The Experts below are selected from a list of 303 Experts worldwide ranked by ideXlab platform
Yang Zhang - One of the best experts on this subject based on the ideXlab platform.
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Lifted and reattached behaviour of laminar premixed flame under external Acoustic Excitation
Experimental Thermal and Fluid Science, 2018Co-Authors: Lukai Zheng, Yang ZhangAbstract:Abstract The flame chemiluminescent emission fluctuations and the vortex structure of the lifted jet flame under Acoustic Excitation were studied in this investigation. By employing high-speed visualization and DFCD (Digital Flame Colour Discrimination) image processing method, the fluctuation of the instantaneous mixture fraction has been found highly correlated with the lifted height variations. It has been observed that during the flame drifting to downstream, there is no obvious shifting on the mixture fraction. However, when the flame travels back to upstream, the fuel mixture has been evidently diluted. In addition, the stabilisation mechanism can be further explained by analysing the velocity fluctuation of the vortices in the shear layer via PIV. Measurements show that, the turbulent stretching at the shear layer generated by the Excitation leading to the flame lift-off. On the other hand, the Kelvin-Helmholtz vortices in the unburn part play a key role in preventing flame lift-off. But, the excessive external Acoustic Excitation leads to blow-off due to over dilution and increased lifted height.
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Flow characterisation of diffusion flame under non-resonant Acoustic Excitation
Experimental Thermal and Fluid Science, 2013Co-Authors: Li-wei Chen, Qian Wang, Yang ZhangAbstract:Abstract An experimental investigation has been carried out to study the flow characteristics of a diffusion flame under Acoustic Excitations. High speed direct imaging, high speed schlieren visualisation and phase-locked PIV techniques were applied to investigate the flow field induced by Acoustic Excitation in a tube and its effect on the flame. Non-resonant frequencies which are between the first and the second resonant frequency of the test chamber were selected in the experiments. Results indicate that the flame pattern and luminosity of the Acoustically excited diffusion flames were very different in each Excitation case. It was observed that the lower Excitation frequency of 90 Hz has the strongest effect on the flame dynamics. Both the fuel jet flow and ambient air motion were strongly affected. At the Excitation frequency of 150 Hz, the external Excitation effect on the surrounding area was low and the buoyancy-driven flickering started to dominate the flame oscillation behaviour. Under the higher Acoustic Excitation frequency of 200 Hz, only the fuel jet was slightly affected by external forcing. The frequency analysis also showed that flame/Acoustic interaction results in a nonlinear coupling effect between the buoyancy driven instability and the Acoustic disturbance.
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Nonlinear response of buoyant diffusion flame under Acoustic Excitation
Fuel, 2013Co-Authors: Qian Wang, Hua Wei Huang, Hao Jie Tang, Min Zhu, Yang ZhangAbstract:Abstract Using nonlinear theory as guidance, the expected (both reported and unreported) nonlinear response modes of a laminar diffusion flame to external Acoustic Excitation are investigated at different frequencies (6–100 Hz). The flame oscillating frequency is analyzed using digital signal and image processing techniques, while the hot gas dynamics are visualized by a high speed schlieren imaging system. The natural flame flickering frequency of the burner is 7.8 Hz. It has been observed that both the Excitation frequency and amplitude play a role in the resultant dominant flame oscillation frequency. In this study, four nonlinear modes are identified, including frequency division, frequency doubling, sum/subtraction of the Excitation and natural buoyancy frequency, and frequency amplitude increasing with enhanced Excitation signal. From the schlieren images, it is found that all the nonlinear phenomena observed are due to the coupling between buoyancy and Acoustic Excitation near the nozzle exit field. The resultant change in hot gas flow structure and evolution affects the flame frequency behaviour subsequently.
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Characterisation of external Acoustic Excitation on diffusion flames using digital colour image processing
Fuel, 2012Co-Authors: Hua Wei Huang, Qian Wang, Hao Jie Tang, Min Zhu, Yang ZhangAbstract:Abstract The experimental investigation of the dynamic flame flickering properties of Acoustically excited combustion process has been conducted. In particular, the variations of flame oscillation frequency response in diffusion-like sooty and premixed-like chemiluminescence flame colour entities under external Acoustic perturbation were extracted, analysed, and compared using digital colour image processing technique. The technique employed makes use of both the RGB and HSV colour modelling principles to identify and tag digital image data that conforms to the different flame colour distribution regimes. It was found that the bluff-body stabilised diffusion flame considered in this experiment has a natural low-oscillation frequency range of below 20 Hz; the typical diffusion flame frequency response without external Acoustic Excitation. External Acoustic Excitations of 15–100 Hz were applied to the burner. The processed frequency spectrum from the diffusion-like digital colour signals presented in the captured high-speed video data showed that external Acoustic Excitation did little to shift the dominant frequency location away from the well-known low-frequency flame flicker, and only minor peak distributions can be found to match the exact point of external frequency in the spectrum. On the other hand, the frequency spectrum processed using digitally isolated premixed-like flame colour signals showed that the dominant frequency in the processed FFT spectrum is located at the exact frequency of Acoustic Excitation from 15 to 35 Hz. Between 40 and 100 Hz, the presence of external Acoustic Excitation corresponds to the location of the dominant distribution in the secondary group of peaks identified from the frequency spectrum. Within this range of forced Acoustic Excitation, the dominant frequency returned to the natural convective response of the flame at below 20 Hz.
