Investigation on the performance of nanofluids due to preparation effects and operational conditions.

Abstract

Nanofluids are advanced type of fluids that are produced by dispersing nanoparticles within a non-dissolving liquid. In heat transfer applications, these suspensions have shown to be superior to conventional heat transfer fluids in terms of thermal performance because of their enhanced effective thermal properties. The effective thermal conductivity of nanofluid depends on several factors, such as the preparation method employed, particles concentration, colloidal stability, thermal conductivities of both basefluid and solid particles used, … etc. Furthermore, the suspension effective thermal conductivity can only have a value within the range of the added nanoparticles (highest) and the hosting fluid (lowest) thermal conductivities. Thus, to obtain an optimum effective thermal conductivity for a certain mixture with minimum degradation in the aforementioned property, the nanofluid needs to be homogeneously dispersed while sustaining its short and long-term stability. This is one of the main challenges seen today with such type of advanced fluids. Moreover, the nanofouling effect associated with these suspensions in operational conditions is another important factor that needs to be focused on, as it tends to change the surface wettability behaviour depending on the fluid and deposited surface properties, and hence can increase or decrease the heat transfer performance of the system. To address the previous challenges, the thesis at hand investigates the effect of nanofluid fabrication approach on its stability and pH value, and explores the influence of deposited particles of similar surface materials on the wettability behaviour of the surface. In order to achieve this, a two-step controlled temperature approach was used to fabricate the nanofluids at different set of fixed temperatures using a bath type ultrasonicator. The as- prepared suspensions were then characterised in terms of changes in pH value and stability using a pH meter and the sedimentation photograph capturing method, respectively. In addition, an electron beam physical vapour deposition technique was used to form nanoscaled layers on surfaces of similar materials to the evaporant source, so that a reflection of the nanofouling build-up on surfaces can be obtained, after which the wettability was examined, through a goniometer device, by varying the extracted liquid conditions. The results have shown that increasing the nanoparticles concentration had caused the fluid alkalinity level to increase, while the rise in nanofluid sonication temperature had led to a decrease in its pH value, and vice versa. Furthermore, a general correlation was developed to predict the changes in pH value for similar fabricated suspensions, which illustrated an overall accuracy of ~92% in its prediction capability. The shelving-life evaluation of aluminium – water dispersion has showed that the nanofluids fabricated via the two-step controlled temperature approach at 30ᴼC had better short and long-term stabilities than the ones produced by the conventional method. Moreover, the wettability behaviour of aluminium surfaces was seen to depend on the deposited aluminium film thickness, surface characteristics, and water properties; but in general, the water of pH 7 has demonstrated a tendency to enhance the hydrophilicity of the surface, while water of lower and higher pH values were seen to have the opposite outcome. On the other hand, the wettability behaviour of copper or stainless steel surfaces has shown to greatly depend on the surface topographical structure compared to the attached liquid properties.