Experimental Study On Modulated Phase Distribution Of Condensation In Tubes And On Surfaces

This doctoral dissertation involves condensation heat transfer in tubes and on surfaces.The phase distribution is the key factor for condensation heat transfer in tubes.However,the available condensation heat transfer enhancement techniques are similar to those for single-phase heat transfer enhancement,majorly based on extended surface area or vortex generation.It is necessary to develop new methods to enhance condensation heat transfer by modulating the phase distribution in tubes.On the other hand,the dropwise condensation on surfaces generates a huge number of droplets,which have a wide range of size distributes from nanometer to millimeter.The distances between neighboring droplets also vary too much.In fact,the droplet arrays with uniform size are more popular in medical,biological,electronic and optical engineering.Thus,it is meaningful to create droplet arrays by modulating the droplet distribution.In this study,as a functional material,mesh screen was used to modulate the phase distribution for condensation heat transfer enhancement in tubes and drop arrays generation on surfaces.Two types of experiments were performed to investigate the phase distribution modulation in tubes: the air-water two phase flow visualization,and the R245 fa condensation heat transfer performance.The visualization experiment shows that phase distributions of stratified flow and intermittent flow modulated by mesh cylinder inserts behave the characteristics and potentials to enhance condensation heat transfer.The cross section of tested tube is divided into an annular region and a core region by the mesh cylinder.For stratified flow,the hollow mesh cylinder sucks liquid from annular region to core region by capillary force.The liquid level drops in the annular region.Thus,the tube surface area exposed in gas phase is increased.For intermittent flow,the mesh cylinder prevents slug or plug from entering into core region by capillary pressure of interface in mesh hole.But liquid can flow through the mesh screen.As result,the elongated slug or plug is limited to flow in annular region with thinner liquid film on tube wall.The R245 fa condensation experiment verified the ability of phase distribution modulation to enhance heat transfer by mesh cylinder insert.The condensation heat transfer enhancement factors(EF)are in the range of 1.12 ~1.87.Due to the small surface tension of R245 fa and large tube diameter,wavy stratified flow and mist annular flow are the major flow patterns observed during the experiment.The mesh screen captures droplets entrained in the mist annular flow,increasing the vapor mass qualities in the annular region(xan)to enhance heat transfer.The smaller the mesh wire diameter,the larger capillary force is to spread liquid,increasing the droplet capture efficiency.The mesh screen insert reduces the liquid level of stratified wavy flow to enhance heat transfer.The smaller the mesh pore size(dp),the larger liquid suction capability from the annular region to the core region is to reach higher EF.What is more,the ratio of mesh pore size dp with respect to fractional hole area ?,determines the leakage mass fluxes Gleak through mesh pores,affecting the heat transfer enhancement magnitude.Heterogeneous surface is the key to modulate the condensed droplets.Two novel heterogeneous surfaces were prepared based on mesh screen and metal sintering process.The first method is to sinter mesh screen on copper surface.Non-uniform residue stress is generated along weft wires,with larger stress on weft wire top location than elsewhere.Oxidation of the sintered package forms micro pits with hydrophobic behavior on weft wire top location,due to the stress corrosion mechanism.Nanograsses elsewhere have superhydrophobic behavior.Thus,a heterogeneous surface is formed on the mesh screen.For the second method,mesh screen was used as a mask.The sintering of the combined package of mesh screen and copper block forms the welding junction arrays,which prevent the periodic local area from exposing in the chemical solution during the oxidation and polymeric modification.Thus,after separating mesh screen from copper block to expose the arrayed welding area,a heterogeneous surface with arrayed hydrophilic cavity on superhydrophobic nanostructures was generated.Cooling the heterogeneous mesh screen surface in wet air,drop arrays grow on weft wire top locations.The condensed droplet size can be controlled by substrate temperature,air humidity and cooling time.The weft wires were welded on the copper substrate to reach the similar temperate as the copper substrate.Alternatively,the warp wires did not contact the copper substrate,and there is a large conductive thermal resistance between warp wires and weft wires.Thus,the warp wires had higher temperature than weft wires,explaining overwhelming majority of condensed liquid generated on the weft wires instead of warp wires.Besides,the condensed liquid on weft wires moves automatically towards the top locations to form droplet arrays,due to the surface energy gradient.Cooling the heterogeneous surface on copper substrate in wet air,the drop dynamics is mainly dependent on the surface orientation.For horizontal surface,the self-propelled jumping drops generated on superhydrophobic nanograsses would fall back due to the gravity force.If a returning droplet impacts a main droplet on the hydrophilic cavity,the coalescence of droplets makes the main droplet larger to yield non-uniform size distribution.Besides,the coalescence induced droplet jumping will cause the defect droplet area.Several mechanisms including the domino effect,rotating effect and billiard effect,cause severe defect of droplet pattern.Therefore,it is impossible to prepare drop arrays on the horizontal heterogeneous surface.However,the self-propelled jumping drops depart from vertical surface without return.Thus,the main droplets on micro cavities are not disturbed to form perfect droplet arrays with uniform size.In summary,the mesh cylinder was successfully used to enhance condensation heat transfer by phase distribution modulation in tubes.The visualization experiment and theoretical study identified the heat transfer enhancement mechanisms for various flow patterns.The effect of mesh screen parameters such as pore size on the heat transfer was analyzed.The new method of condensation heat transfer enhancement is beneficial to reduce metal consumption and investment of condensers in organic Rankine cycles(ORC)systems.Thus,the study is meaningful to promote ORC systems utilizing in recovery of waste heat and exploitation of new energy sources.Based on mesh screen and metal sintering process,two novel methods of heterogeneous surfaces were proposed to generate drop array.The study revealed the self-organization mechanism of condensed drops on heterogeneous surfaces by phase distribution modulation.The study also discussed the effect of various factors on the controllable drop sizes.The heterogeneous surfaces and drop arrays can be prepared in large scale.Thus,this study indicates that the dropwise condensation,which previously only applied in heat transfer engineering,can form droplet arrays to extend many other industry applications such as biochemistry detection,optical and electronic devices fabrication,military stealth,etc.