实验流体力学

Experimental prediction of lean blowout: a review

Huang Jianqing, Li Lei, Cai Weiwei

2018, 32(2): 1-9. doi: 10.11729/syltlx20170165

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Abstract:
Lean Blowout (LBO) is a special kind of instable combustion phenomena which can lead to catastrophic consequences. Thus, it is critical to accurately predict and control the occurrence of LBO. In this work, we summarize the methods developed since 2000 for the prediction of LBO based on flame chemiluminescence, color, temperature, acoustic, and ion signals. How to collect these signals is described as well as five methods of analyzing the collected signals are introduced and compared against each other. Finally, conclusions are provided and future research perspectives are proposed.

Development of scale-controlled premixed turbulent burner and the flame structure analysis

Yu Qianqian, Wang Jinhua, Zhang Weijie, Zhang Meng, Huang Zuohua

2018, 32(2): 10-17. doi: 10.11729/syltlx20170150

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Abstract:
The structure-variable premixed turbulent burner is developed to investigate the effects of single turbulence parameters on flame structure, and to broaden turbulence intensity and scale range. Hot-wire anemometer measurements of cold flow indicate that the burner can utilize different geometry structures to produce scale-controlled flow field and realize the investigation of the effects of single turbulence parameters on flame structure. Fifteen representative structures were selected for the premixed turbulent combustion experiment. OH-PLIF images show that high turbulence intensity enhances flame surface wrinkling, as well as increasing the number of island structures. Data are reported at 1 < u'/S_{L, 0} < 10 for CH₄/air flames with equivalence ratio of 0.7 in the thin reaction zones. Increasing integral scale decreases the turbulent burning velocity at high Reynold number. There may exist a critical Re_c which can represent the degree of how inertial forces are dominant to determine the effect of the integral scale on the turbulent burning velocity. Increasing the integral scale can also enlarge the flame volume, due to larger vortex containing higher energy. However, intensive turbulence intensity can wrinkle the flame surface much more remarkably, resulting in superposition of small scales on large scales. Therefore, the increasing turbulence intensity increases the flame volume more significantly, covering up the impact of integral scale on flame volume. These results indicate that the effect of integral scale (represent large scale) on the flame heat release rate is less significant than the effect of turbulence intensity (represent superposition degree of small scales on large scales).

Experimental analysis on the longitudinal high frequency combustion instability of a single-element model engine

Wang Di, Nie Wansheng, Zhou Siyin, Wang Haiqing, Su Lingyu

2018, 32(2): 18-23, 73. doi: 10.11729/syltlx20170162

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Abstract:
In order to understand the influence of injector indentation length and combustion chamber length on high frequency combustion instability of oxygen/kerosene rocket engine, the combustion experiment of single-element model engine was designed and carried out. The gas-liquid coaxial centrifugal single-element was used in the experiment. In order to test the combustion stability, the length of the combustion chamber and that of the injector were used as experimental variables, and the data were collected by high frequency pressure sensor, using the central supply of oxygen and the axial rotation of liquid kerosene through the tangential hole into the injector. Based on the pressure signal, the experimental results, especially the longitudinal high frequency combustion instability, are studied in detail. The results show that:under the conditions studied in this paper, with the increase of the indentation length, the longitudinal high frequency combustion instability is damped, but the longitudinal high frequency combustion instability is not eliminated. The length of combustion chamber is between 516mm and 356mm. The effect of the injector indentation length on the combustion stability is negligible. With the increase of combustion chamber length, the first order longitudinal acoustic frequency decreases gradually. These phenomena are due to the phase difference between the combustion pressure oscillation and the acoustic wave. In addition, the influence of the combustion chamber length on longitudinal high frequency combustion instability is more obvious than that of the indentation length.

