PhD Dissertations

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    Effect of pH on particle agglomeration and radiative transfer in nanoparticle suspensions
    (2018-08) Al-Gebory, Layth Wadhah Ismael; Mengüç, Mustafa Pınar; Mengüç, Mustafa Pınar; Başol, Altuğ; Ertunç, Özgür; Koşar, A.; Şendur, K.; Department of Mechanical Engineering; Al-Gebory, Layth Wadhah Ismael
    Nanoparticle suspensions (NPSs) are solid-fluid mixtures where small dielectric or metallic particles (with sizes <100 nm) used in a base fluid. NPSs have unique and tunable thermo-optical properties, and for that reason they can be used extensively to improve the thermal efficiency of different systems where they show remarkable enhancement in heat transfer compared with those of a base fluid. The effectiveness of solar thermal systems used for photo-thermal energy conversion is measured by their ability of absorb radiative energy by the working medium; for such applications NPSs are much better choice than traditional fluids. NPSs have also been used in coatings as they can be tuned to improve or alter the appearance of an object, as radiative and optical properties play significant roles. Although NPSs are considered very promising for these applications, there is some concern about their stability and their long-term use. Particle agglomeration in NPSs remains one of the most important challenges faced in terms of their usage. In all of these applications, the pH value and its effects on the particle agglomeration may have significant impact on the nanoparticles stability behavior, and consequently on the radiative transfer of energy. Steric and electrostatic stabilization methods are among the two approaches used for particulate suspensions to avoid such problems. In thermal applications, especially in high temperature ones, electrostatic stabilization method is usually preferred. In this dissertation, both experimental and theoretical investigations were carried out to determine the stability and optical properties of individual (water/TiO2 and water/Al2O3) and hybrid (water/TiO2+Al2O3) nanoparticle suspensions. The experimental studies include the preparation, characterization, and optical property measurements of the nanoparticle suspensions. The impact of the electrostatic stabilization (zeta potential and pH values) on the size and structure of particles due to agglomeration behavior are explored. The particle size distribution and the average (effective) particle agglomerate size for the nanoparticle suspensions in different conditions (the pH and particle volume fraction) were measured by using the dynamic light scattering (DLS) technique. The effects of the different particle agglomerates under different pH values on the dependent and independent scattering and their boundaries are investigated and demarcated for different conditions, where the relationship between the distance between particle to particle surface and the incident wavelength for different particle types are explored. The effects of particle agglomeration (similar and dissimilar particle agglomerations), particle size distribution and their contributions to the radiative properties of the nanoparticle suspensions are determined using the UV-Vis spectroscopy technique. The numerical part included the study of the optical and radiative properties and thermal radiation transfer based on the average (effective) particle agglomerate size obtained from the experimental studies. The optical and radiative properties of nanoparticle suspensions are calculated based on the Lorenz-Mie theory applying the single-scattering approximation technique. The influence of the particle size distribution on the scattering coefficient of nanoparticle suspensions is studied theoretically to account for the effect of compact particle agglomerates. The thermal radiation transfer in the nanoparticle suspensions is assessed by solving the radiative transfer equation using the discrete ordinates method, where the volumetric radiative heat flux and the thermal flux efficiency are calculated. The results show the impact of pH value on the stability of individual and vi hybrid nanoparticle suspensions. The different particle agglomerate types, sizes, and shapes yield different behavior of suspensions, including their stability or sedimentation rates, which help formation of optically thicker media. Light scattering in such media is significantly different as a function of the proximity of particles to each other. If they are closer to each other roughly less than dominant wavelength of the radiation, then their behavior is defined as dependent scattering, which is explored in this study. It is shown that a significant enhancement in the radiative properties, specifically in the UV/Vis spectrum, can be observed , which has an important effect on the thermal radiation transfer of the incident solar radiation. The demarcation of dependent and independent scattering regimes is explained for the individual and hybrid nanoparticle suspensions based on their pH value. NPSs with different effective particle agglomerate sizes have a considerable effect on the volumetric radiative heat flux, where the losses in radiative energy were decrease in comparison to those of pure water. The results also show the effects of composite particle agglomerates in the hybrid nanoparticle suspensions on the radiative properties, which are produced from dissimilar suspended particles. The results of this dissertation show that the pH value has a dominant effect on the radiative transfer involving nanoparticle suspensions, compared to other parameters. Adjusting the pH value based on the isoelectric point of the nanoparticle is an efficient method when specific radiative properties are required for specific applications. Such impact of pH value on optical and radiative properties of NPSs is studied for the first time in the literature.