颗粒—壁面作用的声发射检测及其在气力输送过程中的应用

发布时间：2018-05-11 10:54

  本文选题：气力输送 + 颗粒-壁面作用　； 参考：《浙江大学》2016年博士论文

【摘要】：随着气力输送过程研究的深入,特别是密相输送应用的愈加广泛,人们对于认识气力输送过程流动本质的需求也愈加迫切。从气力输送过程的研究现状来看,无论是对于流型的识别、压降的预测还是固体流量的测量,都需要从颗粒运动及受力分析,特别是颗粒-壁面相互作用分析的层面出发,更进一步地挖掘气力输送过程流动的本质。由于输送管径一般较小,因此颗粒与壁面作用的壁效应就显得尤为重要。但是,颗粒-壁面相互作用在以往的研究中却没有得到足够的重视,而且局限在柱塞流压降预测模型中应力转变系数的离线测定。究其原因,是没有找到合适的方法来实现颗粒-壁面作用的在线检测。气力输送过程中,颗粒-壁面的相互作用究竟是如何影响管道内气固两相流体力学行为、输送压降及输送稳定性的呢?这是气力输送领域中亟待解决的关键科学问题,具有重要的理论意义和实际应用价值。本研究利用对颗粒运动极为敏感的声发射检测技术,借助小波分解和功率谱分析等信号处理手段,建立了颗粒-壁面作用的在线检测方法。在此基础上,利用该方法,结合压降、摄像和静电等多种检测手段,在实验室条件下,系统研究了竖直管稀相输送过程、稀相到密相流型转变过程以及密相柱塞流中颗粒-壁面作用对管道压降、输送流型和输送稳定性等流体力学行为的影响规律。最后,在实验室研究的基础上,基于小波分解和V统计分析,提出了气力输送声发射信号的多尺度划分标准,阐明了各尺度声发射信号的物理意义,进一步研究了工业装置粉煤高压密相输送中颗粒-壁面作用对输送稳定性的影响,并建立了粉煤质量流量的声发射检测方法。研究结果有助于解决如何有效提取声发射信号中的有效信息这一声发射检测技术发展过程中的难题。同时,对于如何保证在低速下进行稳定输送这一气力输送领域科研工作者和工程师关心的关键问题具有重要的指导意义。论文的主要研究内容及结论如下：1.发现了颗粒-壁面的碰撞(法向作用)和摩擦(切向作用)作用这两种不同的声信号产生机制,提出了颗粒-壁面碰撞和摩擦作用的声发射检测方法。考察了颗粒粒径、颗粒速度以及法向压力对碰撞和摩擦声发射信号主频和能量的影响,建立了声发射信号的主频和能量模型。实现了稀相气力输送固体质量流量和颗粒-壁面碰撞角的声发射检测。(1)颗粒-壁面碰撞和摩擦作用检测的步骤为：首先,通过实验研究确定颗粒-壁面碰撞和摩擦信号的主频范围和小波能量分率分布等频域特征。实验研究发现,对于不同Geldart分类的颗粒,其与壁面碰撞声发射信号的主频均显著高于摩擦声发射信号的主频。其次,通过小波分析方法对声发射信号进行分解,并将其中代表颗粒-壁面碰撞和摩擦成分的小波尺度进行重构,提取声发射信号中包含的颗粒-壁面碰撞和摩擦成分。(2)基于Hertz接触理论和分段塑性模型,建立了考虑颗粒塑性形变条件下的颗粒-壁面碰撞声发射信号主频模型,相比于基于完全弹性碰撞下推导的模型,该模型的预测精度更高。引入接触时间数的概念,建立了颗粒-壁面摩擦声发射信号的主频模型。模型可以很好地解释碰撞和摩擦声发射信号的主频随颗粒粒径的增大而降低,摩擦信号主频随着颗粒速度的增大和法向压力的增大而降低的实验现象。(3)在竖直管稀相气力输送过程中,颗粒的呈现形式有分散悬浮的单个颗粒和颗粒聚团两种,这两种类型的颗粒与壁面的作用方式是不同的。在此基础上,建立了颗粒-壁面碰撞和摩擦声发射信号的能量模型。基于声发射信号的能量模型,建立了竖直管稀相气力输送固体质量流量和颗粒-壁面碰撞角的检测方法。碰撞声发射信号能量分率与固气比的关系为采用该模型测得的固体颗粒质量流量与实验值之间的平均相对误差为3.78%。颗粒-壁面碰撞角a与碰撞和摩擦声发射信号能量比的关系为2.采用声发射、压降和静电多种检测手段相结合的方式,研究了最小输送速度附近操作时的流型转变现象,揭示了颗粒-壁面作用对流型转变的影响规律,建立了基于声能量和静电荷累积量的流型转变图。(1)单一检测手段无法准确地识别最小输送速度附近的流型。恒定质量流率下,随着表观气速的降低,压降、声能量和静电荷累积量均呈现先减小再增大的趋势,但是曲线转折点所对应的转变速度各不相同。声发射能量得到的转变速度(7.0 m/s)略小于压降最小输送速度(7.5-8.0 m/s),而静电荷累积量的转变速度(6.0 m/s)明显小于前两者。(2)压降最小输送速度是气体与壁面摩擦、颗粒-壁面相互作用以及颗粒浓度的变化共同作用的结果。当气速大于压降最小输送速度时,气相压降的影响占主导；当气速小于流型转变速度时,气相压降的影响几乎可以忽略,颗粒浓度的变化和颗粒-壁面相互作用对压降的影响占主导；当气速介于压降最小输送速度和流型转变速度之间时,上述三种因素对管道压降的影响相当。(3)当气速等于流型转变速度时,颗粒-壁面的碰撞声能量分率最小,摩擦声能量分率最大,颗粒-壁面的碰撞角最小。这是由于流型转变时颗粒的聚集形态发生了变化,使得颗粒-壁面的碰撞及摩擦能量分率的变化曲线出现转折点。当气速大于流型转变速度时,颗粒-壁面的作用以悬浮单颗粒的形式为主,随着气速的减小,颗粒-壁面碰撞速度降低,颗粒-壁面碰能量降低,导致碰撞能量减小,摩擦能量分率增大；当气速小于流型转变速度时,颗粒-壁面的作用以回落颗粒与柱塞的作用为主,随着气速的减小,回落颗粒和柱塞之间的相对速度增大,碰撞能量增加,因此颗粒-壁面碰能量分率增大,摩擦能量分率减小。(4)基于声能量和静电荷累积量与压降的关系,建立了新的流型转变图。基于声能量与压降的流型图不仅可以实现压降最小输送速度的识别,还能够反映不同流型下压力损失方式的不同。基于静电信号与压降的流型图可以同时识别最小压降速度和柱塞流转变速度。3.研究了密相柱塞流中最基本的流动结构柱塞与壁面的作用及其对管道压降的影响,发现当柱塞长度较大或者表观气速较小时,柱塞流输送时可能出现压降二次上升的现象。这一现象的出现是由于回落颗粒与柱塞前端的碰撞导致了额外的压力损失。通过在模型中引入颗粒与柱塞的碰撞作用,建立了柱塞流输送的声能量模型,能够准确预测柱塞流输送过程中颗粒壁面作用的变化,说明颗粒与柱塞的碰撞作用是柱塞流的典型特征。4.将声发射技术和多尺度分析方法相结合,用于粉煤高压密相气力输送过程中气固两相流体力学行为的研究,给出了声发射信号微尺度、介尺度和宏尺度的划分标准,并进一步对各个尺度代表的物理意义进行了探讨。在此基础上,建立了粉煤质量流量的声发射检测方法,并将其应用于工业装置中。(1)结合V统计分析和小波分解,提出了粉煤密相输送的声发射信号的多尺度划分原则。研究发现,声发射信号的d1-d4尺度(18.75-300 kHz)和d8-d10尺度信号(0-2.34 kHz)具有一个典型的周期性成分,而d5-d7尺度信号(2.34-18.75 kHz)具有多个周期性成分。基于信号频率及其复杂程度的不同,将粉煤密相输送声发射信号的d1-14尺度、d5-d7尺度和d8-d1o尺度分别划分为微尺度、介尺度和宏尺度。进一步研究揭示了微尺度、介尺度和宏尺度声信号的物理意义。三者分别代表颗粒-壁面的碰撞和摩擦作用、气固两相的相互作用产生的非均匀结构以及气体与管壁的摩擦。其中,介尺度信号是粉煤密相输送声发射信号中的主要成分。随着粉煤质量流量的增加,微尺度信号能量分率下降,介尺度信号能量分率上升,宏尺度信号能量分率基本不变,说明在高浓度下粉煤更倾向于以聚团的形式运动。(2)基于声发射信号的多尺度分解理论,结合信号的时域特征和偏最小二乘回归方法,建立了粉煤质量流量和粉煤浓度的检测模型。当粉煤质量流量变化范围为8000-12000 kg/h时,粉煤质量流量检测值与实际值的平均相对偏差为4.15%,粉煤浓度检测值与实际值的平均相对偏差为4.78%。利用该模型对粉煤质量流量进行预测,发现流量在5800 kg/h附近变化时预测值与实际值的平均相对偏差为10.37%,表明该模型具有一定的预测能力。
