【A journey of a thousand miles begins with a single step – Centrifugal Pumps】1.1 Understanding the concepts: head loss and friction loss

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This post was last edited by 3983596_FPPZ on 2018-11-14 at 15:28. 1.1.9 Frictional Losses and Local Losses: Today we will discuss a concept that is commonly encountered in fluid dynamics – frictional losses and local losses. Members often ask about the pressure required of a pump to transport fluid over a certain distance through pipes; this relates to these concepts. Due to the viscosity of liquids, there is inevitably friction between the flowing liquid and the pipe walls, which creates a certain velocity gradient and leads to a certain amount of frictional resistance, resulting in losses known as frictional losses. These frictional losses are related to the length of the pipeline and represent the main type of loss in long-distance pipeline transportation. Just like the frictional loss of the impeller disk, I classify it under mechanical losses. Due to the viscosity of liquids, there are interactions between liquids, including effects such as impacts and vortices, which result in certain local resistances and lead to losses; these are known as local losses. Compared to the frictional loss along the pipeline, local losses are generally small, and they often occur at the beginning and end of pipelines at locations such as elbows, diameter changes, and valves. Just like the hydraulic loss of the pump, I classify it under the category of hydraulic loss. Frictional losses and local losses can also be understood in terms of long-distance high-voltage power transmission: the losses incurred in long cables are much greater than those that occur at connections, switches, and other such components. Reynolds number: Laminar flow and turbulent flow: The flow state of liquids in pipes can be broadly classified into laminar flow and turbulent flow. Laminar flow is a state of flow in which the liquid at a certain distance from the pipe wall moves in an orderly manner according to specific patterns, resulting in a velocity gradient. Turbulence is a state of chaotic and disordered flow of the liquid in various parts of a pipe. Liquids with low flow velocities and high viscosity generally flow in a laminar state, while liquids with high flow velocities and low viscosity usually flow in a turbulent state. One can imagine a gently flowing river and floods that suddenly burst their banks and surge forth. Reynolds number: a parameter used to determine the flow state of a liquid in a pipe; for circular pipes in engineering applications, a Reynolds number of 2000 indicates that the fluid flow is typically turbulent. In fact, in most turbulent flows, the region near the center of the pipe is turbulent, while the region close to the pipe walls is laminar. Except for high-viscosity crude oil and fuel oil transported in laminar flow channels, the flow state of most liquids transported through pipelines that we observe is turbulent. Friction loss: hf = λ* L * v2 / (2gd). Here, λ is the friction coefficient, which depends on the Reynolds number (whether flow is laminar or turbulent) as well as the smoothness of the pipe walls; specific values can be found in specialized tables and charts related to fluid mechanics. L represents the length of the pipe, v is the average flow velocity, and d is the equivalent inner diameter of the pipe. From the formula for friction loss, it can be seen that the amount of loss is proportional to the square of the flow velocity – a slight increase in velocity results in a significant increase in loss. This is why there is such a concept as the \"optimal flow velocity\" when determining pipe dimensions; if the flow velocity is too low, the pipe must be very large, resulting in high costs for the pipe itself ； The flow rate is too high, resulting in excessive friction losses and high costs for the pump. Local loss: The local loss hj is given by hj = ζ* v2 / 2g. Here, ζ is the local loss coefficient; this coefficient takes into account factors such as contraction and expansion in the pipeline, as well as bends. There are also detailed and specific values related to filters, flow meters, grids, nozzles, and various valves. These values can be obtained by contacting relevant manufacturers or by referring to specialized charts and materials on fluid dynamics. v—average flow velocity. Methods to reduce local losses include not only lowering the flow velocity but also reducing the number of fittings and minimizing sudden changes in cross-section, as these are all measures that help to decrease losses. Especially in the pump inlet pipeline, minimizing local losses as much as possible can reduce or even prevent cavitation in the pump. In actual engineering applications, both frictional losses and local losses exist in pipelines simultaneously; it is merely the design and purpose of the pipeline that determine the relative importance of these two types of losses. In fact, there are many examples in everyday life that correspond to frictional losses and local losses: for instance, with running shoes, after running 600 kilometers the soles wear out, which is an example of frictional loss ； The newly bought soccer shoes were damaged after just one kick – it was a localized damage. After-class questions (multiple choice): Which of the following statements about frictional losses and local losses is incorrect? A. Reducing the frictional losses and local losses in the pipes of a device can increase the net positive suction head available (NPSHa) of that device. B. To reduce frictional losses, we should choose the largest possible pipe diameter. C. To reduce frictional losses, we should use pipes with a smoother inner wall. D. To reduce localized losses, we should clean the clogged filters in a timely manner. E. To reduce local losses, we should use welded reducer tubes with cross-sectional transitions. F. Frictional losses and local losses occur only in pipelines, whereas in fluid handling equipment such as centrifugal pumps, compressors, and metering pumps, these two types of losses are not present. BEF