Investigation of turbulent spray disintegration characteristics depending on the nozzle configuration

The experimental measurements were carried out to examine turbulent disintegration characteristics ejecting from a counter-flowing internal mixing pneumatic nozzle under variable conditions of swirl angles and air pressures. The air injection pressure was varied from 60 kPa to 180 kPa and four counter-flowing internal mixing nozzles with axi-symmetric tangential-drilled holes at swirl angle of 15°, 30°, 45°, and 60° to the central axis have been specially designed. The experimental results were quantitatively analyzed, focusing mainly on the comparison of turbulent atomization characteristics issuing from an internal mixing swirl nozzle. To illustrate the swirl phenomena, the distributions of mean velocities, turbulence intensities, volume flux, and SMD (Sauter Mean Diameter, or D₃₂) were comparatively analyzed.     

旋芯喷嘴高效雾化特性测量研究

分析了旋芯喷嘴的雾化机理,运用三维相位多普勒粒子测速仪(3D-PDPA)设备测量了旋芯喷嘴喷出的细水雾雾滴速度及索太尔平均液滴直径(SMD)。试验结果表明,压力低于2 Mpa时,喷雾状况符合表面波破碎理论规律。压力高于2 Mpa时,喷雾在短距离内由旋芯强制呈空心锥形,最终变为引射方式下的实心锥形。随着喷雾压力升高,空心锥形形状保持的距离缩短。在空心喷雾阶段,随着喷射距离的增大,细水雾的轴向速度和径向速度在喷雾锥边缘处降幂下降。喷雾锥内部雾滴均系从边缘散溅所得,因此在各截面中心处,轴向速度服从普通射流规律,不存在稳定的径向扩散速度。雾滴SMD在各截面均远小于100 μm。     

Effects of ALR and configuration ratio on turbulent structures in swirling flows

To assess the significant physics associated with the increase of ALR and configuration ratio of the nozzle tip in pneumatic swirling flows, comprehensive observations using a 3-D PDPA system were experimentally carried out. Profiles of mean velocities, turbulence intensities, SMD variations, and correlations between droplet size and turbulence components were quantitatively acquired. As discussed in a previous literature, axisymmetric swirl angle of 30° is selected for this investigation because of its strong turbulence levels in the flowfield and finer droplet disintegrations. Various ALRs (Air-to-Liquid Mass Ratio) as well as the length-to-diameter ratios of nozzle tip as parameters were chosen. Due to the complex interactions in swirling flows under these variables, this experimental observation will be of fundamental importance to the understanding of turbulence structures. From the observations, it indicated that increasing the ALR causes the spray development to be positively fluctuated on the atomization in both axial and tangential RMS velocities. Also, it can be concluded that the SMD decreases continuously with increase of ALR, substantiating the fact that the fluctuations are inversely proportional to the SMD variation. Meanwhile, the spray behavior is characteristic with a reduction of length-to-diameter ratio; smaller the configuration ratio, the higher the turbulence intensities and smaller SMD variations in the flowfield.     

Experimental investigation of mixing-enhanced swirl flows

The experimental objective was to compare disintegration characteristics from the internal mixing pneumatic nozzles under the different operating conditions in terms of swirl angles. For this investigation, supplied air pressures and nozzle configuration ratios were fixed. This experimental comparison is of fundamental importance to the understanding and modeling of turbulent atomization because the axisymmetric swirling flows involve relatively complex interactions. For the measurement, four internal swirl mixing nozzles with axisymmetric holes at swirl angles of 15°, 30°, 45°, and 60° to the central axis were employed, which is responsible for the enhancement of mixing in pneumatic jets. To illustrate the swirl phenomena quantitatively, the distributions of mean velocities, turbulence intensities, and SMD (Sauter mean diameter, or D₃₂) variations with different configuration ratio were comparatively analyzed. It indicated that the atomization characteristics are performed well in the case of 30° of swirl angle, and that turbulence intensities are gradually degenerated with the increase of radial distances, showing a slight increment of SMD at downstream region. In particular, measurements showed that nozzle configuration is one of the significant geometrical parameters affecting the spray trajectories. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park     

Intermittent atomization characteristics of multi-hole and single-hole diesel nozzle