[Abstract]:With the deepening of the research on the process of pneumatic conveying, especially the wider application of dense phase transport, the demand for the understanding of the essence of the flow of the pneumatic conveying process is becoming more and more urgent. From the research status of the pneumatic conveying process, the particle motion is required, whether for the identification of the flow pattern, the prediction of the pressure drop or the measurement of the solid flow. And the force analysis, especially the particle wall interaction analysis, further excavate the essence of the flow of the pneumatic conveying process. Because the pipe diameter is generally small, the wall effect of the particle and wall action is particularly important. However, the particle wall interaction has not been sufficiently weighed in the previous study. The off-line determination of the stress change coefficient in the prediction model of the plunger flow pressure drop is considered, and the reason is that there is no suitable method to realize the on-line detection of the particle wall action. In the process of pneumatic conveying, how the interaction between the particle wall surface affects the gas-solid two-phase fluid mechanics behavior, the pressure drop and the transmission in the pipeline. It is the key scientific problem to be solved in the field of pneumatic conveying, which has important theoretical significance and practical application value. In this study, the on-line detection of particle wall action is established by means of signal processing, such as wavelet decomposition and power spectrum analysis, which are very sensitive to particle motion. On the basis of this method, using this method, combined with pressure drop, camera and static electricity and so on, under the laboratory conditions, the fluid mechanics behavior, such as the dilute phase transport process of vertical tube, the transition process of the thin phase to the dense phase flow pattern and the pressure drop of the particle wall surface, the flow pattern and the transport stability in the dense phase plunger flow, are systematically studied. Finally, based on the laboratory research, based on the wavelet decomposition and V statistical analysis, the multi-scale division standard for the acoustic emission signal of the pneumatic conveying is proposed, the physical meaning of the acoustic emission signals of each scale is clarified, and the effect of the particle wall action on the transport stability in the high pressure dense phase transport of the industrial equipment is further studied. Sound emission detection method is established, and the results are helpful to solve the problem of how to effectively extract the effective information of acoustic emission signals, the development process of acoustic emission detection technology. At the same time, the research workers and engineers on how to ensure the stable transport of the pneumatic conveying field at low speed are closed. The key issues of the heart have important guiding significance. The main contents and conclusions of this paper are as follows: 1. the two different sound signal generation mechanisms are found, the particle wall collision (normal action) and the friction (tangential action) effect are found. The acoustic emission detection method for particle wall collision and friction is proposed. The particle size is investigated. The influence of particle