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The average rate of flow Situation and Development
Published on:2014-12-19 9:49:17 Hits:

Abstract: The number and location for the average rate of flow of the total pressure and back pressure detection hole sectional shape of the other issues were discussed detection lever. Details of the structure of the average rate in several forms, gives formulas using flow measurement and analysis of the impact of various factors on the measurement accuracy, the final product of the development of an idea put forward.


Keywords: average rate of flow measurement applications Factors


Average speed meter measuring element tube - the average rate (foreign countries, said Annubar, literally Annubar), is based on the principle of early pitot speed up development, is the late 1960s to develop a new type of differential pressure flow measurement devices, and start the application with China's industrial scene, the mid-1970s there were more than 30 manufacturers to develop and produce. All the advantages of the speed of the tube is; relatively simple structure (Figure 1), a small pressure loss, installation and easy removal, little maintenance.



The meter due to low production costs, low prices, it is more popular in the market, occupies a place in many of the flow meters. Especially because of the pressure loss is small (compared with the plate, only 5% below the orifice), greatly reducing the power consumption, energy-saving effect is remarkable, that in today's energy crisis, has its special significance. Because the meter to adapt to a wide range of long-term stability (Figure 2) has made great progress in recent years, there have been several different meter structure. But due to improper use, resulting in the application of a number of issues, making the objective requirements of the status quo and development has created a great contradiction, many people expect their application problems can be solved, for the tireless efforts of a lot of people, so that the average rate this flowmeter flowmeter both old and young, in energy, environmental protection and other measurement test has been more widely used.


A measuring principle of the average rate of flow sensor


The average rate of flow sensor, known by its structure diagram, which is inserted along a diameter of the pipe in the hollow metal rod, in the direction of fluid flow Looking pairs of pressure taps, generally two pairs, but there are one or more pairs of the outer shape of the flute. Towards multi-stream surface pressure taps measure is the total pressure and total pressure tube communicated, leads the average total pressure p1, back at the center of the flow surface generally has opened a hole, and static tube connected, leads to static pressure p2 . The average rate is the difference between the full use of measuring fluid pressure and static pressure measured flow rate. The average rate of the output of the differential pressure (△ p) and the average fluid velocity (v), can be drawn based on the classical Bernoulli equation

 (1) 

Where; difference △ P-- pressure and static pressure of the whole, Pa

ρ-- fluid density, kg / m 3

k-- correction factor.

If the flow is represented by its basic formula for calculating the flow


Wherein qv - volume of fluid flow, m 3 / s;

qm-- fluid mass flow, kg / s;

α - flow coefficient working condition of the average rate;

Under ε-- working state of the fluid stream flows through the beam expansion coefficient when the detection lever;

A-- working condition within the next pipe cross-sectional area, m 2

For different compressible fluid: ε = 1; for compressible fluids: ε <1

Total pressure hole location, according to decile area method to strike. Thus, in the case of the average rate of flow changes can have a better ability to adapt, as reflected in the smaller error. The so-called decile area method, is to split the pipe section into the inner circle and outer ring of the average flow rate equivalent points, which is the total pressure hole locations shown in Figure 3.

 


Total pressure hole opening position available Chebyshev numerical integration of the solution obtained, shown in Figure 4, Figure r1 = ± 0.4597R, r2 = ± 0.8881R, r1, r2 to take pressure away from the center of the pipe hole center distance, R is the inner radius of the pipe.

For this election point method, either the number or location, academia and international standardization organizations in recent years have raised an objection that the flow tube should be divided into three regions, selected according to the number of points - Chebyshev (Log- conduct Jchebycheff) method, therefore, the position of the total pressure detecting holes should be; r1 = ± 0.03754R; r2 = ± 0.7252R; r 3 = ± 0.9358R. This approach has been the International Organization for Standardization (ISO) in closed conduits flow measurement Committee (TC30) confirmed, in view of the above reasons, the experimental study of people, both the number of total pressure hole pitot tubes or recommend two or three pairs appropriate.


