Investigation On Heat And Mass Transfer Characteristics At The Interface Of Small Bubbles

The gas-liquid two-phase flow and its heat and mass transfer are extensively used in the fields of energy power,petrochemical industry and biomedicine and play a vital role.With the improvement of computing power and computational efficiency,computational fluid dynamics?CFD?method has been widely implemented in the scientific research and engineering optimization,and it also shows great advantages and potentials.However,the numerical research and application on the gas-liquid two-phase flow,such as boiling heat transfer,gas-liquid chemical reaction and absorption,have always been enormous challenges.In recent years,a series of advances have been made on the model of interface reconstruction and interfacial mass transfer which provide the possibility to study the complex multiphase flow numerically.At the same time,commercial CFD solver has been widely used and has played an important role in engineering and academic research because of its higher practicability.In order to promote the application and development of the numerical investigation on the complex gas-liquid two-phase flow,mathematical models for the heat and mass transfer of multiphase flow are established by using the custom algorithm based on the commonly used commercial CFD solver in the present study,and the numerical simulations on a variety of complex gas-liquid two-phase flow progresses including nucleate boiling,flow boiling,physical dissolution and chemical absorption are carried out.First,the boiling process of the cryogenic fluid and normal fluid are investigated experimentally.The heat transfer characteristic and bubble dynamics are studied,and the bubble evolution is found to be dominant during the heat transfer.In order to study the boiling process in detail,a comprehensive model applied to the VOF method is developed based on the CFD solver improved by implementing external functions.The comprehensive model includes the smoothed evaporation model,modified Height Function algorithm and the micro-layer model.The modified Height Function yields more accurate interface normal vector than the Youngs method and also presents smaller spurious velocities.Moreover,the Height Function shows good convergence with mesh refinement.In order to validate the comprehensive model,the growth of a spherical bubble in superheated liquid is studied for three kinds of working fluids,and the numerical results show very good agreement with analytical results.Afterwards,the nucleate boiling processes of water and R113 on a heated surface with constant superheat are investigated.Based on the numerical results,the evolution of bubble size and local heat transfer at the contact line are quantified.The bubble shapes predicted in the numerical investigation agree well with experimental results;however,the departure time is longer than the experimental result.By considering the heat transfer within the bubble,it is shown that the vapor phase in the vicinity of the heating wall is superheated with a homogeneous gradient.Moreover,a detailed insight into bubble dynamics and local phenomena affecting the heat transfer during nucleate boiling is discussed.However,due to the complexity of algorithm,the comprehensive model is only applied to two dimensional studies,and the restriction lead to deviations from the experimental data.Herein,we develop a contour based tracking method which can estimate the local curvature and normal vector by the iso-suface of?=0.5.The numerical results are compared with PLIC method and present better performance and more accuracy.Afterward,the method is used to study the flow boiling of liquid nitrogen in two dimensions.A significant increase of bubble growth rate after the bubble confinement is observed.In addition,the liquid film between the bubble and heated wall is found to play a very important role in the heat and mass transfer during the flow boiling.In order to further improve the computational efficiency of the numerical model,a phase change model based on the interfacial temperature gradients is developed and implemented in the three dimensional numerical study of heat and mass transfer of gas-liquid two-phase flow.The phase change model is derived from the jump condition at interface and the corresponding interfacial mass transfer is based on the net heat transfer across the interface.The phase change model is coupled with CLSVOF method and the numerical approach is first validated by the experimental and numerical results in the literature.A fabricated cavity in lieu of the traditional methods of seed bubble or local superheat is treated as the nucleation site,where boiling occurs exclusively and periodic stream of single bubble emerges.The transitions of the flow patterns from bubbly flow to slug flow and annual flow are observed,and the numerical wall temperatures agree with the experimental results.The growth rates of the radial diameter and axial length of vapor bubble are compared with the experimental data,and good agreement is found.Meanwhile,the local heat transfer coefficients are found to be significantly influenced by the flow patterns.The variations of the local vapor qualities at five representative locations along the flow direction are also studied quantitatively and the fluctuations are found to be consistent with the local flow patterns.Finally,we present a numerical attempt on flow boiling with two nucleation sites,which shows a good potential of the present numerical approach to deal with the complicated flow boiling process.The information presented here is very useful to the design and operation of the mini-/micro-reactors.Afterwards,the interfacial mass transfer model is modified and used to solve the dissolution process during the gas-liquid flow.The numerical method is also based on the VOF method and coupled with species conservation equation applied to multiphase flow.A series of benchmark tests for the numerical approach are also carried out and confirm the accuracy.Finally,the dissolution processes of CO₂ bubbles in ethanol and methanol are studied numerically and compared with the experimental results from literature,which shows good agreement.It is shown that the Taylor bubbles are formed at the mixed region and taking the bullet-shape,and the profiles of streamlines and the corresponding CO₂ concentration in the liquid slug present apparent vortices.The relationship between the mass transfer and the thickness of liquid film is studied quantitatively in a series of slices.It is shown that the mass transfer through the thin liquid film around the Taylor bubble is dominant during the dissolution while the dominance disappears with the thickening of the liquid film,and the mass transfer peaks at the top end of the caps within the cap region.The overall mass transfer coefficients are estimated by the bubble volume change and compared with the empirical correlations in the literature,and the results present that the volume change due to the dissolution can no longer be ignored.The chemical absorption of bubbles is much more complex than the process of pure physical dissolution,because there is a series of problems have to be dealt with including the chemical reaction,the interfacial mass transfer,the bubbles shrinkage and the influence of contamination\surfactant.In this study,the numerical model for above complex process is developed which can provide a better understanding of the complicated chemical absorption and is helpful for engineering design optimization.The numerical approach is first validated by a series of benchmark tests including the dissolutions of a static bubble and a rising bubble,and the numerical results for the concentration fields,the interfacial mass transfer coefficients as well as the drag coefficients are compared with the results from literatures and show very good agreements.Afterwards,the numerical method is utilized to study the chemical absorption process of a rising spherical CO₂ bubble in the alkaline solution of NaOH.The shrinkage of the bubble and the decrease of bubble velocity during the chemical absorption process are taken into account,and the accumulation of the contamination on the bubble surface is also considered based on the stagnant cap model.The profiles of the specie concentration are presented in detail,and it can be found that the cap angle and the flow separation significantly influence the distribution of the concentration layers.The local mass transfer rate and reaction rate around the bubble surface are studied quantitatively,and the local mass transfer is found to be dominated by the chemical reaction directly.In additional,the supplement of the hydroxyl ion significantly affects the mass transfer which is mainly determined by the local velocity.