velocity and normal pressure on the main frequency and energy of collision and friction acoustic emission signals, the main frequency and energy model of acoustic emission signals are established. The acoustic emission detection of solid mass flow and particle wall collision angle in dilute phase pneumatic conveying is realized. (1) the steps of particle wall collision and friction detection are: first, pass through The experimental study determines the frequency domain characteristics of the main frequency range and the wavelet energy distribution of the particle wall collision and the friction signal. It is found that the main frequency of the acoustic emission signal with the wall collision is significantly higher than the main frequency of the friction acoustic emission signal for the particles with different Geldart classification. Secondly, the acoustic emission signal is analyzed by the wavelet analysis method. The number is decomposed, and the wavelet scales representing particle wall collision and friction component are reconstructed to extract the particle wall collision and friction components included in the acoustic emission signal. (2) based on the Hertz contact theory and the piecewise plastic model, the main frequency of the particle wall collision acoustic emission signal under the condition of granular plastic deformation is established. The model, compared to the model based on the complete elastic collision, has a higher prediction precision. The main frequency model of the particle wall friction acoustic emission signal is established by introducing the concept of contact time. The model can explain the main frequency of the collision and friction acoustic emission signal with the increase of particle size, and the main frequency of the friction signal. The experimental phenomenon decreases with the increase of particle velocity and normal pressure. (3) in the process of dilute phase pneumatic conveying of vertical tubes, there are two kinds of particle and particle cluster in the form of dispersed suspended particles, which are different between the two types of particles and the wall surface. On this basis, a particle wall collision is established. The energy model of the acoustic emission signal. Based on the energy model of the acoustic emission signal, a method for detecting the mass flow of solid mass and the collision angle of the particle wall in a vertical tube is established. The relationship between the energy fraction of the acoustic emission signal and the solid gas ratio is between the mass flow of solid particles and the experimental value measured by the model. The average relative error is the relationship between the 3.78%. particle wall collision angle A and the energy ratio of the collision and frictional acoustic emission signals. The flow pattern transition in the vicinity of the minimum transport velocity is studied by the combination of acoustic emission, pressure drop and various electrostatic detection methods. The effect of the convective transformation on the particle wall surface action is revealed. The flow pattern transformation diagram based on the sound energy and the static charge accumulation is established. (1) the single detection method can not accurately identify the flow patterns near the minimum transport velocity. Under the constant mass flow rate, with the decrease of the apparent gas