半封闭与开放空间中大气扩散的观测及模拟研究

发布时间：2018-01-10 00:19

本文关键词：半封闭与开放空间中大气扩散的观测及模拟研究　出处：《兰州大学》2017年博士论文　论文类型：学位论文

【摘要】：危险物质意外(或有意)释放形成的非传统大气环境污染事件(例如,化工厂危险气体泄漏、恐怖分子的毒气袭击等)往往会造成严重的人员伤亡和巨大的经济损失。然而,相比于传统意义上的空气污染,我们对这类问题的研究和关注明显不够。因此,本论文围绕半封闭空间(如工业厂房、候机厅和地铁站等)和开放空间(如城市商业区、化工厂区等)两类不同场景下发生的非传统大气环境污染事件,以工业厂房和实际城市中实施的两次外场示踪试验作为基础,通过资料分析和数值模拟相结合的研究手段,揭示了两类不同场景下的流场和湍流以及污染物扩散特征,验证了计算流体动力学(CFD)模式对工业厂房内重气扩散过程的模拟能力,构建了能够模拟城区流场和扩散过程的单向耦合模式WRFFluent。研究得到的主要结论如下:(1)基于“室内危险气体扩散试验”的观测数据揭示了工业厂房内的风场、湍流和温度层结以及示踪物浓度的时空变化特征。结果显示,自然通风条件下厂房内的风速和湍流十分微弱,而强制通风能够显著增强厂房内的水平风速和垂直速度方差以及湍流动能。再者,白天厂房内为强的逆温层结所控制,下午2:00左右逆温强度(即温度垂直递减率)达到最大值-0.68℃/m,而且烟雾扩散试验显示这种稳定的热力结构会抑制污染物的垂直混合;夜间厂房内的大气层结为弱不稳定。此外,自然通风条件下重气(SF6)的垂直分布呈现出明显的分层特征,低层平均浓度约为高层的5倍,而强制通风条件下低层浓度降低,垂直分布趋向均匀。对于轻气(NH3)示踪试验,浓度的垂直分布主要取决于厂房内温度层结和强制通风作用的综合效应。(2)借助CFD模式Fluent模拟了厂房内的重气扩散过程,并且结合观测的SF6浓度数据对模式的模拟性能进行定量评估。结果表明,模拟的底层呼吸区浓度值与观测值吻合较好,其中standard k-ω和SST k-ω模型的模拟性能最好,模拟性能评估指标FAC2=78%(这说明有78%的浓度模拟值是位于观测值的0.5至2倍的区间内);Realizable k-ε次之(FAC2=72%);而standard k-ε最差(FAC2=43%)。但这四个模型对厂房内呼吸区上部重气扩散的模拟效果都不太理想,尤其是难以准确模拟浓度的剧烈波动特征。进一步使用验证的SST k-ω模型再现了室内重气扩散的时空演变特征,模式抓住了重气扩散的一些主要特征。最后,数值试验结果显示呼吸区平均浓度随着通风率的增加呈指数递减,这也就意味着增加通风率能够改善呼吸区的空气质量。(3)利用“城市-气象示踪试验”在榆中县城区的观测数据,分析了城区内的风和湍流特征、反照率和地表能量收支状况以及污染物的时空演变过程。结果显示,市中心城市冠层顶风速较城市边缘平均偏小2 m/s,而且城区内相邻各观测点的风速风向也存在较大差异;城区内湍流会随着边界入流风速的增大而增强,并且给出一些简单的参数化关系式用于估算湍流特征量;白天大气为中等不稳定层结,而夜间则接近于中性。其次,城市下垫面地表反照率日变化呈现非对称U型结构,日出日落时刻反照率较大,白天较小且接近于常数0.14。白天能量输送以感热为主,感热和潜热占净辐射的比例分别为23%和2%。值得注意的是白天热储量占净辐射的比例高达75%,而夜间两者大小相当。此外,城市冠层顶部的污染物输送和扩散中平流起主导作用,浓度高值区位于释放源下风方向±20°的扇形区域内,而且主导风向两侧的浓度分布接近于正态分布。而地面浓度的分布明显偏离正态分布。(4)将中尺度模式WRF与微尺度模式Fluent相结合,构建了单向耦合的WRF-Fluent模式,并且利用该模式对榆中县城区内的风场和污染物扩散过程进行精细化模拟。结果表明,考虑了随时间变化的边界条件和城市建筑结构的耦合模式WRF-Fluent可以较好地刻画复杂的城市特征流型(例如,分流、汇合、涡旋和峡谷效应等)。而且该模式也能够很好地捕捉到高架源排放情景下污染烟羽随时间的总体变化特征,但模拟的浓度高值中心位置和强度与实测结果之间还存在一定偏差。
[Abstract]:Hazardous substances (accident or intentional) release of the formation of non conventional atmospheric pollution events (e.g., chemical hazardous gas leak, terrorist gas attacks etc.) often cause heavy casualties and huge economic losses. However, compared to the traditional sense of the air pollution, we research and attention to this kind of problem obviously not enough. Therefore, this paper focuses on the semi enclosed space (such as industrial workshop, lounge and subway stations) and open space (such as city business district, chemical plant, etc.) of two different scenarios of non conventional gas environmental pollution incidents, two field tracer test in industrial plants and in the actual city the implementation of the research methods as the basis, through the combination of data analysis and numerical simulation, reveals the flow and turbulence and pollutant diffusion characteristics of two kinds of different scenarios, to verify the computational fluid dynamics (CFD) model The ability to simulate the diffusion process of heavy gas in industry building, construction of the main research conclusion one-way coupled model WRFFluent. can simulate the flow field and the diffusion process of the city are as follows: (1) based on the "indoor hazardous gas diffusion test" data reveals the industrial plant room in the wind field, the temporal and spatial variation characteristics of turbulence and temperature node and tracer concentration. The results showed that the wind speed and turbulence in the plant under natural ventilation condition is very weak, and the forced ventilation can significantly enhance the level of wind speed in the building and the vertical velocity variance and turbulent kinetic energy. Moreover, the day workshop for strong inversion control, inversion intensity (i.e. around 2:00 p.m. the temperature lapse rate) reached the maximum value of -0.68 DEG /m, and smoke diffusion vertical mixing test showed that the thermal structure of this stability will inhibit the pollutants; the plant within the atmosphere at night For the weak unstable. In addition, the natural ventilation conditions of heavy gas (SF6) vertical distribution shows obviously stratified features, the