A Photographic Study of Flash-Boiling Atomization

To gain an insight into the mechanisms of flash-boiling atomization, heated water was injected from a single-hole orifice into heated air (steady injections, liquid pressure 697 kPa, air pressure ambient, test temperatures from 300 to 426 K, orifice diameter 0.34 mm, length 1.37 mm). The breakup regime of interest in the study was that where the spray divergence starts at the nozzle exit. Short-duration backlit photographs and laser diffraction dropsize measurements showed that these flashing jets comprise an inner intact core which is surrounded by the diverging fine spray. These details about the spray structure are not visible in conventional photographs of flashing sprays that use scattered light illumination. The present results cast doubt on a previously proposed theory of flash-boiling atomization that attributes the divergence of the spray cone to the expansion processes that occur in an underex-panded compressible flow, since that theory implies that the liquid is already atomized upon leaving the nozzle. Instead, the photographs show that drops are expelled from the unbroken liquid jet starting at the nozzle exit (presumably by rapid vapor bubble growth within the jet). The core region remains intact for some distance downstream of the nozzle exit, and its breakup eventually produces relatively large drops. As the liquid temperature approaches boiling, the intact length and the core drop size decrease. Thus operation close to boiling is desirable for effective atomization. However, the nozzle mass flow rate decreases and practical difficulties are found (owing to “vapor-lock”) as the liquid is heated near boiling.     

Influence of Nozzle Parameters on Spray Pattern and Droplet Characteristics for a Two-Fluid Nozzle

In this study, a two-fluid nozzle, as, e.g., used in fluidized-bed or spray drying applications, is comprehensively characterized regarding the spray pattern and droplet size. To analyze the spray cone, the spray cone angle and the radial mass distribution of the nozzle were measured at varied liquid flow rate, spray air pressure, liquid insert bore diameter, and air cap position. Additionally, droplet size distributions were recorded at different spray settings. In general, the overall spray cone and single droplets are significantly influenced by the spray parameters, especially the spray air pressure, as well as the nozzle geometry.     

Influence of Atomizing Parameters on Droplet Properties in a Pulse Combustion Spray Dryer

A pulse combustor employed in a spray-drying system offers a new approach for liquid atomization that yields high-quality powders at low cost. Using a pulse combustion atomizer, there is no need for any form of nozzle dispersion and its atomization mechanism differs from those of conventional atomizers, such as rotary atomizers and pressure and pneumatic nozzles. In this work, based on the analysis of atomization mechanism, experiments of unsteady pulsating atomization were carried out in an experimental system of a Helmholtz-type pulse combustor. An optical analyzer was used for measuring the mean diameter of atomized droplet and droplet distribution. The effects of liquid feed rate, air flow oscillatory frequency, and liquid viscosity on atomized droplet size and size distribution were investigated and analyzed. The results indicate that the uniform droplet size distribution can be obtained under the conditions of a low feed rate, high-frequency pulsating flow, and moderate viscosity. The range of the droplets' Sauter mean diameter (SMD) is between 50 and 80 µm. The pulsating air flow from the pulse combustor can be used to atomize liquid or slurry without a nozzle and the atomizing quality can meet the requirements of spray drying.     

Influence of Atomizing Parameters on Droplet Properties in a Pulse Combustion Spray Dryer

A pulse combustor employed in a spray-drying system offers a new approach for liquid atomization that yields high-quality powders at low cost. Using a pulse combustion atomizer, there is no need for any form of nozzle dispersion and its atomization mechanism differs from those of conventional atomizers, such as rotary atomizers and pressure and pneumatic nozzles. In this work, based on the analysis of atomization mechanism, experiments of unsteady pulsating atomization were carried out in an experimental system of a Helmholtz-type pulse combustor. An optical analyzer was used for measuring the mean diameter of atomized droplet and droplet distribution. The effects of liquid feed rate, air flow oscillatory frequency, and liquid viscosity on atomized droplet size and size distribution were investigated and analyzed. The results indicate that the uniform droplet size distribution can be obtained under the conditions of a low feed rate, high-frequency pulsating flow, and moderate viscosity. The range of the droplets' Sauter mean diameter (SMD) is between 50 and 80 µm. The pulsating air flow from the pulse combustor can be used to atomize liquid or slurry without a nozzle and the atomizing quality can meet the requirements of spray drying.     

