RU2152593C1 - Flow-rate measurement method - Google Patents

Abstract

FIELD: measurement of mass flow rate of medium. SUBSTANCE: method is used for measuring of mass flow rate of variable-compound medium by means of thermoconvective transducer which includes heater and two thermoresistors positioned on external surface of tube. Time τ_g of thermal mark transfer between heater and fixed (control) section time τ_k of mark transfer along control section and measured. Density of measured medium is judged by difference (τ_g•τ_k). Value τ_g serves for calculation of volume flow rate. EFFECT: enhanced accuracy of measurement. 3 dwg

Description

 The invention relates to the field of instrumentation, in particular to methods for measuring the flow of substances.

 Among thermal methods for measuring flow, contact (calorimetric) and non-contact (thermoconvective) methods of measurement are distinguished. The advantage of both methods is the ability to measure mass flow, provided that the heat capacity of the measured substance is constant. Another advantage of thermoconvective flow meters is the lack of contact with the measured substance. The disadvantage of both is the large inertia. To improve performance, various methods are used, one of which is tagged.

 Known non-contact methods for measuring the velocities and flow rates of liquids and gases are based on measuring the transfer time of the heat mark between two sections of the measuring section in which the mark is recorded / Kremlevsky P.P. Flow meters and quantity counters. L., Mechanical Engineering, 1989 .-- 701 p. /.

 The closest in its methodology is a non-contact way of measuring flow rates based on heat marks, in which, in order to introduce corrections on the composition of the controlled medium and determine the mass flow rate, the mark cooling rate is additionally measured / USSR Copyright Certificate N 832341, class. G 01 F 1/70, (prototype) /.

 The main disadvantage of this non-contact method is the limited accuracy due to the following factors.

1. An unambiguous relationship between the speed (rate) of regular cooling of the label during its transfer by the flow with the value of the thermal diffusivity coefficient (α _c ) of the measured medium takes place only if there is no heat loss to the external medium surrounding the flowmeter transmitter or if they are constant. In real conditions, when measuring the flow rate by the non-contact thermal method, heat loss to the environment cannot be practically avoided, moreover, their value depends on the flow rate, which leads to an increase in the measurement error of α _c and, therefore, to a decrease in the accuracy of determining the mass flow rate.

где T_м -температура метки;
T_п - температура потока (измеряемой среды);
τ - время,
сопряжена с достаточно сложными совокупными измерениями, включающими операции логарифмирования и дифференцирования, что также ограничивает точность измерения.2. The implementation of measuring the speed (pace) of label cooling according to the formula:

where T _{m is the} temperature of the label;
T _p - flow temperature (measured medium);
τ is the time
It is associated with rather complex aggregate measurements, including the operations of logarithm and differentiation, which also limits the accuracy of the measurement.

 Therefore, the urgent problem of improving the accuracy of flow measurement by the precision non-contact thermal (thermoconvective) method.

 The solution to this problem is achieved by additionally measuring the transfer time of the heat label between its source and a fixed (control) area.

 In non-contact thermoconvective flow converters, when implementing the label method of measurement, the processes of heat transfer from the label source (heater) to the substance stream and from the stream to thermal converters are carried out through the channel wall through heat conduction and convection.

To eliminate the inertia of these thermal processes, the label transfer time is determined in the control area between two thermal converters at the moments when their reaction maxima are reached when the heat label passes. This value of the label transfer time (τ _k ) over the control (fixed) section is uniquely related to the volumetric flow rate and does not depend on the properties and composition of the medium being measured. It is registration by maxima of the reaction to the label that ensures the invariance of the flow meter readings to the properties and composition of the medium being measured. This is confirmed experimentally (see Fig. 2), as well as the following analytical calculations.

где T_п(x,t) - температура потока жидкости;
T_о - начальная температура тепловой метки;
x - линейная координата;
α_c - коэффициент температуропроводности жидкости;
2 • l - начальная длина тепловой метки;
следует, что при достижении максимумом метки зоны регистрации (то есть при ∂T_п/∂τ = 0) имеет место соотношение:

справедливость которого возможна только при выполнении условия x = v • t или t = x/l, то есть в рамках принятых допущений время переноса метки определяется только координатой регистратора (термопреобразователя) и средней скоростью (объемным расходом) жидкости и не зависит от ее свойств.From the analysis, the solution of the idealized problem of the propagation of a heat pulse (label) in a fluid stream:

where T _p (x, t) is the temperature of the fluid flow;
T _about - the initial temperature of the heat label;
x is the linear coordinate;
α _c is the thermal diffusivity of the liquid;
2 • l is the initial length of the heat label;
it follows that when the maximum reaches the mark of the registration zone (i.e., at ∂T _n / ∂τ = 0), the relation

whose validity is possible only if the condition x = v • t or t = x / l is fulfilled, that is, within the framework of the accepted assumptions, the label transfer time is determined only by the coordinate of the recorder (thermoconverter) and the average speed (volumetric flow) of the liquid and does not depend on its properties.