Experimental study on ignition process for ethylene high speed jet

Liu Bing, He Guoqiang, Qin Fei

2018, 32(2): 24-27. doi: 10.11729/syltlx20180003

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Abstract:
In order to investigate the ignition process with fuel-rich gas of scramjet combustor, a simplified burner with fuel-rich hot coflow and sonic nozzle jet was designed. Ignition process of sonic ethylene jet issuing into a coflow of hot exhaust products of a rich premixed ethylene/air flat flame was examined using the chemiluminescence. Three cases with the equivalence ratio of coflow varying from 1.4 to 1.6 and the injection pressure varying from 2atm to 3atm were examined. The results indicate that:(1) the ignition process of sonic ethylene jet in fuel-rich gas may be divided into four steps:(a) jet and coflow mixing; (b) strong chemical reactions between jet and surrounding air; (c) occurrence of extinction in the downstream of flame; (d) steady flame; (2) the equivalence ratio of coflow 1.4 was more effective than 1.6 for ignition process; (3) with the increase of jet velocity, flame luminosity is lowered due to incomplete burning in the higher speed steady flame.

Experimental study on the effects of successive supercooled large droplets impacting

Xu Hong, Kong Weiliang, Wang Fuxin, Liu Hong

2018, 32(2): 28-34. doi: 10.11729/syltlx20170112

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Abstract:
The understanding of the impacting-icing mechanism of supercooled large droplet (SLD) is still limited to a single SLD. The interactions between droplets have not been taken into consideration. A high-speed camera is used to record the impacting-freezing process of two SLDs that are offset a certain distance away from each other and successively impact on the substrate. The experiment studies the relationship between the offsets of two SLDs, the time intervals of successive impacts and the freezing morpholog as well as the freezing speed. The experiment shows that the interactions between SLDs can inhibit the SLD shrinkage behavior and prolong the freezing time. The size of the offset affects the shrinkage rate which decides the final freezing morphology. The freezing time of two SLDs decreases rapidly with the slight increase of the offset of two SLDs in a small range. The time interval of the successive impacts is inversely proportional to the freezing time and has a weak effect on the formation of the freezing morphology.

Experimental study for effect of mean volumetric diameter on ice adhesion strength

Liu Zhen, Kong Weiliang, Liu Hong, Wang Fuxin

2018, 32(2): 35-39. doi: 10.11729/syltlx20170118

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Abstract:
Ice adhesion strength is the interaction between the ice and the solid substrate. Research about how experimental parameters influence ice adhesion strength is important to the understanding of the mechanism of ice adhesion strength, the developing of anti-ice method and the analysis of the Ice shedding. In the experiments, the same substrate and the liquid water content are used, and the ice adhesion strength is measured at different temperatures (-25~0℃) and different mean volumetric diameters (40, 80 and 250μm). Different from freezer ice, there exists a turning point for impact ice as the pattern of the ice adhesion strength changes due to the variation of temperature, which can be called the critical temperature. As the temperature decreasing from 0℃, above the critical temperature the ice adhesion strength does not depend on whether it is freezer ice or impact ice. But below the critical temperature, the ice adhesion strength of impact ice is lower, with the ice regime transforming from glaze ice to rime ice. The critical temperature changes as the mean volumetric diameter changes, so as the variation trend of the ice adhesion strength.

Design of icing wind tunnel experiment system for rotating blades by using natural low temperature

Li Yan, Sun Ce, Guo Wenfeng, Wang Shaolong, Feng Fang, Jiang Yu

2018, 32(2): 40-47. doi: 10.11729/syltlx20170073

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Abstract:
In order to carry out the study on icing wind tunnel experiment for rotating blades of wind turbine, an icing wind tunnel test system with variable cross test sections by using nature low temperature in cold region was designed. Based on the original wind tunnel in the lab, the test section was reformed by adding a non-spry section. Therefore, the test station for the rotating blade of the wind turbine can be set in the non-spry section and the icing test can be finished in the water spry section. The diameter of the wind turbine rotor is not limited by the water spry section size by using this test system. To measure the main parameters and verify the test ability of this system, three main icing parameters of the test section including wind speed, temperature and liquid water content distributions were tested and analyzed. Furthermore, icing experiments on rotating cylinder and rotating blade airfoil were carried out. The test results including the icing shape and the dimensionless icing area are discussed by comparing with the author's previous results of the original icing wind tunnel tests. According to the test results, the distributions of the main parameters of the test section are stable. The icing shape on cylinders and blade airfoils obtained by both test systems agrees in some degree and the values of the dimensionless icing areas are close. The result shows that the new icing wind tunnel system can be used for the research on icing issue for rotating machinery.

