【作者】 陈海焱；

【导师】 陈文梅；

【作者基本信息】 四川大学 ， 化工过程机械， 2007， 博士

【摘要】 纵观气流粉碎与气流分级技术的开发研究现状，大部分集中在应用开发、气流粉碎分级设备的形式、各种操作参数对气流粉碎分级性能的影响方面，而关于气流粉碎和气流分级的机理，气流粉碎和分级流场的特性、甚至结构参数的研究还很不成熟。我国气流粉碎技术研究开始于是在20世纪80年代，并且主要是仿制设备，在气流粉碎和气流分级技术的理论上的研究及成果极少。本论文按气流粉碎分级的基本原理对气流粉碎分级机进行了分类。应用FLUENT流体计算软件，首次对流化床气流粉碎分级机进行整体建模，分析和研究气流粉碎流场和气流分级的流动状态，数值模拟结果表明：喷嘴设计马赫数越高，喷嘴出口的气流速度越大，对直线喷嘴来说，喷射气流的发散情况越明显。超音速喷嘴出口气流速度和最大负压出现在喷嘴出口几毫米处，喷嘴出口气流速度的衰减极快，在喷射气流中心区负压上升迅速。对气流分级区径向速度、切向速度和轴向速度的分析，表明：进入分级轮的气流径向速度不均匀，分级轮的高度越高，径向速度越不均匀，较高的分级轮会有部分气流外溢，外溢位置、大小与分级轮的高度相关，从而引起进入分级轮的径向气流速度反而增大，分级轮的高度与直径之比从数值模拟来看应控制在0.5以下；由于气流在轴向运动过程中逐步进入分级轮，从分级轮底部向上分级区轴向气流流量逐步减少，从而轴向气流速度从分级轮底部向上逐步减少，会造成分级区上下区域的颗粒浓度不均匀和颗粒的粒径大小不均匀；分级轮的气流切向速度在叶片外缘距内几毫米处达到最大，分级轮向外切向速度急剧衰减。当分级轮高径比较大时，形成的外溢气流，对轴向气流速度和分级区的切向速度有明显的影响。这些结轮对一些重要参数(如超音速喷嘴出口流场分布、分级轮高度)具有较大的指导作用。气流粉碎超音速喷嘴的工况参数的分析，表明气流粉碎超音速喷嘴的设计，除要考虑喷嘴速度外，还应考虑喷嘴出口的动压和粉碎区的背压；在相同马赫数喷嘴中加速，喷嘴入口压力越大，粉碎区背压要求越低，喷嘴出口动压越高；为确保获得设计的喷嘴出口速度，粉碎区的背压应控制在喷嘴出口压力左右，若马赫数设计太高，而喷射区的背压达不到要求的负压，则会产生激波，喷射速度会大大降低。从气流粉碎背压的要求、喷嘴出口动能、出口气流的发散程度几个方面来说，气流粉碎喷嘴的设计马赫数应在1.4-2.05范围内较为合理。利用热的压缩空气可以产生更大的气流速度，提高气流粉碎的生产能力，同时高温气体，还能够有效的影响物料的粉碎效果和特性。首次对气流引射式颗粒加速过程进行了分析，有益于高速气体的动能有效地转化为颗粒的粉碎能，喷嘴出口的气流速度衰减极快，颗粒越细，被加速达到的颗粒速度越大，加速到最大速度需要的加速距离也越短；颗粒越粗，颗粒加速到最大速度也越小，需要的加速距离越长，因此，确定流化床喷嘴的最优加速距离，提供最优的粉碎条件是非常重要的。喷嘴之间的加速距离与气固浓度、颗粒的性质(如比重，进料粒度等)和产品的粒度要求有关。同时喷嘴之间的距离还会影响产品的成品率、粒度的稳定性和产品的粒度分布。喷嘴之间的加速距离通常在10-20倍的无因次距离范围内，但对较粗的颗粒粒度(≥500μm)其加速距离会超过20倍的无因次距离。喷嘴喷射气流中心速度较高，喷射气流中心颗粒的碰撞机率也较大，因此，流化床中气流粉碎的设计应让更多的颗粒进入喷射气流中心。作者首创的同轴心喷嘴的实验证明了结论的正确性。不同的碰撞方式，对粉碎效率亦有不同的影响，通过水平圆盘式，流化床式与靶式气流磨的理论分析和实验对比，证明单喷式的粉碎效率比对喷式的低，对喷式的效率又比靶喷式的低。气流粉碎的操作参数(喷嘴入口压力、持料量、入料粒度等)对气流粉碎性能有较大的影响，本文首次给出并验证了气流粉碎分级腔内的持料量的计算方法，探讨了传输区的上升气流速度对气流粉碎和分级的影响。首次设计和研究了新型流化床气流粉碎结构，在下部和上部加入气流，提高了气流粉碎区的上升气流速度，补充喷嘴引射的气流，使底部的颗粒易于被流化沸腾起来，减少涡流强度，从而减少合格颗粒再粉碎的机率，这对较粗、密度高和成品率要求高的颗粒的加工具有极大的价值，并得到广泛的推广与应用，取得了较大的经济和社会效益。首次实验研究了流化床气流粉碎颗粒形貌的控制技术。气流粉碎分级过程中进行颗粒形貌控制，关键是控制颗粒选择性粉碎的机会、颗粒间相互作用的机理、碰撞次数、力度、颗粒在粉碎区停留时间。因此气流粉碎区的结构、喷嘴结构、形状、位置和个数，流体性质(如压力、流量、喷射速度、温度)，颗粒加速距离、位置，气固浓度，分级流场等因素均是影响颗粒形貌的基本因素。实验证明气流粉碎可以在一定程度上改变颗粒的形貌。分级机的转速、流量、二次风大小、分级区的气固浓度和进料粒度组成等操作参数对其分级性能的影响是至关重要的。本文实验研究得出的分级机的操作参数与切割粒径和分级精度的关系，对分级机合理的操作参数的选取和结构设计具有一定的指导作用。新设计的气流分级机可达到对颗粒进行亚微米分级的要求。同时证明缩小分级轮的高径比可获得更高的分级精度。过热蒸汽为介质的流化床气流粉碎技术的理论分析和实验研究，表明蒸汽气流磨的能耗远远小于空气气流磨，蒸汽气流磨的粉碎力大大高于空气气流磨，对气流粉碎能耗的降低和大型设备的开发有较大的意义。为确保过热蒸汽通过喷嘴后不结露，应严格控制喷嘴出口的压力和喷嘴出口马赫数。首次提出并实验了利用电厂低品位过热蒸汽为工作介质，将电厂产生的粉煤灰超细化，结果表明该方法能满足低成本、大规模的粉煤灰应用需求。流化床气流粉碎分级机已在国内外广泛地应用，但针对它的研究却非常少。流化床气流粉碎分级机内部流场极为复杂，难以通过理论及实验手段进行系统研究。本文对流化床气流粉碎分级设备的关键实用技术作了较为全面的分析和研究，对其工业设计和设备应用提供了一定的依据。更多还原

【Abstract】 In terms of the present developing and research conditions of jet grinding and air classifying technology, most of the focus is concentrated on application and development, the forms of the jet grinding and classifying devices, the impact on the jet grinding and classifying performances by various operational parameters. Whereas, it is not very mature in terms of the principles, the characteristics of jet grinding and classification field and even the research on structural parameters. The studies of jet grinding technology in our country were stemmed from 1980s, most of which are limited to modeling other devices; however, the theoretical study and achievements of the jet grinding and classifying technology are extremely fewer.In this dissertation, jet mill and classifier are classified by the principles of jet grinding and classifying. It’s the first time, with the aid of the software—FLUENT, the models of fluidized bed jet mill and classifier are set up as a whole and the flowing condition of jet grinding field and air classifying are analyzed and studies. The result of numerical simulation indicates that the higher the Mach number of nozzle design, the quicker the air speed from the nozzle outlet; and for the straight nozzle, the more diverse the jet spray is. The air speed at the supersonic nozzle outlet and the maximum negative pressure appear at the place about 3-4 mm near the nozzle outlet. The appearance of the shock wave from the nozzle