Effect of bubbles on the turbulence near the exit of a liquid jet

The objective of this paper is to examine the effect of bubbles on the turbulence levels of a water jet. Simultaneous measurements of the axial and radial velocity components were taken in a bubbly jet with a Laser Doppler Velocimeter (LDV) and then compared to the velocities of a single phase jet at the same liquid flow rate. Mean bubble diameters ranged from 0.6 to 2 mm and the void fractions were up to about 20%. The liquid Reynolds numbers were from 5,000 to 10,000 approximately. The measurements extended to from an axial distance of 4–12 cm. It was observed that bubbles did not affect significantly the average velocity profiles in the jet. However bubbles increased the turbulence intensities in the core of the jet near the jet exit. The increase in turbulence intensities was more pronounced at lower Reynolds numbers and at higher void fractions.     

Experimental study of a liquid jet flowing into another immiscible liquid “a local analysis of the interface”

This paper describes an experimental study of a liquid jet leaving a cylindrical nozzle under gravity. A special optical system was used to study the spatial and temporal interface variations between two liquids. A photoelectric cell was used to measure the light intensity and to obtain the physical parameters of the jet. Spatial analysis revealed a continual contraction of the jet from the nozzle exit to the break-up zone. Fluctuations of the interface over time are characteristic of a random signal with a narrow bandpass. The Fourier transform of the different samples shows a bandpass of finite width centered around a characteristic frequency. The distribution of interface amplitude fluctuations was symmetrical to the average diameter, except in the zone in which the jet breaks up. By systematically tracing the main parameters of the jet diameter, we observed three zones with different jet behavior. The characteristic frequency of interface fluctuations increases as a linear function of the distance from the nozzle. The amplitude of interface fluctuations was an exponential function of the distance at which jet diameter fluctuations were measured.     

Effect of Sudden Expansion on Entrainment and Spreading Rates of a Jet Issuing from Asymmetric Nozzles

Turbulent free jets issuing from five different nozzle geometries; smooth pipe, contracted circular, rectangular, triangular, and square, are experimentally investigated by using TSI 2-D laser Doppler velocimetry (LDV) to assess the effect of nozzle geometry and quarl (i.e. a cylindrical sudden expansion) on jet entrainment and spreading. The centerline mean velocity decay and the jet half-velocity width, which are indicators of jet entrainment and spreading rates, are determined for each nozzle’s flow configuration, i.e. with and without sudden expansion. Furthermore, turbulence quantities, such as the flow mean velocities and their mean fluctuating components, as well as Reynolds shear stresses, are all measured along the centerline plane of the jet to facilitate understanding the extent of the effect of nozzle’s geometry (i.e. nozzle’s orifice shape and sudden expansion) on jet’s entrainment and spreading. The main results show that the jet flow with the presence of sudden expansion exhibits higher rates of entrainment and spreading than without. In addition, these results reveal that sudden expansion exercises a greater effect on the asymmetric jet characteristics, especially for the triangular and rectangular nozzles compared to their axisymmetric counterparts (i.e. circular contracted nozzle).     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.     

Measurements of mean and fluctuating temperature in an underexpanded jet using electrostrictive laser-induced gratings

Instantaneous temperature measurements were obtained in an underexpanded jet using electrostrictive laser-induced gratings. Evaluation of the technique under static, low-pressure conditions provided a baseline uncertainty or precision for single-shot temperature measurements of 4.4% of the local mean temperature, which represents the minimum detectable temperature fluctuation. The underexpanded jet was operated at a nozzle pressure ratio of 2.39 and a fully expanded jet Mach number of 1.19. Data were acquired along the centerline and over two radial traverses through the shear layer. Mean temperature data agree well with expectations, describing the shock-cell structure and the compressible shear layer. The growth in shear-layer width with downstream distance can be identified in the mean and fluctuating temperature measurements. Temperature fluctuations are near the baseline detection limit in the jet core and surrounding ambient air, and reach a maximum in the shear layer. The temperature fluctuation measurements compare well with previous computational and experimental work, confirming the application of the technique to a turbulent, supersonic flow.     