Method of determining the location for aircraft icing prober

Yi Xian, Li Weihao, Wang Yingyu, Ma Honglin

2018, 32(2): 48-54. doi: 10.11729/syltlx20170169

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Abstract:
The icing prober is an important component of an aircraft icing protection system. The location of an icing prober affects the ice detection results greatly. It often takes a huge amount of work to determine the location of the icing prober in the process of aircraft design. In this paper, a method of determining the installation position of the icing prober with high efficiency is presented. The basic idea is using numerical method to obtain the volume fraction distribution of water droplets in the flow field around the aircraft without introducing the icing prober firstly. Then the water collection efficiency at the locations where the prober may be installed is yielded according to the results of the volume fraction distribution of water droplets, and it is compared with water collection efficiency on the wing. Finally the locations of the prober are given from the point of view that the prober has an early warning function. The method is then used in an airliner design, and the position suitable for prober location is given. On this basis, the prober is loaded at the corresponding position and a full numerical simulation is taken for the combining configuration of the aircraft and the prober. The simulation results show that the water collection efficiency at the position with prober is similar to that without prober, and both are larger than the value on the wing under the same condition, which means the requirements of the icing protection for early warning are met. The method can be applied to the any transport aircraft and improve the design efficiency of the icing protection system.

Experimental study on droplet size measurement and control of icing cloud in icing wind tunnel

Guo Long, Cheng Yao, Wang Zixu

2018, 32(2): 55-60. doi: 10.11729/syltlx20170096

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Abstract:
Accurately measuring and controlling the droplet size is important for icing wind tunnel tests. To study droplet characteristics and its effects in the 3m×2m icing wind tunnel, a PDI-FP (Phase Doppler Interferometer Flight Probe)system is used for the MVD (Median Volume Diameter) measurement and control test, and the effect factors such as pressure, airspeed, and temperature are analyzed. The experimental results show that there are sensitive relationships between the droplet size and spray air/water pressures. The MVD is inversely proportional to the spray air pressure. The air speed and temperature don't have impact on MVD obviously with the maximum deviation less than ±10%. The MVD is significantly reduced by applying negative pressure.

Cloud calibration method of 3m×2m icing wind tunnel

Wang Zixu, Shen Hao, Guo Long, Guo Xiangdong, Ni Zhangsong

2018, 32(2): 61-67. doi: 10.11729/syltlx20170163

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Abstract:
The cloud calibration has been carried out under SAE ARP5905 standard for 3m×2m icing wind tunnel, results of which can be used for certificating icing wind tunnel performance and supplying data proof for obtaining icing flight certification. Based on the ice accretion mechanism, the phase doppler interference principle, and the hot-wire principle, the calibration work was completed in the main test section by using the icing grid, PDI-FPDR, the icing blade, and LWC-200. Methods and procedures are summarized to increase the efficiency of calibration. Typical cloud calibration results indicate that the icing cloud uniformity is good, 60% of the main test section could remain target LWC within ±20%. The MVD maximum deviation is smaller than ±10%. The LWC stability is greater than ±20%. All the cloud parameters could meet the requirements of ARP5905, which could be used for supplying data proof of 3m×2m icing wind tunnel certification.