Nobuyuki Fujisawa - One of the best experts on this subject based on the ideXlab platform.
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Flow visualization study of a diffusion flame under Acoustic Excitation
Fuel, 2019Co-Authors: Nobuyuki Fujisawa, Koki Iwasaki, Kei Fujisawa, Takayuki YamagataAbstract:Abstract The behavior of a diffusion flame under Acoustic Excitation was experimentally studied with the aid of simultaneous flow visualizations of the flame combining Mie scattering and schlieren/shadowgraph imaging. The Mie scattering imaging showed the periodic formation of a vortex street along the flame axis, while the schlieren/shadowgraph images indicated the formation of a high-amplitude density fluctuation near the burner exit. Furthermore, mean velocity and temperature along the flame axis were measured for evaluating the flame behavior by Acoustic Excitation. These experimental results showed that the Acoustic Excitation led to a density fluctuation near the burner and promoted the flame behavior near the burner. This behavior resulted in enhanced mixing and combustion in the flame near the burner, which grew as the oscillation amplitude of the Acoustic Excitation increased. In addition, the flame height, oscillation amplitude and luminosity decreased as the Acoustic Excitation amplitude is highly increased because of the formation of the increased velocity region with high-temperature near the burner associated with the initiation of laminar-to-turbulent transition of the flame.
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Phase-averaged characteristics of flow around a circular cylinder under Acoustic Excitation control
Journal of Fluids and Structures, 2004Co-Authors: Nobuyuki Fujisawa, G. Takeda, N. IkeAbstract:The influence of internal Acoustic Excitation on the aerodynamic performance of a circular cylinder in a uniform flow is studied by the phase-averaged measurements of fluid forces and flow fields around the cylinder. The experimental results indicate that the mean drag as well as the fluctuating lift are reduced by the Acoustic Excitation control from a slit on a circular cylinder. The phase-averaged PIV measurements at the vortex shedding frequency show that the periodic and fluctuating velocities are suppressed in the wake of the controlled cylinder, which indicates the influence of Acoustic Excitation on the vortex shedding and the corresponding reduction in the fluid forces on the circular cylinder. On the other hand, the phase-averaged flow field at the Excitation frequency of unstable shear layer, which is four times larger than the vortex shedding frequency, indicates the formation of discrete vortices along the shear layer of the cylinder wake. The interaction of these vortices with the cylinder wake causes the reduction in the cross-correlations of velocity fluctuations in the flow field, which results in the attenuation of the vortex shedding phenomenon in the cylinder wake.
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Flow control around a circular cylinder by internal Acoustic Excitation
Journal of Fluids and Structures, 2003Co-Authors: Nobuyuki Fujisawa, G. TakedaAbstract:Abstract The possible drag reduction and the corresponding variation of the flow field around a circular cylinder are studied in a wind tunnel using flow control with Acoustic Excitation, which is supplied internally through a slit to the flow over the cylinder. The drag and the lift force acting on the cylinder are evaluated from the measurement of pressure distributions over the cylinder. The results indicate that the drag is reduced about 30% in comparison with a stationary cylinder, when the control parameters are optimized, such as the slit angle, the forcing Strouhal number and the Excitation amplitude. The corresponding flow field around the cylinder with and without control is measured by particle image velocimetry. It is found that the wake of the circular cylinder is elongated downstream and the streamwise mean velocity is accelerated on both sides of the cylinder by the influence of Acoustic Excitation. On the contrary, the velocity fluctuations and the Reynolds stress in the near wake of the cylinder are strongly diminished under Acoustic Excitation. These variations of the wake flow support the drag reduction observed in the present measurements.
Yuning Zhang - One of the best experts on this subject based on the ideXlab platform.
