WO2017162353A1

WO2017162353A1 - Flow measurement device for measuring a flow parameter of a fluid and method for flow measurement

Info

Publication number: WO2017162353A1
Application number: PCT/EP2017/051832
Authority: WO
Inventors: Ulf Hammerschmidt
Original assignee: Bundesrepublik Deutschland, Vertreten Durch Das Bundesministerium Für Wirtschaft Und Energie, Dieses Vertreten Durch Den Präsidenten Der Physikalisch-Technischen Bundesanstalt
Priority date: 2016-03-23
Filing date: 2017-01-27
Publication date: 2017-09-28
Also published as: DE102016105501A1

Abstract

The invention relates to a flow measurement device (10) for measuring a flow parameter (v) of a fluid (14), in particular a gas, comprising a duct (12) for conducting the fluid (14) in a flow direction (R), an elongated heating element (24), at least one thermometer (26) and a control unit (22) which is connected to the heating element (24) for emitting a temperature impulse and connected to the thermometer (26) for determining a maximum temperature, and which is configured to automatically calculate the flow from the maximum temperature. According to the invention, the thermometer (26) is arranged upstream in relation to the flow direction (R).

Description

Flow measuring device for measuring a flow parameter of a fluid and method for flow measurement

The invention relates to a flow measuring device for measuring a flow parameter of a fluid, in particular a gas, with (a) a passage for passing the fluid in a flow direction, (b) an elongated heating element, (c) at least one thermometer, and (d) a control unit which is connected to the heating element for outputting a temperature pulse and connected to the thermometer for determining a maximum temperature at the thermometer and which is adapted to automatically calculate the flow from the maximum temperature. According to a second aspect, the invention relates to a method for flow measurement comprising the steps of (a) passing a fluid having a temperature T _g in a flow direction through a passage, (b) delivering a temperature pulse by means of an elongated heating element, (c) measuring a maximum temperature means a thermometer and (d) automatically calculating a flow parameter that characterizes the flow from the maximum temperature.

Such flow measuring devices are known from DE 10 2014 010 939 C1 and serve, for example, for measuring the gas flow in household gas pipes. Since these are mass devices, the flowmeter should be particularly simple, small and robust. In addition, the flowmeter should have the highest possible measurement accuracy in order to avoid error calculations.

It is known from DE 10 2014 008 284 A1 to determine the temperature as a function of time in each upstream upstream and downstream thermometer and from the time difference between the dispensing of the temperature pulse and the arrival of the temperature pulse on the thermometer and the distance between the heating element and the thermometer to determine the flow rate. This always requires two thermometers and the measuring accuracy at low flow velocities.

A method in which a periodically fluctuating temperature is used, is known from DE 10 201 1 010 461 A1.

DE 10 2014 008 284 A1 teaches that a thermometer arranged upstream of the heating element measures a temperature profile which is essentially independent of the flow velocity.

The object of the invention is to improve and / or simplify the measurement of the flow parameter.

The invention solves the problem by a generic flow measuring device in which the thermometer is arranged upstream with respect to the flow direction. According to a second aspect, the invention solves the problem by a generic method in which the temperature pulse is emitted with respect to the flow direction downstream of the thermometer. It has been shown that the upstream measurement of the temperature together with the determination of the maximum temperature allows the measurement of the flow parameter with a particularly low measurement uncertainty. This is especially true if - as provided in accordance with a preferred embodiment - a distance between the elongated heating element and the thermometer is less than 250 microns, in particular 150 microns. This is especially true for small flows.

In the context of the present description, the flow parameter is understood in particular to mean a parameter which characterizes the flow through the feedthrough. For example, the flow parameter is the flow velocity v, a flow D of flowing medium, for example liters, molar amount and / or mass per unit time M, or one

Quantity M, in particular a volume, a quantity of substance and / or a mass Medium that has flowed through the passage during a predetermined time unit Δί. Of course, more than one of these sizes can be determined. An elongated heating element is understood in particular to mean a heat source which, in a spatial dimension, has an extent which is at least ten times greater than the dimensions in the other two spatial directions. In particular, the elongate heating element can thus be considered, to a good approximation, as a line-shaped heat source.

The thermometer is understood in particular to mean a device by means of which the temperature and / or the temperature increase relative to an initial temperature can be determined. It is possible, but not necessary, for the thermometer to generate a measured value which indicates the temperature in a temperature unit, for example in degrees Celsius, Fahrenheit or Kelvin. In particular, it is sufficient that the absolute temperature and / or the temperature increase relative to an initial temperature T _g can be determined unambiguously from the temperature signal. Under maximum temperature is the time maximum of the temperature or temperature increase above an initial temperature T _g , in particular that temperature which is applied to the thermometer, when the heating element does not give off heat, understood. By emitting a temperature pulse is meant, in particular, a heating which takes place over such a short period of time that an approximation as Dirac pulse leads to a measurement deviation of at most 5%. For example, heating takes less than a tenth of a second, more preferably less than 1 millisecond.

