| On account of the considerable heat and mass transfer characteristics,the vapor/gas-liquid flow has been widely applied in the nuclear reactor engineering,the chemical engineering and the aerospace engineering.Also,the phase distribution,the pressure drop,the heat transfer and the two-phase flow destabilization are muc h important for the economy a nd safety of the industrial equipments,which have been widely paid attention by researchers.Additionally,more and more narrow/mini channels with large specific surface area are applied to the practical engineering.In the narrow/mini channel,the surface tension and the channel restriction are much important.The bubble dynamic behaviors and the flow boiling phenomena are different from those in the conventional channel.However,the evolution mechanisms of the two-phase interface and its influences on the flow heat transfer and the two-phase flow destabilization characteristics are not clear.Especially for the channels with different sizes,the two-phase flow characteristics,the models of pressure drop,the flow boiling heat transfer,the bubble distribution and the flow destabilization have much potential for the improvement.In this study,the experimental investigations included the two-phase flow/boiling experiment in the single channel,the two-phase flow boiling experiment in the rectangular narrow channel,and the two-phase flow destabilization experiment in the channel with bypass,which were carried out with the deionized water as the working fluid.As for the adiabatic gas-liquid flow,it was found that the Taylor bubble interface morphology was significantly affected by the channel inner diameter and the gas flow rate.The Taylor bubble interface was very smooth and regular.As the channel inner diameter increased,the tail of Taylor bubble was broken due to the vortex and shear effect of wake flow,and this effect became intense with increasing the gas flow rate.Based on the above research results,a model was established to predict the flow pressure drop of the slug flow for the channels with different inner diameters(Din=1.00 mm ~ Din=6.00 mm).In this model,the effects of the surface tension and the interactions between two-phase were taken into account,and a good agreement was achieved with an uncertainty less than ±25%.As for the mini channel(Din=2.15 mm),once the ONB(Onset of Nucleate Boiling)appeared,the FDB(Fully Developed Boiling)and the onset of quasi-stable boiling rapidly appeared,and the flow pattern quickly transferred from the single-phase fluid to the slug flow.In addition,the quasi-stable boiling in the mini channel was caused by the cyclic evolution of vapor slug.A model based on the concept of Chen correlation was established to predict the flow boiling heat transfer coefficients in the channels with different inner diameters(Din=2.15 mm ~ Din=6.88 mm).In this model,the effects of the channel confinement(Nconf),the boiling intensity(Bo),and the interaction intensity between two phases(Xtt)were taken into account,and a good agreement was achieved with an uncertainty less than ±30%.As for the coalescence process of the vapor bubbles in the rectangular narrow channel,the trapping process was caused by the liquid film rupture and the surface tension existing in the newly generated vapor-liquid interface.As the surface tension existing in the triple-phase contact line was larger than the difference between the macrolayer and the surrounding bulk liquid,the macrolayer was inversely trapped into the surrounding bulk liquid,namely the breaking process.Meanwhile,the evolution process from the bubbly flow to the slug flow was dominated by the coalescence process of multiple vapor bubbles,and the macrolayer evaporation could contribute to the heat transfer capacity,especially could maintain the dominated heat transfer mechanism of the nucleate boiling even though the heating wall was covered by the large vapor slug.As for the bubbly flow in the the rectangular narrow channel,the densities of the contacting areas between the vapor bubbles and the heating wall were mainly affected by the nucleation rate and the coalescence rate of vapor bubbles.As for the equivalent diameter of the contacting area,a modified correlation based on the distribution function of Inverse Gaussian was established to predict the distribution parameters of ν and λ,and a good agreement was achieved with an uncertainty less than ±30%.Finally,this study investigated the influence mechanism of bubble dynamics and two-phase interface evolution on the flow instability,which took into account of the flow boiling state in the channel with bypass combined with the bubble dynamics theory.It was found that the two-phase flow instability occurred in the parallel channels with different inners diameters(Din=2.15 mm,Din=4.95 mm,Din=6.80 mm),and the corresponding flow patterns were typical annular flow.As for the channel with bypass(Din=2.15 mm),the surface tension and the restriction of the channel were much obvious;the flow pattern presented typical bubbly flow before the onset of oscillation,and the two-phase flow was in a stable state.Once the heat flux reached the onset of oscillation,the flow pattern rapidly transferred to the annular flow,and the two-phase flow instability phenomena quickly happened.Also,the backflow phenomena appeared due to the vapor phase congestion in the heating section.As for the channel with bypass(Din=6.80 mm),the flow pattern at outlet of the heating section presented the bubbly flow,the slug flow,the churn flow and the annular flow.Meanwhile,the two-phase flow gradually transferred from the stable state to the unstable state.A model was established to predict the Npch in the parallel channels with different inner diameters(Din=2.15 mm ~ Din=6.80 mm),which considered the confinement of the flow channel(Nconf),the surface tension and the inertia force(We),as well as the the system pressure(ρl/ρg).Finally,a good agreement was achieved with an uncertainty less than ±30%. |
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