Electrohydrodynamic Boiling Heat Transfer Enhancement at The Evaporator of Refrigeration Plants

This thesis describes experimental work on the effect of an electric field (known as the electrohydrodynamic, or EHD’ effect) on boiling heat transfer in multi-tube shell and tube flooded evaporators and its use in refrigeration plants. A detailed review of the past experimental and theoretical work of boiling heat transfer outside tube bundles is presented. A comprehensive review of the EHD effects on boiling heat transfer is also presented. A mathematical model of a practical refrigeration system was written and used to optimize the specifications of the flooded evaporator. The model was also used to predict the effects of enhancing the boiling heat transfer coefficient in the evaporator on the overall performance of the plant. A six tube three pass flooded evaporator was built and integrated with a water chiller. The facility was instrumented to enable the study of the performance of the refrigeration plant with and without the electric field applied on the shell side of the evaporator. An arrangement to bypass the compressor and the expansion device was made to facilitate the pool boiling experiments planned for this study. Pool boiling experiments for R12 and an R12/1.4% oil mixture without and with up to 25 kV were carried out. Very substantial augmentation of nucleate boiling heat transfer of R12 and R12/oil mixture was found especially at low degrees of superheat. The application of electric stress to a boiling heat transfer surface has also been shown to activate vapour generating nucleation sites at low degrees of superheat thus reducing boiling hysteresis. The construction of an experimental facility to study the effect of the presence of oil on the electrical behaviour of refrigerants is also presented in this thesis. Furthermore, the practical difficulties of incorporating electrohydrodynamics into a real refrigeration plant and the techniques developed for overcoming them are discussed.