What is a Flow Curve?

Two types of graph may be referred to as a flow curve, depending on the application. One application is in the field of fluid mechanics, where a flow curve shows the relationship between the dynamic viscosity and the shear rate of a fluid. Flow curves may also be applied to pumps, in which case they illustrate the relationship between the volumetric flow rate of the pump and the pump head.

In fluid mechanics, dynamic viscosity — also called absolute viscosity — measures a fluid's resistance to flow. In other words, dynamic viscosity indicates the amount of force that is required to move the fluid. Shear is the pressure experienced by a fluid due to forces that are parallel to the direction of flow — mainly the forces exerted by the walls of a pipe or channel. The shear rate is directly proportional to the velocity of the fluid, because the faster it is flowing, the more shear it experiences.

On a fluid flow curve, dynamic viscosity is plotted on the y-axis and shear rate is plotted on the x-axis. The resulting curve shows the relationship between these two fluid properties. For a Newtonian fluid, in which the dynamic viscosity is independent of the shear rate, the flow curve is a straight line, and the slope of the line is the dynamic viscosity. Non-Newtonian fluids, however, behave differently, and their flow curves are generally curved lines with various shapes. Some fluids are even time-dependent, meaning they have memory that affects the shape of the curve.

Pump flow rate is plotted on the x-axis of a pump flow curve, also called a system curve, and pump head is plotted on the y-axis. Pump head is a term used to describe the pressure loss in a pump. It is a combination of the elevation or static head, which is the difference in elevation between the inlet and outlet of the pump, and the head loss in the system, which is mostly due to friction in the pipes and fittings. Though head is a unit of pressure, it is usually measured in feet or meters — the farther the water has to travel, the more pressure is lost.

The pump flow curve can be used to determine the optimal operating conditions for a pump. If the user knows the flow rate they need, they can find the head loss for that flow rate using the system curve. By overlaying the system curve with a performance curve, which shows the performance for pumps of different sizes, the user can find the optimally sized pump for his or her system requirements.