Characterization Of Steam Condensation And Isothermal Injection Flow In Trapezoidal Silicon Microchannels

With the growing trend in miniaturization of various thermal systems, the investigation of microscale fluid flow and heat transfer has been a hot research topic during the past decade. In particular, condensation heat transfer in microchannels has important applications to the design of compact and micro-heat exchangers for cooling of thermal devices. Our present understanding of mechanism and characteristics of condensation heat transfer in microchannels, however, is still in its infancy. Since surface tension dominates over gravitational forces in microchannels, condensation flow patterns in microchannels are greatly simplified as compared to those in macrochannels. For example, stratified flow induced by gravity in conventional channels does not occur in microchannels. Recently, it was found that the condensation flow patterns in microchannel include mist flow, annular flow, injection flow, and plug/slug flow. Of particular interest is the occurrence of the injection flow which does not exist in condensation in marcochannels. Furthermore, surface properties such as geometry, surface roughness and wettability become increasing important in condensation in microchannels as its diameter becomes smaller. Because of the significant differences in the relative magnitudes of gravity, shear and surface tension forces in conventional channels and microchannels, flow regime transitions in microchannels are considerably different from those conventional channels. The correlation equations for condensation in macrochannels, which did not take surface tension into consideration, is not applicable to condensation in microchannels. Thus, the extrapolation of correlations for condensation pressure drop and heat transfer obtained for large round tube to smaller diameters of non-circular channels will introduce substantial errors.The aim of this thesis is to study mechanisms of condensation in microchannels, with particular emphasis on flow regimes, pressure drop and heat transfer. A visualization study, using a high-speed CCD and a microscope, is conducted to observe steam condensation flow patterns and occurrence frequency of injection flow in microchannels. It was found that there are two types of injection flow patterns: bubbling and jetting. The occurrence frequency of injection flow is found to be increasing with mass flux of steam and decreasing with aspect ratio of microchannel section, condensation rate and hydraulic diameter of microchannels. The location of breakup point, which is the dividing point of annular flow and slug flow regimes in microchannels, moves downstream with increase of mass flux or decrease of condensation rate, and quality at this location decreases. This means that annular regime extends and slug flow regime shrinks.Experimental and numerical research on frictional pressure drop of deionized water in silicon microchannels with different aspect ratios are first conducted, which is followed by measurements of pressure drop in steam condensing in microchannels. It is found that pressure drop in the condensing flow decreases with hydraulic diameter of microchannels and increases with mass flux and quality. The existing correlations of pressure drop in mini- and macro-channels overestimate experimental data in microchannels. The data of condensation pressure drop in microchannels is correlated in the form of Lockhart-Martinelli correlation. The Chisholm constant C in the correlation is found to be increasing with the hydraulic diameter of microchannels. The controlling parameters of the Chisholm constant C are properly chosen based on a dimensionless analysis. A new correlation of Chisholm constant C is developed in terms of appropriate parameters by fitting the experimental data.For condensation heat transfer study, integrated Pt thermoresistors are fabricated by MEMS technologies on the wall of the microchannels. These Pt thermoresistors are used to measure the internal-wall temperature of microchannel directly in our experiments. This would reduce the estimated temperature on the wall in previous condensation experiments in microchannels where temperatures were measured by thermocouples embedded on the bottom of the microchannels. The condensation heat transfer coefficient is found to be decreasing with hydraulic diameter of microchannels and increasing with mass flux and quality. Semi-analytical method, based on thermal boundary layer theory of turbulent flow, is used to derive the condensation heat transfer coefficient of annular condensation flow, which is shown in good agreement with experimental data obtained in this thesis. The overestimation of correlations for pressure drop in conventional channels on that in microchannels results in certain overestimating of its corresponding correlations for heat transfer coefficient on heat transfer coefficient in microchannels are analyzed theoretically. This conclusion can be used to verify the consistency or validity of experimental results on frictional pressure drop and heat transfer coefficient in microchannels.Finally, in order to gain a deeper understanding of the occurrence of injection flow in condensing flow in microchannels, the formation of micro air bubble in the co-flowing of air and water in microchannels is investigated with the aid of a high-speed CCD and microscopy. The effects of geometrical parameters and volume flow rate of air/water on bubble formation frequency are investigated experimentally, and the transition curve and flow map of the two phase flow without phase change are illustrated. It was found there are two different bubble formation patterns: bubbling and jetting. Regression analyses were performed on data obtained for bubble formation frequency of these two formation patterns. Two kinds of bubble separation mode, breaking and non-breaking, are observed at T junction downstream. The regions of the two separation modes are distinguished, which are in good agreement with theory.The result of the present study not only contributes to the improved understanding of condensation in microchannels, it also provides data base for optimal design of micro-condensers for cooling of micro-devices in emerging technology.