Air layer flow meter

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Air laminar flow meter, Heilongjiang Provincial Intellectual Property Office, Ji Weiyuan. This article discusses the design methods for specialized laminar flow meters used to measure air flow rates. A laminar flow meter consists of a differential pressure sensor and a laminar flow element; the laminar flow element is the key component that converts the gas to be measured from a turbulent state to a laminar state. Moreover, the design of the diameter and shape of the laminar flow tube within this element is of paramount importance when creating an ideal laminar flow element. Unlike the calibration method used in the design of general-purpose laminar flow meters, optimization is required when designing the laminar flow element here. The so-called design optimization first refers to the fact that the maximum measurable flow rate occurs when the Reynolds number of the laminar flow of the gas being measured is equal to the critical Reynolds number ； Next, choose a laminar flow tube in the form of an equilateral triangle ； Third, choose a differential pressure gauge whose maximum measurable differential pressure value is as low as possible; in this case, a gauge with a range of 0-30 file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DCA.tmp.png can be selected. Since the viscosity and density of air depend on the conditions at the time of measurement, during design it is first assumed that data under standard atmospheric conditions are used as a basis, and then adjustments are made based on the actual measurement conditions. 1. Determination of the maximum flow rate: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DCB.tmp.png The value of the maximum flow rate is determined based on the Reynolds number reaching its critical value when air flows through a laminar flow tube. When a triangular tube is chosen as the laminar flow tube, the formula for calculating the Reynolds number of air flowing within this triangular tube is: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DDB.tmp.png (1) Where: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DDC.tmp.png represents the flow rate of air inside the laminar flow tube ； file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DDD.tmp.png represents the density of air ； file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DDE.tmp.png is the dynamic viscosity of air ； a is the side length of the equilateral triangle forming the cross-section of the laminar flow tube. Under standard conditions: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DDF.tmp.png ； file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DE0.tmp.png; when the Reynolds number reaches the critical value, file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DF1.tmp.png, then it becomes file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DF2.tmp.png. (2) Determination of the side length of the equilateral triangle in the cross-section of a laminar flow tube: Once the length of the laminar flow tube is determined, the flow resistance encountered by air flowing within the tube depends on the size of the triangle’s sides. When the cross-section of the laminar flow tube is an equilateral triangle, the formula for calculating the resistance encountered by air inside the laminar flow tube is: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DF3.tmp.png (3), where l is the length of the laminar flow tube. Since the maximum differential pressure value of the selected micro-differential pressure should occur at the maximum flow rate, we have: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DF4.tmp.png (4) By substituting equation (2) into equation (4), we obtain: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DF5.tmp.png (5) That is: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7DF6.tmp.png (6) With l=10cm ； file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E06.tmp.png ； By substituting file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E07.tmp.png into equation (6), it can be determined that a = 0.333 cm. By substituting the value of a into equation (4), it is possible to determine the maximum flow rate for each laminar flow tube: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E08.tmp.png. 3. Determination of the maximum range: The size of this maximum range depends on the inner diameter of the flow meter as well as the number of laminar flow tubes. Since the laminar flow tubes are in the shape of equilateral triangles, their number can be calculated as follows: taking the center of the inner diameter of the flow meter as the origin, a series of concentric circles are drawn with the side length of the triangle and multiples of that side length as radii. These concentric circles are labeled from 1 to N; thus, 6 triangular tubes can be placed within the 1st circle ； 18 triangular tubes can be arranged between Garden 1 and Garden 2 ； And the total number of equilateral triangles in those N gardens is exactly: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E09.tmp.png. In other words, in a laminar flow element with an inner diameter of file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E0A.tmp.png, it is possible to arrange file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E0B.tmp.png equilateral triangles within such a laminar flow tube. Therefore, the maximum measurement range that this flow meter can handle can be determined as: file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E0C.tmp.png. (7) 4. Fabrication of the laminar flow element: Considering the design parameters determined above, 0.2mm thick stainless steel is selected as the raw material. First, prepare three thin stainless steel sheets with a length of l=10 cm and a width of file:///C:\Users\ADMINI~1.SKY\AppData\Local\Temp\ksohtml\wps7E1E.tmp.png; after folding and cutting grooves in them, they can be assembled together.