Numerical simulation on air-flow field in the millingchamber of hammer mill

Numerical Simulation on Air-Flow Field in the milling Chamber of Hammer Mill

Liying Cao

Mechanical Engineering School

Inner Mongolia University of Science and Technology

Baotou, China

kdcly@https://www.360docs.net/doc/2313680799.html,

Chundong Li

Research Institute

Inner Mongolia NO.1 Machinery Group

Baotou, China

Abstract—A new model hammer mill was developed by Inner Mongolia Agricultural University of China, in which material can be milled circularly and there is no material-circulation-layer effect. In order to improve the working efficiency of the new model hammer mill, the 3-D air-flow field in the mill was numerically simulated and analyzed in computation fluid dynamics (CFD) software Fluent 6.3.Then the simulated results of air flow velocities were compared with that obtained by tests to validate the results of the numerical simulation.

Keywords-Hammer mill; 3D numerical simulation; air flow; Fluent

I.I NTRODUCTION

Hammer mill is widely used in grinding feed for poultry and animals due to its advantages of higher productivity and lower cost. However, a material-circulation layer is existed in the conventional hammer mills (as shown in Figure 1), in which the greater-size particles of materials are distributed at the outer of the circulation layer closing to sieve because of the centrifugal effect, it blocks the sieve and can not be separated even the proper particle size is reached. This results in the problems of higher energy consumption, material over milled and material heated. In order to solve these problems, a new model of hammer mill has been developed in this study, as shown in Figure 2.

The new model hammer mill consists of two parts: a milling chamber and a sieving setup. In the milling chamber, there is no sieve designed but a teeth board used to assist grinding, so the milling productivity is higher than that of the traditional hammer mill. But the rotator with hammers has no difference with the traditional one. When the material is fed into the milling chamber from the inlet, it is milled by the high rotating hammers and further milled by impacting to the fixed teeth board. The milled materials are conveyed to the sieve by the actions of centrifugal force and air flow, thus some of materials with suitable particle size go through the sieve and separated out from the outlet, and the other part of materials with greater particle size, due to its gravity and vacuum in the center of chamber, is back to chamber and milled again. During this process, the material is instantly separated as long as its particle size reaches or is smaller than the hole size of the sieve, so the material-circulation layer can not be generated, thus without the such problems as higher energy consumption, material over milled and material heated.

Figure 1. Material-circulation layer in the conventional hammer mill

Figure 2. The picture of the new model hammer mill Since an air-material two phase flow field in the feed mill and rotating hammers are existed, it is very difficult to detect the fluid performance in the mill by using the physical experimental method. Therefore, in this research, the fluid properties in chamber of the new model hammer mill was numerically simulated using computational fluid dynamics (CFD) method, and software FLUENT was adopted to do the simulation and to analyze the performance of the mill. The purpose is to provide a basis on optimum designing theory for

this new model of hammer feed mill.

II. M ODELING AND NUMERICAL SOLUTION

A. Numerical Modeling

Although the air flow through the hammer-rotor in the chamber of mill is an unsteady flow, it can be regarded as a steady flow by using MRF to simulate it. A standard k-? model was adopted in the simulation, including the governing equations of continuity equation, momentum equation, turbulence kinetic energy k as well as its dissipation rate ?. The governing equations can be expressed as follows:

()()()()S z z y y x x z w y v x u t +?1

·

¨?§??Γ??+??1·¨¨?§??Γ??+?1·¨?§??Γ??=??+??+??+??φφφφρφρφρρφ

where: ? is the generalized variable, representing u, v, w, k

and ?; k is the t XU bulence kinetic energy; ? is the rate of dissipation; ī is the diffusion coefficient, S is the source item; u, v, w are the velocities in the directions of x, y and z (m/s). B. Modeling and Simulation of Air Flow Region

Main parameters for the new model hammer mill are as follows: 16 hammers, rotor diameter 400mm and rated rotating speed 3730r/min. The air-flow space in the grinding chamber of the hammer mill was taken as the domain for simulation. Fluid flows from the inlet to the outlet, passing the hammer rotor at very high speed.

