Fig. 1 PIV experimental equipment for oil liquid

The initial velocity of the oil inlet was set to V₀=0.0362 m/s, the corresponding Reynolds number was 138.3, and the oil in the pipeline was in a laminar flow state. Since the temperature did not change much during an experiment, the influence of temperature was not considered. After the oil samples to be tested were thoroughly mixed, they were fed into the test pipeline through a peristaltic pump. After flowing through the pipeline development section, a uniform liquid-solid two-phase suspension flow was formed in the test section and then entered the oil tank through the tail section to form a circulation. The system was run for 30 min before the PIV test to ensure that the oil and particles were evenly mixed during the PIV measurement period.

The PIV equipment used in the experiment was a 2DPIV system from TSI Corporation of the United States. This system consists of a double yttrium aluminum garnet laser (200 mJ, pulse frequency 15 Hz, laser wavelength 532 nm), light guide arm, sheet light source lens (focal length from 30-3000 mm, two sets of cylindrical lenses 15 mm F.L. and -25 mm F.L., and divergence angles 25° and 14° , respectively), a Nikon CCD camera (15 frames per sec, 50 mm / F1.8 lens, 2K×2K pixel resolution) and TSI Insight4G image acquisition, analysis and display software platform. The coordinate frame of the camera is three-dimensional and is automatically controlled by a microcomputer with a movement accuracy of 0.01 mm. The laser light emitted from the laser passes through the light guide arm and the light source lens to form a 1 mm thick sheet light, which is irradiated from the normal direction (z-direction) of the pipe test section. Then, oil image information is obtained by the camera positioned in the direction of the pipe (y-direction). In addition, the synchronization controller (time resolution 0.25 ns) is set to trigger the laser and the camera synchronously.

As the particle size in oil is less than 100 μm, the particles have a good following behavior and uniform distribution of light scattering, so no tracer particles need to be added during the test. The acquired image adopts the algorithms of multigrid iterative mutual correlation and multiadaptive deformation window, and the image query area adopts pipeline horizontal and vertical initial query windows of 72×64 pixels, a final query window of 64×48 pixels and 50% overlap to obtain the original velocity vector and eliminate the impact of the query window boundary. The method of processing the error vector is to use a local average verification method, and this vector is replaced by the mean in the adjacent 5×5 pixel range. The measurement position is 300 mm from the inlet of the pipeline; the measurement area is (flow direction×normal direction) 85.86 mm×22.20 mm, while the center of the area is (flow direction×normal direction) 300 mm×30 mm. In addition, 200 image frames are captured in a test period.