What is Reynolds number?
Reynolds number is a dimensionless number that can be used to characterize the flow of a fluid. The ratio of the inertial force Fg and viscous force (internal friction) Fm during fluid flow is called Reynolds number. It is expressed by the symbol Re. Re is a factorless quantity. The Reynolds number can be used to distinguish the flow of fluid is laminar or turbulent. It can also be used to determine the resistance to the flow of an object in a fluid.
Instrument constant of flow meters, i.e. Reynolds number
Reynolds number is an infinite parameter used as a criterion to determine the flow pattern of a fluid as it flows through a pipe. It is related to pipe diameter, flow rate and medium viscosity.
The first used to determine the flow pattern of the fluid is turbulent flow a through laminar flow. A side Re greater than 3200 or more can be judged as turbulent flow. Of course, from the laminar flow to turbulent flow of the Reynolds number has a transition. So be specific and say that the Reynolds number must be greater than a certain value to say is to restore turbulent flow. Less than a certain value is said to be restored laminar flow.
Re = ρvd/μ, where v, ρ, μ are the fluid velocity, density and viscosity coefficient, d is a characteristic length. For example, if a fluid flows through a circular pipe, d is the equivalent diameter of the pipe.
When measuring the flow rate of a fluid in a pipe, it is often necessary to understand its flow state, velocity distribution, etc. Reynolds number is to characterize the fluid flow characteristics of an important parameter.
Fluid flow inertia force Fg and viscous force (internal friction) Fm ratio called Reynolds number. Re is a factorless quantity.
The Reynolds number is small, which means that the fluid flow between the viscous force between the masses dominates, the fluid masses parallel to the inner wall of the pipe flow regularly, laminar flow state.
Reynolds number is large, which means that the inertia force dominates, the fluid is turbulent flow state, general pipe Reynolds number Re < 2000 for laminar flow state, Re> 4000 for turbulent flow state, Re = 2000 ~ 4000 for the transition state.
In different flow states, the law of motion of the fluid. The distribution of velocity is different, so the average velocity of fluid in the pipeline υ and the maximum velocity υmax ratio is also different. Therefore, the size of the Reynolds number determines the flow characteristics of viscous fluids.
When the external conditions are geometrically similar (geometrically similar pipes, fluid flowing through geometrically similar objects, etc.), if their Reynolds numbers are equal, the fluid flow state is also geometrically similar (hydrodynamically similar). This law of similarity is the basis for the standardization of flow measurement throttling devices.
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The influence of Reynolds number on the vortex flowmeter
The influence of Reynolds number on the vortex flowmeter in a certain range of Reynolds number. Vortex flowmeter output frequency signal with the volume of flow through the measuring tube between the relationship is not affected by the fluid properties (density, viscosity) and components. That is, the flow coefficient is only related to the vortex generator and the shape and size of the pipe.
Therefore, only in a typical medium in the calibration of its flow coefficient and applicable to a variety of media. This is a major advantage of the vortex flowmeter.
However, if the Reynolds number exceeds this range, it will have an impact.
Vortex flowmeter working principle
In the fluid flow in the pipeline set a vortex generator (resistance fluid). So in the volume downstream of the generator will alternately produce a regular vortex. This vortex is called Kaman vortex street.
The vortex generator is the main component of the detector. It is closely related to the flow characteristics (meter coefficient, linearity, range, etc.) and resistance characteristics (pressure loss) of the meter, and the requirements for it are as follows.
Vortex flowmeter can control the vortex in the direction of the axis of the vortex generator synchronous separation.
In a wide range of Reynolds number, there is a stable vortex separation point, maintaining a constant Strohal number.
The ability to produce strong vortex street with high signal-to-noise ratio of the signal.
Simple shape and structure, easy processing and standardization of geometric parameters, as well as installation and combination of various detection elements.
materials should meet the requirements of fluid properties, corrosion resistance, abrasion resistance and resistance to temperature changes.
The inherent frequency is outside the frequency band of the vortex street signal.
Due to strict environmental requirements, some process industries, especially chemical, paper and water treatment, all placed under the strict monitoring of the agency. This will drive the vortex flowmeter production increase.
In addition, the privatization of municipal water treatment has become the trend, which will generate new investment to update the aging water treatment and sewer systems.
Multi-variable vortex flowmeters, which can measure temperature and pressure in addition to flow, are also in commercial use. Although its output is negligible, growth can be expected throughout the forecast period.
This enhanced measurement capability allows users to purchase fewer meters, requiring fewer process measurement points and thus fewer potential leak paths.
Read more about: How Vortex Flow Meter measure efficiently? For steam, gas, liquid
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