RU2267790C2 - Method of measuring of a gas or liquid flow speed - Google Patents

Abstract

FIELD: oil-and-gas producing industry; methods of measuring of a gas or a liquid flow speed.

SUBSTANCE: the invention is intended to be used at measurements of a liquid or a gas flow speed by means of heat loss anemometers. In a gas or a liquid flow locate a thermo-sensitive transformer for measuring of a flow temperature and a temperature-sensitive transformer with a heating unit (a thermoanemometer) for measuring of the flow speed and conduct the measurements of the ambient temperature and the temperature of the thermoanemometer. If the difference of the temperatures of the temperature-sensitive transformers (overheating) lays within the limits of the preset range, then the power of a heating is constant, and at an alteration of the speed of the flow the overheating varies; if the overheating of the thermoanemometer overcomes the limits of the preset range, then the power of the heating varies before the overheating hits in the preset band. The value of the measured speed of the flow is calculated by a formula considering the measured overheating and the power of the heating. The technical result is an increased accuracy of measurements of a flow speed within a wide range of speeds and temperatures at a high performance.

EFFECT: the invention ensures an increased accuracy of measurements of a flow speed within a wide range of speeds and temperatures at a high performance.

Description

The invention is intended for use in measuring the flow rate of a liquid or gas using hot-wire anemometers.

Known methods for measuring the flow rate and flow rate of a liquid or gas, based on the use of hot-wire anemometers [1; 2].

The aim of the invention is to improve the accuracy of measuring flow rates in a wide range of speeds and temperatures at high speed.

It is known [1; 2] that the power dissipated by the hot-wire anemometer and its temperature in the steady state convective heat transfer are related by the formula

where P is the power

S is the surface area,

ξ is the heat transfer coefficient,

T _t - temperature anemometer,

T _with - the temperature of the medium.

The heat transfer coefficient ξ depends on the flow rate of a liquid or gas. This dependence is described by the formula (1)

where V is the flow rate,

a, b, n are coefficients depending on the thermophysical properties of the medium, the design of the hot-wire anemometer, and the Reynolds number.

One of the known methods for measuring the flow rate using hot-wire anemometers is based on stabilization of the power P (or current) of heating [1]. When the flow rate changes, the temperature of the hot-wire anemometer changes. In this case, the temperatures of the hot-wire anemometer T _t and the medium T _s are measured, the overheating θ = T _t -T _s is determined, then, based on formulas (1) and (2), the speed V is calculated at P = const. The disadvantage of this method is that with an increase in the flow velocity V, overheating θ decreases, which does not allow for high accuracy of measurements in a wide range of speeds.

Another known method of measuring the flow velocity is based on the stabilization of the temperature T _t (resistance) of the hot-wire anemometer [1]. When the flow rate changes, the heating power P, which is measured, changes. In this case, it is also necessary to measure and compensate for the effect of changes in the temperature of the medium, which significantly complicates the application of this method in a wide range of changes in the temperature of the medium.

In a wide range of changes in speeds and temperatures, the best results are provided by the measurement method based on stabilization of the superheat θ = T _t -T _s . This method is closest to the proposed one. Based on it, for example, a thermo-anemometric device is constructed, which is described in patent RU No. 2017157 [3]. In this device, overheating θ is stabilized, and the output value is the heating power of the hot-wire anemometer.

However, this method also has significant disadvantages. Firstly, the error in measuring the flow rate is largely determined by the error in stabilizing the overheating, which in turn depends on the errors of all the links included in the automatic control circuit. In this regard, it is practically impossible to ensure high accuracy of measurements in a wide range of changes in the speed and temperature of the medium.

So, for example, in a hot-wire anemometer device [3], the error in stabilization of overheating is caused by the following reasons:

- not the identity of the conversion functions of the three semiconductor thermal converters (thermistors);

- the difference in a wide temperature range of the real conversion function of these thermistors from the used mathematical model R _t = A · exp (W / T);

- errors of all other links included in the automatic control circuit of overheating.

Secondly, the inertia of the automatic control system for overheating limits the speed of the measuring device.

The essence of the proposed method for measuring the flow velocity is as follows.

The range of permissible overheating [θ _min , θ _max ] is set. When using semiconductor thermal converters (thermistors), this range can be units or the first tens of degrees Celsius. At a fixed heating power P, the temperatures of the hot-wire anemometer T _t and the medium T _c are measured, and the overheating θ is determined. The obtained value of θ is compared with the boundaries of the range [θ _min , θ _max ]. If θ _min ≤θ≤θ _max , then the heating power P does not change (P = const). A change in flow rate causes a corresponding change in overheating. If the superheat leaves the specified range (θ <θ _min or θ> θ _max ), then the heating power changes until the superheat is approximately in the middle of the specified range [θ _min , θ _max ]. After that, measurements of overheating are made with a new fixed value of the heating power. The value of the measured flow rate is calculated by the formula

where V is the measured flow rate;

P is the heating power;

θ = T _t -T _s - overheating;

k, s, d are constant coefficients.