velocity, the pressure drop, the accumulation of acoustic energy and static charge first decrease and then increase, but the curve turning point corresponds to the flow pattern. The transformation speed is different. The transformation velocity of acoustic emission energy (7 m/s) is slightly smaller than the minimum pressure drop velocity (7.5-8.0 m/s), while the transition velocity of the static charge accumulation (6 m/s) is obviously less than the previous two. (2) the minimum pressure drop velocity is the friction between the gas and the wall, the interaction of the particle wall and the change of the particle concentration. The effect of gas pressure drop is dominant when the gas velocity is greater than the minimum pressure drop. When the gas velocity is less than the flow velocity, the influence of the gas pressure drop is almost negligible. The influence of the particle concentration and the particle wall interaction on the pressure drop is dominant, while the gas velocity is between the minimum pressure drop and the flow pattern change. The influence of the above three factors on the pressure drop of the pipe is equal. (3) when the velocity of the flow pattern is equal to the velocity of the flow pattern, the particle wall surface has the smallest sound energy fraction, the maximum friction energy rate and the smallest collision angle of the particle wall. This is due to the change of the particle aggregation shape when the flow pattern changes, making the particle wall face collide. When the velocity of gas is larger than the velocity of the flow pattern, the effect of the particle wall is mainly in the form of a single particle when the velocity of gas is larger than the flow pattern. With the decrease of the gas velocity, the velocity of the particle wall collision is reduced, the particle wall collision energy is reduced, resulting in the decrease of the collision energy and the increase of the fraction of the friction energy; when the gas velocity is less than the gas velocity, the velocity of the friction energy is increased. When the velocity of the flow pattern is changed, the effect of the particle wall surface is dominated by the falling particles and the plunger. With the decrease of the gas velocity, the relative velocity between the falling particles and the plunger increases and the collision energy increases, so the particle wall collision energy fraction increases and the friction energy fraction decreases. (4) the relationship between the sound energy and the static charge accumulation and the pressure drop is based on the increase of the velocity of the particle and the plunger. A new flow pattern transformation diagram is set up. The flow pattern based on the acoustic energy and pressure drop can not only realize the identification of the minimum pressure drop velocity, but also reflect the different pressure loss modes under different flow patterns. The flow pattern based on the electrostatic signal and pressure drop can identify the minimum pressure drop velocity and the plunger flow velocity.3. at the same time to study the dense phase column The most basic flow structure in plug flow and the effect on the wall surface and its influence on the pressure drop of the pipe. It is found that when the plunger length is larger or the apparent gas velocity is small, the pressure drop may rise two times when the plunger flow