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Cryogenic Dielectrics and HTS Power Apparatus: Research at the University of Southampton
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Thermal Bubble Dynamics in Cryogenic Liquids under Non-Uniform Electric Fields on Superconducting Power Apparatus
No full textThermal Bubble Motion in Liquid Nitrogen under Non-Uniform Electric Fields
No full textThis paper describes an experimental study of the influence of a non-uniform electric field on bubble motion and behaviour in liquid nitrogen. The electric field effect on the bubble motion as it rises to buoyancy within applied dc electric fields is quantitatively investigated using a rod-plane gap. Thermal bubble motion and bubble collision with the plane electrode processes were observed in these experiments. The experimental results show that dc non-uniform electric fields have an obvious effect on bubble behaviour; bubbles move closer to the plane electrode away from a higher field region to a lower one, irrespective of field direction. Furthermore, the bubble dynamic mechanism has been analysed and explained. Results show that the gradient force greatly affects the dynamics of bubbles in the gap. In addition a set of differential equations which describe the motion of a spherical bubble in the rod-plane gap with voltage are given and solved numerically to determine bubble trajectory in liquid nitrogen. Compared to the experiment, the theoretical results show a satisfactory agreement
The Effect of an Electric Field on Behaviour of Thermally Induced Bubble in Liquid Nitrogen
No full textThe effect of a dc uniform electric field on the behaviour of bubbles in liquid nitrogen such as their growth, deformation and departure frequency has been experimentally investigated. The experiment was performed using a stainless steel mesh-plate electrode. The experimental results show that (1) prolate spherical bubbles are formed in the direction of electric field and bubble deformation increases with increasing applied voltage, (2) the bubble departure frequency decreases and the departure volume increases with increasing electric field strength, (3) any effect of the electric field polarity is not obvious. For clarifying these phenomena, the electric field distribution around a bubble has been simulated using the finite element analysis (FEA) method and the electric force acting on bubble calculated numerically. In addition, the effect of changes to thermal bubble behaviour on boiling heat transfer is also discussed. Obtained results may lay a foundation for exploring the mechanism of electrohydrodynamic (EHD) enhancement of boiling heat transfer of liquid nitrogen
Electric field effects on boiling heat transfer of liquid nitrogen
No full textThis paper describes an experimental study into the influence of a d.c. uniform electric field on boiling heat transfer characteristics of liquid nitrogen. The electric field effect on the onset of boiling, nucleate boiling curve, hysteresis phenomenon and critical heat transfer were quantitatively investigated using a mesh-plane electrode system. For this purpose, a novel copper block electrode with temperature measurement and vacuum heat insulation was designed and manufactured. Moreover, a heat transfer model based on this electrode geometry has been developed in order to provide some useful data for the electrode design and assist temperature calculations. Detailed heat transfer processes appearing on the copper surface were observed and monitored using a high-speed camera. The experimental results show that electric fields have an obvious effect on boiling heat transfer of liquid nitrogen
Modelling of Partial Discharge Activity in a Cavity within a Dielectric Insulation Material
No full textThe pattern of partial discharge (PD) occurrence at a defect site within a solid dielectric material is influenced by the conditions of the defect site. This is because the defect conditions such as its size and location determine the electric field distributions at the defect site which influence the patterns of PD occurrence. A model for a spherical cavity and ellipsoidal cavity within a homogeneous dielectric material has been developed by using Finite Element Analysis (FEA) software. The model is used to study the influence of different conditions of the cavity on the electric field distribution in the cavity and the PD activity. Also, experimental measurements of PD in spherical cavity and ellipsoidal cavity of different size within a dielectric material are detailed
Boiling heat transfer of liquid nitrogen in the presence of electric fields
No full textAn experimental study has been undertaken to determine the influence of a d.c. uniform electrical field on boiling heat transfer of liquid nitrogen (LN2). A copper block electrode with temperature measurement and vacuum heat insulation was designed and manufactured. The effects of electric fields on boiling hysteresis, nucleate boiling and critical heat flux have been analyzed and discussed. The results obtained may help the design of the LN2 related components for HTS device cooling and also provide an initial perspective on the possible improvements for cryogenic cooling of HTS equipment
A Model for Bubble Motion in Non-uniform Electric Fields
No full textfluids has found positive application for enhanced boiling heat transfer, for example in refrigerators. However the motion of bubbles in columns of dielectric fluid is also of interest where the presence of bubbles may adversely affect the breakdown performance of the liquid dielectric when employed as an electrical insulation. This is the case for superconducting power apparatus designs where liquid nitrogen is used as both coolant and electrical insulation. A model is presented for the bubble motion in a dielectric fluid under the influence of dielectrophoretic force. A model has been developed by analysis of the forces acting upon a bubble within a non-uniform electric field with rod-plane electrode geometry. This set of differential equations can be solved numerically to determine the bubble trajectory for a specified applied voltage. Model results are compared to experimental data for a range of voltages obtained in a column of liquid nitrogen
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