Technical note: Influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies

• Main Authors: G. Li, H. Su, U. Kuhn, H. Meusel, M. Ammann, M. Shao, U. Pöschl, Y. Cheng
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/2669/2018/acp-18-2669-2018.pdf

Coated-wall flow tube reactors are frequently used to investigate gas uptake and heterogeneous or multiphase reaction kinetics under laminar flow conditions. Coating surface roughness may potentially distort the laminar flow pattern, induce turbulence and introduce uncertainties in the calculated uptake coefficient based on molecular diffusion assumptions (e.g., Brown/Cooney–Kim–Davis (CKD)/Knopf–Pöschl–Shiraiwa (KPS) methods), which has not been fully resolved in earlier studies. Here, we investigate the influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies. According to laminar boundary theory and considering the specific flow conditions in a coated-wall flow tube, we derive and propose a critical height <i>δ</i>_c to evaluate turbulence effects in the design and analysis of coated-wall flow tube experiments. If a geometric coating thickness <i>δ</i>_g is larger than <i>δ</i>_c, the roughness elements of the coating may cause local turbulence and result in overestimation of the real uptake coefficient (<i>γ</i>). We further develop modified CKD/KPS methods (i.e., CKD-LT/KPS-LT) to account for roughness-induced local turbulence effects. By combination of the original methods and their modified versions, the maximum error range of <i>γ</i>_CKD (derived with the CKD method) or <i>γ</i>_KPS (derived with the KPS method) can be quantified and finally <i>γ</i> can be constrained. When turbulence is generated, <i>γ</i>_CKD or <i>γ</i>_KPS can bear large difference compared to <i>γ</i>. Their difference becomes smaller for gas reactants with lower uptake (i.e., smaller <i>γ</i>) and/or for a smaller ratio of the geometric coating thickness to the flow tube radius (<i>δ</i>_g ∕ <i>R</i>₀). On the other hand, the critical height <i>δ</i>_c can also be adjusted by optimizing flow tube configurations and operating conditions (i.e., tube diameter, length, and flow velocity), to ensure not only unaffected laminar flow patterns but also other specific requirements for an individual flow tube experiment. We use coating thickness values from previous coated-wall flow tube studies to assess potential roughness effects using the <i>δ</i>_c criterion. In most studies, the coating thickness was sufficiently small to avoid complications, but some may have been influenced by surface roughness and local turbulence effects.