Backpressure takes a long time to detect holes, because people have realized that the average rate is in accordance with norms potential flow, potential flow and former title is the cross section of the pipe static pressure points are equal, there is no lateral flow. From this perspective, a back pressure detection hole is sufficient, in order to prevent obstruction of fluid flow back pressure detecting hole in the detection process, the porous backpressure take pressure, have begun to use the average rate of flow sensor, in short, by the basic formula of flow shows that as long as a valid measure the average rate of the output of the differential pressure △ P, can be measured by the value of the fluid flow, which is the average rate of the flow sensor tube measuring principle.


2 structure the average rate of


Average rate structure is a hollow metal rod, the largest cross-sectional shape is circular and diamond applications, also used the mid-1980s through the wing-shaped cross section.

The average rate of circular cross section, when the Reynolds number Re is between 105 to 106, so that the flow rate coefficient α is unstable, it is in the stable region Reynolds number Re <10 5 and Re> 10 6. This is mainly due to the circular cross-resistance element itself there is a "Drag crisis" caused. When there is fluid flow through the tube caused by the different separation point at different pressure distribution pipe facing the flow of fluid in the pipe caused by, causing the flow coefficient α changes.

Diamond-shaped cross-section of average rate, is to overcome the circular cross section of the flow coefficient unstable region and design. Diamond section regardless of how much the value of Reynolds number Re, the separation point is determined constant, thus solved the average rate of the difficulties in the unstable region of the flow sensor detects gas, steam flow. The average rate of use of diamond section, has been recognized by the total.

The wing-shaped cross section in order to further reduce the resistance to fluid flow through the welcome detection lever, thereby reducing the pressure loss. In fact, the average rate is concerned, whether circular or diamond-shaped cross-section, the pressure loss which can not be restored, are insignificant, accounting for only about 2% of the output of the differential pressure, but in the actual application process, the average rate the lower output differential △ P is one of its major weaknesses, when using the wing-shaped cross-section can indeed reduce some resistance, but smaller output differential pressure, differential pressure and reduce or diamond circular cross-section compared to 50%, due to the differential pressure is too low, it must be the work of supporting the more expensive differential pressure transmitters, work in this case, making the detection of instability, thus affecting its application.


3 meter average rate system composed of


Formula (2) and (3) below. Composition essence average rate of flow systems is a measure of the differential pressure △ P, which is common to all differential pressure flowmeter technology is generic, which uses differential pressure transmitter to △ P into corresponding mechanical signals or electrical signals can also be measured directly △ p and dealt with accordingly, the paper will not elaborate.


4 application process factors affecting measurement accuracy


Average rate of flow in the application process should pay attention to the problem, see national verification procedures JJG640-94 differential pressure flowmeter verification procedures and related literature, this paper will not elaborate.


5 Conclusion


It should be noted, average rate of flow from design, manufacturing to installation are very strict, as long as one part of a little mistake, can cause large errors.

Accurate measurement of flow rate is required for any production department, the issue is no department of general interest. To play its role in the future of measurement applications, improve the level of flow measurement is the way to go in the average rate of flow. Thermal average rate of flow is one way, shown in Figure 5.



Thermal average rate of flow consists of three parts, the detection lever, the display electronics and flow and totalizer. Detecting lever is a hollow metal tube, and on which a number of hot-wire configuration sensing element. Accordingly, each of the electronic circuits and only a combination of hot wire sensing element, the equivalent of a hot wire tachometer.

Thermal flowmeters are formulas for the rate of flow

 (5) 

Wherein; A-- pipe cross-sectional area, m 2

n-- pipeline section and other points;

ρ i - density of i-th feature point, kg / m 3;

υ i - flow rate of the i-th feature point, m / s;

Mass flow rate (p υ) are calculated as

 (6) 

Where: I is the heating current; R is the heater resistor; T is the temperature of the hot wire sensing element; T0 is the gas temperature; ρ is the measured density of the medium; υ is the fluid velocity: α 1, α 2, m is an empirical constant .



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