COMSOL 6.3 - Resin Transfer Molding of a Wind Turbine Blade

Resin Transfer Molding of a Wind Turbine Blade

Resin Transfer Molding (RTM) is a manufacturing process for composite structures. The resin is injected under pressure into the mold cavity. Reinforcement materials like fiber structures can be placed into the empty mold before the resin is injected, which makes it easier to use any kind of reinforcement material and orientation. After the resin has been injected the curing takes place.

To avoid air bubbles that might be trapped within the resin several vents are placed at certain positions of the mold. Simulation can be used to optimize the vent positions.

In this example the RTM process of a wind turbine blade is investigated. The model geometry and setup was inspired by Ref. 1.

Model Definition

The blade consists of five parts of different composites, which have different anisotropic permeabilities. The colors in Figure 1 indicate the different permeabilities. The blade has a thickness of 1 cm. The geometry was constructed using COMSOL Multiphysics and the Design Module. However, in this example we just import the ready-made geometry file.

Figure 1: Geometry of the wind turbine blade. The colors indicate the different porous materials and different permeabilities.

Initially, the blade is filled with air. Resin is injected via the cylindrical inlet pipes at the top of the blade with a pressure of 800 kPa. The air can escape through the outlets which are defined at both short ends of the blade and at four additional vents along the blade rim which are marked by the outgoing arrows in Figure 1. All other boundaries are assumed as solid walls and the No Slip wall condition applies.

The model is set up using the combined multiphysics interface Two Phase Flow, Level Set, Brinkman Equations.

Model Equations

The flow through the porous media within the mold (due to the reinforcement material) is described by the Brinkman Equations for incompressible flow:

(1)

In these equations, where the inertial term has been neglected, μ (SI unit: kg/(m·s)) is the dynamic viscosity of the fluid, u (SI unit: m/s) is the velocity vector, ρ (SI unit: kg/m3) is the density of the fluid, p (SI unit: Pa) is the pressure, εp is the porosity, and κ (SI unit: m2) the permeability of the porous medium.

For the two-phase flow level-set method, an equation for the level-set function ϕ, which describes the interface between the two phases, is solved:

(2)

In this equation, ϕ is a smoothed step function which is zero within one phase and one within the other, ε denotes the porosity, γ determines the amount of reinitialization, and εls describes the thickness of the interface.

Beside defining the interface between the two phases, the level-set function is used in the multiphysics coupling feature node to smoothen the density and viscosity jumps across the interface through the definitions

(3)

Table 1 lists the parameters that are used in the model.

Table 1: Material properties.
Material property	Value
Air Density: ρair	1 kg/m3
Dynamic Viscosity of Air: μair Resin Density: ρresin Dynamic Viscosity of Resin: μresin	10-5 Pa·s 1250 kg/m3 0.195 Pa·s
Porosity, Material 1: ε1 Porosity, Material 2: ε2 Porosity Material 3: ε3 Permeability, Material 1: κmat1,ii Permeability, Material 2: κmat2,ii Permeability, Material 3: κmat3,ii	0.45 0.5 0.5 2.9·10-10, 8·10-11, 2.9·10-10 m2 2.5·10-10, 7·10-11, 2.5·10-10 m2 1.7·10-10, 8·10-11, 1.7·10-10 m2

Results and Discussion

Figure 2: Velocity magnitude in logarithmic scale (color plot) and tangential velocity (arrow plot) along the mold surface.

After about 45 minutes of simulated time, the mold is filled to 95% with resin. Figure 2 shows the velocity magnitude along the mold surface. The highest velocity values appear near the inlets and the small vents, where the flow channel narrows.

Figure 3 shows the pressure at the external surfaces of the blade. The highest pressure appears at the inlets.

Figure 3: Pressure plotted on the external surfaces of the wind turbine blade.

Figure 4: Volume fraction of resin after 15 minutes of simulated time.

The resin front advances as shown in Figure 4 and Figure 5. In Figure 4, the resin front is shown after 15 minutes, in Figure 5, the resin interface is plotted after 5, 10, 20, and 30 minutes of simulated time. After about 45 minutes, 95% of the mold is filled with resin. The position of the vents could be optimized to avoid air bubbles within the molding process.

Figure 5: The Resin front advancing during the filling process. The figures show the resin interface position after 5, 10, 20, and 30 minutes.

Reference

1. Y. Jung, An efficient analysis of resin transfer molding process using extended finite element method, doctoral dissertation, Ecole Nationale Superieure des Mines de Saint- Etienne in joint supervision with Seoul National University, 2013 (tel-00937556, https://theses.fr/2013EMSE0701).

Porous_Media_Flow_Module/Fluid_Flow/rtm_wind_turbine_blade

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select > > > .

Click .

Click

In the tree, select > .

Click

Geometry 1

Start creating this model by importing the model geometry sequence from a file. This file also includes geometry parameters and selections.

In the window, under click .

In the window for , locate the section.

From the list, choose to make sure that the geometry sequence, which uses features of the COMSOL Multiphysics Design Module, can be imported and run properly.

In the toolbar, click and choose .

Browse to the model’s Application Libraries folder and double-click the file rtm_wind_turbine_blade_geom_sequence.mph.

In the toolbar, click

Global Definitions

Some material parameters needed for this model (see Table 1) are stored in an external file. You can load them as follows:

Parameters 2

In the toolbar, click

and choose > .

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file rtm_wind_turbine_blade_parameters.txt.

Materials

Define the materials in the next step. Introduce them as empty material nodes first; as soon as the physics has been defined, the material node menu will show you which properties are needed for the simulation. You can then just fill in the values defined in the list.

Air

In the window, under right-click and choose .

In the window for , type Air in the text field.

Resin

Right-click and choose .

In the window for , type Resin in the text field.

Porous Material 1 (pmat1)

Right-click and choose > .

Select Domains 1 and 8 only.

In the window for , locate the section.

In the εp text field, type epsilon_1.

Porous Material 2 (pmat2)

Right-click and choose > .

Select Domains 4–7 only.

In the window for , locate the section.

In the εp text field, type epsilon_2.

Porous Material 3 (pmat3)

Right-click and choose > .

Select Domains 2 and 3 only.

In the window for , locate the section.

In the εp text field, type epsilon_3.

Brinkman Equations (br)

Using the wall treatment accounts for porous walls without resolving the full flow profile in the boundary layer. Instead, a stress condition is applied at the porous surfaces by utilizing an asymptotic solution.

Inlet 1

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