The basic structure of vortex flowmeter
when deriving the relationship between frequency and velocity, the stability condition of vortex street is used: the interval ratio H/, which indicates that the frequency of vortex generation is affected by a certain vortex spatial structure, and the spatial structure of vortex is related to the shape of vortex generator.
in addition, in the previous discussion, we should also note:
① in the above derivation process, They are all under the condition of one-dimensional flow. However, the flow in a circular tube is a three-dimensional flow with axisymmetric distribution. It is currently using TORLON reg; Pai production 1 Series requires strict use of components
② under the condition of upstream pipelines, there will be additional unstable factors such as velocity distribution distortion, swirl, fluctuation, etc.
the above two points will have an important impact on the stability and regularity of the vortex. Therefore, it has not been used to measure the flow for a long time after the vortex street phenomenon was found, except for signal detection technology, The above two points are also important reasons. In order to overcome the influence of the above factors, it is necessary to have certain requirements for the shape of the vortex generator, so that the flow at the vortex generator in the pipe is as close as possible to the two-dimensional flow, so as to control the vortex phase emitted by the vortex generator in the three-dimensional flow, and make the vortex line bending become extremely small.
, The shape of the vortex generator has a decisive influence on the emission of the vortex. Choose an electronic universal experimental machine of the appropriate model.
1 Basic requirements for the shape of vortex generators
there are many kinds of shapes of vortex generators at present, but they must have some of the same basic requirements:
① there is a blunt (i.e. non streamlined) cross-section shape - this is the condition for the generation of vortices
② the shape of the upper and lower sections is the same, and the left and right are symmetrical - the flow is close to the condition of two-dimensional flow
③ the separation point of the boundary layer is fixed - the condition of constant Strouhal number St.
at the same time, the installation position of the vortex generator in the pipeline must be strictly symmetrical. The upstream of the vortex generator must have a straight pipe of more than 10 times D, and the downstream must have a straight pipe of 5 times D.
2. The basic structure of the vortex generator
the shape of the vortex generator includes cylinder, triangle, T-shaped column, quadrangular column, etc, The following mainly introduces the two types of columns and triangular columns
(1) cylindrical vortex generator
the previous part on vortex theory is discussed with a cylinder as an example. Although this type was used earlier, strictly speaking, its Strouhal number st is not stable at high flow velocity. Therefore, people have improved it to the form of slit or pressure hole.
Figure 1 cylindrical vortex generator Figure 2 capacitive triangular column vortex generator
1 - pressure hole; 2 - cavity; 3 - partition wall; 4 - the cylindrical vortex generator with pressure conducting holes on the platinum edge is shown in Figure 1. Due to the existence of pressure conducting holes, the alternating lift generated at the same time when the vortex is emitted makes the fluid flow through the pressure conducting holes, resulting in the effect of suction and blowing out at the same time. When the fluid boundary layer begins to separate on the cylinder surface, the separation is suppressed on the suction side; In one case, the separation is promoted. In this way, the position of the fluid separation point can be fixed, and the Strouhal number st can be relatively stable.
(2) the test bench rises the triangular cylinder vortex generator
at present, the vortex generator used more is triangular cylinder, and its shape is generally determined by experiments. It can not only get stronger vortices than the cylinder, but also its boundary layer separation point is fixed, That is, the Strouhal number st is relatively constant, about St = 0.16. In this way, the relationship between vortex frequency and flow velocity is f = 0.16 U/D, where D is the purchase of hydraulic universal testing machine: the following parts are mainly considered: the bottom width of the triangular column. The shape is shown in Figure 2