Formula (3) for calculating the flow rate is obtained from equations (1) and (2).

Thus, in the proposed method for measuring the flow velocity, two operating modes of the measuring device are used:

heating power control mode;

measurement mode.

In the control mode of the heating power, the temperature measurements of the hot-wire anemometer and the medium and the assessment of overheating can be performed with low accuracy, because there is no need for an accurate task and determination of overheating. It is enough to ensure that the superheat is in the given range [θ _min , θ _max ]. In this case, simple mathematical models of measuring transducers and simple control methods can be used.

In the measurement mode, the heating power is fixed, overheating and heating power are measured with high accuracy, and then the flow rate is calculated by the formula (3). In this case, more complex and more accurate mathematical models of measuring transducers (for example, thermistors) are used. Since in the measurement mode there is no regulation of the heating power (P = const), this ensures high speed measuring device. If the measured flow rate changes relatively slowly, then power control is rather rare.

Thus, the separation of the procedures for measuring the flow rate and regulating the heating power allows one to obtain a small range of overheating [θ _min , θ _max ] with a wide range of permissible changes in the flow rate and a large range of medium temperatures. Due to this, high accuracy of flow rate measurements is achieved in a wide range of speeds and temperatures, and high speed is also provided.

The proposed method was implemented in a device for measuring the speed of a liquid (water). A miniature thermistor with a negative temperature coefficient was used as a measuring transducer of the medium temperature. The hot-wire anemometer contained a similar thermistor and heater placed in one housing. The resistances of both thermistors were converted to digital code using the Sigma-Delta ADC. Power control was carried out using a digital-to-analog converter. The control of the ADC, DAC and the necessary calculations were performed using a microprocessor.

In the power control mode for a rough estimate of overheating, a simple mathematical model of the conversion function of the thermistors was used:

where R _t is the resistance of the thermistor;

T is the absolute temperature;

A and B are constant coefficients.

In this case, the characteristics of the thermistors in the hot-wire anemometer and the medium temperature transducer were considered identical. These assumptions greatly simplify the procedure for evaluating overheating.

The calculated rough estimate of overheating was used to control the heating power of the hot-wire anemometer, and stepwise step regulation was applied. With this method of regulation, the process of regulating the heating power can be iterative. However, as experimental studies have shown, for a given range of overheating (10 ± 3) ° С, the heating power is regulated before overheating falls into the specified range in one step.

In the mode of measuring the flow velocity for thermistors, more complex mathematical models were used:

1 / T = A + BlnR _t + C (lnR _t ) ² + D (lnR _t ) ³ , (5)

where R _t is the resistance of the thermistor;

T is the absolute temperature;

A, B, C, D are constant coefficients.

In this case, the coefficients A, B, C, D for each thermistor were determined individually, which made it possible to measure the temperature in the range (0-100) ° C with an error of (0.01-0.02) ° C with a resolution of 0.001 ° C. In the range of flow velocities V _max / V _min = 100, the error in measuring the velocity did not exceed 1% in the entire indicated temperature range. At the same time, the medium temperature was measured with high accuracy, which is necessary in many cases of practice.

The proposed method for measuring the flow rate of a liquid or gas can be applied both to measure the local flow rate and to measure flow.

Literature

1. Kremlin P. P. Flow meters and quantity counters: a Handbook. - 4th ed., Revised. and add. - L .: Engineering, Leningrad. Dep., 1989 .-- 701 p.

2. Levshina E.S., Novitsky P.V. Electrical measurements of physical quantities: (Measuring transducers). Textbook manual for universities. - L .: Energoatomizdat, Leningrad. Dep., 1983 .-- 320 p.

3. Dubovsky VV Hot-wire device. RF patent No. 2017157. Bull. No. 14, 07/30/94.

Claims (1)

A method of measuring the flow rate of a liquid or gas, in which a temperature-sensitive transducer for measuring the flow temperature and a hot-wire anemometer are located in the measured flow, measure the superheat as the temperature difference between the hot-wire anemometer and the medium, control the heating power and calculate the flow rate, characterized in that the range of permissible overheating is set and, if overheating is within the specified range, the heating power does not change, and if overheating is outside the specified range, then the heating power is nyayut overheating before reaching the specified range, the value of the measured flow rate is calculated using the formula V = k (P / θ-c) ^d, where V - flow rate P - heating power, θ = T _m -T _c, T _m - temperature of the hot-wire anemometer, T _c - temperature of the medium, k, s. d are constant coefficients depending on the thermophysical properties of the medium, the design of the hot-wire anemometer, and the Reynolds number.