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combination and simultaneous resonances of gas bubbles oscillating in liquids under dual frequency Acoustic Excitation
Ultrasonics Sonochemistry, 2017Co-Authors: Yuning ZhangAbstract:The multi-frequency Acoustic Excitation has been employed to enhance the effects of oscillating bubbles in sonochemistry for many years. In the present paper, nonlinear dynamic oscillations of bubble under dual-frequency Acoustic Excitation are numerically investigated within a broad range of parameters. By investigating the power spectra and the response curves of oscillating bubbles, two unique features of bubble oscillations under dual-frequency Excitation (termed as “combination resonance” and “simultaneous resonance”) are revealed and discussed. Specifically, the amplitudes of the combination resonances are quantitatively compared with those of other traditional resonances (e.g. main resonances, harmonics). The influences of several paramount parameters (e.g., the bubble radius, the Acoustic pressure amplitude, the energy allocation between two component waves) on nonlinear bubble oscillations are demonstrated.
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the secondary bjerknes force between two gas bubbles under dual frequency Acoustic Excitation
Ultrasonics Sonochemistry, 2016Co-Authors: Yuning Zhang, Shengcai LiAbstract:The secondary Bjerknes force is one of the essential mechanisms of mutual interactions between bubbles oscillating in a sound field. The dual-frequency Acoustic Excitation has been applied in several fields such as sonochemistry, biomedicine and material engineering. In this paper, the secondary Bjerknes force under dual-frequency Excitation is investigated both analytically and numerically within a large parameter zone. The unique characteristics (i.e., the complicated patterns of the parameter zone for sign change and the combination resonances) of the secondary Bjerknes force under dual-frequency Excitation are revealed. Moreover, the influence of several parameters (e.g., the pressure amplitude, the bubble distance and the phase difference between sound waves) on the secondary Bjerknes force is also investigated numerically.
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Instability of interfaces of gas bubbles in liquids under Acoustic Excitation with dual frequency.
Ultrasonics sonochemistry, 2014Co-Authors: Yuning Zhang, Haizhen XianAbstract:Instability of interfaces of gas bubbles in liquids under Acoustic Excitation with dual frequency is theoretically investigated. The critical bubble radii dividing stable and unstable regions of bubbles under dual-frequency Acoustic Excitation are strongly affected by the amplitudes of dual-frequency Acoustic Excitation rather than the frequencies of dual-frequency Excitation. The limitation of the proposed model is also discussed with demonstrating examples.
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thermal effects on nonlinear radial oscillations of gas bubbles in liquids under Acoustic Excitation
International Communications in Heat and Mass Transfer, 2014Co-Authors: Yuning ZhangAbstract:Modeling of nonlinear radial oscillations of gas bubbles in liquids under Acoustic Excitation is an important subject for understanding of many Acoustic bubble related phenomena (e.g., sonoluminescence, sonochemistry and sonoporation). In the present paper, numerical simulations of the oscillations of gas bubbles in liquids based on a more complete polytropic model are performed with heat transfer across bubble interfaces considered. By comparing with predictions given in the literature that using a constant polytropic exponent and ignoring energy dissipation through a heat transfer across bubble interfaces, our simulations reveal that the polytropic exponent and thermal dissipation mechanism significantly influence the predictions of nonlinear bubble behavior (e.g., locations and magnitudes of resonances, thresholds of subharmonics and ultraharmonics).
G. Takeda - One of the best experts on this subject based on the ideXlab platform.
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Phase-averaged characteristics of flow around a circular cylinder under Acoustic Excitation control
Journal of Fluids and Structures, 2004Co-Authors: Nobuyuki Fujisawa, G. Takeda, N. IkeAbstract:The influence of internal Acoustic Excitation on the aerodynamic performance of a circular cylinder in a uniform flow is studied by the phase-averaged measurements of fluid forces and flow fields around the cylinder. The experimental results indicate that the mean drag as well as the fluctuating lift are reduced by the Acoustic Excitation control from a slit on a circular cylinder. The phase-averaged PIV measurements at the vortex shedding frequency show that the periodic and fluctuating velocities are suppressed in the wake of the controlled cylinder, which indicates the influence of Acoustic Excitation on the vortex shedding and the corresponding reduction in the fluid forces on the circular cylinder. On the other hand, the phase-averaged flow field at the Excitation frequency of unstable shear layer, which is four times larger than the vortex shedding frequency, indicates the formation of discrete vortices along the shear layer of the cylinder wake. The interaction of these vortices with the cylinder wake causes the reduction in the cross-correlations of velocity fluctuations in the flow field, which results in the attenuation of the vortex shedding phenomenon in the cylinder wake.
