Disclosure of Invention
Technical problem to be solved
In order to avoid the defects of the prior art, the invention provides a method for measuring the direction and the speed of air flow in an air system, and solves the problems of complex measurement, high measurement cost and complex operation of the conventional flow speed measurement method.
Technical scheme
A method for measuring the direction and speed of an air flow in an air system, characterized by the steps of:
step 1: estimating the speed and direction of the fluid to be measured by CFD numerical calculation software to obtain the total speed of the air flow at the measuring point to be measured and the speed in each direction;
step 2: under the condition that a pitot tube total pressure measuring point, namely an L-shaped short end of the pitot tube is aligned to a speed direction needing to be measured, selecting a static pressure measuring point direction with lower speed on the pitot tube as a static pressure measuring point according to a CFD (computational fluid dynamics) calculation result;
and step 3: reserving the total pressure measuring points on the pitot tube and the static pressure measuring points obtained in the step 2, and sealing the rest static pressure measuring points on the pitot tube;
and 4, step 4: fixing a pitot tube at a measuring point position to be measured, aligning a total pressure measuring point of the pitot tube to a speed direction to be measured, and respectively connecting a total pressure leading pipe and a static pressure leading pipe to a pressure gauge;
and 5: during the experiment, the total pressure and the static pressure of the pitot tube are measured, and the airflow direction speed is calculated according to the measured differential pressure and the Bernoulli equation:
in the formula, V is the speed of the airflow direction, m/s; pt-total pressure in the direction of the gas flow, Pa; p0-static airflow pressure, Pa; rho-gas flow density, kg/m3。
Advantageous effects
The method for measuring the direction and the speed of the airflow in the air system can directly obtain the direction and the speed by only measuring two pressures, namely a group of differential pressures. The measurement is simple and convenient, and the measurement cost is greatly reduced. Through comparison, the maximum absolute deviation of the measured value of the direction speed measuring method provided by the invention and the same-direction speed obtained by the analysis of the traditional five-hole needle is 4.72m/s, the maximum relative deviation is 5.96 percent, the accuracy is very high, and the direction speed of the airflow can be accurately obtained. The method solves the problem that the traditional method for measuring the speed in the airflow direction is lacked, a three-dimensional flow velocity measuring instrument is required to be used when measuring the speed, and the speed in each direction is determined according to the three-dimensional flow velocity result. Has the problems of high use cost, strict use conditions, complex operation and the like.
Detailed Description
The invention will now be further described with reference to the following examples and drawings:
the invention adopts the basic idea of measuring the total pressure and the static pressure of the airflow in a certain direction in order to solve the problem of measuring the speed of the airflow in the air system, and then determines the speed of the airflow by using the Bernoulli equation.
Step 1: and (3) estimating the speed and the direction of the fluid to be measured by adopting CFD numerical calculation software to obtain the total speed and the speed in each direction of the airflow at the required measuring point.
Step 2: and (3) pressure measurement of a total pressure measurement point of the pitot tube is controlled, namely the L-shaped short end of the pitot tube needs to be aligned to the direction of a speed incoming flow to be measured, the directions of all static pressure measurement points of the pitot tube are fixed when the direction of the total pressure measurement point is fixed, and the direction of the static pressure measurement point with a lower speed is selected as the static pressure measurement point according to the CFD calculation result. When the speed of the total pressure measuring point is ten times higher than that of the static pressure measuring point, the relative error brought by the method is only 0.5 percent.
And step 3: and (4) reserving the total pressure measuring points on the pitot tube and the static pressure measuring points obtained in the step two, and sealing the rest static pressure measuring points on the pitot tube.
And 4, step 4: and fixing the pitot tube at a measuring point position to be measured, aligning a total pressure measuring point of the pitot tube to the incoming flow direction of the speed to be measured, and respectively connecting the total pressure leading tube and the static pressure leading tube into a pressure gauge.
