WO2019014697A1

WO2019014697A1 - Photoacoustic measuring device having resonator elements

Info

Publication number: WO2019014697A1
Application number: PCT/AT2018/060157
Authority: WO
Inventors: Benjamin LANG; Alexander Bergmann
Original assignee: Avl List Gmbh
Priority date: 2017-07-20
Filing date: 2018-07-20
Publication date: 2019-01-24

Abstract

The invention relates to a device (1) for photoacoustic measurement of the concentration of particles, in particular water in various physical states, in a measuring fluid, comprising a feed conduit section (2) for the measuring fluid, a measuring section (3) with a photoacoustic measuring cell (30) and a drainage conduit section (4) for the measuring fluid. In the region of the feed conduit section (2) and/or in the region of the drainage conduit section (4) at least one resonator element (5a, 5b, 5c, 5d) is provided which is concentric with the feed conduit section (2) and/or drainage conduit section (4) and which is fluidically connected to the feed conduit section (2) or the drainage conduit section (4) via at least one opening (6) in an outer wall (7) of the feed conduit section (2) and/or drainage conduit section (4).

Description

Photoacoustic measuring device with resonator elements

The invention relates to a device for photoacoustic measurement of

Concentration of particles, especially water in different

Physical states, in a measuring fluid, with a supply line for the measuring fluid, a photoacoustic measuring cell and a discharge section for the measuring fluid. Furthermore, the invention relates to the use of such a device for water concentration measurement in fuel cell and / or electrolysis systems.

Photoacoustic detectors provide a very sensitive measurement technique for determining, for example, trace gas or aerosol concentrations in a measurement fluid, such as, e.g. a carrier gas. A common application is the measurement of the concentration of solid particles in gaseous media, e.g. for the determination of the soot mass concentration in vehicle exhaust gases. In this case, an intermittently modulated or pulsed laser beam with a suitably selected operating frequency of the measuring system is directed into a measuring cell with gaseous measuring fluid.

During the radiation phases, the particles irradiated by the laser light in the measuring fluid absorb the laser light, whereby these and indirectly the measuring fluid surrounding the particles are heated. This leads to periodic pressure fluctuations in the gas of the measuring cell, which propagate as sound waves and consequently can be detected with a microphone. The recorded microphone signal is proportional to the particle mass concentration in the measuring cell and is advantageously used for

Suppression of noise and signal noise synchronous to

Operating frequency and evaluated with adjusted phase shift,

for example with a synchronous rectifier.

The measuring cell is preferably designed as an acoustic resonator. To increase the resonance of the acoustic signal to increase the

To be able to use measurement sensitivity, the modulation or pulse frequency of the laser is tuned to a resonant frequency (natural frequency) of the resonator, for which in general the fundamental oscillation in the axial direction of the measuring cell is preferred. By positive interference of the acoustic signals generated by the excited solid particles creates a standing acoustic wave in the axial direction of the resonator. At its axial boundaries occur pressure maxima, However, the resonator cell actually effective in the measuring cell as a resonator can also be limited on the two sides by acoustic blocking filters (notch filters) in the form of a chamber with a larger cross section, whereby the resonator cell becomes one both sides acoustically "open" resonator, which has a pressure maximum in the middle of the resonator cell in the case of the fundamental vibration. In such an arrangement, therefore, the sound pressure-measuring microphone should be placed in the center of the resonator.

Such an arrangement is described, for example, in EP 1 564 543 B1 of the Applicant.

In certain applications, known solutions can cause the occurrence of noise that falsifies the measurement results. This is currently prevented by very low flow rates of the measuring fluid through the

photoacoustic devices are set, since thereby the

Reduce noise. However, there is still the problem of the influence of ambient noise, which is detrimental to the

Can affect measurement results. In addition to the resulting slow response and detection times has also been shown that the known solutions for the measurement of proportions of water in different states of aggregation, especially in gaseous measuring fluids are poorly suited.

It is therefore an object of the invention to avoid the above-mentioned disadvantages of the prior art and to provide a high-quality photoacoustic measurement

Flow rates and good sensitivity for different types of particles, especially water in solid, liquid and gaseous form to allow.

This object is achieved by an initially mentioned device according to the invention in that in the region of the supply duct section and / or in the region of the discharge duct section at least one to Zuleitungs- and / or

Discharge channel portion concentric resonator element is provided, which is flow-connected to the Zuleitungskanalabschnitt via at least one opening in an outer wall of the supply and / or Ableitungskanalabschnitts. Thus, according to the invention, a higher measurement accuracy is made possible, since the resonator elements function as acoustic bandpass filters and suppress disturbing noises from the environment of the measuring device, in particular in the frequency range of the photoacoustic measurement. This damping can be improved in particular by the fact that in the supply duct section, ie before or

upstream of the measuring cell, a plurality of such resonator elements are provided. Conveniently, at least one such resonator element is also provided in the discharge channel section, ie downstream of the measuring cell.

The openings between the inlet and / or outlet channel section and the resonators are designed in the outer wall of the respective channel sections and slit-shaped, wherein preferably the width of the opening in the direction normal to the flow direction of the measuring fluid is greater than the length of the opening in the direction parallel to the flow direction. In other words, it runs

Longitudinal extension of the slot-shaped openings normal to the flow direction of the measuring fluid. In a variant of the invention, the longitudinal axis of the at least one opening at an acute angle, preferably between 25 ° and 65 °, particularly preferably 45 ° to the longitudinal axis of the supply and / or

Discharge duct section inclined. The openings are so against the

Flow direction of the measuring fluid inclined.

