Effect of boiling on deposition of metallic colloids

In this study relationship between heterogeneous nucleate boiling and deposition of metallic colloidal particles, popularly known as crud or corrosion products in process industries, on those heterogeneous sites is investigated. Various researchers have reported that hematite is major constituent of crud which makes it primary material of interest; however the models developed in this work are irrespective of material choice. Qualitative hypothesis on deposition process under boiling proposed by previous researchers have been tested, which lack to provide explanation for several physical mechanisms observed and analyzed. The quantitative model of deposition rate has been developed on the basis of bubble dynamics and colloid-surface interaction potential. Boiling from a heating surface aids in aggregation of the metallic particulates viz. nano-particles, crud particulate etc. suspended in a liquid which helps in transporting them to heating surfaces. Consequently, cluster of particles deposit onto the surfaces with various interactive forces, resulting in formation of porous or impervious layers. Deposit layer grows or recedes depending upon variation in interparticle and surface forces, fluid shear, fluid chemistry etc. This deposit layer in turn affects the rate of bubble generation, formation of porous chimneys, critical heat flux (CHF) of surfaces, activation and deactivation of nucleation sites on the heating surfaces. Several problems are posed due to the effect of boiling on colloidal deposition which range from, research initiatives involving nano-fluids as a heat transfer medium to industrial applications such as light water reactors. In this study, it is attempted to integrate colloid and surface science with vapor bubble dynamics, boiling heat transfer and evaporation rate.;Pool boiling experiments with dilute metallic colloids have been conducted to investigate several parameters impacting the system. The experimental data available in literature is obtained by flow experiments which do not help in correlating boiling mechanism with the deposition amount or structure. With the help of experimental evidences and analysis, previously proposed hypothesis for particle transport to the contact line due to hydrophobicity has been challenged. The experimental observations suggest that the deposition occurs around the contact line of bubble and extends underneath area of bubble microlayer as well. During the evaporation concentration gradient of a non-volatile species is created which induces osmotic pressure. The osmotic pressure thus developed inside the microlayer draws more particles inside the microlayer region or towards contact line. The colloidal escape time is slower than the evaporation time which leads to the aggregation of particles in the evaporating micro-layer. These aggregated particles deposit onto or get removed from the heating surface, depending upon their total interaction potential. Interaction potential has been computed with the help of surface charge and van der Waals potential for the materials in aqueous solution. Based upon the interaction-force boundary layer thickness which is governed by debye radius (or ionic concentration and pH), a simplified quantitative model for the attachment kinetics is proposed. This attachment kinetics model gives reasonable results in predicting attachment rate against data reported by previous researchers. The attachment kinetics study has been done for different pH and particle size for hematite particles. Quantification of colloidal transport under boiling scenarios is done with the help of overall average evaporation rate because generally waiting time for bubble at same position is much larger than growth time. In other words, from larger measureable scale perspective frequency of bubbles dictate the rate of collection of particles rather than evaporation rate during micro-layer evaporation of one bubble. The combination of attachment kinetics and colloidal transport kinetics has been used to make consolidated model for deposit amount prediction and is validated with the help of high fidelity experimental data.;In an attempt to understand and explain boiling characteristics, high speed visualization of bubble dynamics from single artificially large cavity and multiple naturally occurring cavities is conducted. Bubble growth and departure dynamics model is developed for artificially active sites and is validated with the experimental data. The variation of bubble departure diameter with wall temperature is analyzed with experimental results and shows coherence with earlier studies. However, deposit traces after boiling experiments show that bubble contact diameter is essential to predict bubble departure dynamics which is ignored previously by various researchers. The relationship between porosity of colloid deposits and bubbles under the influence of Jakob, Sub-cooling number and particle size has been developed. This further can also be utilized in variational wettability of the surface. Designing porous surfaces can have vast range of applications varying from high wettability such as high critical heat flux boilers to low wettability such as efficient condensers.