转炉一次除尘新OG系统高效喷淋塔喷嘴雾化特性的模拟

利用离散相模型对转炉一次除尘新OG系统高效喷淋塔内喷嘴的雾化特性进行模拟,分析了喷射角度、喷射压力、喷射流量及喷嘴水平间距等因素对雾化场索太尔平均直径(SMD)和蒸发效率的影响. 结果表明,在一定范围内随喷射角度增加,液滴在雾化场中的覆盖面增大,液滴驻留时间变长,蒸发效率增加,雾化场SMD减小,喷射角度大于60o时,SMD值减小缓慢. 随喷射压力增大,液滴蒸发效率增加,雾化场SMD减小,压力大于1.0 MPa时对SMD的影响较小. 随喷射流量增加,液滴蒸发效率减小,雾化场SMD增加,流量小于0.15 kg/s时,SMD增加幅度偏小. 两喷嘴水平间距越大,液滴分布面积越大,但对雾化场SMD影响较小. 在一定条件下,喷嘴间距约为800 mm时,截面速度分布较均匀.     

Atomization of Liquids by Two‐phase Gas‐liquid Flow through a Plain‐orifice Nozzle: Flow Regimes inside the Nozzle

Liquids or suspensions are divided into sprays of small droplets by atomization of two‐phase gas‐liquid mixtures. In this way either an equal distribution of the droplets or the generation of large surface areas of the liquid phase are accomplished, leading to increased heat‐ and mass‐transfer. The spatial and time dependency of the mean droplet diameter is a function of the total pressure upstream of the nozzle, the volumetric flow rate of the liquid and the gas, as well as on the flow regime in the nozzle. Thus the radial and axial profile of the void fraction inside the nozzle are measured with an electrical measurement technique. In addition, the flow in the nozzle is imaged by a high‐speed camera. Three flow regimes are identified. These are bubbly flow, plug flow and annular flow. A continuous flow of the emitting spray is observed for bubbly flow and annular flow only. The distribution of the dispersed bubble phase is given by ratio of the isothermic compression energy needed to pressurize the gas mass flow rate from atmospheric pressure up to the total pressure in front of the nozzle, and the potential energy of the supplied liquid mass flow rate.     

基于图像法的环雾状流液滴参数测量与分析

环雾状流广泛存在于石油化工领域,其内部流场测量具有重要意义。本文结合光学图像法和高速摄影技术对撞针式喷嘴的雾化特性进行了测量分析,以此为基础对基于雾化混合的环雾状流中夹带液滴特性开展了实验研究。利用高速摄影技术对喷雾进行可视化,采用单帧单曝光法对液滴尺寸和速度信息进行提取。研究发现,液滴速度随轴向距离增大呈衰减趋势,且相同轴向距离（约在径向位置10mm处）条件下,速度达到峰值;液滴索泰尔平均直径（SMD）随喷嘴孔径d₀的增大而增大,并与液相质量流量m_l和喷嘴上下游压差?p均呈负相关;另外,在环雾状流环境中,相同气压条件下液滴SMD随气相体积流量Q_g增大而减小,而相同气相体积流量条件下SMD随气压p_g增大而增大。基于实验测量结果,以气相韦伯数We_g和液相雷诺数Re_l为主影响参数,引入相间滑移和压力系数建立了基于量纲分析的环雾状流液滴SMD预测模型,平均绝对百分比误差MAPE为11.4672%。     

Experimental Study of the Effect of High Back-Pressure on the Atomization of a Plain Jet Injector Under Coaxial Air Flow