Under the fluid is understood in particular a gas or gas mixture. Preferably, they are volatile hydrocarbons such as natural gas and / or biogas and / or hydrogen. Alternatively, the fluid may also be a liquid or a liquid mixture. By the feature that the flow rate parameter is calculated from the maximum temperature, it is to be understood in particular that it is possible, but not necessary, that the maximum temperature itself be used directly. In particular, it is also possible for the calculation to use digital signals which encode the corresponding size.

By the feature that the flow parameter is calculated by means of an equation, it is understood in particular that mathematical operations are performed which correspond to this equation. It is possible, but not necessary, for the given equation to be used exactly as written. If, for example, a factor is set equal to 1, then, of course, no multiplication by 1 must be carried out when applying the formula since such a multiplication does not change the result. It is also possible to add another term to the equation that will only marginally change the result. For example, it is possible to add any term that changes the measurement accuracy by less than 1%. It is also possible to use a mathematically equivalent equation for the calculation.

When it is said that a temperature, for example, the maximum temperature, is determined, it is understood that a measured value is determined, which represents the temperature. As a rule, the temperature is represented by an electrical measurand, for example an electrical voltage.

It should be noted that in a flow meter, the direction of flow is indicated. Regardless of any explicit designation of this flow direction, for example by a graphical representation, which is attached to the flowmeter according to a preferred embodiment, the flow direction can also be uniquely assigned to the flowmeter by the control unit being designed to determine the flow parameter calculated using an equation that then gives a correct result when the fluid flows in the direction of flow through the flow meter.

Of course, it is possible and constitutes a preferred embodiment that the flow measuring device is designed so that it can be flowed through in two directions. In this case, the flow measuring device has at least two thermometers, wherein the heating element is arranged between the two thermometers. Since the temperature pulse always moves downstream faster than upstream, the control unit can preferably be designed so that it automatically determines from the measurement results of the two thermometers in which direction the fluid flows and the flow rate based on the measurement of the upstream Thermometer determined. In the following, a derivation of the formulas usable for the evaluation is given.

Mathematical models for the thermal balance of thermal anemometry usually consider technical arrangements of heating element (s) and thermometer (s) that operate downstream. For the propagation of the heat generated by the heating element, this inevitably results in that heat conduction and advection (collective flow of heated fluid particles) are superimposed additively to forced convection. The derivation of working equations for this combined transport creates a complicated system of differential equations which until now can only be solved approximately.

The outlined "Downstream" situation can be formally simplified by considering the upstream heat propagation in the otherwise flowing fluid instead, so the thermometer is located at a not too close mutual distance upstream of the heating element, where heat conduction and advection do not interfere constructively The result of this "upstream situation" can always be particularly simple, if at least mathematically approximated technical realization in microtechnology is present. For both functional elements, very small volumes and correspondingly low heat capacities can then be mathematically determined. Also, the mutual distance of heating element and thermometer may be chosen very small (eg at most

150 μιτι, for example).

At the time f = 0 sets the line-shaped heating element of length L at the location x = 0 instantaneously, ie 5-function-like (for example τ = 100 μηη), the enthalpy Ho free. This propagates in the surrounding, stationary fluid, in particular gas, ("G") of the temperature T = T _G and the thermal conductivity λG and the thermal conductivity aG isotropically and homogeneously by conduction

The maximum temperature is

It occurs at the place ± x at the time? _max = x ² / 4a _G on. For example, tmax = 5 ms. In (2), [pc _p ) _G = λ _g / a _g denotes the volumetric specific heat for the

Density of the fluid, e is the Euler number 2,71828 ....

The heater is now to be overflowed by a fluid with the flow velocity v \ n (positive) x-direction. The fluid withdraws the enthalpy from it by forced convection

Η _κ = A _H t _m ^ (T _H -T _G ) = a A _H t _{mäx (} ^H ° with ⁽³⁾

\ P ^C P) H ^V

Call it "heat transfer coefficient> A _H dievom fluid-stained surface of the heater and tmax the effective Überströmzeit, ie the time that elapses between the temperature pulse and the arrival of the maximum temperature on the thermometer. The temperature maximum of an upstream (x <0) thermometer T1,

one obtains from equations (2) and (3):

(4) where A _H d = V _H. This result is only valid for places x <0 (eg x = -50 μηη). For a known fluid, 4 can be simplified

AT m ^{l a ⁾ x (Vx, 't max

If the considered sensor is located near the wall of the flow channel, then the heat transfer coefficient can be simplified with a mostly good approximation

a «bv ^£ with ε ~ 0.5. (6) With the further approximation

becomes (5) finally:

