The mitigation of crystallization fouling in a falling-film flow: A self-cleaning mechanism driven by flow and wetting behavior

Crystallization fouling (or scaling) on heat transfer surfaces occurs in a wide range of applications in modern industries, such as desalination, food processing, petroleum refinery, and so on. Negative consequences of scaling include deteriorated heat transfer performance due to the formation of the fouling layer, increased flow resistance, and performance/product line interruption due to the cleaning routine. During regular cleaning and pre-or-pro anti-scaling treatment, commonly utilized hazardous chemicals not only add operational costs but also leave a significant environmental footprint, which potentially imposes harmful influences on human health and marine life. Therefore, developing more effective, low-cost, environmentally sound methods for scale control is highly desired. In this work, we studied scale formation on both flat and horizontal tube surfaces under falling film flow conditions. An alternated hydrophilic-hydrophobic patterned surface has been designed to achieve spatial scale-controlling, which promotes nonuniform scale layer distribution, particularly a height step along the wettability boundary. As the height step in the scaling layer increases, both flow mixing and shear stress can be enhanced to the point that the scaling layer can be peeled from the surface, and self-cleaning is then achieved. Surface wetting, scale layer adhesion, and nonuniform scale layer-induced shear force can all contribute to the self-cleaning mechanism, which has been explored in both experimental and modeling studies. The effects of surface roughness, surface wettability pattern, operational hydro-chemical parameters, and exposure time on simultaneous scale deposition and removal were investigated by assessing the scale mass deposition, scale morphology and topography, and heat transfer performance. It is observed that surface roughness largely dictates the initial crystallization; less uniform and larger-size deposition primarily occurs with higher surface roughness, prone to wash-off by shear flow. With the ongoing deposition and removal during prolonged exposure, scaled surfaces were transformed with similar uniformly scaled topography and roughness, regardless of their initial roughness, losing the dictation of further scaling. A drastic drop and delay in scaling and significant heat transfer improvements were observed on both the wettability patterned flat surface and the round tube surface. The mass deposition on the flat surfaces is reduced by a maximum of 87% compared with the non-patterned baseline surface, while the heat transfer coefficient remains unchanged till the end of the test period, which drops by 50% for the baseline. For the falling film flow over the round tube, scaling layer was observed peeling off from the patterned surface after testing for 192 hours, and the heat transfer coefficient maintained a non-falling trend due to enhanced flow mixing, while the baseline tube heat transfer coefficient degraded to 77% of its initial performance at the end of 192 hours testing period. The interaction between surface wetting, heat transfer, and anti-scaling performance was also explored, which is necessary for further design optimization to cope with a wide range of operating conditions. By exploiting the effects of surface features on scale deposition and removal with shear force, this study introduces a novel “green” inline scale mitigation strategy with the potential as a resilient alternative to current scale control methods in practice. This study further provides guidance and recommendations to improve self-cleaning by patterning surfaces with the presented novel scale control method.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Zaman, Mohammad Arafat

Contributors dc:contributor

• Wang, Sophie
• Miljkovic, Nenad
• Feng, Jie
• Zhang, Yuanhui

• Crystallization fouling control
• Wettability pattern

Statement dc:rights

• Copyright 2024 Mohammad Arafat Zaman

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/129356

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/129356