An experimental study was carried out on the effect of high back-pressure on the spray characteristics of a plain jet injector in coaxial air flow. The back-pressures tested ranged from 100 to 1600 kPa, the velocity range of coaxially flowing air is 60–120 m/s. The working fluid was water. The injector hole diameter was 0.5 mm. The key feature of the experiment was to use a convergent-divergent nozzle connected to the atomization test chamber to maintain a high back-pressure in the chamber for atomization, and at the same time, to maintain a high-velocity air flow in the atomization region. Thus, it totally eliminated the air recirculation and droplet slow-settling problem in the commonly used pressurized vessel for high back-pressure atomization study. The results show that the Sauter mean diameter decreases monotonically with the increase of back pressure. At different air velocities, the effect of back pressure is different. The drop size distribution becomes more non-uniform slightly with increasing back pressure.     

Simulation and analysis of group-hole nozzle sprays using a gas jet superposition model

A gas jet superposition model has been recently developed for computing group-hole nozzle sprays in computational fluid dynamics (CFD) simulations. The objectives of this study are: (1) to perform a systematic validation of the comprehensive spray model for group-hole nozzles using a broad range of experimental data; (2) to analyze the dynamics and physical insight of group-hole nozzle sprays based on the simulation results; and (3) to further clarify the impact of included-angle on spray/mixture properties of group-hole nozzle sprays. An updated version of the KIVA-3V Release 2 code, which employs the Lagrangian-Drop Eulerian-Fluid (LDEF) methodology for numerical calculation of two-phase flows, was used in the simulations. Diverging group-hole nozzles with various included-angles were considered. The test conditions included non-evaporating and evaporating ambient conditions, free sprays and sprays impinging on a flat wall. Detailed comparisons were made between the experiments and computations in terms of spray/mixture characteristics. The results show that numerical parameter dependencies are significantly reduced with the new models, and good levels of agreement are obtained in terms of spray structure, liquid/vapor penetration, overall SMD and cumulative vaporized fuel mass. Both experimental measurements and simulations reveal the importance of included-angle in group-hole nozzle sprays. In particular, some important features of group-hole nozzle spray are captured in the computations by the present models: compared to the equivalent single-hole nozzle, smaller local droplet size can be achieved in the near nozzle field, indicating an enhanced fuel primary atomization; the ambient gas entrainment rate is increased during the injection period, implying the better mixing; the spray axis deflection is identified in the case of group-hole nozzles with smaller angles, which is caused by a negative relative pressure region that exists between the sprays; in addition, the asymmetric structure of wall-impinging group-hole nozzle spray is well predicted by the present models through applying the gas jet superposition model in the entire computational domain.     

引射雾化喷嘴性能研究

对新型引射雾化喷嘴的流量系数、雾化角、雾粒分布均匀性进行了研究,提出了喷嘴流量系数、雾化角、雾粒大小计算方法。研究表明,这些方法的计算结果与实际很接近,对新型引射雾化喷嘴的设计计算十分有益,具有重要的工程意义。     

Experimental Study of Spray Drying and Atomization with a Two-Fluid Nozzle to Produce Inhalable Particles

A series of 50 experiments was performed to investigate particle size and yield from three laboratory spray dryers with two-fluid nozzles, using an aqueous solution of egg albumin as the feed. First principles theory suggests that the main parameters affecting final particle size should be atomization gas flowrate and velocity, feed solution flowrate, solution concentration, and inlet and outlet gas temperature. The experiments demonstrated that atomization gas velocity at the nozzle correlated better than volumetric gas flowrate to the final particle size obtained. Increasing atomization gas flow gave a decrease in particle size, levelling off at high nozzle gas velocities approaching sonic values. Liquid flowrate had little effect, and hybrid ratios such as the ALM (atomization gas to liquid flow ratio) did not correlate well to measured final particle size for this system. This seems likely to apply to aqueous systems with temperature limitations, where liquid flowrates will be relatively low compared to gas flowrates (both atomization and main gas flow). Particle size generally increased with solution concentration, and in some cases scaled exactly, assuming a constant initial droplet size and final particle density. All experiments were performed below the boiling point and therefore temperature had little effect. Particles of 2–3 microns were obtained at high atomization gas flows, indicating initial droplet sizes of 4–7 microns at most, while lower flowrates gave substantially larger particles and a wider size distribution.     