The intermittent spray characteristics of a multi-hole and a single-hole diesel nozzle were experimentally investigated. The hole number of the multi-hole nozzle was 5, and the hole diameter of the 5-hole and the single-hole nozzle was the same as d_n=0.32 mm with the constant hole length to diameter ratio (l_n/d_n=2.81). The droplet diameters of the spray, including the time-resolved droplet diameter, SMD (Sauter mean diameter) and AMD (arithmetic mean diameter), injected intermittently from the two nozzles into the still ambient were measured by using a 2-D PDPA (phase Doppler particle analyzer). Through the time-resolved evolutions of the droplet diameter, it was found that the structure of the multi-hole and the single-hole nozzle spray consisted of the three main parts : (a) the leading edge affected by surrounding air and composed of small droplets ; (b) the central part surrounded by the leading edge and mixing flow region and scarcely affected by the resistance of air ; (c) the trailing edge formed by the passage of the central part. The SMD decreases gradually with the increase in the radial distance, and the constant value is obtained at the outer region of the radial distance (normalized by hole diameter) of 7-8 and 6 for the 5-hole and single-hole nozzle, respectively. The SMD along the centerline of the spray decrease shapely with the increase in the axial distance after showing the maximum value near the nozzle tip. The SMD remains the constant value near the axial distance (normalized by hole diameter) of 150 and 180 for the 5-hole and the single-hole nozzle, respectively.     

Study of divergence angle influence for sonic nozzle in non-equilibrium condensation

The condensation happens generally in a nozzle during expansion of compressed steam from convergent to the divergent part of the nozzle. The divergence angle is the angle measured from the throat of the nozzle to the outlet. In this paper, the outlet is kept constant and the throat diameter is varied. In turn, the divergence angle of the sonic nozzle is altered. The effect of divergence angle on condensation phenomena is investigated with wet steam in a sonic nozzle. For analyzing the wet steam properties, the non-equilibrium condensation model is used. This model is the classical nucleation theory coupled with the droplet growth rate equation. The base nozzle is designed with the throat diameter of 4.5 mm and other dimensions are calculated according to ASME nozzle formulas. Furthermore, the chosen divergence angles are 3°, 4.2°, and 6° for which the throat diameters are 4.5 mm, 3 mm, and 1.5 mm, respectively. As the divergence angle is gradually increased, the position of maximum Mach number of the flow moves upstream, the static temperature of the flow near the throat reaches the lower value, and the droplet nucleation rate is increased. The condensation shock gets gradually stronger with decreasing the divergence angle.

     

An investigation of swirling flow in a cylindrical tube

An experimental study was performed for measuring velocity and turbulence intensity in a circular tube for Re=10,000, 15,000 and 20,000, with swirl and without swirling flow. The velocity fields were measured using PIV techniques and swirl motion was produced by a tangential inlet condition. Some preliminary measurements indicated that over the first 4 diameter, two regions of flow reversal were set up (the so called 2-cell structure). At the highest Reynolds numbers, the maximum values of the measured axial velocity components had moved toward the test tube wall and produce more flow reversal at the corner of the tube. As the Reynolds number increased, the turbulence intensity of swirling flow at the tube inlet also increased.     

Jet flow characteristics of sinusoidal wavy nozzles

The flow characteristics of jets issued from a sinusoidal nozzle with in-phase and 180° out-of-phase exit configurations were investigated using PIV (particle image velocimetry) and flow visualization techniques. The experiments were carried out at a Reynolds number of about 6300 based on the mean width of the jet nozzle. Compared to a normal rectangular jet, the sinusoidal nozzle jets have smaller velocity deficits as the flow goes downstream. In addition, the turbulence intensity is suppressed in the horizontal center plane. For the case of in-phase wavy nozzle jet, the length of the potential core exhibits small variations along the lateral direction, while the 180° out-of-phase wavy nozzle jet shows large lateral variation in the length of potential core. The turbulent kinetic energy of the 180° out-ofphase nozzle jet also shows sinusoidal variation in the horizontal planes. Large-scale vortices shed from the sinusoidal edge of the nozzle interact strongly and migrate toward the center plane as the flow develops downstream.     

