A flow curve can refer to either a graph showing the relationship between dynamic viscosity and shear rate of a fluid in fluid mechanics, or a graph showing the relationship between pump volume flow and pump head. The dynamic viscosity measures a fluid’s resistance to flow, while shear is the pressure experienced by a fluid due to forces parallel to the direction of flow. The flow curve for a Newtonian fluid is a straight line, while non-Newtonian fluids have curved lines. The pump flow curve shows the relationship between pump flow rate and pump head, which is the pressure loss in a pump. It can be used to determine the optimum operating conditions for a pump.
Two types of graph can 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 shear rate of a fluid. Flow curves can also be applied to pumps, in which case they illustrate the relationship between pump volume flow and pump head.
In fluid mechanics, dynamic viscosity, also called absolute viscosity, measures the resistance of a fluid to flow. In other words, dynamic viscosity indicates the amount of force required to move the fluid. Shear is the pressure experienced by a fluid due to forces parallel to the direction of flow, mainly 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 flows, 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, where 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 usually curved lines with different shapes. Some fluids are even time-dependent, meaning they have a memory that affects the shape of the curve.
The pump flow rate is plotted on the x-axis of a pump flow curve, also called a system curve, and the 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 elevation or static head, which is the difference in elevation between the inlet and outlet of the pump, and the pressure drop in the system, which is mainly due to friction in the pipes and fittings. Although the head is a unit of pressure, it is usually measured in feet or meters – the more water has to travel, the more pressure is lost.
The pump flow curve can be used to determine the optimum operating conditions for a pump. If the user knows the flow rate he needs, he can find the pressure drop for that flow using the system curve. By superimposing the system curve onto a performance curve, which shows performance for different sized pumps, the user can find the optimum size pump for their system requirements.
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