The effects of electric fields on phase change processes in industry

This thesis describes an experimental and theoretical investigation of the effects of an electric field on two kinds of phase change process in industry: nucleate boiling in heat exchangers and crystallization from industrial melts. The first is called electrohydrodynamic (EHD) enhancement of nucleate boiling heat transfer at low superheat surfaces in heat exchangers. The major emphasis of the work has been to probe the compound effect of EHD and passive enhanced sur-faces on nucleate boiling. Field potential distribution for six different surfaces has been predicted and analyzed using a computational fluid dynamics (CFD) method. The analysis has shown that the shapes of passive enhanced surfaces potentially create considerable increases in local field gradient to produce such favourable con-ditions. The critical bubble radius, bubble nucleation rate and bubble movement in the presence of an electric field have been also studied and discussed. Equations are derived showing that a local electric field gradient will enhance nucleation rate and decrease critical bubble radius, both leading to improved heat transfer rate. In the mean time, experimental work for two typical low superheat surfaces, Hitachi Thermoexcel-HE and Wieland Gewa-T tubes was carried out using a single tube test rig. Furthermore, comparisons for current experimental results and those for other surfaces are made. Major improvements are shown in the performance of heat exchangers fitted with enhanced tube surfaces by applying up to 30 kV between those surfaces and a surrounding electrode. In parallel with this, the effect of an electric field on melt crystallization has been studied. The experimental samples were selected from industry; the simplicity of laboratory preparation, the dielectric properties and the polymorphic behaviour were typical and attractive for the research purpose. A large number of experiments under different conditions for the effects of an electric field on the crystallization of a dielectric with polymorphic behaviour and lower molten temperature has been carried out for the first time. The experiments are assessed by time-temperature measurement and differential scanning calorimetry (DSC), respectively. The results have shown that the crystallization of the melt on cooling is enhanced under the action of an electric field. For the effect of an electric field on the melt mixture, a novel approach of applying the Taguchi method to the experimental DSC results has been used. Also, a theoretical analysis for dielectric crystallization in the presence of an electric field has been carried out