【作者】 张正华；

【导师】 王培红；

【作者基本信息】 东南大学 ， 动力工程及工程热物理， 2019， 硕士

【摘要】 燃煤电站一次风管道中煤粉浓度的准确测量对电厂运行的安全至关重要,煤粉与风的混合物属于典型的稀疏相气固两相流,其不固定的相界面,可压缩的气相,使得煤粉浓度的准确测量一直是工程界的难题。超声波在两相流浓度测量领域多有应用,但测量对象多为液固或气液两相流,利用超声波测量气固两相流浓度的研究并不多。本文围绕燃煤电站煤粉浓度超声检测实验研究这一课题,通过实验的方法,对利用超声法测量一次风煤粉浓度进行了研究,主要的研究内容和创新点如下:(1)设计并搭建了稀疏相气固两相流固相浓度检测实验台和模拟一次风的风粉系统实验台。稀疏相气固两相流固相浓度检测实验台以玻璃珠为物料,用于进行气固两相流中超声衰减的多因素分析实验,可以实现不同颗粒粒径、超声频率和固相浓度下的超声衰减测量。系统的负压设计减少了物料的泄漏,探头吹扫保证了超声探头的整洁,基于Lab VIEW开发的一套控制系统,可以实现实验台的自动化给粉和超声测量,减少了手动操作的误差,提高了实验的效率。风粉系统实验台以煤粉为物料,采用闭式循环设计,主要用于对稀疏相气固两相流固相浓度检测实验台的结果进行验证。(2)气相温度、湿度和流速会对超声衰减的多因素分析实验结果产生影响。其中,气相温度和湿度会影响超声换能器的特性,并改变超声的接收信号幅值;气相流速会改变超声在两相流中的传播方向,影响接收换能器接收份额,进而影响实测衰减率。本文提出了相应的实验标定方法,并分别确定了气相湿度、温度和流速对幅值和衰减率的修正函数。(3)提出了一种基于超声回波的信号处理方法,与传统的先将超声衰减前后的信号幅值相除,然后取对数的方法不同,该方法考虑了超声回波信号的特性,利用接收信号的一、二次波形之间的关联性,有效地消除了背景噪声对超声信号的影响,可以将计算得到的衰减系数与浓度的线性相关系数从0.774提高至0.972,提高了气固两相流中超声衰减系数的测量精度。(4)在稀疏相气固两相流固相浓度检测实验台上进行超声衰减的多因素分析实验,系统地研究了超声波频率、固相浓度以及颗粒粒径变化对超声波衰减系数的影响,得到了不同频率和粒径下,衰减系数与浓度的变化特性,提出了一种稀疏相气固两相流固相浓度的反演算法。并对粒径在106μm-250μm的颗粒,做了反演的误差分析。其中,当粒径为120μm,固相体积浓度在0.65‰-1.43‰之间时,浓度测量的相对误差为-5.35%-3.12%。(5)在风粉系统实验台上,利用稀疏相气固两相流固相浓度反演算法对风粉系统煤粉浓度进行测量,验证该算法在煤粉浓度检测中的适用性。验证结果表明,对提出的稀疏相气固两相流固相浓度反演算法进行系数修正后,可用于粒径为120μm,体积浓度为0.73‰-1.31‰的风粉系统煤粉浓度的测量,测量的相对误差为-8.32%-8.86%。更多还原

【Abstract】 Accurate measurement of pulverized coal concentration in primary air pipeline is critical to the safety of power plant operations,but its complex fluidity and unfixed phase interface make it difficult in engineering field.Ultrasound has many advantages,such as good penetration,non-contact and easy cleaning.It is widely used in two-phase flow measurement.The paper focused on the topic of experimental study on measurement of primary air concentration in coal-fired power plant,through experiments,the measurement of primary air concentration by ultrasonic was studied.The main research contents are as follows:(1)An experimental platform for solid phase concentration measurement of sparse gas-solid two-phase flow and a test bench for simulating primary air were designed and built The experimental platform can measure the ultrasonic attenuation coefficient under different particle size,ultrasonic frequency and solid phase concentration.The negative pressure design of the system makes the material unable to leak,and the purging ensures the cleanness of the ultrasonic transducer.The control system developed by LabVIEW make the automatic powder feeding and ultrasonic measurement of the experimental bench possible,which reduces the error of manual operation and improves the efficiency of the experiment.Pulverized coal is used as material in the test bench,and closed cycle design was adopted to ensure that pulverized coal cannot leak.The test bench was mainly used to verify the results of the experimental platform for solid phase concentration measurement of sparse gas-solid two-phase flow.(2)The main interference factors of the experimental platform were studied,including ambient temperature,humidity and gas-phase velocity.The principle of each factor was explored through designing relevant experiments,and the correction method was put forward based on that.The results show that temperature and humidity can change the characteristics of ultrasonic transducer and the attenuation of ultrasound in two-phase flow.The velocity of gas phase will change the direction of ultrasonic propagation,so that the ultrasonic signal that can be received by the receiving transducer will be reduced and the calculated ultrasonic attenuation will be increased.Through correction,the attenuation coefficients of ultrasound at different temperatures,humidities and gas velocities are basically the same.(3)A signal processing method based on ultrasonic echo was proposed,which discards the general method of subtracting the logarithm of the signal amplitude before and after attenuation.By dividing the first and second waveforms,the influence of background noise on the ultrasonic signal was effectively eliminated,and the ultrasonic attenuation coefficient in the two-phase flow of wind-powder was obtained.Compared with general method,the linear correlation coefficient between the calculated attenuation coefficient and the concentration was increased from 0.774 to 0.972.(4)The effects of ultrasonic frequency,wind-powder concentration and solid particle size on the ultrasonic attenuation coefficient were systematically studied.The variation characteristics of attenuation coefficient and concentration under different frequencies and particle sizes were obtained.A wind-powder concentration inversion algorithm based on experimental data of glass beads was proposed.An error analysis of the inversion was performed on particles having a particle size of 106 μm to 250 μm.Among them,when the particle size is 120μm and the solid phase volume concentration is between 0.65‰-1.43‰,the relative error of the concentration measurement is-5.35%-3.12%..(5)On the experimental platform of the air-powder system,the pulverized coal concentration of the air-powder system was measured by the sparse phase gas-solid two-phase flow solid phase concentration inversion algorithm,and the applicability of the algorithm in the detection of pulverized coal concentration was verified.The verification results show that the proposed method can be used to detect the pulverized coal concentration of the air-powder system with particle size of 120μm and volume concentration between 0.73-1.31‰,and the relative error of the detection is within-8.32-8.86%.更多还原