Optimization research based on local exhaust of oil mist particles in cutting machine tool

Oil particles generated from metalworking fluids(MWFs) in machining process can lead serious health problem to operator. Local exhaust hood is an effective engineering method to capture oil particles and other contamination which is wildly used in manufacture workshop. In this paper, exhaust hood capture efficiency with various height, air volume and particle size was gotten by Computational Fluid Dynamic(CFD) technology. Though further analysis of the CFD result, feature air velocity was introduced. Then an equation of feature velocity and capture efficiency was established by multi regression method. According to this equation one improvement solution was studied: Set to flexible enclosure for up exhaust hood. The solution raised particle capture efficiency on each size significant, the result is equivalent to low down up exhaust hood for 60cm.


Introduction
The further diffusion of oil mist particles can be avoided by placing a local exhaust hood near the source of oil mist particles.The local exhaust system composed of local exhaust hood, air duct and fan is also widely used in industrial buildings due to its characteristics of high capture efficiency and low energy consumption [1].
Oil mist particles of various sizes will be generated in the machining process [2].In this application scenario, the capture efficiency of the local exhaust hood at different heights for different particle sizes is beneficial to the fine design of the local exhaust hood.At the same time, how to optimize the local exhaust hood by using the characteristics of the machine tool structure and improve the capture efficiency is also worth further study.

Research methods
In this paper, the method of computational fluid dynamics (CFD) was used to study the capture efficiency of local exhaust hoods with different heights for oil mist particles with different particle sizes, explore the reasons affecting the capture efficiency, and then improve and optimize the local exhaust hoods.

Physical models and computational domains
After field investigation, this paper establishes the simulation physical model according to the structure of vertical machining center.Fig. 1

Numerical simulation method
Based on the existing studies, this paper adopts the Realizable K-ɛ Turbulence model, uses the second-order upwind format for discreteness, and solves it with the SIMPLE algorithm.
The convergence conditions are judged as follows :(1) continuity residual is less than 1×10 -3 , and energy residual is less than 1×10 -6 ;(2) The wind speed of the exhaust outlet no longer fluctuates greatly;(3) The mass balance of the whole system is less than 1%.

Discussion of grid independence
Local exhaust hood as shown in Fig. 2  From Fig. 3(b), it can be seen that the wind speed on the L5 test line no longer varies with the increase of grid number when the grid number reaches 280,000.
Therefore, it can be considered that the simulation results of the two grid numbers, 340,000 and 470,000, are independent of the grid number, and the grid number of 340,000 is used in this model.

Introduction of simulated working conditions
According to the research conclusion of Wang [2], oil mist particles produced by cutting have the characteristics of multi-dispersion.Four typical particle sizes of 2.5  m, 6.25  m, 12.5  m and 17.5  m were selected for study.The particle density of oil mist is 989kg/m³.DPM model is adopted for particle calculation with single-phase coupling.The effects of gravity, buoyancy, drag force, Saffman force and thermophoretic force are considered in calculation.
The exhaust air outlet is the speed outlet, and other boundaries are pressure inlet.
In this paper, two types of updraft exhaust hood, updraft exhaust hood adds enclosure were calculated with different heights, different air volumes and different particle sizes, and a total of 160 working conditions were simulated.The detailed information is shown in Table 1.Therefore, the feature air velocity is defined, and its calculation method is shown in Formula (1):

Table 1 Simulation working condition table
In the calculation formula: is average exhaust wind speed inside machine tool.It can be obtained by dividing the cross-sectional area of the machine by the amount of air entering from the bottom of the machine,m/s； is free settling velocity of oil mist particles,m/s； D is particle size of oil mist,m.
Due to the small particle size, the Reynolds number of its movement in the air is always Re < 1, and the particle movement is in the Stokes region, which can be calculated by Formula (2).
In the calculation formula: is density of oil mist particles,kg/m³； is air density,kg/m³； is the kinematic viscosity of air,m 2 /s； is acceleration of gravity,m/s².It can be seen from Fig. 5 that the capture efficiency is zero when the feature air velocity is less than 2.8×10 7 , while the capture efficiency is 100% when the feature air velocity is greater than 1.7×10 8 .When the feature air velocity is between 2.8×10 7 ~1.7×10 8 , the capture efficiency is positively correlated with the feature air velocity, and the larger the feature air velocity is, the higher the capture efficiency is.
In terms of distribution characteristics, the relationship between feature air velocity and capture efficiency is shown in Formula (3): In the calculation formula: ℎ and are full degree correction and zero degree correction respectively.50 is the median characteristic drag ratio, that is, the characteristic drag ratio when the efficiency is 50%. is the action width, which is used to describe the action range of the characteristic drag ratio.
Through multiple regression, the median characteristic drag ratio of the exhaust hood studied in this paper is 66.03.The action width is 5.
It can be seen from Fig. 5