Instantaneous measurement of two-dimensional temperature distributions by means of two-color planar laser induced fluorescence (PLIF)

We present a noninvasive technique to measure instantaneously two-dimensional temperature distributions in liquid flows by means of two-color planar laser induced fluorescence (PLIF) of rhodamine B. This technique allows absolute temperature measurements by determining the relative intensities on two adequate spectral bands of the same dye and a reference measurement at a known temperature. Demonstration measurements on a heated turbulent jet injected into a coflow at ambient temperature are presented. The mean temperature field is investigated, as well as the temperature fluctuations. The results are compared to those obtained by means of a usual single-color PLIF technique.     

药型罩锥角对射孔枪射流冲击过程的影响

采用ALE方法对射孔弹射流形成的过程及聚能射流对混凝土靶板的侵彻进行了数值模拟,对比了锥形药型罩的不同锥角对聚能射流形成和侵彻的影响.研究结果表明药型罩的锥角大小对聚能射流的速度和形状、射流头部和杵体的质量、侵彻的宽度和深度有着明显显著的影响.小锥角射流头部比重较小且杵体速度未达到侵彻临界值而无法起到很好的侵彻效果,大...     

Impact pressures of turbulent high-velocity jets plunging in pools with flat bottom

Dynamic pressures created by the impact of high-velocity turbulent jets plunging in a water pool with flat bottom were investigated. Pressure fluctuations were sampled at 1 kHz at the jet outlet and at the pool bottom using piezo-resistive pressure transducers, jet velocities of up to 30 m/s and pool depth to jet diameter ratios from 2.8 to 11.4. The high-velocity jets entrain air in the pool in conditions similar to prototype applications at water release structures of dams. The intermittent character of plunge pool flows was investigated for shallow and deep pools, based on high order moments and time correlations. Maximum intermittency was observed for pool depths at 5.6 jet diameters, which approximate the core development length. Wall pressure skewness was shown to allow identifying the zone of influence of downward and upward moving currents.     

An experimental study of two-phase turbulent coaxial jets

The effect of solid particles on the flow structure of axisymmetric turbulent coaxial jets has been studied. A laser-Doppler anemometer was used to measure the mean and fluctuation velocities of both phases, and a Malvern laser diffraction instrument was applied to measure particle size and concentration. A series of velocity ratios and particle loading ratios were investigated, and the results were analysed for the effects of these ratios on the mixing characteristic and the similarity behavior of the jet. The effects of particle diameter and its distribution were also studied as well as their influence on the coaxial jet behavior.     

The planar jet-plate oscillator

The aeroacoustic noise generated by a high speed, planar gas jet impinging on a flat plate is investigated experimentally. The jet used in this study is typical of those commonly found in industrial applications such as in various coating control and heat transfer processes. Normal jet impingement on the plate is found to generate strong acoustic tones over a wide range of impingement distances and jet velocities. The characteristics of these tones, as a function of the jet velocity and impingement distance, are quantified. Phase and amplitude measurements of the pressure fluctuations on the impingement plate indicate that the acoustic tones are generated by an antisymmetric instability mode of the jet oscillation. The effect of plate inclination in both the transverse and span-wise directions, with respect to the incident jet, is also studied. The jet-plate tone is found to be much more sensitive to changes in the span-wise plate inclination than to changes in the transverse inclination, but in both cases, a complete suppression of the tone is found to be possible.     

Planar fluorescence for round bubble imaging and its application for the study of an axisymmetric two-phase jet