is transported. This phenomenon is due to the additional pressure loss caused by the collision between the falling particles and the front end of the plunger. By introducing the collision between the particles and the plunger in the model, the acoustic energy model of the plunger flow is established. It can accurately predict the change of the effect of the particle wall in the process of the plunger flow. It shows that the collision between the plunger and the plunger is the typical characteristic of the plunger flow..4. combines the acoustic emission technique with the multi-scale analysis method and is used in the coal powder. The study on the mechanical behavior of gas-solid two-phase fluid in high pressure dense phase pneumatic conveying process has given the standard of acoustic emission signal micro scale, mesoscale and macro scale, and further discusses the physical significance of the representative of each scale. On this basis, a method of acoustic emission detection for the mass flow of coal powder is established and applied to the work. (1) combined with V statistical analysis and wavelet decomposition, the multi-scale division principle of acoustic emission signals for dense phase transport of coal is proposed. The research shows that the D1-D4 scale (18.75-300 kHz) and d8-d10 scale signal (0-2.34 kHz) of the acoustic emission signal have a typical periodic component, and the d5-d7 scale signal (2.34-18.75 kHz) has many weeks. Based on the difference of signal frequency and its complexity, the d1-14 scale, d5-d7 scale and d8-d1o scale are divided into microscale, mesoscale and macro scale respectively. Further studies have revealed the physical significance of the microscale, mesoscale and macro scale acoustic signals. The three ones represent the particle wall surface respectively. Collision and friction, the non-uniform structure of gas-solid two-phase interaction and the friction between gas and tube wall. Among them, the mesoscale signal is the main component in the acoustic emission signal of the dense phase transport of coal powder. With the increase of the mass flow of coal, the energy fraction of the microscale signal decreases, the energy fraction of the mesoscale signal rises, and the macro scale signal The energy fraction is basically the same, indicating that the pulverized coal is more inclined to move in the form of cluster at high concentration. (2) based on the multi-scale decomposition theory of acoustic emission signals, combining the time domain characteristics of the signal and the partial least squares regression method, a test model for the quality flow of pulverized coal and the concentration of pulverized coal is established. The change range of the mass flow of the pulverized coal is 8000-1200. At 0 kg/h, the average relative deviation between the measured value of coal quality flow and the actual value is 4.15%. The average relative deviation between the measured value of coal concentration and the actual value is 4.78%. using this model to predict the mass flow of coal powder. It is found that the average relative deviation between the predicted value and the actual value is 10.37% when the flow rate is changed near 5800 kg/h, indicating the model. Have a certain ability to predict.

【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ022
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本文编号：1873658