新型扇形雾化喷嘴的实验研究

在对Y型喷嘴结构和雾化机理研究分析的基础上,设计了一种新型的扇形雾化喷嘴,并对该喷嘴进行了实验研究. 实验测量了不同气、液压力下扇形雾化喷嘴的流量、雾化角和索特尔平均粒径及其在空间上的横向和纵向分布. 通过对液流量用Y型喷嘴设计公式进行拟合,得到了公式中适合扇形雾化喷嘴设计的系数取值：m=0.39, b=0.98. 喷雾雾化角a大约在90o~98o之间,变化不大. 喷雾的索特尔平均粒径在空间横向呈类似"W"的分布,在空间纵向呈逐渐降低并趋于稳定的分布.     

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.     

气泡雾化喷嘴泡状流出口喷雾脉动特征

采用实验和仿真方法,对一特定气泡雾化喷嘴泡状流时混合室内的气液两相混合形态以及喷孔出口喷雾脉动特征进行了研究。研究结果表明,泡状流时气泡尺寸呈近似正态分布,气泡尺寸随液相质量流量和气液质量比增大而减小;喷雾形态和喷孔出口气液流动参数存在较大脉动,喷雾锥角脉动超过20°;气泡数量密度小且气泡直径较大时,喷雾平均锥角相对较小,且喷雾脉动现象比较严重;随着气泡数量密度增加,喷雾平均锥角呈现先快速增大后缓慢增大趋势,而喷雾锥角变异系数先快速增大随后逐渐减小并趋于稳定;复杂的流场结构是喷孔内气泡表观形态发生较大变化以及喷孔出口气液流动参数产生较大脉动的主要原因。     

Experimental investigation and model improvement on the atomization performance of single-hole Y-jet nozzle with high liquid flow rate

Y-jet nozzle, as an efficient multi-hole internal-mixing twin-fluid atomizer, has been widely used for liquid fuel spray in many industrial processes. However, single-hole Y-jet nozzle with high liquid flow rate is indispensable in some confined situations due to a small spray cone angle. In this paper, the atomization performance of single-hole Y-jet nozzles with high liquid mass flow rates ranging from 400 to 1500 kg/h for practical semidry flue gas desulfurization processes was investigated by the laser particle size analyzer, and the effects of spray water pressure, atomizing air pressure and air to liquid mass flow ratio on the liquid mass flow rate and the droplet size distribution were analyzed. Moreover, the secondary atomization model was modified on the basis of previous random atomization model of Y-jet nozzle. The predicted results agreed well with the experimental ones, and the improved atomization model of Y-jet nozzle was well validated to design the nozzle geometry and to predict the droplet size distributions for single-hole Y-jet nozzle with high liquid mass flow rate.     

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.     

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.     

小型二流式喷嘴雾化性能研究

以适用于小型设备的小流量气流式雾化器为研究对象 ,对六种具有不同气体出口长径比的喷嘴分别进行试验 ,讨论其在不同操作参数下的雾场特性 ,并关联出计算雾滴平均直径的准数方程式     

大型喷雾粒径分布的图像法测量

雾化技术在能源、化工等领域应用广泛,研究雾化机理和优化雾化喷嘴性能的前提是对其雾化液滴尺寸及粒度分布进行准确有效的测量和表征。目前常用的雾化液滴粒度测量技术,如基于光散射或衍射原理的激光粒度仪和相位多普勒分析仪等,能够较准确地测量粒径分布比较窄、最大粒度在2000 μm以下的喷雾,但对含特大颗粒且粒径分布很宽的喷雾,往往难以得到可靠结果甚至不可能进行测量。本文提出了用图像法测量这类大流量喷雾,构建了图像法测量系统,编写了图像处理程序,经标定实验后,采用该系统对某喷嘴喷雾液滴粒径分布及规律进行了测量研究。研究结果表明图像法可用于大型喷雾液滴粒度及分布的测量。     

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.     

