稠密气固同轴射流中的间壁效应

发布时间：2018-06-03 17:19

本文选题：稠密气固同轴射流 + 间壁效应　；参考：《华东理工大学》2017年博士论文

【摘要】：本文以单喷嘴水冷壁式粉煤加压气化技术为研究背景,以解决同轴喷嘴因严重磨蚀而影响使用寿命及安全生产为研究目标,对稠密气固两相同轴射流中的颗粒流动特性进行了详细而深入的研究。发现了同轴喷嘴端部的颗粒逆向卷吸现象是导致了喷嘴的磨蚀主要原因,并分析喷嘴间壁厚度对同轴射流远场颗粒弥散的影响。具体内容归纳如下:1.利用高速摄像仪配合高倍变焦镜头,观测了喷嘴端部的颗粒局部流动特性。发现了由于喷嘴壁面厚度而产生间壁效应,引起了同轴气体强烈的回流作用,从而诱导了颗粒逆向卷吸现象的出现并导致喷嘴的磨蚀。通过处理与分析实验图像,揭示了颗粒逆向卷吸随环隙气速、间壁厚度、颗粒质量流率和颗粒粒径的变化规律。发现了颗粒逆向卷吸的临界环隙气速随颗粒质量流率的增加呈线性增加,得到了颗粒流卷吸长度和逆向弥散角的经验关系式。此外,通过数值模拟计算获得了间壁效应中的回流气体流场特性,并呈现了高气速下被卷吸颗粒的流动轨迹。根据气固两相流理论和修正曳力模型,建立了被卷吸颗粒的运动模型,得到了颗粒回流速度,为解决喷嘴磨蚀提供了理论分析。2.研究了间壁效应对稠密气固两相同轴射流中颗粒弥散特性的影响。发现了间壁效应会增加同轴环隙气体射流的涡层厚度,显著削弱对中心颗粒流的剪切作用,进而抑制颗粒弥散。分析了喷嘴间壁厚度对颗粒波状弥散中的扰动波长、振幅和弥散角等特征量的影响。利用不稳定性理论揭示了扰动波长与修正涡层厚度呈线性关系,获得了波动振幅随修正涡层厚度的变化规律。3.考察了稠密气固两相同轴交叉射流的颗粒流动特性。发现了由于同轴交叉气体射流的"挤压"作用,射流近场的中心颗粒流发生收缩现象。通过观测颗粒射流近场中的局部流动特性,得到了颗粒流最小直径和收缩长度都随环隙气速和交叉角的增加而减小。提出了颗粒流最小直径和收缩长度与交叉角、环隙气速和颗粒质量流率之间的数学模型。引入交叉角因子来分析同轴交叉射流对颗粒射流弥散长度的影响,修正了交叉射流中的弥散长度关系式。揭示了颗粒射流弥散角与环隙气速和交叉角呈非线性关系,提出了交叉角对颗粒弥散的影响是由径向挤压扰动和轴向剪切不稳定共同决定的。此外,还发现了同轴交叉射流中的间壁效应诱导颗粒逆向卷吸现象更靠近喷嘴壁面,加剧了喷嘴的磨蚀,并利用交叉角因子修正了颗粒流卷吸长度。
[Abstract]:In this paper, the research background is single nozzle water-cooled wall type pulverized coal pressurized gasification technology. The purpose of this paper is to solve the problem of coaxial nozzle affecting service life and safety production because of serious abrasion. The particle flow characteristics in dense gas-solid two-phase coaxial jet are studied in detail. It is found that the reverse entrainment of the particles at the end of the coaxial nozzle is the main cause of the erosion of the nozzle, and the influence of the thickness of the wall between the nozzles on the dispersion of the particles in the far field of the coaxial jet is analyzed. The details are summarized as follows: 1. The local flow characteristics of particles at the nozzle end were observed by using high speed camera and high power zoom lens. It is found that the interwall effect is produced because of the wall thickness of the nozzle, which leads to the strong reflux of coaxial gas, which induces the appearance of particle reverse entrainment and leads to the erosion of the nozzle. By processing and analyzing the experimental images, the variation of particle entrainment with annular gas velocity, interwall thickness, particle mass flow rate and particle size was revealed. It is found that the critical annular gas velocity of particle reverse entrainment increases linearly with the increase of particle mass flow rate, and the empirical relationship between particle entrainment length and reverse dispersion angle is obtained. In addition, the characteristics of the reflux gas flow field in the interwall effect are obtained by numerical simulation, and the flow path of the entrained-particle at high gas velocity is presented. According to the theory of gas-solid two-phase flow and the modified drag force model, the motion model of entrained-particle is established, and the velocity of particle reflux is obtained, which provides theoretical analysis for solving the erosion of nozzle. The effect of interwall effect on particle dispersion in dense gas-solid two-phase coaxial jet was studied. It is found that the wall effect will increase the thickness of the vortex layer of the coaxial annular gas jet, weaken the shear effect on the central particle flow, and then restrain the particle dispersion. The influence of the thickness of the internozzle wall on the characteristic quantities such as the disturbance wavelength, amplitude and dispersion angle in the particle wave dispersion is analyzed. By using the instability theory, the linear relationship between the perturbation wavelength and the thickness of the modified vortex layer is revealed, and the variation of the amplitude of the wave with the thickness of the modified vortex layer is obtained. The particle flow characteristics of dense gas-solid two phase coaxial cross jet were investigated. It is found that the central particle flow in the near field of the coaxial cross gas jet shrinks due to its "squeezing" effect. By observing the local flow characteristics in the near field of particle jet, it is obtained that the minimum diameter and shrinkage length of particle flow decrease with the increase of annular gas velocity and cross angle. A mathematical model of the minimum diameter of particle flow, shrinkage length and cross angle, annular gas velocity and particle mass flow rate is presented. The cross angle factor is introduced to analyze the influence of coaxial cross jet on the dispersion length of particle jet, and the relation of dispersion length in cross jet is modified. The nonlinear relationship between particle jet dispersion angle and annular gas velocity and cross angle is revealed. The influence of cross angle on particle dispersion is determined by radial extrusion disturbance and axial shear instability. In addition, it is found that the interwall effect of coaxial cross jet induces the reverse entrainment of particles closer to the nozzle wall, which intensifies the erosion of the nozzle, and modifies the particle flow entrainment length by using the cross angle factor.
【学位授予单位】：华东理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ541

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