竖直矩形窄通道内PM_1颗粒沉积的数值模拟

Simulation of PM_1 particles deposition in the narrow vertical rectangular channel

采用数值模拟,在长为1 000mm,截面为20mm×20mm的竖直矩形窄通道内,研究不同温度场内PM1颗粒湍流工况下的沉积规律。对气相采用标准k-ε模型来预测通道内流场与近壁面区域的湍流变化,对PM1颗粒采用DPM(DiscretePhaseModel)离散相模型来跟踪颗粒的运动轨迹。在模拟计算前,对模型进行网格敏感性分析,确定网格节点数为180 000。结果表明,矩形通道内湍流扩散作用、热泳效应和二次流的共同作用使PM1颗粒在近壁面区域富集;矩形窄通道内的二次流使PM1颗粒随主流被"搬移"到拐角和靠近拐角的壁面处,并在此大量富集;在温度场内近壁面区域,热泳力是PM1在冷壁面上发生沉积的最主要因素,但随主流温度的增高,PM1颗粒布朗运动显著增强,致使其无规则运动和扩散加强,不利于形成稳定的富集区,减弱了PM1颗粒在冷壁面上的沉积。

This paper intends to make a numerical simulation to study the regularity of the sedimentation of PM1 particles under the turbulent condition at different temperatures in a narrow vertical rectangular channel.The channel is designed of 1 000 mm long,with the cross-section being 20 mm2.The gas phase should be adapted to the standard κ-ε model so as to be made to predict the flowing field in the channel and forecast the turbulence variation in the near-wall regions.As the PM1 particles can be used to the DPM（Discrete Phase Model） to trace the trajectory of the particles＇ movement,the model in Ansys Fluent can be made to follow the Euler-Lagrange approach.In this case,the fluid phase（i.e.the gas phase） can be treated as a continuum by solving the Navier-Stokes equations,while the dispersed phase can be solved by tracking the mass of particles through the calculated flow field.Since the dispersed phase can exchange the momentum,mass,the energy with the fluid phase（gas phase）,we can use the model for the grid sensitivity analysis to determine the grid nodes as 180 000 before it can be used for the formal simulation.The study helps us to find that since in the vertical narrow rectangular channel,the turbulent diffusion and the thermophoresis with the secondary flow can make the PM1 particles gathering in the channel near-wall regions,it is possible for the narrow rectangular channel of the secondary flow to make the PM1 particles move to the corner and near the corner of the wall with the mainstream assembled there.In the temperature field near the wall areas,it can be found that the thermophoretic force is the most important factor for the PM1 to deposit on the cold wall.With the rise of the main flow temperature,the random motion and diffusion of the PM1 particles tend to be intensified due to the significant intensification of the PM1 particles＇ Brownian motion,which is not conducive to form the stable gathering areas of PM1 particles in the near wall regions,on the contrary,it tends to weaken the deposition of PM1 particles on the cold wall.