fluent dpm(離散相模型)的壁面邊界條件類型介紹

2016-10-07 by:CAE仿真在線來源:互聯網

fluent dpm(離散相模型)的避免邊界條件類型用于描述離散相粒子與墻碰撞后發生的物理模型,在fluent中情況有好幾種:

圖:dpm避面類型

分別有:

1、reflect 反射——dpm顆粒到達后發生彈性碰撞,同時與避免發生熱量交換,離散粒子繼續運動,只是路線改變了

2、trap 捕獲,離散粒子【不再】繼續運動

3、escape 逃逸,消失了:如紗網壁面,如所有的inlet,outlet等

4、wall-jet 噴墻(反射和粘附并存,但不會形成膜)

5、wall-film 形成離散相物質的薄膜模型(墻膜),由于極薄的(最大50微米的)墻膜存在,離散相例子與這種墻撞擊后其反射和散射,傳熱等情況均有特殊考慮,如汽油剛體內部

6、user-defined 自定義

對離散相通過particle tracks 的track功能,可以輸出粒子的狀態情況,其中包括了上述的一些狀態在內,例如:

DPM Iteration ....
number tracked = 20, escaped = 1, aborted = 0, trapped = 19, evaporated = 0, incomplete = 0, incomplete_parallel = 0

24.4. Setting Boundary Conditions for the Discrete Phase

When a particle reaches a physical boundary (for example, a wall or inlet boundary) in your model, ANSYS Fluent applies a discrete phase boundary condition to determine the fate of the trajectory at that boundary. One of several contingencies may arise:

The particle may be reflected via an elastic or inelastic collision.

The particle may escape through the boundary. The particle is lost from the calculation at the point where it impacts the boundary.

The particle may be trapped at the wall. Nonvolatile material is lost from the calculation at the point of impact with the boundary; volatile material present in the particle or droplet is released to the vapor phase at this point.

The particle may pass through an internal boundary zone, such as radiator or porous jump.

The particle may slide along the wall, depending on particle properties and impact angle.

The particle may form a film (wall film model).

You also have the option of implementing a user-defined function to model the particle behavior when hitting the boundary. More information about user-defined functions can be found in the Fluent Customization Manual.

The boundary condition, or trajectory軌道 fate歸宿, can be defined separately for each zone in your ANSYS Fluent model.

For additional information, see the following sections:

24.4.1. Discrete Phase Boundary Condition Types

The available boundary conditions are

reflect

The particle rebounds off the boundary in question with a change in its momentum as defined by the coefficient of restitution. (See Figure 24.24: “Reflect” Boundary Condition for the Discrete Phase.)

Figure 24.24: “Reflect” Boundary Condition for the Discrete Phase

反射

The normal coefficient of restitution defines the amount of momentum in the direction normal to the wall that is retained by the particle after the collision with the boundary [109]:

(24–13)

where is the particle velocity normal to the wall and the subscripts 1 and 2 refer to before and after collision, respectively. Similarly, the tangential coefficient of restitution, , defines the amount of momentum in the direction tangential to the wall that is retained by the particle.

A normal or tangential coefficient of restitution equal to 1.0 implies that the particle retains all of its normal or tangential momentum after the rebound (an elastic collision). A normal or tangential coefficient of restitution equal to 0.0 implies that the particle retains none of its normal or tangential momentum after the rebound.

Nonconstant coefficients of restitution can be specified for wall zones with the reflect type boundary condition. The coefficients are set as a function of the impact angle, , in Figure 24.24: “Reflect” Boundary Condition for the Discrete Phase.

Note that the default setting for both coefficients of restitution is a constant value of 1.0 (all normal and tangential momentum retained).

trap

The trajectory calculations are terminated and the fate of the particle is recorded as “trapped”. In the case of evaporating droplets, their entire mass instantaneously passes into the vapor phase and enters the cell adjacent to the boundary. See Figure 24.25: “Trap” Boundary Condition for the Discrete Phase. In the case of combusting particles, the remaining volatile mass is passed into the vapor phase.

Figure 24.25: “Trap” Boundary Condition for the Discrete Phase

捕獲

escape
The particle is reported as having “escaped” when it encounters the boundary in question. Trajectory calculations are terminated. See Figure 24.26: “Escape” Boundary Condition for the Discrete Phase.

Figure 24.26: “Escape” Boundary Condition for the Discrete Phase

消失

wall-jet 噴射墻

這個模型類似于反射,但是有他特別的地方,即他根據粒子的運動特性,可能會產生反彈或不反彈而粘附在避免上。

適合于撞擊后不會形成膜的情況,如非常熱的墻,但不適合于形成的膜有顯著作用的情況。

The wall-jet boundary condition provides a range of rebound directions and velocities when a liquid droplet collides with a wall. It is suitable for situations where droplets impact a hot wall, where no liquid film is formed, but the droplets reflect or stick on the wall depending on their impact properties. The direction and velocity of the droplet particles are given by the resulting momentum flux, which is a function of the impingement angle, , and Weber number.

