The relationship between Reynolds number and flow meter

The value of the Reynolds number Re depends on three parameters, namely the velocity of the fluid and the characteristic size of the flow stream. When selecting based on flow rate, the Reynolds number must be taken into account.

The Reynolds number is a dimensionless parameter used as a criterion to determine the flow regime of a fluid flowing in a pipe. It is related to the pipe diameter, flow velocity, and fluid viscosity. It is primarily used to determine whether the flow of a fluid is turbulent or laminar. Generally, when Re is greater than 3200, the flow can be considered turbulent. Of course, there is a transition range in Reynolds numbers between laminar and turbulent flow; therefore, to be precise, a Reynolds number must exceed a certain value for the flow to be considered fully turbulent, and it must be below another certain value for it to be considered fully laminar. Only by knowing whether it is laminar or turbulent flow can one determine which flow meter to use How to calculate it? Because laminar flow and turbulent flow follow completely different laws, the formulas and theoretical parameters used are also different.

(ZT) When measuring the fluid flow rate in a pipe, it is often necessary to understand its flow state, velocity distribution, etc. The Reynolds number is an important parameter that characterizes the flow properties of a fluid. 　　?? The ratio of the inertial force Fg to the viscous force (internal friction force) Fm during fluid flow is called the Reynolds number. It is denoted by the symbol Re. Re is a dimensionless quantity. 　　A low Reynolds number means that, during fluid flow, the viscous forces between the various particles play a dominant role; the fluid particles flow in an orderly manner parallel to the inner wall of the pipe, resulting in laminar flow. A high Reynolds number means that inertial forces play a dominant role, and the flow is turbulent. Generally, in pipes, a Reynolds number Re < 2000 indicates laminar flow, Re > 4000 indicates turbulent flow, while Re = 2000–4000 corresponds to a transitional state. Under different flow conditions, the motion patterns of the fluid and the distribution of flow velocities vary, and as a result, the ratio of the average flow velocity υ of the fluid in the pipe to its maximum flow velocity υmax also differs. Therefore, the value of the Reynolds number determines the flow properties 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, then the fluid flow regime is also geometrically similar (hydrodynamic similarity). This similar principle is precisely the basis for the standardization of throttling devices used in flow measurement.

The Reynolds number is a dimensionless quantity that represents the ratio of the inertial forces to the viscous forces in a fluid. It is defined as: Re = Vl/u, where v is the average velocity of the fluid, in m/s； ι —— the characteristic length of the flow velocity; for example, in a circular pipe, it is the inner diameter of the pipe, in meters ； υ —— dynamic viscosity of the fluid, m2/s. If the Reynolds number is low, viscous forces play a dominant role, and viscosity has a significant impact on the entire flow field. If the Reynolds number is high, inertial forces are dominant, and viscosity only plays an important role in the flow within the boundary layer or in areas with large velocity gradients. For various types of throttling elements in standard throttling devices, there are certain limitations regarding the pipe diameter, diameter ratio, and range of Reynolds numbers applicable; these are detailed in the standard GB/T 2624-93 (or the international standard ISO 5167-1). For example, orifice plates can be used in a wider range of pipe diameters than nozzles and Venturi nozzles, while there are significant differences in the pipe diameter ranges among different types of conventional Venturi tubes. 　　The accuracy of various types of throttling elements in standard throttling devices, under the same differential pressure and density measurement accuracy, depends on the uncertainties of the discharge coefficient and the expansibility coefficient. The uncertainties in the discharge coefficients of various throttling elements vary significantly; in comparison, the uncertainty in the discharge coefficient of orifice plates is the smallest, while it is larger for shaped throttling elements (nozzles, venturi tubes). The reason for the larger profiled throttle elements is the poor quality of the fitted database on which the coefficient of discharge formula given in the standards is based. However, high precision can also be achieved by individually calibrating the profiled throttle elements. 　　At the same pressure difference, the pressure loss of classical venturi tubes and venturi nozzles is about 1/4 to 1/6 that of orifice plates and nozzles. And at the same flow rate and the same β value, the pressure loss of the nozzle is only 30%–50% of that of the orifice plate. While ensuring the safe operation of the instruments, efforts are made to improve their accuracy and energy efficiency. To this end, it is necessary not only to select display instruments that meet the accuracy requirements but also to choose a suitable measurement method based on the characteristics of the medium being measured. For the measurement of main steam flow in power plants, which is crucial for both the safety and economic efficiency of such plants, differential pressure flow meters equipped with standard throttling elements are generally used. Sewage from chemical water treatment processes and fuel fall under the categories of dirty flows and viscous flows with low Reynolds numbers, respectively; therefore, standard throttling elements are not suitable for these applications. For dirty flows, non-standard throttling elements such as elliptical orifice plates are generally used in combination with differential pressure gauges or ultrasonic Doppler flow meters, whereas for viscous flows, positive displacement flow meters, target flow meters, or wedge flow meters can be employed. Parameters such as the water inflow rate to the turbine, the circulating water volume in the condenser, and the regenerative steam from regenerative units are all flow measurement parameters with large pipe diameters (over 400 mm). Due to the difficulties involved in their fabrication and high pressure losses, standard throttling devices are generally not used for these applications. Depending on the characteristics of the medium being measured and the requirements for measurement accuracy, insert-type flowmeters, velocity sensors combined with differential pressure transducers, ultrasonic flowmeters are used, or lossless methods such as the marking method and simulation method are employed to measure flow rate

The Reynolds number is a parameter that characterizes the flow state of a fluid, with flow states including laminar flow, turbulent flow, and transitional flow. Instruments used for measuring flow rate based on the principles of fluid dynamics generally require that the fluid be in a turbulent state; typically, a Reynolds number of over 10,000 indicates turbulent flow.

The Reynolds number is a parameter that characterizes the flow state of a fluid, and the flow meters affected by it are differential pressure flow meters (orifice plates) and vortex flow meters.