Atomization characteristics of intermittent multi-hole diesel spray using time-resolved PDPA data

The intermittent spray characteristics of a multi-hole diesel nozzle with a 2-spring nozzle holder were investigated experimentally. Without changing the total orifice exit area, the hole number of the multi-hole nozzle varied from 3 (d_n=0.42 mm) to 5 (d_n=0.32mm). The time-resolved droplet diameters of the spray including the SMD (Sauter mean diameter) and the AMD (arithmetic mean diameter), injected intermittently from the multi-hole nozzles into still ambient air, were measured by using a 2-D PDPA (phase Doppler particle analyzer). The 5-hole nozzle spray shows the smaller spray cone angle, the decreased SMD distributions and the small difference between the SMD and the AMD, compared with that of the 3-hole nozzle spray. From the SMD distributions with the radial distance, the spray structure can be classified into the three regions : (a) the inner region showing the high SMD distribution; (b) the mixing flow region where the shear flow structure would be constructed; and (c) the outer region formed through the disintegration processes of the spray inner region and composed of fine droplets. Through the SMD distributions along the spray centerline, it reveals that the SMD decreases rapidly after showing the maximum value in the vicinity of the nozzle tip. The SMD remains the constant value near the Z/d_n=166 and 156.3 for the 3-hole and 5-hole nozzles, which illustrate that the disintegration processes of the 5-hole nozzle spray proceed more rapidly than that of the 3-hole nozzle spray.     

An experimental study on swirling flow behind a round cylinder in the horizontal circular tube

An experimental study is performed for turbulent swirling flow behind a circular cylinder using 2-D PIV technique. The Reynolds number investigated are 10,000, 15,000, 20,000 and 25,000. The mean velocity vector, time mean axial velocity, turbulence intensity, kinetic energy and Reynolds shear stress behind the cylinder are measured before and behind the round cylinder along the test tube. A comparison is included with non swirl flow behind a circular and square cylinder. The recirculation zones are showed asymmetric profiles.     

阀芯结构对双流体喷雾粒子特性的影响

为了研究阀芯结构对双流体喷雾粒子特性的影响,提升喷雾效果,运用相位多普勒粒子分析仪（PDPA）对不同阀芯结构的双流体喷雾雾滴粒径、轴向速度以及雾滴数目进行了测试,并对测量结果进行了分析和讨论。结果表明：随着轴向距离的增大,雾滴索特平均直径(SMD)、算术平均直径(AMD)呈先增大后趋于平缓的趋势,轴向速度以及湍流脉动速度均呈减小趋势,雾滴数目呈先增大后减小的趋势;随着气液压力比的增大,SMD呈先增大后减小的趋势,而AMD、轴向速度以及雾滴数目均呈减小趋势;阀芯的喉口直径、出口直径的减小均有利于喷雾效果的提升,但同时导致速度稳定性变差;当喉口直径为1.5mm、出口直径为2.5mm时,与原始阀芯结构相比,雾滴数目和雾滴轴向速度分别增大了82.43%和22.31%,SMD和AMD分别减小了52.18%和21.47%,综合喷雾效果得到了大幅提升。     

Estimation of spray flow characteristics using ensemble Kalman filter

Spray flows are widely used in several industrial applications, such as combustion engines. Accurate measurement of spray flow characteristics requires sophisticated equipment and techniques. In recent years, the discrete droplet method (DDM), which analyses droplet scattering, has become a mature technique and has been applied to various analyses. We propose an estimation system based on particle image velocimetry (PIV) measurements and an ensemble Kalman filter, together with DDM, to efficiently investigate spray flow characteristics. The proposed method performs data assimilation on the velocity distribution in a two-dimensional cross-section obtained by PIV to estimate the characteristics of the spray flow in three dimensions. In this study, the system was constructed so that droplet particle is ensembled during data assimilation to estimate the droplet diameter distribution indirectly. The proposed method can be used to estimate the spray velocity and droplet size distribution. The numerical solution obtained using DDM was used as a criterion for assimilation and validated by conducting twin experiments. The results showed that, in terms of spray velocity, the estimation error for the velocity component parallel to the main flow was 2% and that for the velocity component perpendicular to the main flow was around 10%. Finally, the velocity and particle size distributions of the spray stream and the three-dimensional droplet distribution were estimated by assimilating the velocity distributions measured by PIV. This technique predicts the spray angle and droplet size distribution from the two-dimensional velocity field of the PIV only and is expected to contribute to the development of injectors and atomizers.     

