【作者】 赵文军；

【导师】 孙剑飞； 曹福洋；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2012， 博士

【摘要】 雾化器的结构对射流流场会产生很大的影响，因而雾化器设计也是喷射成形及液体雾化领域至关重要的核心技术。本文运用理论分析、数值模拟和试验研究相结合的方法，对单级和双级雾化器流场及流场中液体的波动与破碎状态进行了系统研究分析，以期为实现对单相及两相流场的有效控制奠定基础。采用计算流体力学软件Fluent提供的k-ε湍流模型和RSM湍流模型对四个不同出气口交角的单级紧配合式雾化器气体流场分析，结果表明雾化器在10atm的工作压力下随着出气口交角从5°增大到65°，k-ε湍流模型计算出的主射流区间散射角为15.0°到17.4°，而RSM湍流模型给出的主射流阶段张角为17.2°到19.0°，两个模型的计算结果相近。将模拟结果与实测的气体流速对比发现，计算结果与实验数据吻合良好，其中RSM模型相对精确。喷嘴出气口喷出的气体经导液管保护套反射后会在导液管前方形成一个气体流速的环峰。两种模型给出的结果均显示：随着出气口交角的增大，环形速度峰转变为一个速度单峰的合并位置会逐渐远离导液管，并在导液管前方形成一个涡流区域，且该区域中心气体反向流向导液管。同时，在该区域内沿轴线相对流动的气流交汇处会出现气流速度接近于零的“驻点”。同时，改变雾化器出气口交角会对涡流区域的位置产生微弱的影响。流场动态分析表明：对于不同工作压力下处于稳定状态的气体流场，增加气体压力时不改变气体流场结构，涡流区域的范围与位置也不发生变化，气压的增加仅促使气体流速的增加；从中心入口引入微弱的气体扰动将会在整个流场中产生周期性的气体射流波动。对于双级雾化器，通过模拟与实验确定了上下雾化器的装配关系：上、下雾化器之间的装配距离不大于15mm，下雾化器中心孔径为65mm。上、下雾化器同时工作时，上雾化器喷出的气流可以有效地抑制上下雾化器之间的涡流区。当下雾化器处于摆动极限位置时，可以有效地促使射流角度发生偏转：上、下雾化器工作压力均为10atm且下雾化器摆动5°时，射流主段的偏转角约为5°；只有下雾化器工作压力为10atm且其摆动角度为5°时，射流主段的偏转角约为6°。对雾化过程的研究发现：在一次雾化过程中液柱的Weber数在130.1~160.9范围内时为波动破碎向膜状破碎的转变区间。“波动破碎”源于Kelvin-Helmholtz不稳定影响，气液界面的波动导致大液滴的脱落和液柱的破碎。初级雾化中的“膜状破碎”受到涡流区域的气体静压力与动压力的共同作用并使液柱变形为液膜继而破碎成小液滴。对破碎粉末观察表明空心粉末的形成是液膜破碎震动和自身表面张力共同作用的结果。文中还对不同直径的入射液滴运动轨迹进行了分析。入射液滴会受到导液管前方涡流区域的作用，当液滴入射速度为50m/s时，直径大于35μm的液滴会顺利穿越导液管前方的涡流区域，其运动受涡流区的影响较小。直径相同的液滴，入射速度越大则越容易穿越涡流区域。文中计算了50μm、100μm和150μm的液滴在相对气体流速为100m/s与200m/s的流场中液滴内部的温度场分布，在给定条件下液滴的冷却速度范围集中在104~105数量级。三个直径的液滴在100m/s的相对气体流速中，当温度降到液相线以下时，液滴内部温度场内温度波动不超过2℃，而在200m/s的相对气流中降到液相线以下时温度场内温度波动不超过1℃。更多还原

