Measurements and modeling of instantaneous particle orientation within abrasive air jets and implications for particle embedding

• Measurement of instantaneous orientation angle of particles.
• Prediction of particle velocity and instantaneous orientation angle.
• Influence of process parameters on particle orientation was investigated.
• Percentage of oriented particles and embedded particles were correlated.

Previous theoretical studies indicate that the material removal mechanism and the likelihood of particle embedding in solid particle erosion processes strongly depend on instantaneous particle orientation at impact. The present study utilized pulsed laser shadowgraphy to measure the size, shape, and instantaneous orientation distributions of particles within a micro-abrasive jet at various pressures, particle sizes, and nozzle standoff distances. A particle orientation angle was defined as the angle between the particle linear velocity vector (approximately parallel to the jet centerline axis) and the line connecting the center of mass to the furthest downstream particle vertex. Particles were considered to be ‘oriented’, i.e. in a configuration favorable to embedding, if this angle was between 0 and 10°. The measurements revealed that, at all pressures (i) between 24% and 29.5% of the particles were oriented at the nozzle exit; (ii) the percentage of oriented particles increased slowly with standoff distance; (iii) larger particles had a stronger tendency to orient than smaller ones; (iv) particles having larger aspect ratios had a stronger tendency to aerodynamically align with the jet, and (v) although not all particles that were oriented actually embedded, the measured percentage of embedded particles on Al 6061-T6 surfaces nevertheless correlated with the percentage of oriented particles. A model capable of predicting the instantaneous particle orientation and velocity within and downstream of the nozzle was presented and shown to agree well with measurements. To the knowledge of the authors, these are the first measurements of the instantaneous orientation of abrasive particles under conditions that are typical of abrasive air jets. The results may have important implications for optimizing solid particle erosion tests and in abrasive jet machining applications.