Study on length of mixed region during gas purging in pipeline

The low Reynolds number k-e turbulence model was applied for the 2-dimensional numerical simulation of the length of mixed region of the gas purging in natural gas pipeline conveying. The result indicates that the concentration distribution profile in the mixed region appears asymmetric along the axial direction, which has such a characteristic called "short head and long tail". Purging velocity, pipe diameter and length of purged pipe are three key factors to the length of mixed region. The mixture length increases very fast in the initial purging state and then changes slowly with the increasing of length of purged pipe. Purging velocity is one of the key parameters of control in the process of the purging. The length of mixed region is shorter during the turbulent flow than that during the laminar flow. Increasing the velocity of turbulent flow has less effect on shortening the length of mixed region. Under the same turbulent velocity, the length of mixed region increases with the increasing of the pipe diameter. But the impact of the pipe diameter on the length of mixed region is weakening while the turbulent intensity is high. The gas mixing length of nitrogen-natural gas is 1%-2% more than that of nitrogen-air.