In order to take into account the change in the properties (composition) of the medium being measured and to determine the mass flow rate under the conditions of this change, it is proposed to additionally measure the transfer time of the mark between the mark source (heater) and the fixed (control) section. The value of this time (τ _d ) will consist not only of the time of label transfer by the flow, but also of the duration of the conductive-convective heat transfer from the heater to the liquid flow and from the flow through the channel wall (metal pipe) to the thermal converter. It is the inertia of the convective component of the processes of label formation in the substance flow and its registration on the outer surface of the channel (pipe) wall in the zone from the heater to the fixed (control) section significantly depends not only on the volumetric flow rate, but also on the properties (composition) of the medium. If, for simplicity, we take the option when the length of the fixed section is equal to the distance from the heater to the specified fixed section (l ₁ = l ₂ ), then by the difference (τ _d -τ _k ) one can judge the properties of the medium (for example, its density) and provide measurement mass flow rate of liquid with variable properties, while obtaining higher accuracy, by minimizing the disadvantages of the known method.

 In FIG. 1 shows a block diagram of a device for implementing the proposed method.

 The flow meter contains: a channel (metal pipe) 1, on the outer surface of the wall of which a heater 2 (for example, wire) is placed; 3 and 4 - measuring thermocouples; 5 and 6 - compensation thermal converters (film thermistors) included in the unbalanced DC bridge circuit 7 and 8 with amplifiers; a heater control unit 9 and a computing (microprocessor) unit 10.

×-ρ = 1115 кг/м³.
Экспериментальная проверка предлагаемого способа и его сравнение с известным проводилась в диапазоне расходов 0 - 40 кг/ч на потоках водных растворов солей NaCl и Na₂CO₂, с концентрацией от 4 до 16% вес. Использовался первичный преобразователь со следующими параметрами:
- внутренний диаметр патрубка - d_вн = 5 мм;
- толщина стенки патрубка - δ_cт = 0,3 мм;
- материал патрубка - нержавеющая сталь;
- напряжение, подаваемое на нагреватель в момент импульса - U_н = 36B;
- длительность импульса - τ_н = 0,2 c.
Границы распределения погрешности определения τ_д (как дополнительного информативного параметра о свойствах среды) составляют не более 1.8%, а при определении скорости охлаждения метки (темпа) - порядка 3%. В итоге, как показали эксперименты, погрешность определения массового расхода предлагаемым методом, в условиях изменения свойств среды, на 1 - 1.2% ниже, чем у прототипа /Авторское свидетельство СССР N 832341, кл. G 01 F 1/70/.The method is as follows. The control unit 9 periodically turns on the heater 2, generating heat marks into the stream. When the heater is turned on, a command is issued to the microprocessor unit 10 to start the time measurement. Upon reaching the maximum value of the reaction on the thermistor 3, from the passage of the heat label, the amplified output signal from the bridge 7 fixes the transfer time τ _d and the countdown starts for the transfer of the label between the thermistors 3 and 4. When the maximum signal occurs on the thermistor 4, through the bridge 8 s amplifier, unit 10 is determined by the transfer time of the label on the control section τ _to , and therefore the volumetric flow rate G ₀ ; the difference (τ _d -τ _k ), also determined by block 10, judges the flow properties (for example, density) and then determines the mass flow rate (G _m = G ₀ • ρ). A graphic illustration of these operations is shown in FIG. 2 and 3, where
τ $\genfrac{}{}{0ex}{}{\text{ism}}{\text{to}}$ - the measured time of label transfer by the flow over the control section;
G _о ^ISM - volume flow rate determined by block 10;
τ $\genfrac{}{}{0ex}{}{\text{ism}}{\text{d}}$ - measured label transfer time from the heater to the control section;
ρ ^ISM - the density value found by block 10;
1-τ _k = f (G ₀ );
2,3,4,5-τ _d = f (G ₀ , ρ);

× -ρ = 1115 kg / m ³ .
An experimental verification of the proposed method and its comparison with the known one was carried out in a flow range of 0-40 kg / h on streams of aqueous solutions of NaCl and Na ₂ CO ₂ salts, with a concentration of 4 to 16% by weight. The primary converter was used with the following parameters:
- the inner diameter of the pipe - d _VN = 5 mm;
- pipe wall thickness - δ _ct = 0.3 mm;
- pipe material - stainless steel;
- voltage supplied to the heater at the moment of impulse - U _n = 36B;
- pulse duration - τ _n = 0.2 s.
The boundaries of the distribution of the error in determining τ _d (as an additional informative parameter on the properties of the medium) are no more than 1.8%, and when determining the label cooling rate (pace), it is about 3%. As a result, as shown by experiments, the error in determining the mass flow rate of the proposed method, in conditions of changing the properties of the medium, is 1 - 1.2% lower than that of the prototype / USSR Copyright Certificate N 832341, cl. G 01 F 1/70 /.

Claims (1)

A method for measuring the flow rate using a thermoconvective flow transducer, based on measuring the time τ _to transfer the heat mark in a fixed area, with a correction in the composition of the controlled flow, characterized in that it additionally measures the time τ _{d of} transfer of the mark between its source and the fixed area and the difference (τ _d -τ _k ) judge the composition of the medium.

Images