Taylor Flow Absorption for Low Pressure CO2 in A Meandering Microchannel

Author:Liang Zuo Qing

Supervisor:ma xue hu

Database:Doctor

Degree Year:2018

Download:18

Pages:165

Size:15795K

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Microfluidic platforms have promising applications in gas-liquid mass transfer,heat transfer and chemical reaction,due to its great specific surface area,precise control of residence time,and stability of the transport process.The Taylor flow absorption for low partial pressure CO2 is very significant for the gas purification and CO2 enrichment of gas mixture.This dissertation focuses on the transport mechanism during low partial pressure CO2 Taylor flow absorption process in a meandering microchannel,based on experimental platform for determination of the micro-flow hydraulics and mass transfer performance.The hydraulic properties of the Taylor flow for six different gas-liquid systems with different surface tension and viscosity at low CO2 partial pressure were investigated,aiming at the bubble formation characteristics,evolution period and size regulation during the bubble formation process.The characteristics of bubble formation process and bubble formation mode transition in different gas-liquid phase systems were observed.It was found that the competition of inertial force,interface stress and viscous force greatly influenced the bubble formation mode,bubble/liquid slug length during the bubble formation.Considering physical properties of the liquid phase and cross-section configuration of the rectangular mcirochannel,novel scaling laws for bubble/slug length in terms of Garstecki-type model were respectively established whether for surface tension variation systems or viscosity variation systems.This work provided an extensive study for scaling the bubble/slug length in Taylor flow in a rectangular T-junction meandering microchannel,which plays an important role in the size regulation of the bubbles/droplets in the microchannel.Based on the physical model for liquid film distribution around Taylor bubble during the bubble transport process,a more applicable correlation for the liquid film thickness was proposed.The influnces of surface tension and viscous forces on the liquid film thickness were analyzed as well.Meanwhile,the transport properties of the bubble were also systematically examined,including the bubble velocity,the gas holdup and the two-phase flow pressure drop.The novel prediction equationcorrelation for two-phase Taylor flow pressure drop based on Kreutzer model has been proposed for various gas-liquid systems in this work.Here,the leakage flow was quantified in Taylor flow for the rectangular channel,and the effect of surface tension and viscosity on the leakage flow were also analyzed.And the predictions for the leakage flow were also made for various gas-liquid systems.The liquid volumetric mass transfer coefficients(kLa)of the physical absorption process for different gas-liquid phase systems for low partial pressure CO2 were respectively measured.It was found that the liquid volumetric mass transfer coefficient increased with the increasing of gas-liquid flow rate and decreased with the increasing of gas-liquid ratio,whether it is a surface tension variation system or a viscosity variation system.Moreover,the effect of surface tension on kLa was more significant at the fixed liquid velocity at low gas velocity,while the effect of viscosity on kLa was more remarkable at the fixed gas velocity and high liquid velocity.Since shorter bubble,longer the liquid slug,lower gas void fraction and higher two-phase flow pressure could result in a higher liquid volumetric mass transfer coefficient,the experimental results showed a higher kLa(~0.08/s)was obtained the surface tension vairiation systems than the viscosity variation systems(~0.03/s).The hydrodynamic characteristics of micro-flow modes,i.e.,the internal circulation of liquid slug,the secondary flow of curved channels and leakage flow on micro-mass transfer during physical absorption were analyzed.The novel semi-empirical model for the liquid volumetric mass transfer coefficient based on hydraulic properties of Taylor flow and micro-flow modes was raised,displaying favorable prediction performance.Additionally,on the basis of conventional empirical correlation for the liquid volumetric mass transfer coefficient,considering the effects of the basic hydraulic characteristics and mass transfer characteristics of Taylor flow,a new empirical correlation for the physical absorption process under low CO2 partial pressure in Taylor flow for the meandering rectangular microchannel was also obtained.It was found that chemical reaction systems in this work almost got the same hydraulic properties as the CO2-H2O system.It was also found that chemical absorption process can obtain a higher CO2 absorptivity(-85%)than that of physical absorption process(-60%).At the same time,it was found that when the liquid concentration is 5%,kLa decreased with the increase of the liquid velocity at a fixed gas velocity,while kLa increased with the increasing gas velocity at a fixed liquid velocity.Although the chemical reaction rate was much higher than the mass transfer rate,indicating that gas-liquid transfer was dominant in the chemical absorption process,the liquid volume mass transfer coefficient for chemical absorption(0.3-1.14/s)was still higher than that of physical absorption process(0.08/s).Based on the empirical model of physical absorption process,combined with the influence of chemical reaction on the absorption process,a novel empirical model for the liquid volumetric mass transfer coefficient under low CO2 partial pressure chemical absorption in meandering rectangular microchannel were obtained,demonstrating excellent prediction.