1. A gas-particle mixed flow numerical calculation method suitable for coarse particle flow is characterized by comprising the following steps:

step 1: based on Computational Fluid Dynamics (CFD) theory, introducing a void ratio alpha and a gas-solid exchange source item S_pEstablishing a gas phase continuous equation and a momentum equation, namely:

in the formula: tau is_gIs the gas viscous stress tensor, p_gIs gas density, u is gas velocity, g is gravitational acceleration, p is gas pressure;

step 2: calculating the gas phase volume fraction in each CFD grid unit by a method of extracting sample points, namely the void ratio alpha, and specifically comprising the following steps:

step 2-1: uniformly distributed sample points N are extracted for each particle unit_sample；

Step 2-2: each sample point is examined to determine the sample point S where the particle i is located in the CFD grid cell_i；

Step 2-3: calculating the volume fraction alpha of the particles in each CFD grid cell according to equation (3)_s；

In the formula, V_particleIs the particle velocity, V_fluidIs the gas velocity, n is the number of particles;

step 2-4: calculating the gas phase volume fraction alpha in the CFD grid unit by the formula (4);

α＝1-α_s (4)

and step 3: for the case that a single coarse particle occupies a plurality of CFD grids, based on a free flow drag force model, the drag force F borne by the single particle is calculated by adopting the formula (5)_d；

F_d＝0.5C_Dρ_gA_local|u_g-u_p|(u_g-u_p) (5)

In the formula, A_localIs the projected area of the particle in the CFD grid, u_gIs the gas velocity u_pIs the speed of movement of the particles, C_DIs the drag coefficient of single particle, Re_pIs the Reynolds number of the particle, d_pAnd μ represents the diameter and hydrodynamic viscosity of the particle sphere, respectively;

and 4, step 4: calculating gas-solid momentum exchange source term S in single CFD grid by formula (8)_p：

Wherein V is a single CFD mesh volume;

and 5: using a finite volume method and a second-order windward format discrete gas phase control equation, taking a calculation result of pure gas flow under the same discrete grid as an initial condition, and solving the formulas (1) and (2) by adopting a SIMPLE algorithm;

step 6: solving the particle motion by adopting a discrete unit method;

and 7: coupling the CFD and a discrete unit method DEM, iteratively calculating the corresponding physical quantity of the coarse grain flow to the starting time of the next CFD calculation time step in each CFD calculation time step, and stopping calculation until the ending time, wherein the specific steps are as follows:

step 7-1: initializing a flow field, wherein the time T is 0;

step 7-2: calculating a flow field to be converged by the CFD at the current time T;

and 7-3: judging whether the current moment is a termination moment, if so, finishing the calculation, and if not, performing the next step;

and 7-4: calculating drag force, calculating particle motion by adopting a discrete unit method, updating particle speed and position, and performing iterative calculation to a CFD time point;

and 7-5: calculating the momentum exchange source item and the gas phase volume fraction, and updating the moment: and T is T plus delta T, and the step 7-2 is carried out.

2. The method for calculating the gas-particle mixed flow numerical value of the coarse particle flow according to claim 1, wherein the step of solving the particle motion by using the discrete unit method comprises the following steps:

step 6-1: particle search, contact judgment, and calculation of correlation and physical quantity;

step 6-2: calculating a motion equation and updating unit physical quantity;

step 6-3: calculating equivalent physical quantity;

step 6-4: a time increment is calculated.

Background

The gas-particle mixed flow is embodied in natural phenomena such as avalanche, sand wind, dense fog, smoke dust and the like; and is widely used in industrial processes such as drying, spraying, pelletizing, combustion, gasification, and catalytic cracking. In recent decades, with the rapid development of computing technology and the continuous improvement of numerical methods, numerical methods based on Computational Fluid Dynamics (CFD) have become another important means for studying gas-particle mixed flow.