FACTORS INFLUENCING FLOW PATTRENS, TEMPERATURE FIELDS AND CONSEQUENT DRYING RATES IN SPRAY DRYING

The PSI-Cell model is used to predict flow patterns and drying rates for laboratory-scale spray dryers. The liquid feed is water, with atomization producing a spectrum of droplet sizes. An analysis of the cause-and-effect relationships determining droplet trajectories, temperature fields, the location and magnitude of the backflow recirculation effect, and drying rates provides insight into the factors governing spray-air contacting and dryer performance. Independent effects are assessed for spray angle, dryer column diameter, initial droplet size distribution and droplet velocity (separately and as linked through atomizer pressure), air/water flow ratio, and liquid volatility. Inferences are made for effective dryer design, selection of operating conditions, and logic for scale-up.     

生物柴油浸没喷吹的雾化特性

以生物柴油替代柴油进行浸没喷吹熔池熔炼是发展低碳铜冶炼的重要途径。针对该过程建立生物柴油浸没喷吹雾化流动过程计算模型,本文模拟计算了喷枪在水模型中浸没深度为20mm时不同油气比条件下生物柴油雾化颗粒的雾化特性,并实验验证了计算模型。研究结果表明：油滴颗粒的贯穿距随着空气流速的增加而增大;在气泡内油滴颗粒以一定的雾化半角向前扩散;当油滴颗粒到达气泡底部时,油滴颗粒以气泡底部平面为中心向整个空间扩散;气泡内距离喷枪轴向越远的观察面内大颗粒数目越多;油滴颗粒的索特平均直径（SMD）沿喷枪轴向先增大后减小;气泡内距喷枪口越远油滴颗粒SMD越大,进入水区域的油滴颗粒SMD逐渐减小;雾化空气流速越大,油滴颗粒SMD最大值的位置距喷口越远。     

Influence of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME)

This paper describes the effects of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME) both experimentally and numerically.In this investigation, the droplet atomization of DME fuel affected by ambient flow conditions was studied in terms of droplet mean size and detected droplet percentage under elevated ambient pressures and temperatures. In order to predict the DME spray atomization, the hybrid breakup model combined with KH-RT (Kelvin-Helmholtz and Rayleigh-Taylor) and KH-DDB (Kelvin-Helmholtz and Drop Deformation Breakup) models was applied in this study.It was revealed that the spray arrival time of DME fuel under a high ambient pressure increased in accordance with the increase in ambient pressure in the spray chamber. It can be seen that more small droplets are distributed at high ambient flow pressure conditions than at atmospheric conditions. This is a consequence of enhanced atomization of DME fuel. On the other hand, when the ambient pressure increases to 2 MPa, the Sauter mean diameter (SMD) increases only slightly compared with that at 1 MPa of pressure. The SMD value of droplets is increased as ambient temperature is increased. Under the high temperature condition in the chamber, the small droplets of DME fuel evaporate quickly and mix with the ambient air. As a result, it promotes the air-fuel mixing in a combustion chamber.     

LEAFLASH SPRAY DRYER: INVESTIGATION OF THE ATOMIZATION PROCESS

The atomization process of the LEAFLASH spray dryer has been investigated by laser diffractometry and video imaging. The LEAFLASH atomizer is a particular pneumatic nozzle where the pressurized gas is the hot drying air. The drying and the atomization processes are interrelated in the spray dryer. Consequently the mixing of the liquid droplets and the drying medium is very efficient, permitting to work with short drying duration and reduced chamber volume. The mean diameter and the drop size distribution are measured at the outlet of the nozzle by laser diffraction. We studied the coupling effects of the air temperature (150 to 300° C) and pressure (1.20 to 1.50 bars abs) and of the dry matter content of the liquid feed (aqueous maltodextrin solution, 20 to 55% w/ w) on the droplets/ size distribution and on the general pattern of the spray. Analyses of video images of the generated spray give the general pattern of the spray and an estimation of the liquid velocity     