Optimization with the goal of capture efficiency
Considering the objective conditions and limitations, this paper tries to optimize the exhaust hood by changing the structure and location of the exhaust hood.It can be seen that with the addition of enclosure, the particle capture efficiency of 6.25  m~17.5  m particle size has been greatly improved.

Conclusion
In this paper, a numerical model which has been verified by experiment is used to study the local exhaust air trapping effect and its optimization, aiming at the capture efficiency of oil mist particles in cutting process.
Based on the influence of exhaust height and exhaust air volume on the capture efficiency, the paper puts forward that feature air velocity is the key parameter affecting the capture efficiency, and gives the regression equation between feature air velocity and capture efficiency.
In order to improve the feature air velocity, the paper proposed a way to optimize the particle capture efficiency of the exhaust hood by increasing the enclosure on the updraft exhaust hood.Finally, it was concluded that the capture efficiency of the oil mist particles of 6.25m, 12.5m and 17.5m were improved at different exhaust speeds under the condition of increasing the enclosure on the exhaust hood.The capture efficiency of the updraft exhaust hood with the enclosure increased is equivalent to that of the exhaust hood with the height reduced by 60cm.

E3SFig. 2 Fig. 3
Fig. 2 Actual model drawing of local exhaust hood Gambit 2.4.6 was used to model and generate the grid, and the velocities on the L1 test line (with the central axis as 0, horizontal lateral position: -0.55~0.55m)and the L5 test line at the distance from the axis of the exhaust hood (with the ground as 0, vertical position: 1.26~2.26m) in Fig. 2(b) were used as the basis for the grid-independent test.The velocity variations on the two test lines for grid numbers of 140,000, 280,000, 340,000 and 470,000 are given in Fig. 3.

1
Fig.4a, 4b, 4c and 4dshow the influence of exhaust hood height on the capture efficiency of oil mist particles with different sizes at different exhaust air volumes.For fine particles with particle size less than 2.5  m, the capture efficiency of the exhaust hood is always maintained at 100%, so there is no data of such particle size in the figure.For oil mist particles with 6.25  m, 12.5m and 17.5m particle sizes, the height of exhaust hood has a different effect on them.When the exhaust air volume is 200m³/h, the particle capture efficiency of 6.25  m decreases obviously with the increase of the exhaust hood height.The average capture efficiency decreases by 10% for every 10cm increase.For oil mist particles of 12.5m and 17.5  m, the height of exhaust hood has a great

Fig. 5
Fig. 5 Relationship between feature air velocity and capture efficiency

4. 1
Updraft exhaust hood adds flexible enclosure.In order to make the comparison more intuitive, the particle capture efficiency of 2.5  m, 6.25  m, 12.5  m and 17.5  m with the height of 50cm and 80cm was compared.The results are shown in Fig. 6.Lines of the same color and dots of the same shape represent the same particle size.Solid lines and solid figures represent increased enclosures, dotted lines and hollow figures represent no enclosures.

Fig. 6
Fig. 6 Comparison of capture efficiency between adding enclosure and no enclosure (solid line is adding enclosure)As can be seen from Fig.6, when the height of the exhaust hood is 50cm and 80cm, the capture efficiency of the three particle sizes of 6.25  m, 12.5  m and 17.5  m can be improved by adding the enclosure to different degrees.However, as 2.5m particles are easy to be captured, the capture efficiency has little change for this particle size.