【作者】 朱小林；

【导师】 刘焕彬；

【作者基本信息】 华南理工大学 ， 制浆造纸工程， 2010， 博士

【摘要】 废纸已成为我国主要的造纸原料,在废纸制浆造纸过程中胶粘物的处理与控制一直是国内外研究的热点,而作为主要技术手段的热分散技术的分散作用机理的研究则是一个难点。深入了解胶粘物热分散的机理,对降低热分散的能耗、提高废纸制浆热分散处理后的质量具有重要意大。另外,分散也是废纸制浆过程中的碎浆、浮选、打浆、热分散等工段的主要物理作用过程。因此,分散技术的研究与改进提高,有利于废纸造纸和循环利用,从而为实现造纸工业的循环经济提供技术支撑。本文针对盘式热分散,采用理论分析、可视化实验和数值模拟相结合的方法研究盘式热分散机内部浆料流场和胶粘物分散混合特性,主要研究胶粘物的分散特性、盘式热分散机工作原理及能耗、胶粘物剥蚀和破裂分散的过程与规律,浆料流场中的剪切作用和空化效应,以及不同结构和运行参数对浆料流场及热分散特性的影响。实验和数值计算所获得的数据与结论对于盘式热分散机的结构设计和优化具有较大的参考价值,本文虽然以热分散过程为主要研究对象,但其研究方法和理论同样适用于制浆造纸过程中其它浆料分散过程。本文的主要研究内容和及创造性成果如下:(1)对胶粘物在浆料流场中发生的分散历程,从理论上进行了定性分析。认为盘式热分散是对胶粘物的高剪切作用历程,并可描述为分散混合与分布混合两个主要阶段。(2)应用多相流理论,建立了胶粘物粒子受力的数学模型。把胶粘物粒子视为一种团聚体,分析出它在分散混合过程中所承受的各种作用力,从而得到胶粘物粒子受到的分散力Fc为:(?)(3)对胶粘物在盘式热分散机工作区域内的分散特性进行了分析,提出盘式胶粘物热分散的基本原理为:作为一种团聚体的胶粘物,在工作区域内因受浆料流场剪切和空化效应作用,会以破裂分散和剥蚀分散两种不同的方式发生分散,并形成不一样的分散效果;剥蚀分散在整个热分散过程中都存在,而破裂分散只有在分散力FC大于胶粘物的聚集强度σc,即Fc >σc时,胶粘物颗粒才会被破裂成多个小颗粒;长时间作用下,剥蚀分散可使胶粘物颗粒不断变小;引起胶粘物颗粒在盘式热分散工作区域内发生破裂分散和剥蚀分散的4种主要作用是破碎作用力、层流剪切力、湍流剪切力和空化效应。(4)对胶粘物粒子热分散的能耗进行了分析。以热分散磨片的磨齿作为固相,以高速流态化的纸浆作为液相,利用多相流理论,得出盘式热分散机的能耗为:(?)(5)研制了可模拟多工况情景、且动/静盘片间隙可调的可视化热分散实验装置,设计加工了透明热分散磨片。采用高速摄像技术,对胶粘物在纸浆中的热分散混合过程进行了实验研究。实验较清楚地记录下浆料和胶粘物的流变过程,证明了胶粘物在盘式热分散过程中的破碎与剥蚀分散原理;实验还观察到层流剪切、湍流剪切和空化现象。(6)经生产实验表明:虽然废纸浆中胶粘物粒径分布不匀,但经过热分散后,粒径分布会越来越集中,小粒径胶粘物不再变小,大粒径胶粘物的颗粒面积和数量变化最为强烈;盘齿间隙和盘径的大小都会影响到热分散效果,其中间隙的影响最为直接。(7)采用FLUENT软件,结合RNG k-ε湍流模型、Discrete Phase模型和Euler-Lagrange方法的颗粒轨道模型,对5种分散片方案下的胶粘物分散过程进行数值模拟,并与高速摄像实验测量结果进行对比,验证了计算模型的可靠性和准确性,并获得了实验手段所不能测量到的盘齿近壁区流场、胶粘物颗粒运动轨迹和压力梯度、湍动能量损失、剪切速率等参数。(8)对盘式热分散机内浆料的非定常湍流进行了数值模拟,从不同层面揭示了热分散区内的速度、静压、总压、湍动能、壁面剪切应力场和剪切速率分布规律;并从吸入流场的产生、压力场、速度场、剪切率、湍流强度与能量耗散率等方面分析了湍流场对胶粘物的分散作用与规律;揭示了热分散片结构和运行参数对胶粘物分散效果的影响。(9)实验测量和非定常数值计算结果表明,盘齿间隙和盘径是影响热分散效果的2个最明显的因素,随着盘齿间隙的减小或盘径增大,压力梯度和速度梯度均增大,引起热分散发生的湍流剪切、层流剪切、空化效应相应增强,导致热分散作用强度提高。(10)非定常数值计算结果还表明,①动盘齿槽内的压力远高于静盘齿槽,动/静盘齿间隙处的压力梯度,沿径向方向持续增大;②浆料经过动/静盘齿槽时,在径向和周向产生较大的压力梯度;③动静盘间形成的空化现象是胶粘物颗粒发生分散作用的一个重要原因;④湍流剪切主要发生在齿槽处,而层流剪切主要发生在动/静盘齿之间;⑤剪切率在整个工作区域是不同的,与半径大小相关,半径越大的地方,剪切率越大,同时存在动/静盘齿间的剪切率>动盘齿槽内的剪切率>静盘齿槽内的剪切率的现象。更多还原

【Abstract】 In china, the waste paper has become the major paper-making raw materials at the present time. In the process of wastepaper pulping and papermaking, Treatment and control of stickies has always been a focus of investigation by worldwide researchers. But for the heat dispersion technology, a major means of dispersion technology, its dispersion mechanism is very difficult to understand deeply. It is of great significance for understanding of heat dispersion mechanism to reduce the energy consumption of heat dispersion, and improve the quality of waste paper pulp dealt by heat dispersion. On the other hand, the dispersion is also a major physical process and section during wastepaper pulping, such as pulping process, flotation, beating and heat dispersion process. Therefore, Investigation and improvement of dispersion technology is propitious to wastepaper pulping and recycling, and provide technical support for the realization of the paper industry recycling economy. As for disk heat dispersion, both internal slurry flow field and stickies dispersion mixing characteristics are investigated through theoretical analysis, visualization experiments and numerical simulations in this project. The main research content include the rheological properties of stickies, working principle and energy consumption of disc heat disperser, process and disciplinarian of stickies erosion and ruptuer dispersion, shear and cavitation in pulp flow field, and the effects of various