average concentration of low layer is about 5 times higher, and the lower concentration of forced ventilation conditions decreased, vertical distribution tends to uniform. For light gas (NH3) tracer test, the comprehensive effect of vertical the concentration distribution mainly depends on the workshop temperature stratification and forced ventilation. (2) using CFD mode Fluent diffusion process of heavy gas plant were simulated and the simulation performance of combined mode SF6 concentration observation data are quantitatively evaluated. The results show that the simulation of underlying breathing zone values are in good agreement with the observed concentration the value simulation performance of standard k- and SST k-. The Omega model best index FAC2=78% to evaluate simulation performance (which means 78% of the concentration of the simulation value is 0.5 to 2 times in the range of observations within); Realizable k- e times (FAC2=72%); Standard k- (FAC2=43%) was the worst. But the simulation results of the four models of heavy gas diffusion on the upper region of plant respiration are not ideal, especially the volatility characteristics it is difficult to accurately simulate the concentration. Further use of SST k-. Model validation reproduces the dispersion of the indoor air evolution model captures the some of the main features of gas diffusion. Finally, the numerical experiment results show that the breathing zone average concentration decreased exponentially with the ventilation rate, which means that the increase of ventilation rate can improve breathing zone air quality. (3) the city meteorological observation data - Tracer Test in Yuzhong County, analyzed the wind and the turbulence characteristics in the city, spatio-temporal albedo and surface energy balance and process for pollutants. The results showed that the canopy wind speed is the center of the city edge of the city average 2 m/s, and the city The wind in the adjacent observation points are quite different; in the city of turbulence will increase with the boundary inflow wind speed, and gives some simple parametric formula for estimating turbulent features; daytime atmosphere for secondary instability stratification, and the night is close to neutral. Secondly, ground surface city the diurnal variation of albedo under present asymmetric U type structure, sunrise and sunset albedo is large, the day is small and close to the constant 0.14. daytime energy to heat, sensible heat and latent heat of the net radiation ratio were 23% and 2%. respectively. It is worth noting that the heat in the day of net radiation reserves accounted for the proportion as high as 75%, and both the size of the night. In addition, the top of the canopy and the city transportation pollutant diffusion advection plays a dominant role, the concentration of high value sector release source area is located in the downwind direction + 20 degrees, but the dominant wind direction on both sides of the The concentration distribution near the Yu Zheng normal distribution. And distribution of ground concentration significantly deviate from the normal distribution. (4) the WRF mesoscale model and micro scale model combined Fluent, built a one-way coupled WRF-Fluent model, and the model of wind field and the pollutant of Yuzhong County in the diffusion process of the elaborate simulation results. That, considering the WRF-Fluent coupled model with time dependent boundary conditions and city building structure can well describe the city characteristic complex (e.g., shunt, confluence, vortex and Canyon effect). And the model is also able to capture well to the overall features of elevated emission scenarios of pollution plume over time but, there is a certain deviation between the high values of the simulated concentration center position and intensity and the measured results.

【学位授予单位】：兰州大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：X51

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