Characteristics and mechanism of turbulent drag reduction in microgrooves in natural gas transportation pipelines

In order to reduce the resistance generated at the wall in the process of natural gas transport in the pipeline and, thus, improve the transport efficiency, a 426 mm diameter pipeline was used as the simulation object, and based on the turbulence reduction technology, by constructing a semi-circular groove structure with a longitudinal distribution of s (width) = h (depth) = 0.540 72 mm and alternating width semi-circular groove structures on the inner wall of the pipeline, and using the large eddy simulation method, the numerical simulation of methane gas is carried out under the working conditions of incoming flow velocity from 5 to 25 m/s. Based on the drag reduction effect at different Reynolds numbers, the evolution of vortex structure, the velocity strip structure at different y+ (dimensionless wall heights), and the root-mean square of the spreading, normal, and flow pulsation velocities, the near-wall region of the flow field characteristics of the smooth surface and the groove surface were compared and analyzed, so as to derive the mechanism of drag reduction in the groove. The results show that with the increase in the Reynolds number, the drag reduction rates of both types of microgroove structures showed a trend of first increasing and then decreasing, and the maximum drag reduction effects of 17.99% and 15.93% were observed when the s+ (dimensionless groove width) was between 16.96 and 20.84, respectively. The longitudinal microgroove structure has a drag reduction effect by suppressing the rising process of the flow vortices and the spreading motion of the vortex structure.