【Abstract】 The design of the atomizer is a key technology in both spray forming and theatomization of the liquid, and affects the gas-only and the gas-liquid flow field.This research about the gas flow field is based on the atomizer designed byourselves. Theory analysis is combined with numerical simulation and experimentto analyze the flow fields generated by the single layer atomizer and the doublelayer atomizer, and the fluctuation and breakup of the liquid in the gas flow field.All these are the foundation to control the gas-only and gas-liquid flow field.Both k-ε turbulence model and Reynolds stress turbulence model (RSM)provided by FLUENT, a commercial CFD software, are applied to calculate the gasflow field formed by four atomizers with different intersection angles. Thescattering angle of the main part of gas spray increased from15.0°to17.4°givenby k-ε model and17.2°to19.0°given by RSM model, as the nozzle intersectionangle increased from5°to65°at the operating gas pressure of10atm, and the twomodels give similar results. Comparing the simulation results with the experimentaldata, they agree well with each other, and RSM model is more accurate. In the gasflow field, the gas sprayed from the nozzle is reflected by the out surface of theprotector of the delivery tube, and an annular speed peak comes up downstream ofthe delivery tube. The results by both k-ε model and RSM model show that theposition where the annular speed peak merged into one single peak moves fartherfrom the delivery tube as the nozzle intersection angle increased. There is arecirculation zone in front of the delivery tube, and the gas moves towards thedelivery tube in the center of this area. The confluence of opposite gas flow is theso-called ‘stagnation point’ of the gas speed, where the gas speed nearly drops tozero. Increasing the intersection angle affects the position of the recirculation, butthere is only a limited effect. Dynamic analysis has also been done to the gas flowfield. Observing different gas flow fields under different gas inlet pressures, it’sfound that the increasing of the gas inlet pressure doesn’t change the structure ofgas flow field and the position of the recirculation zone, but only makes the gasmove faster. A slight disturbance is introduced from the center inlet, which leads toa periodic jet fluctuation. In the study about double layer atomizer, the assembly relation between uplayer and under layer nozzles is determined by both simulation and experiment. Thedistance between up layer and under layer nozzles is no more than15mm, and thediameter of under layer central opening is65mm. When both two nozzles areworking together, the gas spray from up nozzle can restrain the scope of the largerecirculation between the two nozzles. When these two nozzle are working under10atm operating pressure and the scanning angle of the under nozzle is5°, thedeflection angle of gas flow in the main part is5°from the axis. But when onlyunderlayer nozzle is working under10atm operating pressure and the scanningangle of the under nozzle is also5°, the deflection angle of gas flow in the mainpart is6°.The primary and secondary breakups have been analyzed in the paper. Duringthe primary breakup process, when the Weber number is in the range of130.1~160.9, vibration breakup is transforming into sheet breakup. The vibrationbreakup is based on the Kelvin-Helmholtz instability, and the disturbances on theinterface make big droplets separate from the liquid column and lead the liquidcolumn to break into fragments. The results indicate that sheet breakup is the resultof gas static pressure and dynamic pressure, which made the liquid columntransform into liquid sheet then break into little droplets. Those droplets generatedafter sheet breakup are smaller than those after vibration breakup. Observing thepowders after breakup, it’s found that hollow powder is out of the vibration of thelocal liquid sheet after breakup and its own surface tension.The trajectories of droplets with different diameters have been calculated inthe paper. The movements of droplets are affected by the recirculation. When theadmission velocity of droplets is50m/s, the ones with diameters larger than35μmcan go through the recirculation zone easily without being influenced. For thedroplets with same diameter, the faster get into the flow field, the easier go throughthe recirculation zone. It’s analyzed that the whole flow field and temperature fieldin the droplets with50μm,100μm and150μm diameters moving within the gasflow of100m/s and200m/s relative velocity. The cooling speed of the droplets areof the order of104~105just in the context of the given situation. When thetemperature of droplets is below the liquidus, the differences of the temperaturewithin droplets are no more than2℃in gas flow with100m/s relative velocity, and no more than1℃in gas flow with200m/s relative velocity.更多还原