structure parameters and operating parameters wake on pulp flow field and heat dispersion characteristics are also analyzed. The data and conclusions from the experimental measurements and numerical simulations are valuable for the design and optimization of the disk heat disperser. Although the heat dispersion process is treat as the main object, its research method and theory can apply equally to other pulp dispersion workshop section of pulping and papermaking process. Main contents and conclusions of the investigation are described in the following:(1)The very possibility of stickies dispersion process occurred in pulp flow field is analized theoretically. Analysis results show that high shearing process occurred in disc stickies heat dispersion can be described as two main two main phases, namely dispersion mixing and distribution mixing.(2)With the help of multi-phase flow theory, amathematical model stickies particle force is established. As a kind of aggregates, various forces received by stickies particle in the course of dispersion mixing are analyzed. When all of these forces are treated as dispersion force Fc, it can be described Fc as follows :(?)(3)The stickies dispersion characteristics occurred in the working region of Disc heat disperser is analyzed theoretically, and the basic principles of disc type stickies heat dispersion is put forward. It can be described as follows: As a type of aggregate, the stickies dispersion occur in two different way of rupture and erosion under the action of both shear and cavitation inside working region. Two different dispersion way will result in unlike dispersion effect. Erosion dispersion occurs in the whole course of heat dispersion, but rupture dispersion occurs only when dispersion force Fc is more than gathered strength of stickiesσc, namely Fc>σc. When Fc>σc, stickies particles will be broken into multiple smaller particles. under long time action, erosion dispersion changs constantly stickies particle into smaller. The cause of stickies heat dispersion within working region, which occurs in two way of rupture and erosion, is 4 major role. Thry are crushing force, laminar shear stress, turbulent shear stress and cavitation effect respectively.(4)Energy consumption of stickies particle heat dispersion is analyzed When heat dispersion grinding teeth is treated as a solid phase, and high speed fluidization pulpas liquid phase, with the help of multi-phase flow theory, the energy consumption model of disc heat disperser is obtained as fellow: (?)(5) A set of visualization experiment device is designed and manufactured, which can simulate multiple loading conditions, and its clearance of static and dynamic disk can be adjusted. According to the visualization experimental conditions, a set of transparent heat dispersion disk is also specially designed and manufactured. Using high-speed photography technology, several experiments of stickies heat dispersion mixed process in pulp are carried out. Both pulp and stickies movement process in disc heat disperser is recorded distinctly. Experimental results has proved that rupture and erosion dispersion principle put forward by this dissertation is fit for stickies dispersion occured in disc heat disperser. Experimental results has also recorded laminar shear, turbulent shear and cavitation phenomenon.