Reynolds knowledge introduction
Published on:2015-1-19 9:10:51 Hits:

Often must understand their liquidity status, velocity distribution of fluid flow within the measuring tube. Reynolds is the characterization of fluid flow characteristics is an important parameter.

Inertial force and viscous force F g (internal friction) than F m of fluid flow when the Reynolds number is called. Re indicated by symbol. Re is a dimensionless quantity.


Wherein the dynamic viscosity η with kinematic viscosity υ be replaced by η = ρυ, then



l υ - average fluid velocity;

l l - setting the size of the stream;

l ρ, η eleven in the working state; fluid kinematic viscosity and dynamic viscosity

l ρ - measured fluid density;

From the above equation, the size of the Reynolds number Re depends on three parameters, namely the speed of the fluid, the viscosity of the stream shape and size of the work state.

Transmission fluid with a pipe, when calculating the Reynolds number, size and generally setting the pipe diameter (D), then


Transmission fluid with a square tube, pipe size to take shape equivalent diameter (D d). Equivalent diameter equal to four times the hydraulic radius of. For any cross-sectional shape of the pipe, the hydraulic radius is equal to the ratio of the area of the pipeline stamp perimeter. So the length and width of A and B, respectively


Rectangular pipe, its equivalent diameter pipe for any cross-sectional shape

The equivalent diameter, and can be four times by a cross-sectional perimeter than the cross-sectional area calculation, therefore, the Reynolds number is calculated as


Reynolds number is small, which means when the viscous fluid flow between the force of each particle is dominant, fluid lines parallel to the inner wall of each particle flow rules, was laminar flow state. Reynolds number is large, which means that the inertial forces dominate, was turbulent fluid flow conditions, general pipeline Reynolds number Re <2000 for laminar flow, Re> 4000 as a turbulent state, Re = 2000 ~ 4000 for the transition state. Under different flow conditions, the movement of the fluid. Distribution of flow rate and the like are different, and thus the ratio of the average fluid velocity υ pipe υ max and the maximum velocity is different. Therefore determines the size of the Reynolds number viscous fluid flow characteristics. The following figure shows a smooth pipe Reynolds number ReD and the speed ratio V / Vmax relationship.


Pipe Reynolds number Re p and speed ratio smooth tube V / Vmax relationship

Tests showed that when the external conditions of geometric similarity (geometrically similar tubes, the fluid flow through the geometric similarity of objects, etc.), if the Reynolds number are equal, the fluid flow state is geometrically similar (similar fluid dynamics). This is similar to what the law of flow measurement throttling device standardized basis. Visible, Reynolds accurately reflects the flow characteristics of the fluid flow measurement is commonly used parameters.

Reynolds number flow expressed as:


M - mass flow measured media kg / h:

Q - measured medium volume flow m / h;

D - pipe diameter mm;

v - the working state of dynamic viscosity measured medium Pa · S

p - the working state of the kinematic viscosity measured medium m 2 / s

Wherein the constant value, varies depending on where the unit of each parameter. When using a non-specified unit type, constant value should be amended accordingly.

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