Author

Kevin Ng

Date of Award

9-2010

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Mechanical Engineering

Supervisor

J.S. Cotton

Language

English

Abstract

The objectives of this research is to determine the mechanisms involved in the development of electrohydrodynamic (EHD) flow structures, such as twisted liquid cones, twisted liquid columns and entrained liquid droplets, in a two-phase flow and to determine their impact on convective condensation heat transfer. EHD involves the application of high voltage to a dielectric fluid flow to induce electric forces that can create additional convection currents in single-phase and two-phase flows and also the redistribution of the different phases within the channel in a two-phase flow. The EHD phenomenon was investigated inside a smooth horizontal tube with a concentric rod electrode used to apply high voltage to a flow of refrigerant R-134a.

The EHD flow structures were initially observed to be a transient phenomenon that appeared during the initial step input of high voltage and were not present after steady state. Investigations to sustain the EHD two-phase flow structures using various ±8kV pulse width modulated waveforms (PWM) are performed for a range of mass flux between 45 to 110 kg/m^2 s and a quality of 50%, conditions which correspond to a stratified/stratified wavy flow pattern. The effect of sustaining flow patterns consisting of these flow structures on condensation heat transfer and pressure drop is measured. The purpose of this research is to evaluate the potential of sustaining specific EHD two-phase flow patterns as a means of enhancement and control of heat transfer and pressure drop for industrial heat exchange applications.

An evaluation of the mechanisms involved in the development of the EHD flow structures was first performed by determining the effect of applying positive and negative polarity high voltage to a single-phase flow to determine the mechanisms of charge injection in this particular geometry. A negative polarity high voltage applied to the concentric rod electrode was found to result in a larger heat transfer enhancement due to the charge injection occurring at the center electrode, whereas a positive polarity high voltage applied to the rod electrode resulted in negative charge injection at the grounded tube wall. This method of charge injection was used to explain the difference in the development of the twisted liquid cone and twisted liquid column flow structures that arise due to an applied positive and negative voltage respectively.

An analysis of the development and sustainability of the EHD two-phase flow structures using various PWM waveforms was investigated using a high speed camera to visualize the flow. An image analysis of these high speed videos determined the suitable pulse width, duty cycle and voltage polarity conditions for the development of the twisted liquid cones/columns and in sustaining the structures during the pulse ON period. The sustainability of these flow structures were determined to be mainly influenced by the charge distribution within the flow and a means to sustain these flow structures for a range of duty cycles between 10%-90% was found using waveforms consisting of both positive and negative high voltage pulses. For duty cycles of 100%, an inverse annular flow pattern was observed, where liquid is extracted and encircles the concentric rod electrode. For the entrained droplets produced using EHD, the required high voltage pulse conditions for the production of the droplets was determined and the effect of EHD on the coalescence of droplets was also observed.

Experiments were performed to measure the condensation heat transfer and pressure drop performance of flow patterns consisting of EHD flow structures. The results show that flow patterns consisting of different EHD flow structures exhibit different heat transfer and pressure drop characteristics. The heat transfer enhancement was determined to be a result of the extraction of liquid from the heat transfer surface and into the central core, the increase in convection within the phases and a disruption of the thermal boundary layer. The increase in pressure drop is due to the increase in frictional pressure drop between the liquid and electrode surfaces and an increase in the interfacial area resulting in greater liquid-vapour shear. The maximum enhancement of heat transfer and the maximum increase of pressure drop by using the EHD technique were determined to be 2.9-fold and 5.0-fold respectively. The results also indicate that EHD can be used to independently control the heat transfer and pressure drop as it was shown that for a fixed heat transfer or pressure drop performance, a range of corresponding pressure drop or heat transfer conditions can be achieved depending on the flow pattern established using PWM waveforms. The findings in this study show the promise in utilizing the EHD technique as an effective means of enhancement and control of heat transfer and pressure drop performance in advanced heat exchange systems.

McMaster University Library

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