Measurement of particle/bubble motion and turbulence around it by hybrid PIV

Characteristics of bubble flow are influenced by bubble motion, liquid flow and interactions between bubbles, and between a bubble and liquid phase. Thus because behavior of a single bubble and liquid around it is regarded as one of the basic elements characterizing bubble flow, the single bubble motion in stagnant water was investigated experimentally by using flow visualization and image processing methods. The bubble motion is influenced by several factors, that is, bubble size, density difference between gas and liquid, bubble shape and deformation in motion. In order to separate the effect of each factor, some solid particles with different size, shape and/or density were also measured and the characteristic of each factor was discussed. Two-dimensional water velocity field and the motion of a rising particle/bubble in the water were simultaneously measured by PIV (Particle Image Velocimetry) and PTV (Particle Tracking Velocimetry), respectively (Hybrid PIV). The experimental results showed that the large density difference between a particle and water caused high relative velocity and induced zigzag motion of the particle. Furthermore, the turbulence intensity of a bubble was about twice in the case of the spherical solid particle of similar diameter.     

The effects of injector nozzle geometry and operating pressure conditions on the transient fuel spray behavior

Effects of injector nozzle geometry and operating pressure conditions such as opening pressure, ambient pressure, and injection pressure on the transient fuel spray behavior have been examined by experiments. In order to clarify the effect of internal flow inside nozzle on the external spray, flow details inside model nozzle and real nozzle were also investigated both experimentally and numerically. For the effect of injection pressures, droplet sizes and velocities were obtained at maximum line pressure of 21 MPa and 105 MPa. Droplet sizes produced from the round inlet nozzle were larger than those from the sharp inlet nozzle and the spray angle of the round inlet nozzle was narrower than that from the sharp inlet nozzle. With the increase of opening pressure, spray tip penetration and spray angle were increased at both lower ambient pressure and higher ambient pressure. The velocity and size profiles maintained similarity despite of the substantial change in injection pressure, however, the increased injection pressure produced a higher percentage of droplet that are likely to breakup.     

Atomization characteristics in pneumatic counterfiowing internal mixing nozzle

In an effort to illustrate the global variation of SMD (Sauter mean diameter, orD ₃₂) and AMD (Arithmetic mean diameter, orD ₁₀) at five axial downstream locations (i. e., at Z=30, 50, 80, 120, and 170 mm) under the different experimental conditions, the radial coordinate is normalized by the spray half-width. Experimental data to analyze the atomization characteristics concerning with an internal mixing type have been obtained using a PDPA (Phase Doppler Particle Analyzer). The air injection pressure was varied from 40 kPa to 120 kPa. In this study, counterfiowing internal mixing nozzles manufactured at an angle of l5^o with axi-symmetric tangential-drilled four holes have been considered. By comparing the results, it is clearly possible to discern the effects of increasing air pressure, suggesting that the disintegration process is enhanced and finer spray droplets can be obtained under higher air assist. The variations inD ₃₂ are attributed to the characteristic feature of internal mixing nozzle in which the droplets are preferentially ejected downward with strong axial momentum, and dispersed with the larger droplets which are detected in the spray centerline at the near stations and smaller ones are generated due to further subsequent breakup by higher shear stresses at farther axial locations. The poor atomization around the centre close to the nozzle exit is attributed to the fact that the relatively lower rates of spherical particles are detected and these drops are not subject to instantaneous breakup in spite of the strong axial momentum. However, substantial increases in SMD from the central part toward the edge of the spray as they go farther downstream are mainly due to the fact that the relative velocity of droplet is too low to cause any subsequent disintegration.     

Optimization of a subsonic wind tunnel nozzle with low contraction ratio via ball-spine inverse design method

The goal of wind tunnel design is to generate a uniform air flow with minimum turbulence intensity and low flow angle. The nozzle is the main component of wind tunnels to create a uniform flow with minimal turbulence. Pressure distribution along nozzle walls directly affects the boundary layer thickness, pressure losses and non-uniformity of flow velocity through the test section. Although reduction of flow turbulences and non-uniformity through the test section can be carried out by nozzles with high contraction ratio, it increases the construction cost of the wind tunnel. For decreasing the construction cost of nozzle with constant test section size and mass flow rate, the contraction ratio and length of nozzle should be decreased; that causes the non-uniformity of outlet velocity to increase. In this study, first, three types of nozzle are numerically investigated to compare their performance. Then, Sargison nozzle with contraction ratio of 12.25 and length of 7 m is scaled down to decrease its weight and construction cost. Having scaled and changed to a nozzle with contraction ratio of 9 and length of 5 m, its numerical solution reveals that the non-uniformity of outlet velocity increases by 21%. By using the Ballspine inverse design method, the pressure distribution of the original Sargison nozzle is first scaled and set as the target pressure of the scaled down nozzle and geometry correction is done. Having reached the target nozzle, numerical solution of flow inside the optimized nozzle shows that the non-uniformity just increases by 5% in comparison with the original Sargison nozzle.     