The wall-jet type boundary condition is appropriate適合 for high-temperature walls where no significant liquid film is formed, and in high-Weber-number韋伯數 impacts where the spray acts as a jet. The model is not appropriate for regimes where film is important (for example, port fuel injection in SI engines, rainwater runoff, and so on).

A more detailed description of underlying theory is available in Wall-Jet Model Theory in the Theory Guide.

weber-number:韋伯數代表慣性力和表面張力效應之比,韋伯數愈小代表表面張力愈重要,譬如毛細管.高韋伯數表示慣性力占主流

wall-film 撞擊后形成薄膜,墻膜

這個情況比較復雜,考慮了極薄的液態薄膜中的撞擊,以及產生的傳熱、散射、蒸發、剪力、薄膜分離等多種情況

Figure 16.3: Mechanisms of Splashing, Momentum, Heat and Mass Transfer for the Wall-Film

This boundary condition consists of four regimes: stick, rebound, spread, and splash, which are based on the impact energy and wall temperature. Detailed information on the wall film model can be found in Wall-Film Model Theory in the Theory Guide.

For a list of limitations that exist with wall-film boundary conditions, see Limitations on Using the Lagrangian Wall Film Model.

interior

This boundary condition means that the particles will pass through the internal boundary. This option is available only for internal boundary zones, such as a radiator or a porous jump.

It is also possible to use a user-defined function to compute the behavior of the particles at a physical boundary. More information about user-defined functions can be found in the Fluent Customization Manual.

Because you can stipulate any of these conditions at flow boundaries, it is possible to incorporate mixed discrete phase boundary conditions in your ANSYS Fluent model.

Discrete phase boundary conditions can be set for boundaries in the dialog boxes opened from the Boundary Conditions task page. When one or more injections have been defined, inputs for the discrete phase will appear in the dialog boxes (for example, Figure 24.27: Discrete Phase Boundary Conditions in the Wall Dialog Box).

Figure 24.27: Discrete Phase Boundary Conditions in the Wall Dialog Box

Select reflect, trap, escape, wall-jet, wall-film, interior, or user-defined from the Boundary Cond. Type drop-down list under Discrete Phase Model Conditions, as shown in Figure 24.27: Discrete Phase Boundary Conditions in the Wall Dialog Box. (In the Walls dialog boxes, you will need to click the DPM tab to access the Discrete Phase Model Conditions.) If you select user-defined, you can select a user-defined function in the Boundary Cond. Function drop-down list. For internal boundary zones, such as a radiator or a porous jump, you can also choose an interior boundary condition. The interior condition means that the particles will pass through the internal boundary.

If you select the reflect type at a wall (only), you can define a constant, polynomial, piecewise-linear, or piecewise-polynomial function for the Normal and Tangent coefficients of restitution under Discrete Phase Reflection Coefficients. See Discrete Phase Boundary Condition Types for details about the boundary condition types and the coefficients of restitution. The dialog boxes for defining the polynomial, piecewise-linear, and piecewise-polynomial functions are the same as those used for defining temperature-dependent properties. The applied wall heat transfer model assumes that a liquid is getting in contact with the wall. See Defining Properties Using Temperature-Dependent Functions for details.

24.4.1.1. Default Discrete Phase Boundary Conditions

ANSYS Fluent makes the following assumptions regarding boundary conditions:

The reflect type is assumed at wall, symmetry, and axis boundaries, with both coefficients of restitution equal to 1.0

The escape type is assumed at all flow boundaries (pressure and velocity inlets, pressure outlets, and so on)

The interior type is assumed at all internal boundaries (radiator, porous jump, and so on)

The coefficient of restitution can be modified only for wall boundaries.

24.4.1.2. Particle-Wall Impingement Heat Transfer
To enable the particle-to-wall heat exchange for the reflect, wall-jet, or wall-film boundary conditions: In the Wall dialog box, under the DPM tab, enable the Particle-Wall Heat Exchange option. The option is available only for wall boundary conditions and unsteady particle tracking when the energy equation is enabled.

Figure 24.28: The Wall Dialog Box: the Particle-Wall Heat Exchange Option

Note that when a particle is reflected from a wall with the wall film DPM boundary condition, the particle-to-wall heat exchange is calculated directly between the particle and the wall. Any wall film present is not taken into account.

The model is applied for all inert, droplet, and multicomponent particles impinging on the wall. The specific model is not applied for the splashed particles. When the wall film DPM boundary condition is active, the model is applied in the Rebound Regime, and it is assumed that the particles hit the wall irrespective of the presence of a film. In the Splashing regime, the mass fraction that is deposited mixes with any existing film, and the splashed particles retain the impinging droplet temperature.

For combusting particles, the wall heat transfer is calculated only if the Wet Combustion Model option is enabled in the Set Injection Properties dialog box, and the particle liquid fraction is nonzero, otherwise the Particle-Wall Heat Exchange option has no effect.

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