1 Experimental apparatus and method
The PIV test apparatus for oils containing different particle sizes and particle concentrations is shown in Fig. 1. 25# transformer oil is used as the test fluid (colorless and transparent for the PIV test). First, the oil is treated by a vacuum filter to obtain an initial oil sample. Then, Cu, Fe and SiO2 powders with different particle sizes are selected as the particulate matter. According to the ISO4406 standard of oil pollution degree, 0.02 g of Cu, Fe and SiO2 powder are mixed with 1 L 25# transformer oil and oscillated evenly in an ultrasonic oscillator for 8 h (temperature 30~60°C). Then, the mixed solution was separated using filter paper having different pore diameters to obtain oil samples having a medium diameter of 5 μm, 15 μm, 25 μm and 50 μm and a concentration of 14.40 ppm. Next, the initial oil sample is added to the oil sample at a certain volume ratio, and ultrasonic vibration is evenly distributed over 8 h (temperature 30~60°C). Oil samples containing particulates having a gradient concentration of 6.58, 4.57, 3.00, 0.971 and 0.648 ppm of different medium-diameter particles were eventually obtained.
The pipeline containing granular oil is a square tube with a total length of 500 mm and a section of 40×40 mm. The bottom and rear walls of the pipe are made of steel plates, while the front and top walls are made of transparent glass and are detachable for PIV laser incidence and testing.