Multi-scale analysis on particle dynamic of vertical curved 90° bend in a horizontal-vertical pneumatic conveying system

To reveal the particle dynamic characteristic in the bend, high-speed particle image velocimetry (PIV) and wavelet transform were used to measure and analyze the particle velocity in a horizontal-vertical pneumatic conveying system. The pressure drop and particle velocity are analyzed to elucidate the macroscopic motion properties of particles in the different radius ratio bend firstly. Then the methods of continuous wavelet transform and one-dimensional discrete orthogonal wavelet transform are used to analyze the particle dynamic characteristic in the different regions of the bend pipe in terms of time–frequency characteristics of particle fluctuation velocity, fluctuation energy distributions of wavelet components, and auto-correlation of various frequencies. The results show that the particles are mainly small-scale motion in the rapidly decreasing region, while the large-scale motion increases in the accelerating region near the inlet and the stable region near the outlet. And the results of the wavelet component show that the acceleration and deceleration of particles in the bend will decrease the proportion of high-frequency fluctuation energy. The auto-correlation coefficient of the high-frequency component decays slower and has a longer period at the critical position of the three regions.