【作者】 吴学成；

【导师】 方梦祥； 骆仲泱； 岑可法；

【作者基本信息】 浙江大学 ， 热能工程， 2007， 博士

【摘要】 多相流在能源、环境和化工等领域应用广泛，但是由于多相流的复杂性，目前的测试技术还远远不能满足对多相流运动特性以及工业在线测量的需求。本文针对多相流研究和测试中的几个难点，对若干激光测试技术及其应用进行了研究。首先，把高速数字摄像技术应用到了冷态循环流化床稀相区固相颗粒的旋转和碰撞等微观层面运动特性的研究中。探讨了该测试系统进行实际气固流场中颗粒旋转测量可行性，并提出了四种颗粒转速的判别方法：旋转角度人工量取法，数圈法，基于转轴重建的转速算法和基于相关性分析的计算机自动识别法。其中转轴重建算法可确定流场中部分颗粒的转轴三维方向，并比较精确地测定其转速。而计算机自动识别算法主要针对旋转轴垂直或近似垂直于拍摄平面颗粒转速的测定；比较详细地分析了颗粒转速测量过程中的影响测量准确性和可信度的若干不确定因素；理论预测了流化床典型工况下颗粒最大转速；用颗粒轨迹验证法对所测颗粒转速的合理性进行了验证；提出了双帧频拍摄方法，有效地提高了颗粒转速测量范围。对一截面为200×200 mm高为4 m的冷态循环流化床稀相区的气固两相流进行了颗粒旋转和碰撞特性的实验研究。1)用统计的方法研究了影响颗粒旋转的因素，结果表明：在相同的流场区域中，小颗粒的平均转速高于大颗粒；同一粒径档的颗粒，平均水平横向运动速度大，颗粒的平均转速就高，但是平均垂直运动速度对颗粒转速影响较小；同一条件下，不规则颗粒的平均转速明显高于规则球形颗粒；随着流场区域颗粒碰撞频率的增加，颗粒的平均转速升高；2)对稀相区的颗粒平均转速空间分布特性进行了统计分析。研究表明，在同一工况下，稀相区横向截面上边壁区域的颗粒平均转速高于中心区域，在垂直高度方向，随着高度的增加，颗粒平均转速有降低的趋势。运行参数对颗粒平均转速空间分布的影响有：随着表观气速的增加，截面边壁区域的颗粒平均转速明显增加，但截面中心区域则无明显变化；循环物料量对颗粒平均转速分布的影响恰恰相反；而随着床料静止高度的增加，截面边壁和中心区域的颗粒平均转速均有所提高。3)对颗粒的碰撞率进行实验研究，结果表明，颗粒的碰撞率近似与颗粒浓度的平方成正比，其规律与其他学者通过分子动力学理论类比所得的碰撞率公式相符，但是需要附加一个修正系数，根据本实验的结果分析，该修正系数可取为0.42-0.68。另外，还对颗粒碰撞参数进行实验研究。其次，提出了一种便携式的和可用于在线测量的基于近后向散射的激光多普勒测量系统BLDMS，该技术基于多普勒原理测速和近后向散射光强测量粒径的原理。为了研究近后向散射测量不规则颗粒粒径分布的性能，分别进行了理论和实验研究。在理论模拟中，用散射响应带描述了实际不规则颗粒的散射响应特性，并用浮动系数k进行了表征。通过对单峰和双峰分布的多分散不规则颗粒群的详细模拟测试，表明了基于近后向散射的多普勒测量系统应用于不规则颗粒粒径分布的测试是可行的。在此基础上，自行搭建了实验系统，并利用该系统对玻璃珠和石英砂颗粒流进行了研究，主要实验内容包括转换矩阵和颗粒平均散射响应曲线的标定实验和已知颗粒群的粒径分布测试实验。结果表明，BLDMS能够很好地用于多相流中透明或者不透明不规则颗粒的速度、粒径和浓度分布的测试。最后，利用基于成像的激光相关测速技术ILCV对脉冲喷嘴出口附近高浓度的气液两相流速度场进行测试。用广义米氏理论的程序理论分析了ILCV光学系统，研究了颗粒经过测量体时产生的光信号的特点以及频谱分布，重点研究了ILCV测量体在光轴方向的有效长度。对ILCV信号处理进行研究，比较系统地研究窗函数、信号预平移、插值等辅助方法对数据处理的作用，相关性计算中各种参数的选取，以及信号的噪声、疏密等参数对计算结果的影响；搭建了实验测试平台和喷雾系统，对低速脉冲喷雾进行了详细测试，研究了不同脉冲宽度下的喷雾速度场以及喷嘴出口附近喷雾速度轴向和径向分布规律。一系列测试研究表明，ILCV可以很好地应用于喷嘴出口附近区域喷雾速度场的测量。更多还原