AT

m ^{l a ⁾ x (Vx, 't max V-,' max, '/

From this one obtains the flow velocity to:

, 2 v [T (V = O) -TU (V)] ^: (9)

^{[PP, H} ftV max [LΔΓ m «ax (Vν = 0) / 1 J ²

While the parameter b generally depends on the fluid and the flow boundary conditions, (pc _p ) _H and c / can be considered as sensor constants. Applying the respective temperature maxima above the root from the flow velocity results in a straight line with a negative slope. Therefore, in a preferred embodiment, the control unit is configured to automatically calculate the flow using an equation that calculates the maximum temperature, a maximum rest temperature, and a factor that depends on the flow rate, where the maximum rest temperature is the maximum temperature at the thermometer is applied when the flow rate of the fluid is zero and the heating element is energized in the same manner as in the determination of the maximum temperature. In this case, it is only necessary to calibrate the factor, which depends on the flow rate, and to measure the maximum rest temperature. Thereafter, the flow parameter, in particular the flow rate, can be determined solely from the respective maximum temperature.

Preferably, the control unit is therefore configured to automatically calculate the flow or the flow rate using the formula

ΔΓ «(- χ, ν) = ΔΓ« (- χ, ν =

That's it

AT ^ _x (-x, v) the maximum temperature,

AT ^ (-x, v = 0) the maximum resting temperature,

- the (positive) distance of the thermometer from the heating element,

tmax the effective overflow time,

b is a proportionality factor,

v ö \ e (unknown) flow rate,

p d \ e density of the fluid,

{pc _p ) _{H is} the volumetric specific heat of the heating element

d a sensor constant.

The sensor constant d is typically calculable as the volume of the heater divided by the cross-sectional area of the heater.

The characteristic that the flow parameter is calculated on the basis of the given formula is understood in particular to mean that a mathematical Operation is performed, which corresponds to this formula. Of course, it is possible to calculate further measured values from the measured value thus obtained, for example the total flow through integration over time under the mass flow through multiplication by the density of the fluid.

It is particularly favorable if the control unit is set up for automatically calculating the flow parameter on the basis of the formula (9). This formula is based on the approximation that the flow velocity in the vicinity of the heating element and the thermometer corresponds to the root of the total quasi integrable flow velocity.

According to a preferred embodiment, the heating element and the thermometer are arranged at a distance of at most 3 mm, in particular at most 1 mm, from an inner wall of the passage.

In the following the invention will be explained in more detail with reference to the accompanying drawings. It shows

1 shows a schematic cross section through a flow measuring device according to the invention and

2 shows a schematic perspective view of a measuring chip on which a heating element and the at least one thermometer are integrated.

1 shows schematically a flow measuring device 10 according to the invention, which has a passage 12 for passing a fluid 14. The flow measuring device 10 also has a measuring chip 16, which is fastened, for example, on an inner wall 18 of the passage 12. Via cable 20, the measuring chip 16 is in communication with a control unit 22. On the measuring chip 16, a heating element 24 and a first thermometer 26 (referred to above as T1) and a second thermometer 28 are formed. In the present case, the heating element 24 and the thermometers 26, 28 are etched out of a substrate 30. It can be seen that the thermometers 26, 28 are arranged upstream with respect to a flow direction R. It is possible, but not necessary, for the flow measurement device 10 to have a transport size determination device 32 arranged, for example, in a branch 34 of the passage 12. The branch 34 contains the same fluid as the passage 12 itself, but a flow rate v is, to a good approximation, zero, since the fluid 14 is greatly decelerated, for example by means of a frit 36. By means of a further chip 38, the thermal transport size, for example the thermal conductivity λ G and / or the thermal conductivity α G of the fluid 14 is determined. Such processes belong to the state of the art and will not be described further here.

FIG. 2 shows the measuring chip 16 in an enlarged schematic view. It can be seen that out of the substrate 30, a channel 40 has been worked out, which the thermometers 26, 28 span. A distance Χ2β between the heating element 24 and the first thermometer 26 is in the present case according to 26 = 50 μιτι. A second distance X28 between the second thermometer 28 and the heating element in the present case is X28 = 400 μιτι.

The control unit 22 is configured to automatically apply a temperature pulse of duration r. In addition, the temperature is continuously

first thermometer 26 measured. After dispensing the temperature pulse, the temperature T2 & rises and goes through a temporal maximum

From the formula given above, the flow rate v can then be determined if the other parameters of the equation are known.

The flow rate v is also calculated from the maximum temperature that the second thermometer 28 measures. For example, the arithmetic mean of the two measured values determined in this way is output as the measurement result. Alternatively, the second thermometer 28 may be used to measure the gas temperature Tg to determine the temperature increase and thus the maximum temperature.

