In order to study the motion law of droplet flow under the airflow action of long-range air-blast sprayers,a CFD-based 3D model was established for the air-blast sprayer duct and its external airflow field,and the dis...In order to study the motion law of droplet flow under the airflow action of long-range air-blast sprayers,a CFD-based 3D model was established for the air-blast sprayer duct and its external airflow field,and the discrete phase model was introduced to simulate the motion of droplet flow in the airflow.The simulation data of the droplet flow trajectory,droplet flow parameters and droplet deposition were obtained by establishing the monitoring sections and bilateral coupling calculation in the airflow field.Results showed that gravity had an obvious effect on droplets and large droplets settled faster.Some of the larger droplets were formed by polymerization in droplets motion.The smaller droplets were transported further along with the airflow,and the long-range sprayer has a significant effect on the directional transport of small droplets.Besides,the spraying swath in the direction perpendicular to the range enlarged gradually with the increase of the spraying range.At the end of the range,the diffuse and drifting of the droplets were dominant.Given that the outlet airflow velocity of the sprayer duct was 25.01 m/s and the spray pressure 1.8 MPa,the maximum motion distances of aerosol,mist,fine mist and coarse mist in the airflow field were 18.5 m,19.5 m,17.5 m and 10.5 m,respectively.Droplet size and number as well as number density and volume density of droplet flow on all monitoring sections showed a regression function with changes in the distance of the spraying range.The simulation results of the model adopted in this paper were verified by Chi-square test between the simulation results of the droplet deposition and the spray measurement results.Research results provide a new method for the study of orchard air-blast spraying technology and references for the optimization of spraying technology.展开更多
Modelling and simulations are conducted on velocity slip and interfacial momentum transfer for super-sonic two-phase (gas-droplet) flow in the transient section inside and outside a Laval jet(L J). The initial velocit...Modelling and simulations are conducted on velocity slip and interfacial momentum transfer for super-sonic two-phase (gas-droplet) flow in the transient section inside and outside a Laval jet(L J). The initial velocity slipbetween gas and droplets causes an interfacial momentum transfer flux as high as (2.0-5.0) × 104 Pa. The relaxationtime corresponding to this transient process is in the range of 0.015-0.090 ms for the two-phase flow formed insidethe LJ and less than 0.5 ms outside the LJ. It demonstrates the unique performance of this system for application tofast chemical reactions using electrically active media with a lifetime in the order of 1 ms. Through the simulationsof the transient processes with initial Mach number Mg from 2.783 to 4.194 at different axial positions inside theLJ. it is found that Mg has the strongest effect on the process. The momentum flux increases as the Mach numberdecreases. Due to compression by the shock wave at the end of the L J, the flow pattern becomes two dimensionaland viscous outside the LJ. Laser Doppler velocimeter (LDV) measurements of droplet velocities outside the LJ arein reasonably good agreement with the results of the simulation.展开更多
The drying of aqueous poly(ethylene oxide) (PEO) droplet on a substrate at different temperatures was studied. It was found that the contact line receded when the substrate was at a temperature above 60 ℃. Differ...The drying of aqueous poly(ethylene oxide) (PEO) droplet on a substrate at different temperatures was studied. It was found that the contact line receded when the substrate was at a temperature above 60 ℃. Different nucleation behavior and surface profiles of PEO films were found in different droplets drying processes. The rheological properties of aqueous PEO solutions were studied to understand the mechanism of contact line recession and micro-flow in drying aqueous PEO droplets. It was found that at low temperature, the contact line was static because of great viscous stress; while at high temperature, it receded because of great Marangoni force and the decrease of viscous stress. It was indicated that Marangoni convection was inhibited by the outward capillary flow and viscous stress at low temperature, whereas it became dominant at high temperature. Two types of mechanism for surface profiles and nucleation of PEO film from drying droplets are proposed, providing a theoretical guide for polymer solution application in oil and gas foam flooding technology.展开更多
A combined experimental and numerical study is undertaken to investigate the hydrodynamic characteristics of singlephase droplet collision in a shear flow. The passing-over motion of interactive droplets is observed, ...A combined experimental and numerical study is undertaken to investigate the hydrodynamic characteristics of singlephase droplet collision in a shear flow. The passing-over motion of interactive droplets is observed, and the underlying hydrodynamic mechanisms are elucidated by the analysis of the motion trajectory, transient droplet deformation and detailed hydrodynamic information(e.g., pressure and flow fields). The results indicate that the hydrodynamic interaction process under shear could be divided into three stages: approaching, colliding, and separating. With the increasing confinement, the interaction time for the passing-over process is shorter and the droplet processes one higher curvature tip and more stretched profile. Furthermore, the lateral separation ?;/R;exhibits larger decrease in the approaching stage and the thickness of the lubrication film is decreased during the interaction. As the initial lateral separation increases, the maximum trajectory shift by the collision interaction is getting smaller. During the collision between two droplets with different sizes, the amplitude of the deformation oscillation of the larger droplet is decreased by reducing the size ratio of the smaller droplet to the bigger one.展开更多
