Particle deposition in ventilation duct with convex or concave wall cavity

Abstract

A numerical study is carried out on particle deposition in ducts with either convex or concave wall cavity. Results show that, if compared with smooth duct, particle deposition velocities (V_d⁺) increase greatly in ducts with wall cavities. More specifically, for τ⁺<1, V_d⁺ increase by about 2–4 orders of magnitude in the cases with the convex and concave wall cavities; for τ⁺>1, V_d⁺ grows relatively slower. Enhancement of particle deposition with wall cavities is caused by the following mechanisms, i.e., interception by the wall cavities, expanded deposition area, and the enhanced flow turbulence. In general, addition of wall cavities is contributive for particle deposition, so it provides an efficient approach to remove particles, especially with small size, e.g., PM2.5. Moreover, the convex wall cavity leads to a larger increment of V_d⁺ than the concave wall cavity. However, taking pressure loss into account, though V_d⁺ is relatively lower, duct with the concave wall cavity is more efficient than that with the convex wall cavity.

摘要

本文采用数值模拟手段对带内凸肋板、外凹槽的通风管内颗粒物的沉降规律进行了研究。对于 0.1~50 μm 粒径的颗粒物, 采用修正的RSM 和DPM 模型能准确地模拟通风管道中颗粒物的沉降特性。 与光滑管道相比可知, 内凸、外凹槽通风管道中颗粒沉积速度(V_d⁺)随弛豫时间(τ⁺)或颗粒物粒径的增大而大幅度提高; 更具体来讲, 在τ⁺<1 的湍流扩散区和涡扩散碰撞区, V_d⁺ 可以增强 2~4 个数量级 并在τ⁺≈0.3 (dp=2 μm)时达到最大值, 增设凸肋板和外凹槽能分别使颗粒物沉降能量增强7770 和1320 倍。在τ⁺>1 的惯性力区, 颗粒物的运动主要受惯性力主导, V_d⁺ 增幅较缓。总体而言, 增设内凸、外 凹槽可以强化颗粒物的沉积, 且对小粒径颗粒物作用更显著, 因此, 这为减少PM2.5 (d_pm>2.5 μm)提 供了一种好的选择。此外, 相比于外凹槽, 增设凸肋板的通风管道近壁区空气流动更加复杂, 产生更 高的湍动能, 有利于颗粒物沉积, V_d⁺ 增幅更大。然而, 若考虑压力损失, 带有外凹槽的通风管道虽 然V_d⁺ 相对较低, 但效率更高。