Thesis
Mobility of charge carriers, particle charging and electro-hydrodynamic processes in dielectric liquids and nanofluids
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2021
- Thesis identifier
- T16086
- Person Identifier (Local)
- 201566683
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- For over a century, different dielectric liquids are used as insulators and coolants in highvoltage systems. Currently, with stricter environmental regulations and developments of morecompact and elevated voltage apparatuses, the power and pulsed power industries requireenvironmentally friendly dielectric liquids with better insulation and cooling ability, andhigher adaptability. Among these liquids, ester liquids, including synthetic and natural esters,are introduced in the past decades: first synthetic ester liquids were introduced in the 1970sand now are utilised in high voltage power systems as liquid insulators. Apart from the puredielectric liquids, nanofluids, which are developed by adding nanoparticles into dielectricliquids, started to generate significant interest among researchers and practitioners in highvoltage technology. This is because the nanofluids may have greater dielectric strength andbetter heat conduction properties than pure dielectric liquids. However, physical mechanismswhich result in this potential increase in breakdown strength of nanofluids are not fullyunderstood and require further investigation.In the present work field, both the dielectric liquids and the nanofluids hosted by thesedielectric liquids were studied experimentally.Three types of dielectric liquids were studied in this thesis: mineral oil, synthetic ester, andnatural ester. The investigation is focused on the mobility of charge carriers in these dielectricliquids stressed with an external electric field with different magnitudes. The obtained resultsshow that the mobility of charge carriers in all tested dielectric liquids has the same order ofmagnitude under the same electric field. Thus, it is concluded that the space charge density inthe ester liquids, which have greater electrical conductivity than the conductivity of themineral oils, is significantly higher than that in the mineral oil.Nanofluids have been developed using these three types of dielectric liquids and two types ofnanoparticles: TiO2 and BN nanoparticles. This study included developing an analyticalmodel of field charging processes in nanofluids stressed with the external electric field andexperimental investigation of the mobility of charge carriers and electrohydrodynamic (EHD)behaviours in nanofluids. The analytical modelling is based on the Maxwell-Wagner relaxation theory. The surfacecharge distribution across the surface on a nanoparticle placed in an insulating liquid stressedwith a step external electric field has been analytically obtained. The obtained results showthat the surface charge density is governed by the electric conductivity and the dielectricpermittivity of the dispersed particles and the hosting liquid. Furthermore, the Coulomb forcebetween two particles immersed in a liquid was obtained analytically, enabling the analysisof the force acting between particles suspended in the host liquid, laying the foundation forfurther investigation of the EHD effects based on the experiment results.The experimental investigation of the EHD effects in the nanofluids demonstrated that boththe TiO2 and BN nanoparticles acquired a net negative charge in both ester liquids whenstressed with the external electric field. However, in the case of mineral-oil-based nanofluid,TiO2 the BN particles become only polarised under the action of the external electric field,leading to the formation of ‘particle chains’ in the host liquid. The appearance of the ‘particlechains’ observed in the experiments was explained by the mathematical model developed inthis work.The results obtained in this work will be of interest to researchers and practitioners workingin the field of insulation liquids and their practical applications in high voltage power andpulsed power systems.
- Advisor / supervisor
- Wilson, Mark
- Timoshkin, Igor
- Resource Type
- DOI
Relações
Itens
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PDF of thesis T16086 | 2021-11-19 | Público | Baixar |