Airborne Dust Capture and Induced Airflow of Various Spray Nozzle Designs

Water spray characteristics, including droplet size and velocity, airborne dust capture potential, and induced airflow quantity for various spray nozzle designs were evaluated to provide basic information for improving spray applications. Water droplet size and velocity characteristics were initially measured by a Phase Doppler Particle Analyzer (PDPA) for hollow cone, full cone, flat fan, and air atomized spray nozzles at similar operating parameters. Airflow inducement and dust capture experiments were also conducted under the same operating parameters to examine any salient features of the spray nozzle type, droplet characteristics, induced airflow, and airborne dust capture.

Test results indicate that there are trade offs between airflow inducement and dust capture efficiency. A spray nozzle with a wider discharge angle was observed to induce more airflow, but at reduced dust capture efficiencies. Increasing spray nozzle fluid pressure(s) generally reduced water droplet sizes with concurrent increases in droplet velocity, airflow inducement, and airborne dust capture. Placing a three-sided barrier around the spray nozzles normally reduced spray air induction and increased dust capture efficiency. A direct relationship between airborne dust capture efficiency and spray input power normalized per unit of airflow induced was observed. This information can be utilized to improve the performance of water sprays for reducing airborne dust levels.     

Experimental Study and CFD Modeling of Wall Deposition in a Spray Dryer

The wall deposition phenomenon in a pilot-scale spray dryer was investigated based on mathematical modeling and experimental trials. For this purpose, the governing equations were obtained and solved numerically by applying a mathematical modeling technique and an open-source computational fluid dynamics (CFD) software. The wall deposition, velocity distribution of the existing phases, and droplet trajectory in the drying chamber were determined. The effect of the operating parameters including the feed flow rate, inlet concentration of dissolved solid, and initial droplet diameter on the air flow pattern, droplet trajectory, and wall deposition was investigated. Through the experiments, the wall deposition of powder product in different positions of the drying chamber was measured. In modeling part of this study, we attempted to determine the effect of particle diameter on the percentage of wall deposition and the position where it occurred.

The model results obtained for wall deposition were compared with collected experimental data and good agreement was observed.     

Effects of co- and counter-swirl on the droplet characteristics in a spray flame

This paper reports measurements of droplet characteristics and flow field in a spray flame with inner and outer swirling air streams. The spatial distribution of droplet characteristics produced by the burner's airblast atomizer was measured using dual-phase Doppler anemometry (PDA). The spray flame was operated near the lean blow-out limit at two flow conditions: co-swirling (flow rotation in the same direction) and counter-swirling (flow rotation in opposite directions). In both cases, the flame exhibited a U-shaped form and was marked by a large central recirculation zone. Based on the measurements of the droplet velocity components, differences between both configurations appeared for the counter-rotational setup mainly in the near burner region, where the decrease of total swirl causes deeper penetration of the droplets from the inner duct into the combustion chamber, resulting in a much more homogeneous distribution than the other one. The droplet size in terms of the Sauter mean diameter (SMD) shows little variation in the change of the direction swirl condition. Application of counter-swirl results in more turbulent droplet motion.     

A simplified analysis method for correlating rotary atomizer performance on droplet size and coating appearance

The dominant method of atomizing automotive paint is through the use of rotating bell sprayers. For this class of atomizer, the problem of paint thickness across the bell has been theoretically solved on a representative geometry that includes factors such as fluid flow rate, bell speed, bell cup radius, and fluid properties. It was assumed that the paint film eventually forms uniform ligaments at the bell cup edge that break due to hydrodynamic stability during the paint spray process; thus, creating a characteristic particle size distribution for the spray. These particle size distributions will vary as the spray parameters, specifically fluid flow rate, bell speed, and bell cup radius, vary. The theoretical model that has been developed strongly correlates to the literature data available for paint droplet size from rotary bell atomizers. Expansion of the correlation of the theoretical model to paint appearance wavelength measurements, Wc and Wd, in place of droplet size provides further understanding of the effect of paint spray parameters on paint appearance. Use of these correlations can help to optimize paint appearance and improve paint spray simulation results.     

