Research on Sand-dust Separation Technology Based on Gas-solid Two-phase Numerical Simulation

. The separation technology in the large-scale sand-dust environment ground simulation test system applicable to the environmental adaptability and reliability verification of aerospace electromechanical products is studied. The gas-solid two-phase numerical simulation method is adopted, and the possible cyclone separation, inertial separation methods are used to study the separation efficiency and regularity technology, which provides a basis for the separation design and test of the sand-dust environment simulation of large electromechanical products.


Introduction
In the ground simulation test system of large sand-dust environment, after the sand-dust particles are injected by means of gravity or gas-solid jet, the floating and migration of sand-dust are realized by the wind tunnel. The migration and distribution of sand-dust in the circulating wind are important factors influencing the performance of simulating system. In order to ensure the concentration control accuracy when the sand airflow hits the testing product, it is necessary to recover the sand-dust particles in the airflow downstream of the injection section and the test zone, to avoid the concentration increase caused by the dust particle circulation operation, and to reduce the erosion of internal moving parts such as fans by dust particle. So the space separation in the testing system is critical. Because it is difficult to realize the design and optimization of the separation method by means of direct solution, the gas-solid two-phase numerical simulation is used to study the separation efficiency and regularity of various separation methods that may be used.

Gas-solid two-phase mathematical model
There are generally two methods for solving multiphase flow: the Euler-Eulerian method and the Euler-Lagrangian method. Since the volume fraction of the particle phase in the gas stream is extremely small, the influence of the sand-dust on the air flow field is extremely small, so the DPM (Discrete Phase) model is used in this calculation. The gas (air) is calculated as a continuous medium, and its flow law is described by the two-phase coupled Navies-Stokes equation. The particles (sand-dust) are used as discrete media to describe the motion orbit of each single particle in the Lagrange coordinate system [1][2].
(1) Motion control equation of single particle The forces that a single particle moves in the sanddust separation flow field are: drag force, pressure gradient force, additional mass force, Basset force, Saffman lift, Magnus force and gravity. The equation of motion of a single particle can be directly from Newton's second law: In the above formulas, m p is the particle phase mass. Although the forces acting on the particles are quite complex, not all forces are equally important. In the gassolid two-phase flow, since the density of the gas is much smaller than the density of the particles, the buoyancy, pressure gradient force, and additional mass force are small in magnitude compared with the inertial force of the particles themselves, which can be ignored.
(2) Particle random orbit model The key of multiphase turbulence simulation is to determine the particle phase model. Due to the influence of turbulence, the fluid has pulsation velocity in different directions at different times. The position and velocity of the particles in flow field will change due to the pulsation velocity. Therefore, it is necessary to consider the influence of turbulent pulsation and adopt random orbit model. The random orbit model of the particle is: When the turbulence equation is a two-equation model, μ can be obtained by turbulent kinetic energy. The calculation formula is: Where ξ is a random number and -1≤ξ≤1, the pulsation velocity obtained by the above formula satisfies the Gaussian distribution. When the turbulence model is the Reynolds stress model, the turbulent pulsation velocity is directly obtained from the three normal stress components of the Reynolds stress.

Modelling and analysis of cyclone separation methods
Cyclone separation has always dominated in the machinery, metallurgy, chemical, petroleum and other industries due to its simple structure, low energy consumption and convenient maintenance. The principle is to use the centrifugal force of the airflow to pry the particles to the wall to realize the separation. In the separation process, it is necessary to consider the airflow, the characteristics of the particles themselves, and the complex interaction between the two phases, such as static electricity and adsorption. The internal flow of the cyclone separation is a complex three-dimensional strong swirl flow, which is accompanied by a local secondary flow such as longitudinal circulation, short-circuit flow, and back-mixing in addition to the main flow vortex [3][4][5]. The three sizes of sand-dust particle was simulated. The particle size of the sand-dust was 3μm, 8μm and 80μm respectively. The inlet gas flow rate and particle phase velocity were both 18m/s, and the outlet pressure was atmospheric pressure.

Track of sand-dust particle
In order to study the effect of the cyclone separation method, the particles of different diameters were simulated. The particle size of the particles was set to 3μm, 8μm, and 80μm, and the mass flow rate was 0.05 kg/s. It can be seen from the trajectories of three different particle sizes that when the particle size is 3μm, the particle follows the gas flow better, and some particles reach the taper of sand-dust outlet and then spiral upward with the internal ascending airflow to enter the exhaust pipe, some of the particles are also directly discharged from the sand-dust outlet. When the size of the sand-dust particles is 8μm and 80μm, the two large-diameter particles are smashed toward the wall due to the large centrifugal force, and are spiraled down to the sand-dust exit position along the wall surface. It can be seen the inertia of 80μm particle is larger and it is easier to maintain the original motion trajectory.