And 5: and (3) carrying out an experiment, reading the total pressure and the static pressure measured by the pitot tube, and calculating according to the measured differential pressure and a formula 1, namely a Bernoulli equation to obtain the speed of the airflow in the direction.
In the formula, V is the speed of the airflow direction, m/s; pt-total pressure in the direction of the gas flow, Pa; p0-static airflow pressure, Pa; rho-gas flow density, kg/m3。
In an air system, when the airflow direction to be measured is the circumferential airflow speed, the selection of the static pressure measuring point can be based on the following principle: when the airflow is axially pre-rotated, the airflow has higher circumferential speed and axial speed, and the radial speed is lower, and pressure measuring holes arranged in the radial direction can be selected to replace static pressure; when the air flow is radially pre-rotated to be fed, the air flow has larger circumferential speed and radial speed, and the axial speed is smaller, so that the static pressure can be replaced by arranging pressure measuring holes in the axial direction.
In a word, the method for measuring the speed in the airflow direction is to measure the total pressure in the direction of the airflow required to be measured, select and reasonably arrange static pressure holes according to the speed distribution condition to approximately replace the static pressure of the airflow, and further obtain the speed in the airflow direction according to the Bernoulli equation.
To control the error of this directional velocity measurement method, the applicable conditions of this method need to be given. When only the error caused by the static pressure measurement point replacement is considered without considering the systematic error, the following error analysis can be performed:
suppose that the true static pressure at the measurement point is P0And the total pressure in the direction measured at the total pressure measuring point is as follows:
in the formula, Pt-the measured total directional pressure, Pa; p0The true static pressure at the measurement point, Pa. Vt-real direction velocity of air flow at the direction total pressure point, m/s.
And the approximate static pressure measured at the static pressure measurement points is:
in the formula, P(0)Measuring the static pressure, Pa, V at the measuring pointS-static pressure measurement point air flow velocity, m/s; .
The measured airflow direction velocities are known as:
in the formula V(t)-the velocity of the air flow direction measured at the measuring point, m/s.
From equation (5) it can be inferred that:
when in use
Then, 0.995V was found
t≤V
(t)≤V
t (6)
Therefore, when the measuring point speed size is satisfied:
in the method, the relative deviation between the airflow direction speed calculated by the method and the real airflow direction speed is not more than 0.5%.
The relative error under different conditions can be listed according to equation (5), table 1:
| airflow direction velocity/static pressure point velocity (V)t/VS)
|
Relative error
|
| 10
|
0.5%
|
| 5
|
1.2%
|
| 3
|
6% |
Example 1:
with reference to FIG. 1, the air flow is axially fedAfter reaching the static
rotational flow disk 1 shown in the figure, the direction of the airflow is changed through the
inclined hole 2, and the airflow has circumferential speed
Total velocity V
outAnd the included angle between the axial direction and the axial direction is theta. The size of the airflow deflection angle theta is related to the size and direction of the incoming flow speed, the deflection angle of the pre-rotation hole and the length-diameter ratio of the pre-rotation hole, and when the size and direction of the incoming flow speed are determined and the length-diameter ratio of the pre-rotation hole reaches a certain value, the airflow deflection angle theta is equal to the deflection angle of the pre-rotation hole.
As can also be seen from FIG. 1, the velocity of the air flow direction, e.g. the circumferential velocity, is obtained
The size of the flow coefficient is important for obtaining the air flow circulation characteristics, and the air flow rotation ratio, the air flow and axis angle and the like can be calculated, so that the size of the flow coefficient can be predicted.
Rotation ratio calculation formula:
wherein, the beta-rotation ratio;
-gas flow circumferential velocity, m/s; w-rotor angular velocity, rad/s; r-local rotor radius, m.
The air flow angle is shown in fig. 1, and the calculation formula is as follows:
in the formula, the angle between theta-airflow and an axis is included; vX-gas flow axial velocity, m/s.