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Flow control around a circular cylinder by internal Acoustic Excitation
Journal of Fluids and Structures, 2003Co-Authors: Nobuyuki Fujisawa, G. TakedaAbstract:Abstract The possible drag reduction and the corresponding variation of the flow field around a circular cylinder are studied in a wind tunnel using flow control with Acoustic Excitation, which is supplied internally through a slit to the flow over the cylinder. The drag and the lift force acting on the cylinder are evaluated from the measurement of pressure distributions over the cylinder. The results indicate that the drag is reduced about 30% in comparison with a stationary cylinder, when the control parameters are optimized, such as the slit angle, the forcing Strouhal number and the Excitation amplitude. The corresponding flow field around the cylinder with and without control is measured by particle image velocimetry. It is found that the wake of the circular cylinder is elongated downstream and the streamwise mean velocity is accelerated on both sides of the cylinder by the influence of Acoustic Excitation. On the contrary, the velocity fluctuations and the Reynolds stress in the near wake of the cylinder are strongly diminished under Acoustic Excitation. These variations of the wake flow support the drag reduction observed in the present measurements.
Ann Karagozian - One of the best experts on this subject based on the ideXlab platform.
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Laminar Microjet Diffusion Flame Response to Transverse Acoustic Excitation
Combustion Science and Technology, 2020Co-Authors: Hyung Sub Sim, Andres Vargas, Dongchan Daniel Ahn, Ann KaragozianAbstract:The present experiments focused on the response of burning gaseous fuel jets to prescribed transverse Acoustic Excitation as a means of exploring the coupling of reactive, Acoustic, and flow proces...
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droplet combustion in the presence of Acoustic Excitation
Combustion and Flame, 2014Co-Authors: Cristhian I Sevillaesparza, Owen Smith, Jeffrey Wegener, Sophonias Teshome, Juan Rodriguez, Ann KaragozianAbstract:Abstract This experimental study focused on droplet combustion characteristics for various liquid fuels during exposure to external Acoustical perturbations generated within an Acoustic waveguide. The alternative liquid fuels include alcohols, aviation fuel (JP-8), and liquid synthetic fuel derived via the Fischer–Tropsch process. The study examined combustion during Excitation conditions in which the droplet was situated in the vicinity of a pressure node (PN). In response to such Acoustic Excitation, the flame surrounding the droplet was observed to be deflected, on average, with an orientation depending on the droplet’s relative position with respect to the PN. Flame orientation was always found to be consistent with the sign of a theoretical bulk Acoustic acceleration, analogous to a gravitational acceleration, acting on the burning system. Yet experimentally measured Acoustic accelerations based on mean flame deflection differed quantitatively from that predicted by the theory. Phase-locked OH∗ chemiluminescence imaging revealed temporal oscillations in flame standoff distance from the droplet as well as chemiluminescent intensity; these oscillations were especially pronounced when the droplets were situated close to the PN. Simultaneous imaging and pressure measurements enabled quantification of combustion-Acoustic coupling via the Rayleigh index, and hence a more detailed understanding of dynamical phenomena associated with Acoustically coupled condensed phase combustion processes.
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Acoustic Excitation of droplet combustion in microgravity and normal gravity
Combustion and Flame, 2005Co-Authors: Srinivasan Dattarajan, A. Lutomirski, R. Lobbia, Owen Smith, Ann KaragozianAbstract:This experimental study focused on methanol droplet combustion characteristics during exposure to external Acoustical perturbations in both normal gravity and microgravity. Emphasis was placed on examination of Excitation conditions in which the droplet was situated (1) at or near a velocity antinode (pressure node), where the droplet experienced the greatest effects of velocity perturbations, or (2) at a velocity node (pressure antinode), where the droplet was exposed to minimal velocity fluctuations. Acoustic Excitation had a significantly greater influence on droplet-burning rates and flame structures in microgravity than in normal gravity. In normal gravity, Acoustic Excitation of droplets situated near a pressure node produced only very moderate increases in burning rate (about 11–15% higher than for nonAcoustically excited, burning droplets) and produced no significant change in burning rate near a pressure antinode. In microgravity, for the same range in sound pressure level, droplet burning rates increased by over 75 and 200% for droplets situated at or near pressure antinode and pressure node locations, respectively. Observed flame deformation for droplets situated near pressure nodes or antinodes were generally consistent with the notion of Acoustic radiation forces arising in connection with Acoustic streaming, yet both velocity and pressure perturbations were seen to affect flame behavior, even when the droplet was situated precisely at or extremely close to node or antinode locations. Displacements of the droplet with respect to node or antinode locations were observed to have a measureable effect on droplet burning rates, yet Acoustic accelerations associated with such displacements, as an analogy to gravitational acceleration, did not completely explain the significant increases in burning rate resulting from the Excitation.
