Effect of Channel Roughness on Micro Droplet Distribution on Internal Minimum Quantity Lubrication

Abstract

This research studied the effect of channel roughness on micro droplet diameter and distributions for two commercially available lubricants in internal minimum quantity lubrication (MQL). The effect of increased channel roughness in the nozzle of a MQL system was tested. Chemical etching was studied to control the roughness of external surfaces of tungsten carbide tools. Mixtures of oils and air were flown though internal channels with different simulated roughness: as fabricated, partially threaded, and fully threaded. Resultant micro-droplet sizes and counts were collected on a glass grid for evaluation. Droplet density was calculated to characterize the dispersion patterns of lubricant exiting the MQL system. The results were compared with outcomes of air flow simulations using a computational fluid dynamic approach. Nonparametric statistical analysis was conducted to further analyze the results of the MQL droplet characterization experiment. Chemical etching of tungsten carbide was done using a reagent of hydrogen peroxide and nitric acid. Hand agitation and a combination of hand agitation and ultrasonic pulsation were used to compare the rates of material removal. Scanning electron microscopy images were used to further analyze the surface structure of substrates before and after etching. For low viscous lubricant, the rough channel surface helped to break large droplets in the boundary layer into smaller droplets and reintroduce them into the main downstream flow. The opposite trend was found for lubricant with high viscosity, as increased lubricant wall adhesion inhibited the breakdown of droplets within the channel, leading to the dispersion of larger droplets. In chemical etching experiments, the synergy of hand and ultrasonic agitation successfully roughened a carbide surface within twelve minutes. Hand agitation alone was less successful, and resulted in smoother surface finishes. Scanning electron microscopy examination showed that the addition of ultra-sonic pulsation enabled deep etching that removed all grinding marks on a WC-Co cutting tool surface.