outlet induces the greatly reducing of the jet speed. The air speed at the jet spray center increases rapidly. The analysis of the radical velocity, the tangential velocity and the axial velocity in the air classifying area indicates that the radical velocity into the classifier is uneven; the higher the classifier, the more uneven the radical velocity; the higher classifier has partial air leaking and the position and size of leaking are related with the height of the classifier, which leads to the increase of radical velocity into the classifier. According to the numerical simulation, the ratio of the height of the classifier to the diameter should be controlled under 0.5. Because air gradually enters the classifier in the axial process, the air amount gradually decreases from the bottom to the top of the classifier; thus, the axial air speed gradually decreases from the bottom to the top of the classifier, which causes uneven in particle concentration and its diameter size in different areas of the classifier. At the spot several millimeters directing inside from the outside of blades, the tangential velocity comes to the maximum and the tangential velocity directing outside decays rapidly. When the ratio of the height to the diameter is higher, the leaking air formed has obvious impact on the axial velocity and the tangential velocity in the classifying area. These conclusions have a guiding role to some important structural,parameters, such as the fluent-field distribution of supersonic nozzle outlet and the height of classifier.Through analyzing the operating parameters in supersonic nozzle, what should be considered include not only the velocity in nozzle, but also dynamic pressure of nozzle outlet and grinding pressure in the grind district when supersonic nozzle is designed for jet mill. In the nozzle, to accelerate air at the same Mach number, the higher the pressure of the nozzle inlet, the lower requirements of grinding pressure and the higher the dynamic pressure of nozzle outlet. To ensure the requirements of the designed nozzle outlet velocity, grinding pressure should be approximately equivalent to the pressure of nozzle outlet. If the Mach is too high, and the grinding pressure cannot meet the required negative pressure, which will appear shock wave and jet speed is reduced greatly. In terms of the requirement of grinding pressure, dynamic pressure of nozzle outlet and the diverse degree of outlet air, the designed Mach number of air grinding nozzle should be 1.4-2.05. The use of heat compressed air could get higher air speed and improve the productivity of jet mill. At the same time, high temperature air can effectively affect the grinding effect and characteristics.It’s the first time to do the analysis on the particle progress which is accelerated by the high-speed gas in the nozzle is conducive to the kinetic energy of high-speed gas can change into grind energy of particles efficiently. And the research is proved that the exit velocity of nozzle decayed extremely fast. The finer the particles, the faster the speed and the shorter the accelerated distance, vice versa.Whereas, it’s important to choose the best accelerated distance of fluidized bed nozzle for the best grinding condition. Accelerated distance of nozzles is related to solids load, particles characteristics(e.g. density, injecting feed size) and the product size requirements. Accelerated distance of nozzles is also related to production rate, size stability and size distribution of product. Accelerated distance of nozzles is 10-20 times than the dimensionless distance, but for tougher particles(≥500μm), accelerated distance is over 20 times than the dimensionless distance.Given the air speed which is in the center of the nozzle is high and the probability of particle collisions is frequent, more particles need to add into the jet center for fluidized bed jet mill designed. The author adopts