In many applications, a moving fluid carries a suspension of droplets of a second phase which may change in size due to evaporation or condensation. Examples include liquid fuel drops in engines and raindrops or ice-c...In many applications, a moving fluid carries a suspension of droplets of a second phase which may change in size due to evaporation or condensation. Examples include liquid fuel drops in engines and raindrops or ice-crystals in a thunderstorm. If the number of such particles is very large, and, if further, the flow is inhomogeneous, unsteady or turbulent, it may be practically impossible to explicitly compute all of the fluid and particle degrees of freedom in a numerical simulation of the system. Under such circumstances Lagrangian Particle Tracking (LPT) of a small subset of the particles is used to reduce the computational effort. The purpose of this paper is to compare the LPT with an alternate method that is based on an approximate solution of the conservation equation of particle density in phase space by the method of moments (MOM). Closure is achieved by invoking the assumption that the droplet size distribution is locally lognormal. The resulting coupled transport equations for the local mean and variance of the particle size distribution are then solved in conjunction with the usual equations for the fluid and associated scalar fields. The formalism is applied to the test case of a uniform distribution of droplets placed in a non homogeneous temperature field and stirred with a decaying Taylor vortex. As a benchmark, we perform a direct numerical simulation (DNS) of high resolution that keeps track of all the particles together with the fluid flow.展开更多
The asymmetric breakups of a droplet in an axisymmetric cross-like microfluidic device are investigated by using a three-dimensional volume of fluid(VOF) multiphase numerical model. Two kinds of asymmetries(droplet lo...The asymmetric breakups of a droplet in an axisymmetric cross-like microfluidic device are investigated by using a three-dimensional volume of fluid(VOF) multiphase numerical model. Two kinds of asymmetries(droplet location deviation from the symmetric geometry center and different flow rates at two symmetric outlets) generate asymmetric flow fields near the droplet, which results in the asymmetric breakup of the latter. Four typical breakup regimes(no breakup, one-side breakup, retraction breakup and direct breakup) have been observed.Two regime maps are plotted to describe the transition from one regime to another for the two types of different asymmetries, respectively. A power law model, which is based on the three critical factors(the capillary number,the asymmetry of flow fields and the initial volume ratio), is employed to predict the volume ratio of the two unequal daughter droplets generated in the direct breakup. The influences of capillary numbers and the asymmetries have been studied systematically in this paper. The larger the asymmetry is, the bigger the oneside breakup zone is. The larger the capillary number is, the more possible the breakup is in the direct breakup zone. When the radius of the initial droplet is 20 μm, the critical capillary numbers are 0.122, 0.128, 0.145,0.165, 0.192 and 0.226 for flow asymmetry factor AS= 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5, respectively, in the flow system whose asymmetry is generated by location deviations. In the flow system whose asymmetry is generated by two different flow rates at two outlets, the critical capillary numbers are 0.121, 0.133, 0.145, 0.156 and 0.167 for AS= 1/21, 3/23, 1/5, 7/27 and 9/29, respectively.展开更多
In the framework of the two-continuum approach, using the matched asymptotic expansion method, the equations of a laminar boundary layer in mist flows with evaporating droplets were derived and solved. The similarity ...In the framework of the two-continuum approach, using the matched asymptotic expansion method, the equations of a laminar boundary layer in mist flows with evaporating droplets were derived and solved. The similarity criteria controlling the mist flows were determined. For the flow along a curvilinear surface, the forms of the boundary layer equations differ from the regimes of presence and absence of the droplet inertia deposition. The numerical results were presented for the vapor-droplet boundary layer in the neighborhood of a stagnation point of a hot blunt body. It is demonstrated that, due to evaporation, a droplet-free region develops near the wall inside the boundary layer. On the upper edge of this region, the droplet radius tends to zero and the droplet number density becomes much higher than that in the free stream. The combined effect of the droplet evaporation and accumulation results in a significant enhancement of the heat transfer on the surface even for small mass concentration of the droplets in the free stream.展开更多
基金The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China(Grant No.31671591)Guangdong Provincial Special Fund for Modern Agriculture Industry Technology Innovation Teams(2020KJ108)+1 种基金Guangdong Provincial Department of Agriculture 2018 Provincial Rural Revitalization Strategy Special Fund(YUE CAI NONG[2018]No.125)and Special Fund for the Construction of Modern Agricultural Technology System(CARS-27).
