【作者】 李建锋；

【导师】 吕俊复；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2006， 硕士

【摘要】 中储式煤粉锅炉系统中,三次风中含有一定量的超细煤粉,利用一般的分离方法已经很难对这种超细粉作进一步的分离。在本文中提出了利用高压离心排粉风机对三次风煤粉进行浓缩,根据煤种,富集后的超细煤粉作为再燃燃料以降低NOx排放,或者作为一次风以提高燃烧效率。本文利用9-26型高压离心风机试验以及数值模拟了排粉风机叶轮高速旋转时产生的强大离心力对这部分超细粉进行分离的效果。在实验过程中,将风机出口分成大小均匀的五个出口,利用空心玻璃微珠模拟煤粉,用布袋收集器来收集经过五个出口的玻璃珠以及通过确定每个出口的空气流量来计算出不同出口的玻璃珠平均浓度,从而可以得出风机出口的浓度分布。实验结果表明,在风机出口固相颗粒的浓度分布是不均匀的,其分布趋势是越往外侧,相应越高,五个出口中最高浓度可以达到平均浓度值的2-2.5倍;而最低的浓度仅有平均浓度的30%左右。但是在靠近蜗舌的区域却出现了一个浓度偏高区。为了全面研究风机的气固两相流动特性,本文利用Fluent对风机内部的气固两相流进行了数值模拟,与实验结果进行的比较表明模拟结果是可信的。计算表明,在风机出口的前后方向上固相颗粒浓度分布也不均匀:在风机出口的后侧附近区域浓度明显偏高,而在风机出口的前侧面附近,浓度却很低;浓度最大值出现在贴近风机出口外侧后侧的角落处。计算结果预示了排粉风机内部固相颗粒浓度分布规律、叶轮的易磨损部位以及磨损方式,比如叶片压力面进出口部位的滑擦磨损与中间部位的撞击磨损等,这与实际结果相符。根据实验与模拟结果,本文提出了一种更好地分离方法:即考虑将风机出口沿前后方向(轴向)与径向两个方向进行划分浓淡分离,按这种方式:如将风机出口分成面积相等的两个部分时,高浓度部分气流中所含固相可以占到固相总量的80%以上;而按高浓度部分中固相可以占到固相总量的50%来划分时,此时高浓度区面积仅占风机出口总面积的1/8,这为利用风机进行含粉气流的浓淡分离提供了一个很好的依据。利用Gambit建立了风机流场的计算平台,以此能够对风机结构改进作出性能预测。计算表明,通过在风机叶片通道内增加小叶片,可以改善风机叶片通道内流场,风机的流量最高可以提高接近5%,而出口全压平均提高近10%。更多还原

【Abstract】 In the in-direct mill system of a pulverized coal fired boiler, a certain amount of ultra-fine coal particles contains in the tertiary air and is difficult to be separated from the gas-solid flow by using the conventional separation methods. In this study, the enrichment of particulate concentration in the tertiary air is proposed, based on kinds of the coal, such that the tertiary air can be used to form fuel re-burning for NOx emission reduction or to be used as the primary air to increase combustion efficiency. A high speed, centrifugal dust exhaust fan (DEF) with high pressure head was also proposed to achieve the gas-solid separation in the tertiary air.Experimental and numerical studies were conducted on the separation efficiency of solid particles from the fine-particle-containing two-phase flow for a DEF with the model of 9-26. The outlet of the fan was in rectangular shape. During the experiments, it was evenly divided into five rectangular sub-outlets with equal area in the radial direction. Hollow glass beans were used to simulate the coal powder. The glass beans exiting from the sub-outlets were collected individually by the bag house type dust collectors. In the same time, the flow rate of the air stream from every outlet was independently measured. With the solid particle collection rate and the air flow rate, the local particulate concentration at each sub-outlet was calculated, and the distribution of the solid concentration at the entire outlet was obtained. The experimental results showed that the distribution of solid concentration in a cross-section at the DEF outlet was uneven. The closer to the outer side of the exit, the higher the particulate concentration. The maximum local particulate concentration was 2-2.5 times of the averaged one, while the minimum particulate concentration was only 30% of the averaged one. However, near the tongue of the volute, there was a high particulate concentration region.Numerical simulation of the internal two-phase flow inside the DEF was conducted by using the Fluent software. The simulation results agree wellwith the experimental data. Calculation further shows that particulate concentration distribution is uneven along the front-back (axial) direction of the fan exit. The concentration near the back wall of the outlet is remarkably higher than that in the region near the front wall. The maximum concentration of glass bean appears in the region near the sidewall of the rear section of the outlet. The simulation predicts the distribution of solid-phase concentration inside the fan, wearing position and manner of the impeller. The predictions are consistent with the practice.According to the results of experiments and numerical simulation, an advanced enrichment method was proposed, by considering the particulate concentration distribution not only in the radial direction but also in the front-back direction (i.e. axial direction). In general, about 80% of the particles in the incoming gas-solid flow can be separated into the outer half channel. The enrichment effect can be further improved since 50% of the particles can be collected in a small portion of the exit, counting 1/8 of the outlet crossing area. The practicability of the particulate enrichment of the tertiary air using the DEF method was validated.Additionally, a computational fluid dynamic (CFD) platform for the DEF simulation was developed based on the Gambit software, and used to improvement the fan performance. It wad found that placing a small vane in the vane channel can improve the flow characteristics in the vane channel, resulting in a mass flux increment up to 5% and a pressure drop reduction near 10% .更多还原