Damage caused by erosion has been reported in several industries for a wide range of situations. In the present work, a new method is presented to improve the erosion resistance of machine components by biomimetic method. A numerical investigation of solid particle erosion in the standard and biomimetic configuration blade of axial fan is presented. The analysis consists in the application of the discrete phase model, for modeling the solid particles flow, and the Eulerian conservation equations to the continuous phase. The numerical study employs computational fluid dynamics (CFD) software, based on a finite volume method. User-defined function was used to define wear equation. Gas/solid flow axial fan was simulated to calculate the erosion rate of the particles on the fan blades and comparatively analyzed the erosive wear of the smooth surface, the groove-shaped, and convex hull-shaped biomimetic surface axial flow fan blade. The results show that the groove-shaped biomimetic blade antierosion ability is better than that of the other two fan blades. Thoroughly analyze of antierosion mechanism of the biomimetic blade from many factors including the flow velocity contours and flow path lines, impact velocity, impact angle, particle trajectories, and the number of collisions.
Wear is one of the main reasons for failures of mechanical parts. [
Nature is a school for scientists and engineers; after billions of years of evolution, creatures in nature possess almost perfect structures and functions [
The dorsal surface of the scorpion: (a) scanned data using a laser scanner; (b) the groove of scorpion back; (c) the convex hull of scorpion back; (d) bionic groove pattern; (e) bionic convex pattern.
Many of the factors which control the rate of erosion, such as particle velocity or particle mass flow rate, particle diameter, impact angle, and particle distribution can be studied at different flow conditions of the system. A lot of practical examples may be found when a change in flow conditions has greatly increased or decreased erosion.
The experimental study on the dynamic behavior of solid particles requires special equipment and methodology to pursue this goal. Also, the erosion process is a complex problem to obtain a mathematical formula to account for some of the factors which control the rate of erosion of the blade. This paper presents a numerical study of the erosion process of biomimetic axial fan blade, applying computational fluid dynamics (CFD).
The numerical study of the erosion process applying CFD considers a mathematical model with Eulerian conservation equations in the continuous phase and a Lagrangian frame to simulate a discrete second phase. The dispersion of particles in the fluid phase can be predicted using a stochastic tracking model. This model includes the effect of instantaneous turbulent velocity fluctuations on the particle trajectories.
The computational domain considers the mass conservation and momentum equations for incompressible flow in a 3D geometry in a steady state. The mass conservation is
This model permits us to simulate a discrete second phase in a Lagrangian frame of reference, where the second phase consists of spherical particles dispersed in the continuous phase. The coupling between the phases and its impact on both the discrete phase trajectories and the continuous phase flow is included. The turbulent dispersion of particles is modeled using a stochastic discrete-particle approach. This approach predicts the turbulent dispersion by integrating the trajectory equations for individual particles, using the instantaneous fluid velocity. The prediction of particle dispersion makes use of the concept of the integral time scale,
The trajectory of a discrete phase particle can be predicted by integrating the force balance on the particle, which is written in a Lagrangian reference frame. This force balance equates the particle inertia with the forces acting on the particle:
The relative Reynolds number,
To incorporate the effect of the discrete phase trajectories on the continuum, it is important to compute the interphase exchange of momentum from the particle to the continuous phase. This exchange is computed by examining the change in momentum of a particle as it passes through each control volume in the computational domain.
This momentum change is computed as
Finally, to evaluate the erosion rate at the wall of the blade, applying the discrete phase model, it is important to define parameters such as the mass flow rate of the particle stream,
Geometric construction and meshing were performed with UG and GAMBIT. Bionic configuration created on the curved surface created by the projection along the direction of the normal surface, surface bias, and so forth. Figure
The geometrized structure graph of axial fan blades.
Different mesh type and size were used in each region, owing to the structure of each part of the axial fan, and the flow patterns are different. Longer segment structure of the inlet and outlet is simple, and the flow is relatively stable; hence, hexahedral grid was selected in the two regions. Strong rotation in the blade regional airflow, the flow is quite complex. Meanwhile, the structure of the leaves is more complex, especially the bionic blade. Therefore, selection of unstructured grid geometry structure strong adaptability, and leaves of the surface of the grid are encrypted. Table
Each computational grid information of centrifugal fan.