Experimental investigation of multi-scale turbulent eddy adjustment in personal gasper flow with wedges

Cui Xujia, Tang Zhanqi, Jiang Nan, Liu Junjie

2018, 32(2): 68-73. doi: 10.11729/syltlx20170110

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Abstract:
In aircraft cabin, the personal gasper plays a significant role in the environment control system. It improves the thermal comfort of passengers and enhances the ventilations and heat exchanges. However, the personal gasper currently in use always brings about uncomfortable draft sensation to the passenger's head. In this paper, the personal gasper is optimized by fixing wedges with different sizes at the nozzle, so that it can adjust and control the multi-scale turbulent eddy structures in the jet flow filed and reduce the uncomfortable draft sensation. The constant-temperature anemometer with the hot-wire probe was employed to finely measure the turbulent jet flow field with high time resolution and evaluate the turbulent level of the personal gasper with wedges. The acquired time series of the instantaneous velocity are decomposed by wavelet analysis to extract the multi-scale turbulent eddy structures, and the effects with and without wedges of different sizes are compared. The 5mm wedges structure is highlighted, which has the most optimal effect and the most comfortable sensation to passengers. By the adjustment and control of wedges, the momentum, energy, mass exchanges of the turbulent jet flow are enhanced, the turbulent level is reduced, the draft sensation deceases and the comfortability is improved.

Temperature and heat flux measurement technologies in scramjet

Jin Xinhang, Ma Binghe, Qiu Tao, Deng Jinjun, Luo Jian

2018, 32(2): 74-81. doi: 10.11729/syltlx20170013

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Abstract:
The accurate measurement of temperature and heat flux field in scramjet provides strong support for characterization, assessment and optimization of scramjet engine. Measurement methods are reviewed and analyzed, including sheathed and thin film sensors, and photoelectric method and radiation method. Researches of thin film sensors are introduced, and the applications of thin film thermocouple and heat flux sensors in scramjet are pointed out.

Flicker compensation method based on scale-time equalization for videogrammetric measurement images in high speed wind tunnels

Fan Jinlei, Zhang Zhengyu, Huang Xuhui

2018, 32(2): 82-88. doi: 10.11729/syltlx20170128

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Abstract:
The complex illumination conditions in the high-speed wind tunnels can easily lead to the flicker of the videogrammetric measurement(VM) test image sequences, which affect the accuracy of the measurement result. As the flicker contains both the global change of the gray level caused by the illumination and the local change caused by the object motion, deformation and so on, it is difficult to apply the compensation methods based on affine transformation model (linear or non-linear) between pixels or between blocks of the images. Based on the scale space theory, the scale-time equalization(STE) method was proposed by Delon(2006) and applied to correct flicker in video and movie. The STE method performs the Gaussian convolution in the time dimension of each gray value in the histograms to obtain the target histogram of each image, and then uses histogram matching to get the flicker compensated images. Three experiments with different flicker and interference were conducted:the camera calibration plate image under the fluorescent lighting, the deformation test image of the wind tunnel model and the cavity oil film test image with diffusion. The experimental results show that the STE method is suitable for the global flicker compensation of the VM test image sequence. Compared with the model-based methods, the flicker compensation effect is not dependent on the reference image, and the robustness to the interference factors such as image jitter, local motion or deformation is strong. Meanwhile, the method has the advantages of simple procedures and small computation cost, and therefore it has high engineering application value.

Lightweight design of low speed wind tunnel model

Wang Biling, Liu Chuanhui, Sun Pengfei, Zhou Rui, Zhang Caicheng, Han Songmei, Zhao Changhui

2018, 32(2): 89-93. doi: 10.11729/syltlx20170125

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Abstract:
Design and manufacturing of wind tunnel models are related directly to the accuracy of wind tunnel test data, and also influence period and cost of aircraft development. Lightweight design of low speed wind tunnel models is one of key points to obtain reliable aerodynamic data and lower testing cost. With the structure of frame and beam strengthened composite skins and the additional manufacturing parts, the integral and lightweight model of civil aircraft is designed. The finite element analysis is used to calculate the structure intensity and vibration analysis is conducted. It concludes that the weight of the model is reduced by 50% compared to that of the conventional metal model, and meanwhile the structure intensity of the model satisfies design requirements. The irregular shaped part is designed with additional manufacturing method, the weight of which is reduced by 20% and the manufacturing period is shortened by more than 50% compared to that for composite model. The inherent frequency of the model-support system is increased compared to that of metal model.

2018 Vol. 32, No. 2