An atomization model for splash plate nozzles

A model for the atomization and spray formation by splash plate nozzles is presented. This model is based on the liquid sheet formation theory due to an oblique impingement of a liquid jet on a solid surface. The continuous liquid sheet formed by the jet impingement is replaced with a set of dispersed droplets. The initial droplet sizes and velocities are determined based on theoretically predicted liquid sheet thickness and velocity. A Lagrangian spray code is used to model the spray dynamics and droplet size distribution further downstream of the nozzle. Results of this model are confirmed by the experimental data on the droplet size distribution across the spray. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Numerical simulation of cavitation in the conical-spray nozzle for diesel premixed charge compression ignition engines

The conical-spray injector is capable of achieving lean mixture with high homogeneity in the cylinder for diesel Premixed charge compression ignition (PCCI) engine with advanced injection timing. To better understand the cavitating flow inside the conical-spray injector, numerical simulations have been conducted by using a mixture multiphase model and a full cavitation model in this study. The results indicate that the cavitation evolution significantly affects the liquid sheet thickness and velocity at nozzle exit, which further change the spray angle and droplet Sauter mean diameter (SMD) dramatically. Based on the cavitation distribution inside the nozzle, the cavitating flow inside the conical-spray nozzle can be classified into four regimes with no cavitation, cavitation inception at inlet, developing cavitation at nozzle exit and super cavitation respectively. The extension of cavitation to nozzle exit in the super cavitation regime significantly improves the fuel atomization by increasing the injection velocity and decreasing the thickness of the liquid sheet. A cavitation map for the conical-spray injector has been developed by sweeping the ambient pressure and injection pressure simultaneously. It is found that the phenomenon of super cavitation only occurs in a narrow region where ambient pressure is very low. Therefore, the start of injection timing should be kept well before top dead center (TDC) to ensure the occurrence of super cavitation inside the nozzle in order to provide more homogeneous fuel/air mixture for diesel PCCI engines.     

HAN基液体推进剂喷雾场液滴尺寸分布的简化模型

为了建立HAN基液体推进剂喷雾场液滴尺寸分布的简化模型,基于最大熵原理,在质量守恒与概率和为定值的约束条件下,用拉格朗日因子法则推导了液滴尺寸分布函数,将该函数应用于HAN基液体推进剂模拟工质喷雾场液滴尺寸的预估;采用相位多普勒粒子动态分析仪(PDA)测量了HAN基液体推进剂模拟工质在1.8~2.6MPa喷射压力下对撞式喷嘴雾化液滴尺寸分布;通过最小二乘法优化Nukiyama-Tanasawa分布函数式中的q值对数目微分分布进行了修正,拟合得到q值与喷射压力p之间的函数关系为q(p)=-0.344p2+1.525p+1.268。结果表明,计算得出的液滴尺寸数目微分分布与HAN基液体推进剂喷雾场实测数据变化趋势相符,表明建立的液滴尺寸分布模型是合理的。     

Concentrating dilute sulfuric acid by spray evaporator

Large quantities of dilute spent sulfuric acid are released in many chemical processes. Recovering the dilute acid is not only profitable to the manufacturer but also imperative to environmental protection. This paper proposes a spray evaporator with a Venturi-type nozzle to concentrate the dilute sulfuric acid. Both hot air and dilute acid flow concurrently upwards through the nozzle. Water involved in the droplets is vaporized in the chamber and the dilute acid is concentrated. The bench-scale experimental results show that the dilute acid with initial concentration 18 wt% can be easily concentrated to 40–75 wt%. The measured parameters, such as concentration of outlet sulfuric acid, outlet air temperature and total pressure drop, are in accordance with those estimated from a mathematical model incorporating momentum, mass and heat transfer between the acid and air. The model is also applied to simulate the performance of the concentrator, including variations of droplet diameter, droplet velocity, droplet temperature, air temperature, air absolute humidity as well as pressure drop along the concentrator.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.