Theory of Drag Reduction
Frictional pressure drop, or drag, is a result of the resistance encountered by flowing fluid coming into contact with the pipe wall. There are generally two types of flow – laminar and turbulent. The friction pressures observed in laminar flow cannot be changed unless the physical properties of the fluid are changed. The current class of DRA does not change fluid properties and hence is effective only in turbulent flow. In most petroleum pipelines, the liquid flows through the pipeline in a turbulent regime. Therefore, current DRA can perform very well in most pipelines.
In a turbulent flow regime, the fluid molecules move in a random manner, causing much of the energy applied to them to be wasted as eddy currents and other indiscriminate motion. DRA works by an interaction of the polymer molecules with the turbulence of the flowing fluid.
In order to understand how drag reducers decrease the turbulence, it is necessary to describe the structure of turbulent flow in a pipeline. Turbulent flow in a pipeline that has three parts to the flow. In the very center of the pipe is a turbulent core. It is the largest region and includes most of the fluid in the pipeline. This is the zone of the eddy currents and random motions of turbulent flow. Nearest to the pipeline wall is the laminar sub layer. In this zone, the fluid moves laterally in sheets. Between the laminar layer and the turbulent core lies the buffer zone.
Drag reduction occurs due to suppression of the energy dissipation by turbulent eddy currents near the pipe wall during turbulent flow.
There still is much to be learned about polymeric drag reduction, as there still is much to be learned about the complex phenomenon of turbulence. Recent research into this area tells us that the buffer zone is very important because this is where turbulence is formed first. A portion of the laminar sub layer, called a “streak”, will occasionally move to the buffer region. There, the streak begins to vortex and oscillate, moving faster as it gets closer to the turbulent core. Finally, the streak becomes unstable and breaks up as it throws fluid into the core of the flow. This ejection of fluid into the turbulent core is called a turbulent burst; the bursting motion and growth of the bursts in the turbulent core results in wasted energy.
Drag reducing polymers interfere with the bursting process and reduce the turbulence in the core. The polymers absorb the energy in the streak, like a shock absorber, thereby reducing subsequent turbulent bursts. As such, drag-reducing polymers are believed to be most active in the buffer zone.
For an evaluation of whether LSPI’s drag reducing products can aid in your application,
contact your LSPI area representative.