Mobility of charge carriers, particle charging and electro-hydrodynamic processes in dielectric liquids and nanofluids

Student thesis: Doctoral Thesis

Abstract

For over a century, different dielectric liquids are used as insulators and coolants in high voltage systems. Currently, with stricter environmental regulations and developments of more compact and elevated voltage apparatuses, the power and pulsed power industries require environmentally friendly dielectric liquids with better insulation and cooling ability, and higher 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 1970s and now are utilised in high voltage power systems as liquid insulators. Apart from the pure dielectric liquids, nanofluids, which are developed by adding nanoparticles into dielectric liquids, started to generate significant interest among researchers and practitioners in high voltage technology. This is because the nanofluids may have greater dielectric strength and better heat conduction properties than pure dielectric liquids. However, physical mechanisms which result in this potential increase in breakdown strength of nanofluids are not fully understood and require further investigation. In the present work field, both the dielectric liquids and the nanofluids hosted by these dielectric liquids were studied experimentally. Three types of dielectric liquids were studied in this thesis: mineral oil, synthetic ester, and natural ester. The investigation is focused on the mobility of charge carriers in these dielectric liquids stressed with an external electric field with different magnitudes. The obtained results show that the mobility of charge carriers in all tested dielectric liquids has the same order of magnitude under the same electric field. Thus, it is concluded that the space charge density in the ester liquids, which have greater electrical conductivity than the conductivity of the mineral oils, is significantly higher than that in the mineral oil. Nanofluids have been developed using these three types of dielectric liquids and two types of nanoparticles: TiO2 and BN nanoparticles. This study included developing an analytical model of field charging processes in nanofluids stressed with the external electric field and experimental 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 surface charge distribution across the surface on a nanoparticle placed in an insulating liquid stressed with a step external electric field has been analytically obtained. The obtained results show that the surface charge density is governed by the electric conductivity and the dielectric permittivity of the dispersed particles and the hosting liquid. Furthermore, the Coulomb force between two particles immersed in a liquid was obtained analytically, enabling the analysis of the force acting between particles suspended in the host liquid, laying the foundation for further investigation of the EHD effects based on the experiment results. The experimental investigation of the EHD effects in the nanofluids demonstrated that both the TiO2 and BN nanoparticles acquired a net negative charge in both ester liquids when stressed 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 'particle chains' observed in the experiments was explained by the mathematical model developed in this work. The results obtained in this work will be of interest to researchers and practitioners working in the field of insulation liquids and their practical applications in high voltage power and pulsed power sys
Date of Award23 Aug 2021
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorIgor Timoshkin (Supervisor) & Mark Wilson (Supervisor)

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