RU2244310C1 - Method for measuring liquid flow velocity - Google Patents

Abstract

FIELD: measurement technology; measuring velocity of single-phase laminar and turbulent flow of liquid.

SUBSTANCE: proposed method includes pulsed power delivery from ac power supply to metering thermocouple junction; heating of metering thermocouple junction; measurement of power N and pulse time τ; measurement of junction temperature on descending section of T _j = f(τ) curve; evaluation of junction cooling rate on descending branch of T _j = f(τ), m = -(dT _j /dτ)/(T _j - T _l ) curve, where T _j and T _l are junction and liquid temperatures, respectively; calculation of heat-transfer coefficients α _as and α _des from dependencies α _des = mcρV/F and α _as = N/F(T _j - T _l ), where F is junction surface; V is junction volume; c and ρ are thermal capacity and density of junction material, respectively; calculation of mean value of heat-transfer coefficient α _m = (α _as + α _des )/2; calculation of liquid flow velocity W from W = f(α _m ) dependence; pulse time τ _m is chosen from condition of regular junction heating time τ _r , τ _m ≥ τ _r found from condition that m _as = -(dT _j /dτ)/(T _j - T _l ) = idem with time.

EFFECT: enhanced flow velocity measurement accuracy.

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Description

The invention relates to measuring technique and can be used to determine the speed of a single-phase fluid flow in laminar and turbulent flow regimes.

A known method for determining the flow rate of a liquid is that the thermocouple junction is heated from a separate heater, the junction temperature of the thermocouple is measured, the fluid flow rate is determined based on the previously obtained dependence W = f (T), where W is the flow velocity, T is the junction temperature thermocouples (B.I. Leonchik, V.P. Mayayakin. Measurement in dispersed flows. M: Energoatomizdat. 1981. P.92).

The disadvantages of the method are as follows: temperature measurement in this way has a large inertia, including due to the presence of an electrical insulating layer between the junction of the thermocouple and a separate heater. The design of the sensor is complicated due to the presence of electrical insulation elements, a separate heater. The operating temperature of the sensor is limited by the properties of electrical insulation and does not exceed 200-250 ° C.

The closest in technical essence and the achieved result to the proposed method is a method for determining the speed of a single-phase fluid flow, which consists in the fact that a fixed power is supplied from the AC source to the junction of the measuring thermocouple, the junction of the measuring thermocouple is heated, the useful signal formed by the junction of the measuring thermocouple is separated, from a signal generated by an alternating current source, measure the junction temperature of the measuring thermocouple, determine the fluid flow rate at new previously obtained dependence W = f (T),

where W is the flow velocity, T is the temperature of the thermocouple junction (B.I. Leonchik, V.P. Mayakin. Measurement in dispersed flows. M: Energoatomizdat. 1981. P. 92).

In the known method, the separation of the useful signal generated by the junction of the measuring thermocouple from the signal generated by the AC source is achieved using a low-pass filter consisting of a capacitor and a choke. The main disadvantage of this method is that with the help of a low-pass filter it is not possible to completely tune out interference from the AC source. Due to the passage of interference and the impossibility of quantifying them, the measurement of temperature and, accordingly, speed is carried out with a large error.

определяемое на основе условия - m_в=-((dTсп/dτ)/(Т_сп-Т_ж)=idem с течением времени, а максимальная температура спая не превышает температуру насыщения при давлении теплоносителя в сечении измерения скорости, где с, ρ, V, F - соответственно теплоемкость, плотность, объем, поверхность спая -определяются предварительно в процессе отладочных опытов, N - мощность импульса, W - скорость потока жидкости в месте размещения спая, м/с, Т_cп, Т_ж - соответственно температура спая (средняя по объему), температура жидкости °С, m=-(dT_cп/dτ)/(T_сп-Т_ж) - темп охлаждения (нагрева) спая, α_в и α_н коэффициенты теплоотдачи на восходящей и нисходящей участках кривой Т_сп=f(τ)The technical result, the achievement of which the present invention is directed, is to increase the accuracy of determining the fluid flow rate, which is ensured by the fact that the power is supplied to the thermocouple junction pulsedly, the power and pulse time are measured, the junction temperature in the descending section of the dependence T _sp = f (τ ), determine the cooling rate of the junction on the descending branch of the dependence T _cn = f (τ), m = - (dT _cn / dτ) / (T _cn -T _x ), determine the heat transfer coefficients α _in and α _n , based on the dependences α _n = mcρV / F and α _in = N / (F (T _cn -T _w )), determine the average e value of the coefficient of heat transfer α _cp = (α _a + α _n) / 2 is determined fluid flow rate on the basis of beforehand obtained dependence W = f (α _cp), wherein the pulse time τ _m is chosen from the condition of the regular heating mode τ _p junction,

determined on the basis of the condition - m _in = - ((dTsp / dτ) / (T _cn -T _l ) = idem over time, and the maximum junction temperature does not exceed the saturation temperature at the coolant pressure in the velocity measurement section, where c, ρ, V, F - heat capacity, density, volume, junction surface, respectively, are determined previously in the process of debugging experiments, N - pulse power, W - fluid flow rate at the junction location, m / s, T _cp , T _W - junction temperature, respectively ( volume average), fluid temperature ° С, m = - (dT _cp / dτ) / (T _cn -T _w ) - cooling rate (heat eva) junction, α _in and α _n heat transfer coefficients in the ascending and descending sections of the curve T _sp = f (τ)

