(c) 25 μm (d) 50 μm
Fig. 2 The distribution of streamwise mean intensity along the normal direction
Fig. 2 (a) shows the distribution of the flow pulsation intensity of the oil along the normal direction with different particle concentrations for a particle size of 5 μm. The figure shows that the flow pulsation intensity of the PIV acquisition oil field is relatively slow to change along the normal distribution in the central region due to the presence of particulate matter, while the change is steep in the near-wall area, and this trend becomes increasingly obvious as the particle concentration increases. On the other hand, when the particle concentration is lower, the flow pulsation intensity of the oil field is larger in the near-wall area and gradually decreases toward the center of the pipe; however, this trend becomes less obvious as the particle concentration increases. On the whole, with increasing particle concentration, the flow pulsation intensity of the oil successively decreases, and if the particle concentration is too large (such as 6.58 ppm and 14.40 ppm), the difference in flow pulsation intensity is smaller. The reason is that the particles in oil in the near-wall area increase the flow velocity of the oil. To maintain mass conservation, the oil flow velocity in the central region decreases. Additionally, coupled with the blocking action of the oil particles, the flow pulsation intensity of the oil is steeply distributed in the near-wall region along the normal direction, while this intensity is relatively gentle in the center area(Ling and Zhong, 1999). In addition, with increasing particle concentration in oil, the effect of near-wall particles on oil flow velocity growth is attenuated due to decreasing viscous shear force, which weakens the tendency of oil flow pulsation intensity growth in the near-wall area. Therefore, as the particle concentration reaches 3.00 ppm or more, the tendency of the flow pulsation intensity distribution along the normal direction becomes slower. This finding also indicates that the flow pulsation of the flow field in the pipeline is greatly affected by the wall surface.
According to the effect of the 15 μm particle size on the flow pulsation intensity of oil shown in Fig. 2 (b), the effect of different particle concentrations on the intensity distribution along the normal direction is basically consistent with that of the 5 μm particle size shown in Fig. 2 (a). The difference is that in the case of low particle concentration, the maximum value of the flow pulsation intensity is larger for the case of the 15 μm particle size. For example, the maximum value of the flow pulsation intensity for the 5 μm particle size under 0.65 ppm particle concentration is 0.233, while the maximum value of the flow pulsation intensity for the 15 µm particle size is 0.236. The reason is that the larger the particle size is, the greater the viscous shearing force of the oil, so the particles in the near-wall region increase the role of oil flow velocity growth.
Fig. 2 (c) shows the effect of different particle concentrations on the flow pulsation intensity of the oil when the particle size is 25 μm. The effect of the 25 μm particle size on the flow pulsation intensity of the oil is similar to the trends in Fig. 2 (a) and (b). However, for the case of the particle size of 25 μm, the flow pulsation intensity is larger in the near-wall region and has a value of 0.256, and then, the 5 μm and 15 μm amplitudes are increased by 9.95% and 8.56%, respectively. In addition, when the particle concentration is 6.58 ppm, the flow pulsation intensity of oil is larger, and this finding indicates that the particle size in the oil has less attenuation effect on the viscous shear force of oil when the particle size is 25 μm. However, the effect of the particles in the near-wall area on the oil growth rate is enhanced when the particle size exceeds 25 μm, as shown in Fig. 2 (d). When the particle size is 50 μm, the amplitude of the flow pulsation intensity under each particle concentration decreases greatly, except for the particle concentration of 0.65 ppm. Moreover, when the particle concentration is 6.58 ppm, the flow pulsation intensity is basically the same as that for 14.40 ppm.
The results in Fig. 2 show that the flow pulsation intensity decreases with increasing particle concentration, and the difference decreases gradually for oil with different particle concentrations.