Direct Numerical Simulation Of Gas-liquid Two-phase Flows And Heat Transfer & Fluid Flows In Deep-penetration Laser Welding

Gas-liquid two-phase flows widely exists in material processing engineering A thorough study of the heat transfer and fluid flow mechanism in gas-liquid two-phase flows is of great theoretical and practical significance to understanding and improving the material processing technologies.Direct numerical simulation(DNS)of gas-liquid two-phase flows has achieved great success in the past few years due to its advantages of high accuracy and universality.However,gas-liquid two-phase applications in material processing usually involves complex flows,heat & mass transfer,phase transition,and complex interfaces phenomena,which result in great challenges for applying DNS in material processing.In this paper,several DNS methods of gas-liquid two-phase flows are deeply invesitgaed,the corresponding DNS models and solution procedures are studied.The corresponding DNS models are applied to the simulation of heat transfer & fluid flow in deep penetration laser welding.An incompressible two-phase flow DNS model is firstly developed and then solved based on the finite volume method in the paper.In order to verify the accuracy and validity of the numerical model,the paper validates and analyzes it with widely used flow benchmark tests.Numerical results show that the incompressible two-phase flow DNS model developed in the paper has high accuracy,strong numerical stability and good numerical convergence,which is suitable for DNS study of Gas-liquid two-phase flow in material processing.In order to solve the interface problems,the paper proposes a coupled THINC and LS method with high-order interface representation.The THINC/LS method can effectively improve the numerical accuracy of VOF calculation.At the same time,the method ensures rigorous conservation of LS fields by retrieving higher-order surface from smooth LS field in the form of higher-order polynomials.To verify the accuracy and validity of proposed THINC/LS method,the paper validates and analyzes the THINC/LS method with widely used interface reconstruction and advection benchmark tests.Numerical results show that the present THINC/LS method has great advantages,such as high-order curvature accuracy,sub-grid resolution,interface sharpness,geometric fidelity,easy programming and so on.In a word,the THINC/LS method is suitable for solving the gas-liquid two-phase flow problem in material processing.A two-phase heat transfer DNS model coupled with material phase change is developed and solved based on the finite volume method ro solve the heat transfer problem in material processing.The accuracy and validity of the two-phase heat transfer DNS model has been verified with widely used heat transfer benchmark tests.Numerical results show that the two-phase heat transfer DNS model coupled with phase change of materials present in this paper has high accuracy,strong numerical stability and good numerical convergence.It can accurately simulate the temperature changes caused by phase transformation of materialsThe numerical accuracy and robustness of the two-phase heat transfer and fluid flow DNS model is investigated by studing the realistic two-phase flow problems commonly seen in nature and engineering applications.Numerical results show that the two-phase heat transfer and flow DNS model established in this paper can not only accurately obtain the physical parameters of the two-phase heat transfer and flow process,but also effectively predict the gas-liquid two-phase flow process under other experimental conditions.The DNS application in deep penetration laser welding was carried out.Some special physical phenomena involving heat transfer and fluid flow effects in deep penetration laser welding was considered,and its accuracy and effectiveness were investigated.Furthermore,the effects of different model parameters,physical factors and material properties on the heat transfer and fluid flow are discussed.Numerical results show that the thermal capillary force and the recoil pressure are the key factors causing the instability of free surface.Surface tension and viscous force can improve the stability of free surface.Increasing the thermal conductivity of the gas phase or the viscosity of the liquid phase are beneficial to maintain the keyhole stability.These conclusions provide important theoretical basis and data support for the study of two-phase heat transfer and flow DNS in material processing.Based on the background of gas-liquid two-phase flow in material processing and aimed at the breakthrough of DNS indeep penetration laser welding a,this paper has studied DNS technology of gas-liquid two-phase flow and heat transfer & fluid flow in deep penetration laser welding in detail.The results and analysis in this paper provides useful basis for the successful application of two-phase DNS technology in material processing.