Drop transport in air paint sprays

Paul Garraway Hicks, Purdue University

Abstract

The goal of this research is to develop an improved understanding of paint transfer in air-spray processes for which there is considerable margin to improve appearance and reduce both paint usage and solvent emissions. The approach taken is a synthesis of (1) analysis of published experiments in order to provide a basis for simulation development and verification, (2) detailed simulation using stochastic separated flow methods in order to examine the mechanisms controlling paint transport, and (3) detailed measurement of spray structure using modern optical diagnostic methods in order to confirm the simulation assumptions and to provide greater insight into air-spray structure. The two-dimensional simulation uses measured spray conditions to incorporate the complex flow near the nozzle and models the region where the workpiece affects the flow. The model accurately simulates paint transfer and illustrates that turbulent velocity fluctuations are responsible for deposition of the smaller drops ($<$20 ${\rm\mu m})$ which are prevalent in fine finish application. The concept of drop transfer efficiency developed in this work is shown to be a powerful tool for simulation verification and for separating the paint transfer effects of atomization from those of drop transport. Only painting distance has a significant effect on drop transport. The reduced paint transfer that accompanies an increase in air supply pressure is due to atomization, as opposed to the common misconception of drop transport. The simulation predicts the measurements well, revealing that drop transfer efficiency increases rapidly with drop diameter. This indicates that significant improvements in transfer efficiency are possible with minor shifts in atomization. Detailed spray measurements have required development of a phase Doppler particle analyzer configuration methodology to minimize over-sizing of small drops. These detailed measurements illustrate the significant influence of shaping air. Under normal shaping air operating conditions, the two-dimensional model assumptions are valid over broad ranges in air pressures and liquid flow since shaping air augments mixing and creates a high aspect ratio spray.

Degree

Ph.D.

Advisors

Senser, Purdue University.

Subject Area

Mechanical engineering

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