A correlation-based processing algorithm for bubble identification by a planar fluorescence for bubble imaging (PFBI) technique is presented in this paper. The algorithm includes procedures to identify bubble positions and sizes, as well as to track bubbles and correct bubble displacement vectors. Moreover, several schemes for calculation time optimisation were realised to achieve a reliable calculation time. The developed algorithm identifies and tracks overlapping bubble images or images with non-uniform intensity distributions. The employed correlation and iterative passing approach provides sub-pixel accuracy of bubble displacement estimation. In addition, the presented algorithm for bubble ring detection can be easily applied to shadow photography images of bubbles, after the application of a derivative filter. The PFBI technique, combined with the particle image velocimetry and particle tracking velocimetry algorithms, was applied for the experimental study of bubbly free jet two-phase flows at Re = 12,000. Four cases of volumetric gas content in the jet core were studied: 0, 1.2, 2.4 and 4.2%, with the same mean bubble diameter—0.85 mm. The developed technique measures two-dimensional distributions of instantaneous void fractions, as well as both gaseous and liquid-phase velocities. Consequently, the mean void fraction and velocity fields and a set of second-order statistical moments were obtained, including correlations of void fraction and velocity pulsations. It was shown that the increase in volumetric gas content leads to the suppression of liquid-phase velocity fluctuations in the jet mixing layer.     

超临界CO2磨料射流流场影响因素的模拟分析

为了揭示超临界CO₂磨料射流流场特性,利用计算流体动力学模拟软件,对超临界CO₂磨料射流结构及不同因素对射流流场的影响规律进行了研究。结果表明:超临界CO₂磨料射流轴向速度和冲击力随着喷距的增大,先增大后减小,即存在最优喷距,喷射压差为10~30 MPa时最优喷距为3~6倍喷嘴直径;喷射压差一定时,围压由10 MPa增至30 MPa对射流速度场及液相冲击力会造成较小的负面影响。通过超临界CO₂射流破岩实验对上述2因素进行了辅助对比验证;流体温度由333 K增至413 K,固液两相轴向速度增大,而流体密度降低,导致液相冲击力减弱;磨料浓度由3.0%连续增至11.0%,射流固液两相轴向速度逐渐降低,降幅逐渐减小。     

Heat generation at the tip of a moving crack

The high energy concentration at the tip of a moving crack causes irreversible deformations and produces heat as a consequence. The resulting temperatures were calculated by consideration of the crack tip as a moving heat-source of rectangular shape. In brittle materials with very small plastic zones and high crack velocities, these temperatures are predicted to be higher than 1000 K. For the experimental verification of these calculations, a very sensitive radiation thermometer was developed. It registers the intensity of the radiation at four wavelengths. By comparison of these intensities with that of black body radiation, the temperature was determined as 3200 K for glass and 4700 K for quartz.     

Swirl effects on variable-density jet mixing in confined flows

An experimental investigation on swirl effects on inhomogeneous confined jet mixing in a combustor configuration is reported. The confined swirling flow was simulated by a swirler with a central jet mounted in a cyclindrical tube. Helium and air jets set at different velocities were injected into the confined swirling air flow. The resulting flow fields due to two vane swirlers with constant vane angles of 35° and 66° were compared. Results show that the 35° vane swirler produces a solid-body rotation core with a slope about twice that created by the 66° vane swirler. It is the behavior of this solid-body rotation core that determines jet mixing rather than the swirler vane angle. Consequently, the coaxial jet decays much faster, the mixing is more intense, and the turbulence intensities are higher for the 35° vane swirler. In view of these results, combustor designers should be more concerned with behavior of the solid-body rotation core produced by the swirler, instead of the swirler vane angle.     

Effect of particle size on a two-phase turbulent jet

The effect of particle size on two-phase turbulent jet flow structure is studied in the present experimental investigation. Polystyrene solid particles of 210, 460, and 780 μm were used. The particles' mass loading ratios ranged from 0 to 3.6. The flow Reynolds number was 2 ‘ 10⁴, which was based on the pipe nozzle diameter and the fluid-phase centerline velocity at the nozzle exit. A two-color laser-Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed for measurement. The measurement range of the jet flow was from the initial pipe exit to 90D downstream. Results are presented for the mean velocities of particle and fluid phases, the flow's turbulent intensities and the flow's Reynolds stresses. The energy spectra and the correlation functions of the two-phase jet flow were also obtained by using another one-component He-Ne LDA system.     

