Introduction
Oil will inevitably pulsate during circulating processes of employed oil equipment due to the functional action of the equipment, and this effect will affect the action accuracy of the equipment. A variety of measures are often employed in industry to reduce the effects of such pulsations. However, the pulsation of oil is not only related to the mechanical function but also the characteristics of the oil itself, and the particle size and concentration of particles in oil also play significant roles in the pulsation of oil. The effect of particle size and concentration on the pulsation characteristics of oil belongs to a sparse solid-liquid two-phase flow field, on which domestic and foreign scholars have carried out theoretical simulation and experimental research and obtained many research results(Abiev and Galushko, 2013; Eschmann et al., 2015; Wang et al., 2016; Yuan et al., 2016). These results have a certain reference value to investigate the influence of particle size and concentration in oil with higher viscosity on the pulsation characteristics of the oil.
Papadopoulos et al.(2016) conducted a direct numerical simulation of sinusoidal pulsation turbulence in a straight tube with a low-volume Reynolds number and a high-frequency, indicating the evolution of the average velocity and fluid pulsation over time. Tian et al.(2016) obtained the variation trend of vibration displacement and velocity of a pipeline system by establishing an analysis method of pressure pulsation in the channel and the coupling interaction of tube flow. Yan et al.(2012) studied the interaction between quasiperiodic large-scale vortex structures and flow pulsation in a rectangular channel, and the conclusions indicated that the pulsation of the vortex structure and the flow velocity pulsation could be enhanced by an adjacent velocity interface. Zhao Hui et al.(2016) studied the unsteady flow field in a channel using direct numerical simulation and found that the contributions of the low-frequency vortices to the flow direction and normal pulsation velocity increase with increasing normal height; the pulsation of the spanwise velocity at the near-wall surface exhibited a large pulsation strength in the logarithmic layer and the viscous bottom layer. Ghadi et al.(2016) conducted jet oscillating flow experiments and showed that pulsating flow forms a coherent periodic structure and that the pulsation frequency has a significant influence on the formation, size, and dynamics of a vortex structure. Hsu et al.(2014) measured a jet field using high-speed particle image velocimetry (PIV) and found that with the change in the jet exit velocity, the jet produced vibration within a pulsating period and induced a periodic wavy flow structure in the downstream region. Yang bin et al.(2013) believed that the change in frequency of wind-sand flow was at least greater than 100 Hz, and the effect of atmospheric turbulent pulsation on sediment concentration was related to the sand grain size; the smaller the particle size was, the greater the effect of wind speed fluctuation.
Two-phase flow in pipelines has been studied for the generation and development of flow field pulsation and the interaction of vortex structures in flow fields at home and abroad. Due to the random nature of oil movement and the complexity of its interaction with particles, the understanding of the pulsation characteristics of oil with different particle sizes and concentrations remains in the qualitative or semiquantitative stage. There is little literature that analyzes the pulsation characteristics of oil with higher viscous fluids. Therefore, it is necessary to investigate the pulsation characteristics of oil in the flow direction and for the normal pulsation strength as well as the instantaneous velocity and the average velocity based on the vector field data measured by the PIV of the oil containing particles in pipelines. Additionally, understanding the pulsation movement of oil at different concentrations and particle sizes reveals the interaction between particles and oil and lays a foundation for understanding the physical essence of the formation and development of oil pollution.