喷水减温阀离心喷嘴雾化性能的优化研究

为了探索喷水减温阀喷嘴结构参数变化对其雾化效果的影响,优化喷嘴结构参数,根据Fluent软件VOF模块对喷水减温调节阀的离心喷嘴进行气-液两相仿真分析。以喷嘴出口直径、旋流槽倾斜角、旋流室收缩角作为优化因素,以雾化锥角、流量系数作为雾化性能的评价指标,进行正交实验设计。基于响应面法建立雾化锥角和流量系数的代理模型,再运用粒子群优化算法对代理模型进行寻优,得到一个最优结构参数。结果表明:当出口直径为2.55 mm,旋流槽角度为40°,旋流室角度为110°时,雾化性能得到最优,雾化锥角比原模型增大17.7％,流量系数增大32.53％,为喷嘴的设计提供了一个新的方案。     

旋流片对喷嘴流场特性影响的数值研究

以水为流体介质,建立了旋流喷嘴的计算流体动力学模型.通过调整旋流喷嘴旋流片的螺旋升角和通流面积等结构参数,对喷嘴雾化的压力、速度场进行计算,推导了喷嘴的雾化角,从而研究旋流片对喷嘴流场特性的影响.将数值模拟的结果与试验结果进行对比,两者基本吻合.验证了数值计算方法的正确性,对旋流喷嘴的设计具有指导意义.     

Geometrical effects on spray characteristics of air-pressurized swirl flows

In an effort to obtain the significant features associated with the ALR and length/diameter ratio of the final discharge orifice in swirling flows, experimental observations using a 3-D PDPA system were carried out. Profiles of SMD distributions depending on l _o /d _o , correlation between SMD and turbulence intensities in terms of l _o /d _o and correlations between droplet size and turbulence components were quantitatively analyzed. As discussed in a previous literature, an axisymmetric swirl angle of 30° was selected for this investigation because of its strong turbulence levels in the flow-field and finer droplet disintegrations. Three ALRs of 0.093, 0.106, and 0.122 as well as the length/diameter ratio of 0.15, 0.45, and 0.60 were chosen as parameters. Due to the complex interactions in swirling flows under these variables, this experimental observation will be of fundamental importance to the understanding of geometrical effects on spray trajectory. From the observations, it is indicated that increasing the ALR causes the spray development to be more dependent on number density and volume flux. The results indicated that the SMD decreases discernibly with smaller l _o /d _o , substantiating the fact that turbulence intensities are inversely proportional to the SMD. But, l _o /d _o is quite proportional to the SMD.     

Experimental prediction of multi-nozzle spray characteristics for optimal design in a lead frame etching process

The objective of this paper is to provide quantitative information of uniform impact forces on the sprayed surface in order to optimize the multi-nozzle spray etching system. Spray characteristics obtained from optical non-intrusive measurements using particle image velocimetry (PIV) and particle motion analysis system (PMAS) are measured in single- and twin-nozzle sprays, and then the multi-nozzle spray characteristics is simulated based on those of measurement data. The influences of the multi-nozzle arrangement, nozzle pitch, and pipe pitch on the spray characteristics such as droplets’ velocity, diameter, number density, impact force and their uniformity are properly evaluated. The experimental cases E1 and E2 represent single-spray nozzle A and B, respectively. For twin-spray tests, three nozzle combinations, namely E3 (nozzle A-A), E4 (nozzle A-B) and E5 (nozzle B-B) are considered with different nozzle pitches. The multi-spray simulation cases S1 and S6 represent the multi-spray cases with a homo-nozzle arrays which is consisted in all nozzles of nozzle A or B. For cases from case S2 to S5, the multi-spray cases with a hybrid-nozzle arrays which is consisted in all nozzles of nozzle A and B. The results show that the impact force increases approximately twice as much for changing of experimental test cases from E1 to E5 owing to the differences in nozzle characteristics of single-sprays and the overlap region between two adjacent nozzles. For the multi-nozzle spray simulation, the uniformity of impact force (UI) is increased with increasing the number of nozzle B which has larger orifice diameter and a wider spray angle. The optimum multi-nozzle spray arrangement is case S4 with more than 90 % UI, based on the fact that the UI is quite stable with increasing the nozzle pitch ranging from 90 mm to 145 mm.