Thermal bubble behaviour in liquid nitrogen under electric fields.
University of Southampton, School of Electronics and Computer Science,
This thesis describes thermally induced bubble behaviour changes in liquid nitrogen (LN2) under electric fields. Cryogenic liquids such as LN2 have been used not only as the coolant but also as the electrical insulator in superconducting and cryogenic
apparatus. However, bubbles are easily formed in these liquids by even moderate heating because of a narrow liquid temperature range and low boiling point. Bubbles
are considered to be one of the factors causing a reduction of the electrical insulation level. Consequently, bubble behaviour in electric fields is of great interest primarily in the study of prebreakdown and breakdown phenomena in the presence of thermally induced bubbles. In addition, a bubble can appear and its behaviour changes in a fluid under the influence of an electric field, and this is the main reason for boiling heat transfer enhancement which is related to thermal stability and heat transfer efficiency.
Studies including single bubble behaviour and bubble column behaviour as well as boiling heat transfer enhancement due to changes in bubble behaviour under electric fields using different electrodes have been completed. Free thermal bubble motion and related characteristics in LN2 under a conductor-plane electrode have been experimentally studied. A model for bubble motion in this non-uniform electric field has been developed and is described. Compared with theoretical results, the experimental measurements are in good agreement. An rimental study into the behaviour of thermal bubbles between two plane-plane inclined electrodes has been completed. Using a stainless steel mesh-to-plane electrode system, experimental
investigations have been carried out to study the effect of uniform dc electric fields on the behaviour of a single thermal bubble in LN2. Bubble characteristics such as bubble
growth, deformation and bubble departure frequency have been experimentally evaluated. Finally, the electric field effects on boiling heat transfer of LN2 have been experimentally assessed. The obtained data is applicable to the design of LN2 cooled
high temperature superconductor power apparatus for both coolant and electrical insulation issues.
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