(6) The industry experiments show that although particle size distribution of waste pulp stickies is irregularity, after heat dispersion disposal, particle size distribution will be increasingly concentrated, and small size stickies becomes smaller not any longer, but both surface and quantity of large size stickies particle will change in strong degree. experiments also show that both gap size of grinding teeth and disk dismeter will affect the heat dispersion effect, and the gap influence on stickies disersion is the most direct.(7)Combining with RNG k-εturbulence model, Discrete Phase model and particle trajectory model based on Euler-Lagrange method, a series of numerical simulations of stickies dispersion process with FLUENT software are carried out for five types of dispersion disk. The simulations are also compared with the experimental results obtained by high speed photography to validate the reliability of the computational model. The emphases of the numerical simulation are to investigate the flow field near the grinding teeth wall surface, stickies particles trajectories and pressure gradient, turbulence energy loss, shear rate and so on which cannot be got in the experimental measurements.(8)The unsteady pulp turbulent flow field inside disc heat disperser are performed by numerical simulation. Simulation results has revealed some distribution rule of physics parameters from different levels, such as velocity, static pressure, total pressure, turbulent kinetic energy, wall shear stress and shear rates distribution. At the same time, the effects of turbulent flow field wake on stickies dispersion and its rule also analyzed from aspect of inhalation flow field, pressure field, velocity field, shear rate, turbulent intensity and energy dissipation rate and so on. Analysis results has revealed the effects of heat dispersion disk structure parameters and operating parameters wake on stickies dispersion performance.(9)The results of the experimental measurements and the unsteady numerical simulations indicate that both gap size of grinding teeth and disk dismeter are two most obvious factors affecting stickies heat dispersion effect. With the teeth gap reducing or disk diameter increasing, both pressure gradient and velocity gradient will aggrandize. Correspondingly, turbulent shear, laminar shear and cavitation effect, which arouse stickies heat dispersion, will enhance, and will lead to the effect strength of heat dispersion increasing.(10)The results of unsteady numerical simulations also show that:①The pressure inside dynamic teeth channel is far higher than the static teeth channel, and the pressure gradient in the clearance static and dynamic grinding teeth increase continuously along radial direction.②When pulp flows through both static and dynamic teeth channel, a greater pressure gradient will occur along with radial and circumferential direction.③The cavitation phenomenon forming between the gap of static and dynamic disk is one of the important reasons of stickies particle dispersion.④turbulent shear occur mainly in teeth channel, but laminar shear occur mainly in the gap of static and dynamic grinding teeth.⑤The shear rate is various around the whole dispersion region. the bigger the radius, the greater the shear rate. This phenomenon of shearing rate distribution lies in all of the above five types of dispersion disk, namely, the shear rate between the gap of static and dynamic disk> the shear rate in the dynamic teeth channel>the shear rate in the staticdynamic teeth channel.更多还原