the Tunable Diode Laser Absorption Spectroscopy technique (TDLAS) and the computer tomography (CT). It has wide applications in hot, high-speed and toxic gas diagnostics, especially for combustion and propulsions in harsh environments, due to the merits of high sensitivity and noise resistance. Firstly, the fundamentals of the TDLAT are presented. Secondly, the TDLAT system is divided into four stages:the optical measurement system, the data processing system, the reconstruction algorithm and the data presentation. The progressing on the four stages and key technologies are discussed. Thirdly, some examples of applications of TDLAT sensors in practical combustion diagnostics are presented, including scramjet engines, aero-propulsion engine and coal-field combustion. Finally, the development tendency and conclusions are given. This paper can provide reference for the researchers to comprehend the progress and key techniques of TDLAT.

An experimental study on riblet-induced spanwise vortices in turbulent boundary layers

Wang Xin, Li Shan, Tang Zhanqi, Jiang Nan

2018, 32(1): 55-63. doi: 10.11729/syltlx20170092

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Abstract:
Coherent structure is closely related to the high frictional resistance in wall turbulence.Summarizing the influence rule of vertical micro-grooves on spanwise vortex is helpful for understanding the drag reduction mechanism by riblet surface.15998 instantaneous velocity fields over both riblet (s⁺=2h⁺=16.3) and smooth surfaces in turbulent boundary layers (TBL) at a Reynolds number of Re_τ=190 were acquired by using time-resolved particle image velocimetry (TRPIV) in a water tunnel.The swirling strength λ_ci was used to reveal spanwise vortices, and we extracted the cores of spanwise vortices by finding swirling strength's local extremum in whole field.Subsequently, the proportion of the spanwise vortex, the average swirl strength, the average diameter of the vortex and the proportion of the each scale vortex were analyzed statistically in flow field over riblet surface.All experimental results were compared with those over a smooth surface.It is found that (1) the number of prograde spanwise vortices is decreased by riblet in near wall region, while retrograde spanwise vortices'number is increased.(2) The average strength of the spanwise vortex is decreased, whether prograde vortex or retrograde vortex.(3) The proportion of small scale prograde vortices and mesoscale retrograde vortices is increased in near wall region, while mesoscale prograde vortices and large scale retrograde vortices'proportion is decreased.(4) The difference in stream-normal scale of the prograde spanwise vortex becomes smaller in inner boundary, but the retrograde spanwise vortex's scale difference without change.(5) The proportion of small scale prograde vortices is decreased in log-law region, while the proportion of large scale prograde vortices is increased.Impact on the proportion of retrograde vortices is exactly opposite of the prograde vortex.

Measurements of circular cylinder's wake using time-resolved PIV

Wang Yong, Hao Nansong, Geng Zihai, Wang Wanbo

2018, 32(1): 64-70. doi: 10.11729/syltlx20170099

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Abstract:
Time-resolved particle image velocimetry (PIV) with sampling frequency f=1000Hz was used in 0.55m×0.4m aeroacoustic wind tunnel to test the unsteady characteristics of the flow behind a D=20mm diameter circular cylinder in an area of 7.5 times the diameter long and 6.6 times the diameter wide at Reynolds numbers of 2.74×10⁴.The characteristics of the mean and pulsating flow fields, as well as the frequency characteristics of the cylinder's wake have been revealed.A phase-averaged analysis method based on the cross-correlation coefficients among the PIV velocity vectors has been proposed to capture the process of vortex production, shedding, development and dissipation.The results indicate that there exists a low-speed recirculated flow area behind the circular cylinder, where the flow structure changes most intensively.The vortices shed from the upper and lower sides of the circular cylinder cross and correlate around the 1.9D downstream area, where the turbulence is the strongest.Strouhal number corresponding to the vortex shedding frequency of the cylinder's wake is stable around 0.2.The proposed phase-averaged analysis method is simple but efficient to study the spatial-temporal evolutions of the vortex shedding, which can be easily applied to other areas in unsteady flow field measurement.

