1. Investigation of the air boundary layer around a rotating grinding wheel
Hui Wu
Advanced Manufacturing Technology Research Laboratory
General Engineering Research Institute
Liverpool John Moores University
07 November 2008

2. Content of presentation
Introduction and review
Experimental arrangement and method
Experimental results and discussion
Further results from CFD simulation
Conclusions

3. Introduction and review
Problems in grinding process:
High specific energy, which is converted to heat in the grinding zone
High temperature causes thermal damage to the workpiece
- burning, phase transformations
- softening surface layer with possible re-hardening
- unfavourable residual tensile stresses
- cracks, and reduced fatigue strength
- accelerate wheel wear
Contribute to inaccuracies and distortions in the final product

4. Introduction and review
Problems in grinding process:
Grinding fluid is applied in order to limit the high temperatures and thermal damage and wheel wear.
Role of grinding fluid: lubrication, bulk cooling and transport of debris.

5. Introduction and review
Problems in fluid delivery in grinding :
Grinding fluid must be delivered to the grinding zone in sufficient volume.
(for effective lubricating and cooling)
Air boundary layer develops around the periphery of the wheel surface in grinding, which may prevent the fluid reaching the wheel surface.
Two main methods of grinding fluid delivery:
low pressure nozzle (flood)
high pressure nozzle (jet)
Fluid delivery at high pressure (jet) leads to increased friction and wastage of power.
Fluid delivery via a low-pressure system may be insufficient to penetrate this air boundary layer, especially in high-speed grinding.

6. Cutting fluid backing up due to boundary layer effects
Introduction and review
Interest on air boundary layer investigation:
The air boundary layer flow has sufficient energy to prevent the fluid reaching the wheel surface and the grinding contact zone.
This has aroused interest that has led to the studies of the air boundary layer flow around the rotating grinding wheels.
The air velocity distribution has been measured by some researchers.
Cutting fluid backing up due to boundary layer effects
S. Ebbrell etal (1999)

7. Introduction and review
Problems in previous measurement of air boundary layer:
Most of the previous measurements were intrusive measurement method (not sensitive enough to measure the low velocity air flow)
Most previous researchers only measured the tangential component velocity of the air boundary layer (one component velocity)
Different measurement methods and experimental parameters led to conflicting results
Therefore, a comprehensive understanding of the air boundary layer flow has not been obtained

8. Introduction and review
Review of previous measurements:
wheel
Measurement
area
V. N. Serov (1965): air velocity changes along the wheel width reaching a minimum value in the middle section.
Method: unknown
V. Radhakrishnan (1977): Similar to Serov’s results
Method: Pitot tube and hot-wire

9. Introduction and review
Review of previous measurement:
wheel
Measurement
area
J. Shibata (1982): Two peaks around the sides of the wheel close to wheel surface, but tend to shift to the center of the wheel width with the distance to the wheel surface
Method: Hot-wire anemometer
Sven Alenius (1996): Air central peak along the wheel width
Method: Pitot tub

10. LDA measurement system
Experimental arrangement and method
LDA measurement:
Laser Doppler Anemometry (LDA) represents a most effective tool in the fluid velocity measurements.
Advantages of the LDA: non-intrusive measurement with high temporal and spatial resolution
Ideal measurement technique for the investigation of air flow velocity.
(a ) Laser system generator
(c) PC
(b) Burst Spectrum Analyzer (BSA)
LDA measurement system

11. Experimental arrangement and method
Grinder: Abwood Series 5020 surface grinding machine
LDA: Argon+-ion LDA system
(two-component dual-beam LDA , back-scatter mode)
Seeding particle: Smoker generator

12. Experimental arrangement and method
Wheel Parameters:
Material: Alumina
Diameter: 182.5mm
Width: 25mm
Surface roughness: Rt=2mm
Schema of the configuration

13. Experimental arrangement and method
Experimental Method:
Measurement coordinate system and the measurement area
Lattice of measurement points

14. Experimental arrangement and method
Experimental works presented:
Air tangential velocity contour map with different wheel speed (Vs)
Air radial velocity contour map with different wheel speed
Effect of the wheel speed on air velocity
Effect of the wheel roughness on air velocity
Tangential and radial velocity turbulent value map

15. Experimental results Air tangential velocity contour maps:
(Vs = 20m/s)
(Vs = 30m/s)
(Vs = 40m/s)
The air tangential velocity has its highest value close to the wheel surface.
The highest tangential air velocity is always in the wheel middle section along the wheel width.
Close to the wheel edges, the gradient of Vt is high.

16. Experimental results Air radial velocity contour maps:
(Vs = 20m/s)
(Vs = 30m/s)
(Vs = 40m/s)
In one wheel speed, the maximum radial velocity is the distance of about 25mm to the wheel surface in the wheel middle section
the radial velocity is increasing with increasing wheel speed

17. Experimental results Effect of the wheel speed on air velocity:
Vt distribution with distance to wheel surface
Measurement line
Wheel
Vt in one point with increasing wheel speed
Tangential velocity exponentially decreases with distance from the wheel surface.
Tangential velocity is linear proportionally with the wheel speed in one point.

18. Experimental results Effect of the wheel roughness on air velocity:
Measurement line
Air Vt VS wheel surface roughness
The wheel surface roughness is bigger, the air flow velocity is higher.

19. Experimental results
Tangential and radial velocity turbulent value map:
Air velocity turbulent value is the percentage of the RMS velocity value divided by the mean velocity value.
Vt and Vr Percent Turbulent distribution (Vs=30m/s)

20. CFD Simulation: Tangential and radial velocity turbulent value map:
Vt simulation contour (Vs = 30m/s)
Vr simulation contour (Vs = 30m/s)

21. CFD Simulation: Air flow vector (Vs = 30m/s) Simulation result
Experimental result

22. Conclusions
The Laser Doppler Anemometry (LDA) was successfully employed to investigate the grinding air boundary layer around the rotating grinding wheel
The velocity distributions of the air boundary layer flow were fully observed and investigated.
The results of the experiments and simulations clarify the full understanding of the air boundary layer compared to previous research.
These outcomes will help users to optimize the fluid delivery design when thinking about the effect of the air boundary layer.

23. Thank you for your attention!