Effect of Maltodextrin Addition during Spray Drying of Tomato Pulp in Dehumidified Air: I. Drying Kinetics and Product Recovery

This work investigates the effect of maltodextrin addition on the drying kinetics and the stickiness during spray drying of tomato pulp in dehumidified air. A pilot-scale spray dryer was employed for the spray-drying process. The modification made to the original design consisted in connecting the spray dryer inlet air intake to an absorption air dryer. Twenty-seven different experiments were conducted varying the dextrose equivalent (DE) of the maltodextrin, the ratio (tomato pulp solids)/(maltodextrin solids), and the inlet air temperature. Data for the residue remaining on the walls were gathered. Furthermore, the effect of maltodextrin addition on the drying kinetics and the stickiness of the product was investigated using a numerical simulation of the spray-drying process modeled with the computational fluid dynamics (CFD) code Fluent. The code was used to determine the droplet moisture content and temperature profiles during the spray-drying experiments conducted in this work. The stickiness was determined by comparing the droplet temperature with its surface layer glass transition temperature (T_g ). The T_g was determined using a weighted mean rule based on the moisture content profiles calculated by the CFD code and the experimental data of T_g , which were obtained for the different tomato pulp and maltodextrin samples and fitted to the Gordon and Taylor model.     

Analysis of droplet impingement of different atomizers used in spray coating processes

This paper presents numerical studies on spray painting processes using three different atomizers, i.e., a high-speed rotary bell atomizer, an airless gun and a pneumatic air spray gun. A commercial CFD code was applied using an Euler-Lagrangian approach for two phase flow calculations. Special attention was paid to the droplet impingement process on the substrate. Orthogonal impact velocities and size distributions of the droplets depositing on substrates were analyzed in detail. The obtained simulation results show substantial differences of the impacting droplet characteristics of these three atomizers, which will have significant influences on the properties of the forming paint films.     

Atomization and combustion of canola methyl ester biofuel spray

The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 μm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO₂ and O₂ concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames.     

喷嘴雾化参数轨迹图像法测量实验研究

针对喷嘴雾化多参数同步测量问题,提出了基于图像处理的喷嘴雾化角、雾化细度、液滴运动速度及分布参数测量方法,利用背光阴影成像技术搭建了喷嘴雾化参数测量系统,建立了基于轨迹图像法原理的喷嘴雾化参数图像处理流程与算法,利用标准颗粒测量验证了该方法对颗粒粒径测量的精度,并开展了不同孔径与压力下扇形喷嘴雾化参数同步测量实验研究。结果表明:当雾化压力不变,扇形喷嘴孔径从0.66 mm变为1.10 mm时,雾化细度与液滴平均运动速度分别增加26.82%、10.42%,而雾化角随扇形喷嘴孔径增大而减小16.66%;当扇形喷嘴孔径不变,雾化压力从0.1 MPa增加到0.4 MPa时,雾化角与液滴平均运动速度分别增加47.71%、95.10%,而雾化细度随雾化压力增加而减小44.23%。这为雾化液滴特性研究与喷嘴性能评估提供了有效手段。     

Effects of different nozzle materials on atomization results via CFD simulation

Spray drying, as a crucial operation in industrial production, converts solution to fine particle. The spray moiety directly affects the final particle morphology, size and distribution. Compared with the experimental method, computational fluid dynamics (CFD) modeling is a powerful and convenient tool for simulating the spray process. Based on the verified CFD model, different materials of atomizer were simulated to investigate the effect on droplet size and distribution in this work. The modeling result proved that the droplet size and distribution were influenced by the resistance coefficient of materials, wherein the Reynolds number could change the effect of roughness along with the change of mass flow rate on spray process. The results in this work have implication for controlling droplet size through developing new spray nozzle with different materials or surface coating.