Direct numerical simulation of a turbulent axisymmetric jet with buoyancy induced acceleration

Direct numerical simulations of an axisymmetric jet with off-source volumetric heat addition are presented in this paper. The system solved here involves a three-way coupling between velocity, concentration and temperature. The computations are performed on a spherical coordinate system, and application of a traction free boundary condition at the lateral edges allows physical entrainment into the computational domain. The Reynolds and Richardson numbers based on local scales employed in the simulations are 1000 and 12 respectively. A strong effect of heat addition on the jet is apparent. Heating causes acceleration of the jet, and an increased dilution due to an increase in entrainment. Further, the streamwise velocity profile is distorted, and the cross-stream velocity is inward for all radial locations for the heated jet. Interestingly, the maximum temperature is realized off-axis and a short distance upstream of the exit of the heat injection zone (HIZ). The temperature width is intermediate between the scalar and velocity widths in the HIZ. Normalized rms of the concentration and temperature increases in the HIZ, whereas that of streamwise, cross-stream and tangential velocities increases rapidly after decreasing. Both mass flux and entrainment are larger for the heated jet as compared to their unheated counterparts. The buoyancy flux increases monotonically in the HIZ, and subsequently remains constant.     

The relation between the high speed submerged cavitating jet behaviour and the cavitation erosion process

In order to accurately and reliably evaluate the cavitation erosion resistance of materials using a cavitating jet generator, the effects of the hydrodynamic parameters and the nozzle geometry on the erosion process were investigated. Since the behaviour of a high speed submerged cavitating jet is also depending on the working conditions; their influence is also discussed based on the evaluation of cavitation erosion process. The erosion rate was used as an indicator for cavitating jet behaviour. Specimens of commercial-purity copper were subjected to high speed submerged cavitating jets under different initial conditions, for certain time periods. The force generated by jet cavitation is employed to initiate the erosion in surface. The tested specimens were investigated with a digital optical microscope and a profilometer. It was found that erosion becomes more pronounced with decreasing cavitation numbers, as well as with increasing exit jet velocities. The nozzle configuration and hydrodynamic parameters have strong influences on the erosion rate, eroded area and depth of erosion. A comparison between the obtained results explains some of the mechanisms involved in cavitation and erosion processes and their relation to the tested parameters. Mathematical expressions which combine these parameters with the erosion rate are obtained. These parameters are very important in order to control the cavitation as a phenomenon and also to control the performance of the cavitating jet generator.     

Stereo-PIV measurements of vapor-induced flow modifications in confined jet impingement boiling

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75 mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is compared for a constant jet inlet subcooling of 10 °C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbulence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor generation from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap. Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets.     

Use of a Modified Langevin Equation to Describe Turbulent Dispersion of Fluid Particles in a Channel Flow

A stochastic method to represent the positions and velocities of fluid particles in a nonhomogeneous turbulence was pursued. Spatially varying Lagrangian time scales obtained from direct numerical simulations of turbulent flow in a channel and spatially varying joint Gaussian forcing functions were incorporated into a Langevin equation. The model was tested by comparing calculations of the dispersions and velocities of particles originating from point sources with experiments carried out in a DNS of fully-developed turbulent flows in a channel at Re_τ = 150 and 300. The model captured the dispersions, mean velocities and moments of the velocity fluctuations very well. The use of jointly Gaussian (rather than uncorrelated Gaussian) forcing functions greatly improved the calculation of dispersions and mean velocities in the stream wise direction, as well as turbulence quantities which include stream wise velocity fluctuations. The condition of well-mixedness for the model was verified by considering the behavior of a uniform distribution of sources. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temporal and spatial instability of an inviscid compound jet

This paper examines the linear hydrodynamic stability of an inviscid compound jet. We perform the temporal and the spatial analyses in a unified framework in terms of transforms. The two analyses agree in the limit of large jet velocity. The dispersion equation is explicit in the growth rate, affording an analytical solution. In the temporal analysis, there are two growing modes, stretching and squeezing. Thin film asymptotic expressions provide insight into the instability mechanism. The spatial analysis shows that the compound jet is absolutely unstable for small jet velocities and admits a convectively growing instability for larger velocities. We study the effect of the system parameters on the temporal growth rate and that of the jet velocity on the spatial growth rate. Predictions of both the temporal and the spatial theories compare well with experiment.Dedicated to the memory of Professor Tasos C. Papanastasiou