【Abstract】 Multi-phase flows have wide applications in the fields of Energy, Chemical and Environmental engineerings. Examples are pneumatic conveying, fluidized beds, sprays, vertical risers, particle separation in cyclones, mixing devices, and others. Due to the complexity of themselves, however, the present measurement techniques have not been developed so far to meet all the demands in the dynamical researches of multi-phase flows and on-line and portable measurements. Concerning with some difficulties encountered in this field, this paper aims mainly to carry out studies of several laser-based measurement techniques and their applications in the multi-phase flows.First, a high-speed digital imaging system was used to visualize in a micro-scale level the motion behavior of spherical glass beads with an average diameter of about 0.5 mm in the upper dilute zone of a cold CFB riser with a dimension of 200 mm×200 mm×4 m（H）. The particle rotation and collision motions were investigated. The high-speed digital imaging system is mainly composed of a high-speed digital camera, a high power laser and corresponding digital imaging processing procedures. Four kinds of judgement methods for particle rotation speed were presented. Three of them are manual determinations and the other is digital image processing method based on correlation analysis. Among them, the method based on reconstruction of rotation axis provides one to obtain simultaneously the direction of rotation axis and the relatively correct particle rotation speed. The method based on correltion analysis is only aimed to those particles whose rotation axes are perpendicular or nearly perpendicular to the object plane. The maximal possible rotation speeds for particles in CFB with typical operation conditions are theoretically predicted based on particle collision. The reliability of obtained particle rotation speeds was also verified by a validation method based on particle’s trajectory. Meanwhile, a dual-frequency imaging method was presented, which has been shown to be effective to enlarge the measurable range of rotation speed under finite recording rates of the camera.Combined with statistical analysis, the characteristics of particle rotation and collision were carried out and the results are as follows: 1) the effects on particle rotation speed were studied. It is shown that the average rotation speed of small particles is higher than that of large ones in the same testing area at the same operation condition; for the particles within the same size range, some of which with relative higher horizontal translational velocity components may be found to be with higher rotation speeds statistically, but the particle’s vertical translational velocity component may have a little effect on its rotation speed; the average rotation speed of irregular particles is much higher than that of spherical particles; and the collision rate in the testing area may take great effect on particle rotation; 2) the spatial distribution of average rotation speeds in the upper dilute phase zone of the operating CFB riser has been analyzed. It is shown that in the same working condition, the average rotation speed for particles near wall area is higher than that in the center area. And there is a decrease in the average rotation speed when the height of testing area increases, i.e. in the vertical direction. The effects of operation parameters on the spatial distribution of average rotation speeds have also been investigated. The results show that the increasing superficial gas velocity impoves the aveage rotation speed of particles near wall area but takes nearly no effects on that in the center area. The external solids mass flux, however, takes the opposite effect. And the average rotation speeds of particles in both areas are found to increase as the total amount of bed material increases; 3) the collision rate as well as collision properties is experimentally measured. The collision rate is found to be proportional to the square of particle number density, which coincides with the collision theory derived according to the analogy of kinetic theory of gases. It is also shown that, however, this theoretical model totally overestimates the real collision rates at all particle number densities investigated in the experiment. The author suggests that this discrepancy may be corrected by a coefficient a with values of 0.42-0.68. The measurement results of collision properties based on about 17 particle collision events agrees well with the Walton’s hard-sphere collision model and the three collision parameters, i.e. the average coefficient of frictionμ, the normal and tangential coefficients of restitution e andβ₀, for the glass beads are measured.Then, to be capable of simultaneously measureing the velocity, size distribution, and density for irregular particles embedded in flows, a Backscatter Laser Doppler measurement system （BLDMS） based on near backward scattering mode was presented. The particle velocity is obtained by the well known Doppler Effect, while the near backward scattered powers are used to unfold particle size distribution. The feasibility and performance of particle size distribution measured by near backward scattering mode are intensively investigated both theoretically and experimentally. In the theoretical simulation, the scattering response character of irregular particles is described by using a response band with a floating coefficient K rather than a response curve. The simulation work is done for both typical unimodal and multi-modal size distributions, and the results verify the feasibility of the method. In the experiemental study, a measurement system of BLDMS with near backward scattering mode was established, using which preliminary experiments on both spherical glass beads and irregular quartz sands particulate flows were conducted. The experiments include the calibration experiement which provides the inversion matrix and the average scattered response function for particles and the verification experiment which measures the size distribution of the pre-known particulate flow. The results show that BLDMS is very suitable to conduct size distributuion measurements for irregular particles and therefore BLDMS is attractive as a portable and on-line measurement method.At last, an imaging-based Laser Correlation Velocimetry （ILCV） is presented to conduct velocity measurements of high density gas-liquid two-phase flow at the first break up zone of a pulsed spray. The imaging system is analyzed by using a rigorous generalized Lorenz-Mie theory （GLMT） algorithm and the light signal as well as its frequency spectrum obtained by ILCV system when a spherical particle passes through the measurement volumes is modeled. And also the effective length of the measurement volumes is studied. Signal processing methods concerning with ILCV are intensively investigated with several parameters in considerations. An ILCV experimental setup is established and with which a low-velcoity high-density gas-liquid flow very near to the pulsed injector exit is tested. A series of experiments show that ILCV can be well applied to the velocity measurements of high-density gas-liquid flows, especially for sprays.更多还原