the same axial nozzle to prove the correctness of the conclusion creatively. Different collision ways have different influences on grind efficiency. Through the comparison of experiment and theory among spiral jet mill, fluidized bet jet mill and target jet mill, it’s prove that the single jet efficiency is lower than the opposed jet mill, and the opposed jet mill efficiency is lower than the target jet mill.Operating parameters(e.g. nozzle inlet pressure, feed volume and injecting feed size) have great impacts on the characters of air grinding. It’s the first time that, the computing method of mass hold up in jet mill chamber is given and validated, and the influence of upward currents on air grinding and classification in the transmission zone is discussed in this paper. It’s also the first time to design and study the structure of new typed fluidized bed jet mill. Adding fresh air to the top and the bottom in the fluidized bed jet mill to increase the speed of upward currents, the way is leading fluidized particles to be boiling and reduce the eddy strength. The aim is to reduce the opportunity of regrinding. It is valuable to produce the particles which are rough, high density and high product rate requirement(given size). At present, the technology has been prevailed, which produced lots of benefits for society and economy.And it’s the first time to study the technology which is fluidized bed jet mills to control appearances of particles. In the processing progress, the key factor is to control the probabilities of selective grinding, the principles of particles interactions, collision frequency, power, retention time in the grinding zone. The basic factors affecting particle appearances are that grinding structures, nozzle structures, shapes, locations, fluid properties(e.g. pressure, air flow, jet speed, temperature), particle accelerated distances and locations, gas-solid concentration, classification flow and so on. To some extent, it’s proved that it’s feasible to change appearances of particles by air grinding.The operational parameters, just like the rotating speed of air classifier, air flow, second wind, gas-solid concentration of classification zone and feed size, have great impacts on the characters of air grinding. Through experimental research, it’s proved that the operational parameters are related to the cut size and the grading sharpness. The result is conductive to select parameters and design structures for jet mill. Such as new typed air classifier can meet the requirements of submicron classification. And the research is proved that the ratio of the height of the classifier to the diameter is less, instead the grading accuracy is higher.Theoretical analysis and experimental research to the overheated vapor as media in fluidized bed jet mill, it’s proved that the energy consumption of vapor jet mill is far less than air jet mill, but the grinding power of vapor jet mill is far better than air jet mill. The results are conductive to saving energy in practice and developing larger equipments. To ensure overheated vapor to go through supersonic nozzle under the dry vapor state, and to control the inlet pressure strictly, outlet pressure, outlet Mach of the nozzle. It’s the first time to use low-grade overheated vapor of electric factory as.working media, to superfining the fly ash. It’s proved that this method can meet the requirements of low cost and large scale to apply fly ash practically.At present, fluidized bed jet mills have applied in home and abroad, but the researches in the field are extremely fewer. The internal flow of fluidized bed jet mill is too complicate; it’s too difficult to use theory and experiment to do system research. The dissertation has studied and analyzed application of fluidized bed jet mill in industrial technology comprehensively and provided some useful advices for industrial design and applied equipments reasonably.更多还原