The achievement of the technical result, which consists in increasing the accuracy of determining the speed of a single-phase liquid, is ensured by the fact that the determination of the flow rate is achieved on the basis of values that do not require separation of the useful thermocouple signal from the supply signal going to the input of the measuring circuit. Indeed, the cooling rate m _in , on the basis of which α _n is determined, is determined with the source turned off, and the heat transfer coefficient α _in for its determination does not require the use of a dependence of the form T _cn = f (τ), which is detuned from interference. The technical result is also achieved by the fact that in addition to determining the heat transfer coefficient on the descending part of the dependence T _cn = f (τ), an ascending section is used, on which the average value of the heat transfer coefficient on the ascending part of the dependence T _cn = f (τ) is determined. The use of the ascending and descending branches of the dependence to determine the heat transfer coefficients and, accordingly, the flow rate allows us to eliminate errors associated with the dependence of heat transfer on the direction of the heat flow (heating, cooling) and reduce errors caused by the initial conditions.

Figure 1 shows a diagram for implementing the method.

Figure 2 shows the qualitative dependence of T _SP = f (τ).

Figure 3 shows the calibration dependence W = f (α _cp ), based on which the flow rate is determined by the proposed method. The scheme for implementing the method, figure 1, includes the following elements 1 - Adjustable AC source, 2 - electronic key - is used to control the power source, 3 - separation capacity, 4 - thermocouple, 5 - low-pass filter, 6 - meter AC power, clock generator, 8 - ADC, analog-to-digital converter, 9 - FDA, personal computer.

The method of determining the fluid flow rate is as follows.

The thermocouple junction is placed in the channel, where the fluid velocity is measured. Measure the temperature of the liquid. Power is supplied from the AC source to the junction of the thermocouple. Next, the thermocouple junction is heated. Heating is carried out impulse. The pulse power, heating time and junction temperature are measured. The thermocouple is made according to a 4-wire circuit, i.e. power supply to the junction was carried out using copper insulated wires, and the output signal (measuring) using isolated chromel-kopel wires (an XK type thermocouple was used). After the thermocouple is heated to a certain junction temperature (the junction temperature should not exceed the saturation temperature at the coolant pressure in the velocity measurement section), the source is turned off and the junction is cooled. The heating and cooling times are equal, and this time exceeds the time for reaching the regular heating and cooling mode of the thermocouple junction. Next, the cooling rate of the junction on the descending branch of the dependence T _cn = f (τ), m = - (dT _cn / dτ) / (T _cn- T _l ) is determined, the heat transfer coefficients α _in and α _{N are} determined based on the dependences α _n = mcρV / F and α _in = N / (F (T _sp -T _w )), determine the average value of the heat transfer coefficient αav = (α _in + α _n ) / 2, determine the fluid flow rate based on the previously obtained dependence W = f (α _cp ). Since the determination of speed is valid in stationary modes, the above steps can be carried out both in real time and after receiving the corresponding data.

As an example, consider the definition of water flow in a pipe with a diameter of 10 mm. Performance parameters: pressure 16.0 MPa, temperature 200 ° С, water consumption varied from 180 to 360 kg / h. The dependence W = f (α _cp ) was previously obtained on a pouring (calibration) stand on an identical channel. The speed used for calibration was determined using a pitot tube mounted in the center of the pipe. A thermocouple was installed in the center of the pipe. When making measurements on a measuring stand, the speed in the center of the pipe was determined using a thermocouple and compared with the speed determined using the Pitot tube. A comparison of the speeds in the indicated range of expenses showed a coincidence in the range of 5-7%.

Claims (1)

A method for determining the fluid flow rate, which consists in supplying power from an alternating current source to the junction of the measuring thermocouple, heating the junction of the measuring thermocouple, measuring the temperature of the thermocouple junction, characterized in that the power is supplied to the thermocouple junction pulsed, measuring the power and time of the pulse, measuring junction temperature on the descending section of the dependence T _cn = f (τ), determine the cooling rate of the junction on the descending branch of the dependence T _cn = f (τ), m = - (dT _cp / dτ) / (T _cp -T _w ), determine the coefficients heat transfer α _in and α _n based on the dependences α _n = mcρV / F and α _in = N / (F (T _cn- T _w )), determine the average value of the heat transfer coefficient α _cf = (α _in + α _n ) / 2, determine the fluid flow rate based on the previously obtained dependence W = f (α _cp ), moreover, the pulse time τ _m is selected from the condition of achieving a regular mode of junction heating τ _r

conditional -m _B = - (dT _cn / dτ) / (T _cn -T _w ) = idem over time, and the maximum junction temperature does not exceed the saturation temperature at the coolant pressure in the velocity measurement section, where c is the heat capacity of the junction material; ρ is the density of the junction material; V is the junction volume; F - junction surface - pre-determined during debugging experiments, N - pulse power, W - fluid flow rate at the junction location, m / s, T _cn , T _W - junction temperature (volume average), fluid temperature, ° С, m = - (dT _cп / dτ) / (T _{cп -Т ж} ) - cooling rate (heating) of the junction, heat transfer coefficients α _in , α _n in the ascending and descending sections of the curve T _{cn =} f (τ).

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