文摘In order to study the motion law of droplet flow under the airflow action of long-range air-blast sprayers,a CFD-based 3D model was established for the air-blast sprayer duct and its external airflow field,and the discrete phase model was introduced to simulate the motion of droplet flow in the airflow.The simulation data of the droplet flow trajectory,droplet flow parameters and droplet deposition were obtained by establishing the monitoring sections and bilateral coupling calculation in the airflow field.Results showed that gravity had an obvious effect on droplets and large droplets settled faster.Some of the larger droplets were formed by polymerization in droplets motion.The smaller droplets were transported further along with the airflow,and the long-range sprayer has a significant effect on the directional transport of small droplets.Besides,the spraying swath in the direction perpendicular to the range enlarged gradually with the increase of the spraying range.At the end of the range,the diffuse and drifting of the droplets were dominant.Given that the outlet airflow velocity of the sprayer duct was 25.01 m/s and the spray pressure 1.8 MPa,the maximum motion distances of aerosol,mist,fine mist and coarse mist in the airflow field were 18.5 m,19.5 m,17.5 m and 10.5 m,respectively.Droplet size and number as well as number density and volume density of droplet flow on all monitoring sections showed a regression function with changes in the distance of the spraying range.The simulation results of the model adopted in this paper were verified by Chi-square test between the simulation results of the droplet deposition and the spray measurement results.Research results provide a new method for the study of orchard air-blast spraying technology and references for the optimization of spraying technology.
基金Supported by the National Natural Science Foundation of China (No. 29876022) and Grant of State Key Laboratory of High Speed Hydrodynamics (No. 2007).
文摘Modelling and simulations are conducted on velocity slip and interfacial momentum transfer for super-sonic two-phase (gas-droplet) flow in the transient section inside and outside a Laval jet(L J). The initial velocity slipbetween gas and droplets causes an interfacial momentum transfer flux as high as (2.0-5.0) × 104 Pa. The relaxationtime corresponding to this transient process is in the range of 0.015-0.090 ms for the two-phase flow formed insidethe LJ and less than 0.5 ms outside the LJ. It demonstrates the unique performance of this system for application tofast chemical reactions using electrically active media with a lifetime in the order of 1 ms. Through the simulationsof the transient processes with initial Mach number Mg from 2.783 to 4.194 at different axial positions inside theLJ. it is found that Mg has the strongest effect on the process. The momentum flux increases as the Mach numberdecreases. Due to compression by the shock wave at the end of the L J, the flow pattern becomes two dimensionaland viscous outside the LJ. Laser Doppler velocimeter (LDV) measurements of droplet velocities outside the LJ arein reasonably good agreement with the results of the simulation.
文摘The drying of aqueous poly(ethylene oxide) (PEO) droplet on a substrate at different temperatures was studied. It was found that the contact line receded when the substrate was at a temperature above 60 ℃. Different nucleation behavior and surface profiles of PEO films were found in different droplets drying processes. The rheological properties of aqueous PEO solutions were studied to understand the mechanism of contact line recession and micro-flow in drying aqueous PEO droplets. It was found that at low temperature, the contact line was static because of great viscous stress; while at high temperature, it receded because of great Marangoni force and the decrease of viscous stress. It was indicated that Marangoni convection was inhibited by the outward capillary flow and viscous stress at low temperature, whereas it became dominant at high temperature. Two types of mechanism for surface profiles and nucleation of PEO film from drying droplets are proposed, providing a theoretical guide for polymer solution application in oil and gas foam flooding technology.