The traditional gas-solid two-phase flow numerical calculation method mainly comprises a two-fluid model and an orbit method, wherein the two-fluid model takes particles as pseudo-fluid, the motion of the particles is described by establishing three conservation equations of the pseudo-particle fluid, the method cannot describe the micro-dynamic information of the motion of the particles, and the method is widely applied to the numerical simulation of gas-solid flow in a fluidized bed; the trajectory rule tracks and researches the movement of the particle group, the movement track of the particle group can be given, and the method is suitable for the fine particle flow with the particle volume fraction of less than 10%. Compared with the traditional two-fluid model and orbit method, the numerical method based on the CFD-DEM can provide more abundant particle size information including the motion trail of particles, the collision among the particles, the stress condition of the particles and the like, however, for the coarse particle flow with larger particle diameter, the method has lower solving precision, and has defects on the CFD-DEM coupling model.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a gas-particle mixed flow numerical calculation method suitable for coarse particle flow, aiming at the condition that a single coarse particle occupies a plurality of grid units, and by establishing a suitable gas-particle coupling drag force model, the Computational Fluid Dynamics (CFD) is coupled with a Discrete Element Method (DEM); the method takes an Euler-Lagrange method as a frame, introduces void ratio and momentum exchange source terms into an N-S equation, disperses a solving equation through a finite volume method and a second-order windward format, and solves each physical quantity of a gas phase by adopting a traditional SIMPLE algorithm; simultaneously solving the stress and the movement of the particles by using a discrete unit method; the gas-particle interaction is solved through a momentum exchange source term and a coupling drag force model. The method tracks and researches the stress and the movement of each particle, can fully provide the micro-dynamics characteristic of the coarse particle flow, overcomes the defects of low precision and large error of the common CFD-DEM coupling method for solving the coarse particle flow, and fills the vacancy of the CFD-DEM coupling model in accurately calculating the coarse particle flow.

The technical scheme adopted by the invention for solving the technical problem comprises the following steps:

step 1: based on Computational Fluid Dynamics (CFD) theory, introducing a void ratio alpha and a gas-solid exchange source item S_pEstablishing a gas phase continuous equation and a momentum equation, namely:

in the formula: tau is_gIs the gas viscous stress tensor, p_gIs gas density, u is gas velocity, g is gravitational acceleration, p is gas pressure;

step 2: calculating the gas phase volume fraction in each CFD grid unit by a method of extracting sample points, namely the void ratio alpha, and specifically comprising the following steps:

step 2-1: uniformly distributed sample points N are extracted for each particle unit_sample；

Step 2-2: each sample point is examined to determine the sample point S where the particle i is located in the CFD grid cell_i；

Step 2-3: calculating the volume fraction alpha of the particles in each CFD grid cell according to equation (3)_s；

In the formula, V_particleAre in a granuleParticle velocity, V_fluidIs the gas velocity, n is the number of particles;

step 2-4: calculating the gas phase volume fraction alpha in the CFD grid unit by the formula (4);

α＝1-α_s (4)

and step 3: for the case that a single coarse particle occupies a plurality of CFD grids, based on a free flow drag force model, the drag force F borne by the single particle is calculated by adopting the formula (5)_d；

F_d＝0.5C_Dρ_gA_local|u_g-u_p|(u_g-u_p) (5)

In the formula, A_localIs the projected area of the particle in the CFD grid, u_gIs the gas velocity u_pIs the speed of movement of the particles, C_DIs the drag coefficient of single particle, Re_pIs the Reynolds number of the particle, d_pAnd μ represents the diameter and hydrodynamic viscosity of the particle sphere, respectively;

and 4, step 4: calculating gas-solid momentum exchange source term S in single CFD grid by formula (8)_p：

Wherein V is a single CFD mesh volume;

and 5: using a finite volume method and a second-order windward format discrete gas phase control equation, taking a calculation result of pure gas flow under the same discrete grid as an initial condition, and solving the formulas (1) and (2) by adopting a SIMPLE algorithm;

step 6: solving the particle motion by adopting a discrete unit method;

and 7: coupling the CFD and a discrete unit method DEM, iteratively calculating the corresponding physical quantity of the coarse grain flow to the starting time of the next CFD calculation time step in each CFD calculation time step, and stopping calculation until the ending time, wherein the specific steps are as follows:

step 7-1: initializing a flow field, wherein the time T is 0;

step 7-2: calculating a flow field to be converged by the CFD at the current time T;

and 7-3: judging whether the current moment is a termination moment, if so, finishing the calculation, and if not, performing the next step;

and 7-4: calculating drag force, calculating particle motion by adopting a discrete unit method, updating particle speed and position, and performing iterative calculation to a CFD time point;

and 7-5: calculating the momentum exchange source item and the gas phase volume fraction, and updating the moment: and T is T plus delta T, and the step 7-2 is carried out.

Further, the specific steps of solving the particle motion by using the discrete unit method are as follows:

step 6-1: particle search, contact judgment, and calculation of correlation and physical quantity;

step 6-2: calculating a motion equation and updating unit physical quantity;

step 6-3: calculating equivalent physical quantity;

step 6-4: a time increment is calculated.

