Hydraulic resistance - and how will we flow?

With any movement, there are energy losses - at least it will be a car, at least an airplane, even a liquid in the pipeline. Always part of the energy is spent on overcoming resistance to movement. Reducing the head of the liquid and it is customary to determine how the hydraulic resistance. In fact, there are two types of such resistance - local and linear. Local is associated with energy losses in the valves, valves, bends, extensions and narrowings of the pipe.

It should be noted that the viscosity of the liquid always serves as a source of loss . Local losses or hydraulic resistance, the calculation formula of which is related to the parameters of the valves, pipes and valves, is determined by a special method. But the linear losses largely depend on the nature of the flow of liquid in the pipe.

Investigations of fluid flow regimes were carried out by Reynolds in 1883. In these studies, a stream of water was used, to which paint was added, and the character of the movement of paint and water could be observed in the glass tube. In this case, the pressure, velocity and pressure of the liquid were measured.

The first mode of motion was observed at low water velocity. In this case, the paint and water do not mix with each other and move together along the pipe. The velocity and pressure are constant in time. Such a fluid flow regime is called laminar.

If, however, the speed of motion increases, then at a certain value the picture of the motion of the liquid will change. A stream of paint begins to mix around the entire volume of the tube, vortex formations and rotation of the liquid become visible. The measured values of the velocity and pressure of the liquid begin to pulsate. Such a motion is called turbulent. If the flow rate is reduced, then laminar motion is restored again.

With a laminar flow of fluid, the hydraulic resistance is minimal, with turbulent flow it is much larger. Here it is necessary to clarify that there are still friction losses on the pipe wall. The velocity during laminar flow is minimal at the pipe wall and is maximum along the center of the flow, but the flow of water moves smoothly along the entire pipe. In turbulent motion, the resulting turbulence creates obstacles to water movement and additional hydraulic resistance.

There is another phenomenon that contributes to losses. It is called cavitation. Cavitation is observed when a bottleneck appears in the flow of fluid in the pipe. Then at such a place the speed of motion increases and, according to Bernoulli's law, the pressure decreases. Decrease in pressure leads to the fact that the release of dissolved gases in the liquid starts and the water begins to boil at the current temperature.

After passing through a narrow section, the flow velocity decreases, the pressure increases and boiling disappears. Cavitation causes additional losses due to local laminar flow disturbances. As a rule, it arises in cranes, latches and other similar nodes. Such a phenomenon is considered extremely undesirable, because Can lead to damage to the entire pipeline system.

Thus, it turns out that hydraulic resistance is a concept that is determined by several factors. These include the design features of the pipeline system (length, bends, cranes and latches), including the material from which the pipes are made. The nature of the flow of liquids also affects the loss. This allows us to understand what a pipeline system should be and what should be avoided in its design and operation.

In the presented material such concept as hydraulic resistance in relation to the pipeline system is considered. A description is given of the various flow regimes of the liquid and its behavior in the pipes.