Region of fan | Lengthened segment of air inlet | Lengthened segment of air outlet | Smooth surface | Groove surface morphology | Convex surface morphology |
| |||||
Grid type | Hexahedral grid | Hexahedral grid | Tetrahedron grid refinement | Tetrahedron grid refinement | Tetrahedron grid refinement |
| |||||
Cells | 223816 | 498200 | 2000671 | 2089127 | 2070634 |
Pressure-inlet boundary condition was used in the entrance of fan. Pressure outlet boundary condition was used in the exit of fan. The definition of turbulence parameters is based on turbulence intensity and hydraulic diameter.
Air flows in the tunnel with entrained solid particles at 11.6 m/s velocity. The injection type was set to surface. Solid particles with 1500 kg/m3 density were released from the inlet with an initial velocity of 11.6 m/s assuming no slip between the particle and fluid. The particle diameters were 20
Open the two-phase coupling calculation in the settings panel of discrete phase model. The boundary of reflect was applied for the wall and rebound model by using (
Experimental measurements reported by Hamed et al. [
The simulation results of the erosion rate of the three kinds of blades are shown in the form of histogram in Figure
Numerical simulation results of the erosion rate of three kinds of blades: (a) groove surface morphology; (b) convex surface morphology; (c) smooth surface.
Figures
Flow velocity contours and path lines of the surface region of smooth surface.
Flow velocity contours and path lines of the surface region of convex surface.
Flow velocity contours and path lines of the surface region of groove surface.
The special flow pattern in the groove has significant influence on the erosion resistance of groove surface. The rotating flow in the groove plays an “air cushion” effect. On the one hand, the grooves can enhance fluid turbulence, which lead to the change of the flow field around the groove surface, and the particle motion pattern was changed subsequently. Some of the particles will leave the surface along with air flow without impact, and these particles would impact the surface if the surface was smooth. Therefore, the number of particles impacting the surface was decreased. On the other hand, as a result of the decrease the flow velocity and the velocities of the particles in the two-phase flow were decreased as well. The rotating flow in the groove can absorb particle energy which is used for impacting, and the energy used in impact was correspondingly reduced. These features all help to reduce the particle impact damage on the blade surface and reduce erosion wear [
The particle impact velocity is an important impingement variable which influences the erosion behavior of materials. The dependence of erosion rate (
In the present work, the particle impact velocity distributions on the surface of the three types of blades were analyzed. The particle impact velocities were obtained by the FLUENT postprocessing system. The impact velocities were recorded in the interval value of 2 units. Figure
Particle-surface impact velocity distribution.
Particle impact angle has an important effect on the erosion rate. The maximum erosion of ductile material occurs at angles between 20–30° [
Particle-surface impact angle distribution.
Figures
The trajectories of particle collision smooth surface.
The trajectories of particle collision convex surface.
The trajectories of particle collision groove surface.
It can be seen that the impact times of groove and convex surfaces were lower than that of smooth surface, and the groove surface showed the lowest impact times. Hence, the reduction of impact on the groove surface can lead to the decrease of erosion wear to a certain degree.
In the present work continuous-discrete phase models are used to predict the erosion of the three kinds of blades. Conclusions are as follows.
The groove surface blades showed the best erosion resistance compared to other two kinds of blades. The flow velocities are higher around the surface of smooth surface than those of the convex surface and the groove surface from the velocity contours. The groove surface has a great influence on the airflow. The particle impact velocity of biomimetic groove axial fan blade is less than smooth blade and the convex blade. The impacts on the biomimetic groove axial fan blade occurred at the high impact angles which are relatively less susceptible to impact damage compared with the smooth and convex surfaces, while the surface smooth and convex hull-shaped biomimetic form of axial fan blades collision occurred in apt erosion low-angle region. The impact times of groove and convex surfaces were lower than those of smooth surface, and the groove surface showed the lowest impact times.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Natural Science Foundation of China (nos. 51175220 and 51205161), Specialized Research Fund for the Doctoral Program of Higher Education (nos. 20100061110023 and 20120061120051), China Postdoctoral Science Foundation on the 51th Grant Program (2012M511345), the Projects of Cooperation and Innovation to National Potential Oil and Gas for Production and Research (no. OSR-04-04), the Scientific and the Technological Development Project of Jilin Province (no. 20130522066), and Basic Scientific Research Expenses of Project of Jilin University (450060481176).