The invention has the following beneficial effects:

aiming at the condition that a single coarse particle occupies a plurality of CFD grid units, the computational fluid dynamics is coupled with the discrete unit method by establishing a CFD-DEM coupling drag force model suitable for coarse particle flow, so that the interaction between gas and particles can be accurately described, meanwhile, the mutual collision between the particles can be considered by adopting the discrete unit method, the translation and rotation motion of the single coarse particle can be described, and the micro-dynamics characteristic of the coarse particle flow can be fully given. The method overcomes the defects of low precision and large error of the common CFD-DEM coupling method for solving the coarse particle flow, and fills the vacancy of the CFD-DEM coupling model in accurately calculating the coarse particle flow.

Drawings

FIG. 1 is a flow chart of an algorithm for processing a collision sequence for particle motion within a time step in the method of the present invention.

FIG. 2 is a graph of a standard Lapple cyclone model and grid in accordance with an embodiment of the present invention.

FIG. 3 is a flow chart of a discrete element method for calculating particle impact force in the method of the present invention.

FIG. 4 is a flow chart of CFD coupled DEM calculation in the method of the present invention.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

The invention provides a gas-particle mixed flow numerical calculation method suitable for a coarse particle flow.

The method for calculating the gas-particle mixed flow numerical value is applied to carry out numerical simulation on the gas-particle mixed flow in the standard Lapple cyclone separator, and comprises the following specific steps:

step 1: based on Computational Fluid Dynamics (CFD) theory, introducing a void ratio alpha and a gas-solid exchange source item S_pEstablishing a gas phase continuous equation and a momentum equation, namely:

in the formula: tau is_gIs the gas viscous stress tensor, p_gIs the gas density;

step 2: calculating the gas phase volume fraction in each CFD grid unit by a method of extracting sample points, namely the void ratio alpha, and specifically comprising the following steps:

step 2-1: for eachSampling points N uniformly distributed in each particle unit_sample；

Step 2-2: each sample point is examined to determine the sample point S where the particle i is located in a certain CFD grid cell_i；

Step 2-3: calculating the volume fraction alpha of the particles in each CFD grid cell according to equation (3)_s；

Step 2-4: calculating the gas phase volume fraction alpha in the CFD grid unit by the formula (4);

α＝1-α_s (4)

and step 3: for the case that a single coarse particle occupies a plurality of CFD grids, based on a free flow drag force model, the drag force F borne by the single particle is calculated by adopting the formula (5)_d；

F_d＝0.5C_Dρ_gA_local|u_g-u_p|(u_g-u_p) (5)

In the formula, A_localIs the projected area of the particle in the CFD grid, u_gIs the gas velocity u_pIs the speed of movement of the particles, C_DIs the drag coefficient of single particle, Re_pIs the Reynolds number of the particle, d_pAnd μ represents the diameter and hydrodynamic viscosity of the particle sphere, respectively;

and 4, step 4: calculating gas-solid momentum exchange source term S in single CFD grid by formula (8)_p：

And 5: carrying out CFD (computational fluid dynamics) grid division on a standard Lapple cyclone separator, as shown in figure 1, applying a finite volume method and a second-order windward-format discrete gas phase control equation, taking a calculation result of pure gas flow under the same discrete grid as an initial condition (initial value), and solving by adopting a SIMPLE algorithm;

step 6: creating columnar coarse particles, taking the dead weight, drag force and collision force of the particles into consideration, and solving the particle motion by adopting a discrete unit method, as shown in fig. 2 and 3, the specific steps are as follows:

step 6-1: particle search, contact judgment, and calculation of correlation and physical quantity;

step 6-2: calculating a motion equation and updating unit physical quantity;

step 6-3: calculating equivalent physical quantity;

step 6-4: calculating a time increment;

and 7: coupling the CFD and the discrete element method DEM, and iteratively calculating the corresponding physical quantity of the coarse particle stream, as shown in fig. 4, specifically including the following steps:

step 7-1: initializing a flow field, wherein T is 0;

step 7-2: calculating a flow field to be converged by the CFD at the current time T;

and 7-3: judging whether the current moment is a termination moment, if so, finishing the calculation, and if not, performing the next step;

and 7-4: calculating drag force, calculating particle motion by adopting a discrete unit method, updating particle speed and position, and performing iterative calculation to a CFD time point;

and 7-5: calculating the momentum exchange source item and the gas phase volume fraction, and updating the moment: and T is T plus delta T, and the step 7-2 is carried out.

The invention provides a gas-particle mixed flow numerical calculation method suitable for a coarse particle flow. By carrying out numerical simulation and experimental comparison on the gas particle mixed flow in the standard Lapple cyclone separator, the method disclosed by the invention can be used for accurately solving the gas particle mixed flow and giving an accurate interaction relation between gas and particles in the process of solving the coarse particle flow.