Distribution Cloud Map of Pressure & Velocity
The static pressure distribution of cyclone separation has good symmetry. It can be seen from the figure that the static pressure is larger at the side wall and decreases inward. At the central position (near the axis), the pressure is lower due to the core area in the vortex, where may even be negative pressure, and the stratification is more obvious. The internal flow field of the cyclone separation is a three-dimensional strong rotating turbulent flow field. The gas entering the cyclone separation is downward, and the external vortex flow is gradually formed. After reaching the bottom of the cone, the airflow turns upwards to form an upward internal vortex.

Modelling and analysis of inertial separation methods
The inertial separation method mainly includes a semicircular shape, a flat bottom U-shape, a triangular bottom U-shape, and a semi-circular bottom U-shape structure, wherein the latter two are provided with fins and flanges, and the triangular bottom edge and the fins are formed. 45°, the width and depth of the separating elements are 40 mm, the specific structure is shown in Figure 3

Geometric Model
The inertial separation works as follows: the airflow containing sand-dust enters the separator from the left inlet, passes through separation components with multiple impact, and the energy loses during the collision process. Most of the particles fall into the lower sanddust outlet under the influence of gravity to achieve gassolid separation. The four inertial separation methods are named semi-circular, flat bottom U-shape, triangular bottom U-shape and semi-circular bottom U-shape according to the configuration.

Numerical simulation results
The three sizes of sand-dust particle was simulated. The particle sizes of the sand dust were 80μm, 300μm and 850μm, the particle density was 1550 kg/m 3 , and the particle flow rate was 1e-5 kg/s. The inlet gas flow rate and the particle phase velocity were both 2 m/s, and the outlet pressure was atmospheric pressure.

Track of sand-dust particle
In order to study the inertial separation effect of different bottom U-shapes, numerical simulations were performed on particle diameters of 80μm, 300μm and 850μm respectively. The trajectories of different particles are shown in Figure 4. It can be seen that particles with size of 80μm, a part of which is blocked by the separation element, discharged from the lower outlet, and a large part of the flow enters the right end basin with the gas flow. The majority of the particles with size of 300μm and 850μm are blocked by the separating element and fall into the bottom outlet at the same flow velocity. In addition, comparing the three separation methods, it can be seen that the U-shaped separation method at the bottom of the plane has better separation effect on particles with a particle size of 300μm. (a) flat bottom U-shape particle size:80μm particle size:300μm particle size:850μm (b) triangular bottom U-shape particle size:80μm particle size:300μm particle size:850μm (c) semi-circular bottom U-shape Figure 4. Different trajectories of particle using U-shaped inertial separation method particle size:80μm particle size:300μm particle size:850μm The semi-circular inertial separation method is used to calculate the same boundary conditions. The particle diameters are also 80μm, 300μm and 850μm, which are shown in Figure 5. It can be seen that the semi-circular inertial separation has a better separation effect on sanddust with a particle size of 850μm while only a small fraction of the particles flow into the right region. For sand-dust with particle size of 80μm and 300μm, the separation effect is poor, and a large part of the particles flow into the right end region with the air flow.
By comparison with the three U-type separation methods, the semi-circular separation method has a poor separation effect on small-sized sand-dust particles. The reasons for the analysis are as follows: 1) The semi-circular separator has better conductivity to the airflow than the U-type separator; 2) The semi-circular separator has a smaller entrance boundary area than the U-shaped boundary area, so a considerable part of the sand-dust particles can't enter the separation element area, so that sufficient collision cannot be performed, and the total kinetic energy loss of the particle phase is small, resulting in poor separation effect. Figure 6 below shows the horizontal section pressure (left) and velocity map (right) of the above four separation elements. It can be seen that the airflow uniformity of the U-shaped separation at the bottom of the plane is better than other inertial separation methods, and the pressure drop is smaller.  The pressure in the semi-circular separator decreases from left to right, and the overall pressure loss is about 45 Pa. The pressure loss is significantly smaller compared with U-shaped. It indicates that the gas pressure loss of the semi-circular separator is smaller, and the flow conductivity is better. However, the uniformity of the velocity distribution is worse than that of the flat bottom slot type U-shaped separator.

Conclusion
The several separation techniques applicable to the aerospace electromechanical sand-dust ground environment simulation system are simulated. The separation efficiency and law under different separation conditions are compared and analyzed. The results show that the cyclone separation method can be used to separate sand with smaller particle size. The diameter of the particles suitable for separation is >8μm. The Ushaped and semi-circular inertial separation methods can be used to separate sand-dust with larger particle size. The diameter of the U-shaped separation suitable for separation is >300μm, and the semi-circular separation is suitable for the separation with the diameter of >850μm. Compared with the three bottom-side U-shaped inertial separation methods, the U-shaped separation with flat bottom type is better, while the semi-circular separator has better diversion effect and the overall pressure drop is smaller. The research results can provide technical support for the further development of large-scale sanddust ground environment simulation system.