Referring to fig. 2 and 3, the layout of a typical swirl generator and a measuring device in an air system is shown, wherein 1 is a swirl disk, 2 is a swirl inclined hole, 3 is an outer casing, and 4 indicates the position of a pitot tube. The airflow generates rotational flow after passing through the rotational flow disk, the direction is changed, and the pitot tube is arranged on one side of the airflow outflow rotational flow hole and can be used for measuring the circumferential speed of the airflow outflow rotational flow hole.
And the attached figure 4 is a schematic diagram of a pitot tube structure, wherein 5 is a total pressure measuring point, the direction is aligned with the circumferential speed of the airflow to be measured, 6-1 and 6-2 are static pressure measuring points, 6-1 is a radial position static pressure measuring point, and 6-2 is an axial position static pressure measuring point. 7 leading out a total pressure pipe connected with a total pressure channel of the differential pressure type scanning valve for measuring pressure, and 8 leading out a static pressure pipe connected with a static pressure channel of the differential pressure type scanning valve for measuring pressure. The static pressure measuring points are arranged in four directions, proper static pressure measuring points are selected according to actual conditions, the rest measuring points are sealed, when the relative error is controlled within 0.5%, and the speed in the total pressure direction needs to be more than ten times of the speed in the static pressure direction. After the differential pressure scanning valve measures the differential pressure, the circumferential speed of the air flow can be calculated according to the formula (1).
Example 2:
as shown in the figure, the airflow axially enters, the airflow direction changes after passing through the rotational flow pore plate shown in the figure 2, the circumferential speed with a certain size is obtained, and the circumferential speed of the airflow is obtained when the requirement is met
When the direction speed measuring method is used, the circumferential speed can be obtained
The size of (2). The specific parameters of the swirl orifice plate are shown in Table 2:
table 2: specific parameters of swirl orifice plate
| Diameter of swirl orifice plate
|
304mm
|
| Swirl orifice radial position
|
115mm
|
| Diameter of swirl hole
|
7mm
|
| Axial included angle of swirl hole
|
45° |
Step 1: an appropriate static pressure measuring point needs to be selected on the pitot tube. As the rotational flow pore plate is axially pre-rotated to intake air, the numerical values of the axial speed and the circumferential speed of the airflow are larger, the numerical calculation results of a plurality of working conditions show that the radial speed of the rotational flow pore plate is smaller than 1m/s, the circumferential speeds of the measuring points under different working conditions are larger than 10m/s, and the application condition that the relative error is 0.5% is met, so the rest static pressure measuring points are sealed, and the measuring result of the pressure measuring point 6-1 at the radial position of the pitot tube is selected as the static pressure value.
Step 2: the measuring device pitot tube is arranged. The pitot tube is 10mm away from the axial position of the swirl orifice plate, and the radial position is the center of the swirl orifice. In conjunction with fig. 1 and 2, a pitot tube measurement position is shown at 4. And respectively connecting the total pressure leading pipe and the static pressure leading pipe into a differential pressure type scanning valve.
And step 3: the differential pressure is read, and the airflow circumferential velocity is obtained according to the equation (1).
To verify the correctness of the measurement method proposed by this patent, five holes are utilized herein to measure the circumferential velocity of the air flow at the same location.
The five-hole needle and the pitot tube are arranged at the same radial position and the same axial position behind the rotational flow orifice plate, and the circumferential angle is 180 degrees. And then processing the speed and direction measured by the five-hole needle to obtain the circumferential speed, comparing the circumferential speed with the measured data of the pitot tube, and verifying the correctness of the direction and the speed measured by the pitot tube.
The experimental condition of the measurement is given in table 3, the circumferential speed measured by the five-hole needle is compared with the pitot tube, and the maximum absolute deviation between the circumferential speed measured by the pitot tube and the circumferential speed measured by the five-hole needle is 4.72m/s, the maximum relative deviation is 5.96%, so that the high accuracy of the pitot tube on the measurement of the directional speed is realized, and the pitot tube has engineering application value.
Table 3: comparison of measurement conditions