Pool boiling in electrical systems

Thesis

This thesis presents the results of research into pool boiling in electrical systems, with particular application to stator end windings cooling. Pool boiling heat transfer characterisation of wire-wound motor windings geometry (MWG) surfaces was characterised, with MWG surfaces returning higher heat transfer even with area-compensation. Extant correlations were evaluated against both surfaces; the Rohsenow correlation with curve-fitting parameters was the best descriptor for both surfaces, whilst the Gorenflo-Kenning and unfitted Rohsenow provided good a priori predictive accuracy. Pool boiling of a heated vertical surface parallel to an unheated surface was shown to considerably enhance heat transfer coefficient at low heat flux, and considerably degrade heat transfer coefficient at high heat flux, when Bo < 0.58. Reducing liquid level below a similar level produced a marked deterioration in heat transfer from a horizontal surface. A semi-empirical model was developed to evaluate bulk void fraction for confined and unconfined mini-tube bundles. Heat transfer coefficient was shown to decline where void fraction exceeded a certain threshold. Exceeding the same bulk void fraction threshold anywhere in the column was shown to predict that dryout would occur first at the top tube in the column rather than at the bottom tube. The maximum power density of a mini-tube bundle was obtained around 30 MW m^-3. Bubble nucleator devices were shown to be effective at mitigating against bundle effect by activating dormant nucleation sites. These novel discoveries enable stator end windings geometry and packaging to be optimised to maximise pool boiling heat transfer, in turn allowing the design of machines with higher power density and efficiency and cooled by a system that has the potential for entirely passive operation.

Authors: Bartle, RS

• DOI: 10.5287/ora-8rrmbyojo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Nucleate boiling