If you wear the Eqs. (5) ΔΓ «(- x, v) = ΔΓ« ( ^■ - χ, ν = 0)

above v ^{^} , a straight line is obtained because of a «bv ^E. For example, with ε = 0.5, the straight line is created

(-j, v) = mv + n. This has the intercept

»= Δ7 £ ⁾ (- _Χ , _ν = 0). (1 1) and the slope

To calibrate m and n, proceed as follows: In a given measuring situation (enthalpy of the heater H ₀ = U Ι τ, gas type, temperature), a constant flow velocity between v = 0 and v = v _{fflai is applied} to the flow _{measuring device} and measures the associated temperature maximum x, v) at time t = t _max . The quantity v _max denotes the maximum speed. (The electric power loss of the heater, P = UI, should be set at a given pulse duration (eg τ = 100 μ &) preferably so that even at v _mr , _r a sufficiently clear temperature maximum AT ^ - x, v = v) to measure is.) The obtained discrete temperature maxima ΔΓ ^ (- χ, ν) are applied (usually) to Vv. With the help of a straight line adaptation one finds the searched values for m and n.

LIST OF REFERENCE NUMBERS

10 flow meter

exponent

12 transmission

Ho enthalpy

14 fluid

L length of the heating element

16 measuring chip

R flow direction

18 inner wall

T temperature

20 cables

v flow rate

22 control unit

V _H Volume of the heating element

24 heating element

X distance, space coordinate

26 first thermometer

α heat transfer coefficients

28 second thermometer

Duration of the temperature pulse

30 substrate

32 Transport size determination → Thermal conductivity

direction P density

34 branch £

Time 38 chips

40 channel

time unit

aG thermal conductivity

AH overflowed surface of the heating element

b parameters

dH effective thickness of the heating element

(Sensor constant)

Claims

claims:

Flow measuring device (10) for measuring a flow parameter (v) of a fluid (14), in particular a gas, with

(a) a passage (12) for passing the fluid (14) in a flow direction (R),

(b) an elongate heating element (24),

(c) at least one thermometer (26) and

(D) a control unit (22), the

- Is connected to the heating element (24) for delivering a temperature pulse and

- Connected to the thermometer (26) for determining a maximum temperature, and

is set up to automatically calculate the flow from the maximum temperature,

characterized in that

(E) the thermometer (26) with respect to the flow direction (R) is arranged upstream.

Flowmeter (10) according to claim 1, characterized in that the control unit (22) is arranged to automatically calculate the flow rate using an equation which

the maximum temperature (ΔΓ ^ (- χ, ν)),

- a maximum rest temperature (ΔΙ ^ - χ, ν = θ), which is the maximum temperature applied to the thermometer (26) when a flow rate (v) of the fluid is zero, and

- a factor that depends on the flow rate (v) has. Flow measuring device (10) according to claim 1 or 2, characterized in that the control unit (22) is adapted to automatically calculate the flow parameter, in particular the flow and / or the flow rate (v), based on the formula

where is:

ÄT ^ the maximum temperature,

-x the distance of the thermometer (26) from the heating element (24),

[pc _p ] _{H is} the volumetric specific heat of the heating element (24) for the

Density p,

d is a sensor constant,

tmax the effective overflow time,

b is a constant that depends on the gas and the flow conditions, the flow velocity and

ε an exponent that indicates the flow velocity near the edge in comparison to the flow velocity (v).

Flow measuring device (10) according to any one of the preceding claims, characterized in that the control unit (22) is adapted to automatically calculate the flow parameter using the formula

Flow measuring device (10) according to any one of the preceding claims, characterized in that a thermometer distance of the thermometer (26) from the heating element (24) is smaller than 300 microns, in particular less than 200 microns.

6. Method for flow measurement with the steps:

(a) passing a fluid in a flow direction (R) through a passage (12),

(b) applying a temperature pulse by means of an elongated heating element (24),

(c) measuring a maximum temperature by means of a thermometer (26) and

(d) automatically calculating a flow parameter that characterizes the flow from the maximum temperature,

characterized in that

(E) the temperature pulse with respect to the flow direction (R) downstream of the thermometer (26) is discharged.

A method according to claim 6, characterized in that the flow is calculated from

the maximum temperature,

a maximum rest temperature, which is the maximum temperature applied to the thermometer (26) when a flow rate (v) of the fluid is zero and

a factor that depends on the flow rate (v).

A method according to claim 6 or 7, characterized in that the flow rate and / or the flow rate (v) is calculated using the formula.

9. The method according to any one of claims 6 to 8, characterized in that the flow and / or the flow rate (v) is calculated on the basis of the formula v = d.