Experimental study of the fragmentation behavior of a cylindrical liquid jet into crossflow

Deng Tian, Jiang Shuai, Gao Xuwan

2018, 32(1): 78-83, 97. doi: 10.11729/syltlx20170107

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Abstract:
This paper used the high speed camera to observe a cylindrical liquid jet into crossflow and to breakup under the action of wave developments.A direct nozzle has been used, of which the outlet diameter is 1mm and the aspect ratio is 20.The liquid jet in the experiment is water and the crossflow is air.The test temperature is 293K, and the liquid jet velocities are from 2 to 20m/s, with Reynolds number from 2400 to 22400.The velocities of the crossflow change from 10 to 40m/s, with Weber number of from 1.6 to 25.6, that is to say the momentum ratio between the air and the liquid is from 5 to 127.The frame of the high speed camera is 2000 and the exposure time is 16s.The experimental studies have found that:the differences of Weber number of the crossflow will change the fragmentation behavior of the liquid jet and the dimensionless surface wave length is proportional to the-0.31 power exponent of the Weber number of the crossflow.The distance of the fracture point of liquid column along the direction of the crossflow decreases with the increase of the momentum ratio of the liquid to the air, while it along the initial direction of liquid jet increases with that momentum.The velocity of the droplets produced after fracture on the direction of the crossflow is about 0.1 times as high as that of the crossflow, while it on the direction of initial liquid jet increases linearly with the momentum ratio of the liquid to air firstly, until it becomes 0.8 times of the velocity of initial liquid jet.Based on the above study, this paper had fitted the relation between the wavelength and the Weber number of the crossflow; and also the position of fracture point of the jet, the trajectory of jet, the velocities of produced droplets along with the initial conditions.

Experimental research on the coupling effect of propeller slipstream and flat tail deep stall on aerodynamic characteristics of airplane

Li Xingwei, Li Cong, Xu Chuanbao, Li Shengwen

2018, 32(1): 84-89. doi: 10.11729/syltlx20170068

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Abstract:
The influence of propeller slipstream on aerodynamic characteristics of conventional layout turboprop aircraft is studied at large attack angle by using propeller aircraft dynamic simulation test. The dynamic simulation test is curried out in FL-9 wind tunnel of Aerodynamics Research Institute of AVIC. The propeller is driven by servo motor. The test angle of attack range is 0°~50°, and the wind speed range is 30~50m/s. The conventional single strut is used at the normal attack angle test, while the special prebending strut is used at the large attack angle test. The coupling effect of flat tail deep install and propeller slipstream is studied detailedly. The test results show that the propeller slipstream increases lift and drag forces of the test model, and decreases the longitudinal static stability, and these features also exist at large attack angel test. In addition, the slipstream will make the deep stall effect of flat tail worse, and the flat tail will be harder to get out of stall when the flat tail entering the coupling influence area of slipstream and wing wash flow at the large angle test condition. That means the range of attack angle effected by deep stall will be larger and flat tail deep stall effect is aggravated.

Research on static thrust accurate measurement testing technology of vectoring nozzle

Deng Xiangdong, Song Xiaoyu, Ji Jun, Guo Dapeng, Li Peng

2018, 32(1): 90-97. doi: 10.11729/syltlx20160187

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Abstract:
The principle, test bench and method of the vectoring nozzle static thrust measurement experimental technology are introduced in this paper. The primary simulation parameters are the nozzle pressure ratio (NPR) and the Mach number at the nozzle exhaust. The nozzle model is installed in the vacuum cabin of the thrust test bench, and aerodynamic forces of the nozzle model are measured accurately by the wall balance. After the mass flow correction, installation position correction, and the correction of pressure effect and mass flow effect to the balance caused by the air bridge system based on the rubber membrane, the exact values of the vector nozzle static thrust, thrust coefficient and vector angles etc. are obtained. The experimental results show that the axial thrust coefficient, the normal thrust coefficient and the variation trend of vector angle with NPR, which is measured in the vector nozzle static thrust measurement experiment using the thrust test bench are correct, the test results precision can satisfy the GJB requirements for force-test precision, and the technique can be applied in project test.

Structural design of a small cryogenic wind tunnel compressor rotor

Chen Zhenhua, Nie Xuqing, Yang Wenguo

2018, 32(1): 98-104. doi: 10.11729/syltlx20170094

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Abstract:
The axial compressor is the power equipment of the cryogenic wind tunnel. It has the characteristics of the wide temperature range and high speed of operation. The compressor rotor structural design was studied in this paper. Key technologies of the compressor design were discussed in detail, including shaft-hub connection, hub-blade connection, bearing selection, lubrication method, seal structure, heat protection scheme, et al. The compressor rotor dynamical characteristics under elastic supporting was calculated using the FEM method. The thermal-structure coupling simulation of the rotor was conducted. Calculation results show that the rotor critical speed is much higher than the maximum speed, the maximum stress value of the thrust bearing housing is about 170MPa when the temperature dropping stage ends, and the safety factor is bigger than 1.5. United debugging of the compressor on a small cryogenic wind tunnel was carried out. Experimental results show that when the total temperature of the wind tunnel drops to 110K, the bearing temperature is higher than 5℃, and the bearing vibration value is lower than 3mm/s. They both meet the design requirement, which verifies that this compressor rotor design is reliable.

2018 Vol. 32, No. 1