Mechanism Analysis And Numerical Studies On Interfacial Heat Transfer Of Pipe Flashing Process

Flashing phenomena are commonly encountered in many industrial and manufacturing scenarios,such as metallurgy,chemical industry,food processing and pharmacy,etc.Flashing flow is often characterized by violent mass,momentum,and heat exchange across the gas-liquid interface.Physical aspects involved in the flashing process include nucleation,bubble growth,coalescence,and breakup.These phenomena are strongly affected by interfacial heat transfer,which is characterized by two key parameters,i.e.,heat transfer coefficient（HTC）and interfacial area density（IAD）.Therefore,deep research on mechanism of interfacial heat transfer in terms of HTC and IAD can gain some insight into the kinetic behavior of gas and liquid phases,as well as into the theoretical investigation of flashing process.Besides that,it would be helpful in providing theoretical guidance for industrial application of flashing flows.The thesis focuses on HTC and IAD correlated with interfacial heat transfer models.Numerical simulation and theoretical analysis are carried out for the flashing flow.The main contents and conclusions are as follows:（1）Module development of two-fluid model for numerical simulation of flashing flow based on Open FOAM.A wall nucleation model which includes three sub-models,i.e.,departure diameter model,departure frequency model,and nucleation site density model,and two kinds of phenomenological heat transfer models,i.e.,conduction-convection model and conduction-convection-turbulence model are released in the HZDR-multiphase Euler Foam solver.（2）Analytical solution of interfacial local HTC for an ellipsoid bubble.Based on the momentum and energy equation,the analitical solution of interfacial local HTC for an ellipsoid bubble is achieved,and the distribution of local HTC along the interface of the ellipsoidal bubble is clarified:for spherical bubble,most of the heat transfer is done on the front side,and the local Nusselt number（Nu）decreases gradually from front to tail.The local Nu on the bubble waist increases with increasing aspect ratioc.（3）Direct numerical simulation（DNS）of interfacial heat transfer from a freely rising bubble.Fundamental governing equations describing the interfacial momentum and heat transfer are established.The gas-liquid interface is captured with Level Set（LS）method.A Proportional-integral-derivative（PID）bubble controller is utilized to balance the interfacial forces,and the size of the computational region and the mesh refinement is reduced,which improves the computational efficiency significantly.（4）Effect of bubble deformation on interfacial heat transfer model during flashing process.Based on the current DNS data,an energy-balanced method in estimating the total averaged Nu is proposed.The effects ofcon Nu under different flow conditions in terms of Morton number（Mo）are analyzed.The expression ofcis expressed in terms of Re and Mo:1+exp（-7.04146）Re^{0.24 162}M o^2.02664.The convection Nusselt number in the phenomenological models are corrected with the correction factor:f=1.04378-0.08178c+0.01362c²+0.01348c³,which is valid in the range:100￡Re￡500,1.10′10^-10￡M o￡8.05′10^-7,1￡c￡2.（5）Calculation of interfacial area density using interfacial area transport equation（IATE）.k_{i,m ax} andk_i,min are introduced to correct the algorithm of interfacial curvature,and the numerical stability for solving IATE is improved.Numerical simulation of flashing flow in a converging-diverging nozzle is carried out.It is found that,nucleation occurs at nozzle throat,the nucleated void fraction influenced by the pressure drop of the converging part and the inlet mass flow rate is therefore greatly affected.The axial profile of void fraction is well predicted by IATE and constant bubble number density（CBND）model.However,the mono-dispersed methods are failed to capture the distribution of void fraction along the radial direction.（6）Stability criteria for solving the interfacial area density using population balance model（PBM）in case of phase change.The coupling way between the nucleation source term and PBM is established based on the IAD conservation.A number density flux weighted average Courant number,C_CFL based on bubble growth/shrinkage rate and bubble size discretization,is derived.Then,a stability condition for solving PBM is proposed based on the relationship between theC_CFL and number of outer correctors under PISO and PIMPLE algorithm.（7）Calculation of interfacial area density with PBM.Numerical simulation of transient flashing pipe flow under pressure release is performed.It is observed that,the start and end of flashing corresponding to the pressure course as well as the evolution of void fraction can be accurately predicted by PBM up to a pressure level of1′10⁶ Pa.While the pressure increases to2′10⁶ Pa,the calculated void fraction is lower than the measured one due to the neglect of bulk heterogeneous nucleation.The bubble size distribution of PBM exhibits similar trend to that of the experimental data,but the Sauter mean bubble diameter is underpredicted,which is related to the coalsecence model in PBM.