基金supported by the NSAF(Grants No.U1530260)the National Natural Science Foundation of China(Grant No.51306158)+1 种基金the Natural Science Foundation of Jiangsu Province,China(Grant No.BK20130621)the Special Program for Applied Research on Super Computation of the NSFC–Guangdong Joint Fund(the second phase)
文摘A combined experimental and numerical study is undertaken to investigate the hydrodynamic characteristics of singlephase droplet collision in a shear flow. The passing-over motion of interactive droplets is observed, and the underlying hydrodynamic mechanisms are elucidated by the analysis of the motion trajectory, transient droplet deformation and detailed hydrodynamic information(e.g., pressure and flow fields). The results indicate that the hydrodynamic interaction process under shear could be divided into three stages: approaching, colliding, and separating. With the increasing confinement, the interaction time for the passing-over process is shorter and the droplet processes one higher curvature tip and more stretched profile. Furthermore, the lateral separation ?;/R;exhibits larger decrease in the approaching stage and the thickness of the lubrication film is decreased during the interaction. As the initial lateral separation increases, the maximum trajectory shift by the collision interaction is getting smaller. During the collision between two droplets with different sizes, the amplitude of the deformation oscillation of the larger droplet is decreased by reducing the size ratio of the smaller droplet to the bigger one.
文摘In many applications, a moving fluid carries a suspension of droplets of a second phase which may change in size due to evaporation or condensation. Examples include liquid fuel drops in engines and raindrops or ice-crystals in a thunderstorm. If the number of such particles is very large, and, if further, the flow is inhomogeneous, unsteady or turbulent, it may be practically impossible to explicitly compute all of the fluid and particle degrees of freedom in a numerical simulation of the system. Under such circumstances Lagrangian Particle Tracking (LPT) of a small subset of the particles is used to reduce the computational effort. The purpose of this paper is to compare the LPT with an alternate method that is based on an approximate solution of the conservation equation of particle density in phase space by the method of moments (MOM). Closure is achieved by invoking the assumption that the droplet size distribution is locally lognormal. The resulting coupled transport equations for the local mean and variance of the particle size distribution are then solved in conjunction with the usual equations for the fluid and associated scalar fields. The formalism is applied to the test case of a uniform distribution of droplets placed in a non homogeneous temperature field and stirred with a decaying Taylor vortex. As a benchmark, we perform a direct numerical simulation (DNS) of high resolution that keeps track of all the particles together with the fluid flow.
基金Supported by Major State Basic Research Development Program of China(2012CB720305)the National Natural Science Foundation of China(21376162)
文摘The asymmetric breakups of a droplet in an axisymmetric cross-like microfluidic device are investigated by using a three-dimensional volume of fluid(VOF) multiphase numerical model. Two kinds of asymmetries(droplet location deviation from the symmetric geometry center and different flow rates at two symmetric outlets) generate asymmetric flow fields near the droplet, which results in the asymmetric breakup of the latter. Four typical breakup regimes(no breakup, one-side breakup, retraction breakup and direct breakup) have been observed.Two regime maps are plotted to describe the transition from one regime to another for the two types of different asymmetries, respectively. A power law model, which is based on the three critical factors(the capillary number,the asymmetry of flow fields and the initial volume ratio), is employed to predict the volume ratio of the two unequal daughter droplets generated in the direct breakup. The influences of capillary numbers and the asymmetries have been studied systematically in this paper. The larger the asymmetry is, the bigger the oneside breakup zone is. The larger the capillary number is, the more possible the breakup is in the direct breakup zone. When the radius of the initial droplet is 20 μm, the critical capillary numbers are 0.122, 0.128, 0.145,0.165, 0.192 and 0.226 for flow asymmetry factor AS= 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5, respectively, in the flow system whose asymmetry is generated by location deviations. In the flow system whose asymmetry is generated by two different flow rates at two outlets, the critical capillary numbers are 0.121, 0.133, 0.145, 0.156 and 0.167 for AS= 1/21, 3/23, 1/5, 7/27 and 9/29, respectively.
文摘In the framework of the two-continuum approach, using the matched asymptotic expansion method, the equations of a laminar boundary layer in mist flows with evaporating droplets were derived and solved. The similarity criteria controlling the mist flows were determined. For the flow along a curvilinear surface, the forms of the boundary layer equations differ from the regimes of presence and absence of the droplet inertia deposition. The numerical results were presented for the vapor-droplet boundary layer in the neighborhood of a stagnation point of a hot blunt body. It is demonstrated that, due to evaporation, a droplet-free region develops near the wall inside the boundary layer. On the upper edge of this region, the droplet radius tends to zero and the droplet number density becomes much higher than that in the free stream. The combined effect of the droplet evaporation and accumulation results in a significant enhancement of the heat transfer on the surface even for small mass concentration of the droplets in the free stream.