Figure 3. Geometric Model of Chamber of the Hammer Mill

First, a geometric model for the chamber of hammer mill was established in software Pro/E, as shown in Fig XUH 3, then load this 3-D geometric model into the preprocessing module GAMBIT in FLUENT, and a physical model of computation region in GAMBIT was generated. By meshing the computation region , a CFD computation model was generated. In order to ease the mesh of the model, the inlet of the milling chamber and the material-returning inlet were simplified as an equivalent equal-section cylindrical pipe (as shown in Figure 4) [1, 4].

Figure 4. Simulation Mode of Flow Field in Chamber of Hammer Mill

C. Boundary Conditions

During the simulation, the multi-coordinate system was used, in which the fluid region of the hammer rotor was defined with the moving reference coordinate, and the fluid regions in two inlets, outlet as well as external housing were defined with the fixed reference coordinate. The boundary conditions for the inlet of the milling chamber were: the total pressure in the inlet was set as the boundary condition, which was determined by the measurement, the average total pressure was 100998Pa; The boundary conditions for the outlet: the atmosphere pressure was set as the boundary condition, it was 101325Pa.

D. Numerical Computation Method

3D steady air-flow field in the milling chamber was solved in separating implicit expression based on the dispersing governing equations of finite volume method. The computation was performed using standard k-? model and no-slip wall function. The standard wall function was used to simulate the near-wall air flow. The discretization was second order upwind discretization, and the standard SIMPLE algorithm was employed to solve for the pressure-velocity coupling [2, 3, 5].

Figure 5. Iterations of the simulation

The residual error was calculated to dynamically monitor the convergence during the solving process. When the rotate speed was 3730r/min, the computation was iterated 488 times before the convergence. The Iterations chart of the simulation

was shown in Figure 5.

Figure 6. Testing points in D-D section for total pressure and A-A section

for air velocity

III. E XPERIMENT VALIDATION

In order to validate the correctness of the simulation, the air-flow speed at A-A cross-section of outlet was measured. Also the total pressure at the center of D-D cross-section of material-returning inlet was measured, provided the boundary condition on pressure for the simulation. Considering the cross section of material-returning inlet is very small, only the

IV. R ESULTS AND DISCUSSION

A. Comparison between the simulation and the experiment The simulated and tested velocities at line E(Z=115mm) and line F (X=195 mm) were compared as shown in Fig. 7 and Fig. 8. It can be seen that the velocity distribution trends between the simulation and the experiment were the same, the simulated values were a little bit larger than the tested ones. This is because that the air leaking as well as the frictions between the air flow and the hammers or milling-housing wall were neglected in the simulation. Since the friction resistance near the housing wall was larger than that in other areas, the velocity values between simulation and experiment at the third testing point (as shown in Figure 7) and the fourth testing point (as shown in Figure 8?were not very closed.

B. The Analysis on Air Flow in Chamber

Under the other parameters unchanged, the simulation was conducted at three different rotor speeds 2000 r/min ?2500 r/min and 3730 r/min, the simulated velocities and pressures at two cross sections of Z=100 mm s and X=0 mm were compared. The results indicated that the common points under three conditions were: The velocities near the center of the rotor were very low, the velocities at the rotor swept region were the fastest, and the velocities closing to the interior wall of the chamber were lowered gradually. In the outlet of the

Figure 9. Velocity vector plot at 3730r/min at section Z=0.1 m

Figure 10. Velocity vector plot at 3730r/min at section X=0m

Figure 11. Total pressure plot at 3730r/min at section Z=0m

Figure 12. Total pressure plot at 3730r/min at section X=0.1 m

V.C ONCLUSIONS

Using the interfacial combination of Pro/E and FLUENT, the complicated 3D geometric model can be loaded to Fluent to do the 3D fluid field numerical simulation. In this way, the air-flow field in chamber of the new model hammer mill was simulated, the results showed the air-flow velocity distribution and air pressure distribution, which provides a basis on analysis of the performance and optimum design of the new model hammer mill.

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