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Influence of Low Reynolds Number on Vortex Flowmeter and Countermeasures

Date: 2019-03-19
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According to Karman vortex street theory, vortex street is stable only when h/L =0.2806, which is the most basic condition of vortex flowmeter.


Because only when the vortex street is stable can the vortex street flowmeter work normally.


However, for conventional vortex flowmeter, there should be another important condition, that is, the Strouhal number Sr (i.e. meter coefficient k) should be kept constant within the measurement range.


In fact, Sr and k are both functions of reynolds number ReD.


A large number of tests have proved that when Red≥5000, vortex flowmeter will enter the measurable area.  When Red≥20000, vortex flowmeter will enter the accuracy guarantee area.


For regions with Red≤5000, not only Sr has serious nonlinearity, but also vortex signal is very weak and difficult to detect.


Vortex Flowmeter with Low Reynolds Number Breakthrough in Low Reynolds Number Region.


The lower limit Reynolds number of vortex flowmeter can be reduced to below 5000, and series of products with DN18, DN25 and DN32 specifications can be manufactured with an accuracy of 1.5%


The key problems to be solved for low Reynolds number vortex flowmeter mainly include the following two aspects:


(1) keep Sr constant


It is well known that when Red<2300, the flow in the tube becomes laminar flow, and the velocity distribution also becomes parabolic laminar velocity distribution.


In this case, the vortex can be separated stably, but there is not necessarily a constant Sr.


In order to keep Sr constant, the flow velocity must be as uniform as possible in the radial direction.


The method adopted in document 5 is to add a layer of wire mesh near the upstream side of the triangle generator for rectification.


The wire mesh enables a relatively high flow rate of fluid near the center of the pipeline,


Due to greater resistance, if the flow rate is reduced, part of it will flow away near the pipe wall, thus increasing the flow rate near the pipe wall and making the flow rate distribution tend to be uniform.


Another important measure is to improve the smoothness and coaxiality of the pipe wall.  Because the smoother and straighter the pipe wall, the larger the uniform distribution range of flow velocity.


Due to the rectifying effect of the wire mesh, the flow velocity distribution of the measuring pipeline is relatively uniform in the range of very low Red.  When Red drops to 500, Sr can still be kept constant.




(2) improving the sensitivity of the detection element


The pressure detection technology as shown in figs. 10-9 is adopted.


At a certain distance downstream of the generator, a pressure detecting element is installed to detect the pressure pulsation signal of vortex oscillation wake.


Under the condition of low Reynolds number, the pressure detection method is adopted to effectively solve the problem of vortex street signal detection.


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