This arrangement allows a directed flow through supply and

Ableitungskanal section and through the measuring cell, whereby the separation of the flow and concurrent turbulence, flow-induced pulsations and flow rate dependent resistance and reaction effects can be reduced.

In a variant of the invention, the supply and / or the

Ableitungskanalabschnitt a quadrangular cross section. Preferably, at least three openings are provided per resonator element, wherein in the quadrangular configuration on each of the four transverse sides of the supply and / or

Ableitungskanalabschnitts per resonator at least one, preferably three openings are arranged. These openings are each in

Flow direction of the measuring fluid arranged one behind the other. The device of the invention allows due to their features the

Concentration measurement especially of water in different

Physical states - gaseous, liquid and solid in the form of ice - at for

photoacoustic measurements at high flow rates. While usually only about 1 to 1, 51 / min can be handled, the inventive device allows flow rates of up to 251 / min.

According to the invention, in a device for photoacoustic measurement, an effective damping of disturbing ambient noise by acoustic filters - the resonator elements - in combination with the perforation of the outer wall of the channel sections is achieved by means of slot-shaped openings in the region of the transition to the resonator elements. This minimally disturbs the flow field of the measurement fluid and the particles contained therein (e.g., water in various states of aggregation), and flow-induced pulsations and

flow rate dependent negative effects reduced. Likewise, the particles to be measured can be transported undisturbed to the measuring cell and high flow rates and short reaction times can be realized due to the short distances between supply line and measuring section. Use of the invention is beneficial, for example, in cooling or icing wind tunnel arrangements to make water content measurements. A further advantageous use results in fuel cell or Elektrolyseuranordnungen where the water concentrations are monitored.

The details, features and advantages of the invention will be explained in more detail with reference to the non-limiting embodiments shown in the figures. Show:

Fig. 1 is a first side view of a device according to the invention;

FIG. 2 shows a longitudinal view of a device according to the invention; FIG.

3 shows a second side view of a device according to the invention;

Fig. 4 is a sectional view of the device according to the invention along a line A-A in Fig. 1;

Fig. 5 is a detail view of the sectional view in Fig. 4; 6 shows a perspective, partially transparent view of a device according to the invention; and

Fig. 7 is a schematic view of a measurement setup using the device according to the invention.

In the following figures, like elements are marked with the same reference numerals for reasons of clarity.

The device or measuring device 1 according to the invention according to FIGS. 1 to 6 has a main channel 8, in which the measuring fluid is conducted from an inlet 9 to a measuring section 3 and on to an outlet 10. For this purpose, a connected to the outlet 10 pump device 1 1 is used, which ensures a constant flow through the device 1, in particular the measuring section 3.

Fig. 7 shows schematically the view of an exemplary construction for using the device according to the invention. In this case, the embodiment of the measuring cell 30 is shown, where a measuring cell 30 and the main channel 8 penetrating them through light source 13 - usually an amplitude modulated laser, eg with A = 402nm, P _Pe ak = 20mW -, optionally with radiation trap 14, and a Microphone 15 - for example electret microphones with maximum sensitivity at 4.7 kHz - are provided for recording the excited sound waves. The light source 13 is usually arranged outside the measuring cell 30 and radiates through window elements 17 into it. In this case, desorption rates can be increased if the window elements 17 are designed to be heated.

After the inlet 9, the supply duct section 2 is provided, to which the measuring cell 30 connects, from which the discharge duct section 4 - e.g. in

Direction of flow 100 conically diverging - leads to the outlet 10. In Fig. 4 it can be seen that at the outlet 10, a thread for receiving

subsequent pipe or hose lines is executed. In the illustrated

In the embodiment, the main channel 8, in particular the feed line 2 and the discharge channel section 4, has a quadrangular cross-section (see FIG. square can be formed with side lengths of 1 cm each.

Acoustically short concentric resonator elements 5a, 5b, 5c, 5d are used as acoustic bandpass filters to reduce ambient noise on the frequency to suppress or attenuate the photoacoustic measurement. To one

Preventing the measurement by noise of the pump device 1 1 to prevent this is acoustically separated by a muffler element 12 from the rest of the device 1 (see, for example, Fig. 7).

Two of the said resonator elements 5a, 5b, 5c, 5d are part of the measuring section 3 while further resonator elements are used as damping resonator elements (in the figures, in particular the resonator elements 5a, 5b) in the region of

Supply channel section 2 are provided, wherein the distances between the resonator elements 5a, 5b, 5c are optimized to achieve maximum attenuation.

A resonator element is provided as an amplifier resonator element (resonator element 5c) at a certain distance from the damping resonator elements 5b, 5c

arranged so that a resonator between the amplifier resonator 5c and the last seen in the flow direction resonator downstream of the measuring cell 30 (here resonator element 5d) results.

Conveniently, the distance between the resonator elements 5a, 5b, 5c arranged upstream of the measuring cell 30, in particular between their mutually facing inner walls, is approximately K / 4. This can additionally

Erasure disturbing sound waves inside the channel can be achieved.

This results in the main channel 8, an acoustic resonator or a resonance, which is used for optimum amplification of the photoacoustic signal within this resonator, which corresponds to the measuring section 3 and substantially the region of the measuring cell 30, on the frequency of maximum attenuation by these filtering resonator 5a, 5b, 5c, 5d results. The resonator elements 5a, 5b, 5c, 5d are shown in FIG

Embodiment designed as cylindrical, hollow elements, the

are arranged concentrically to the main channel 8 and connected thereto via openings 6, wherein other shapes are possible. There are

Resonator elements 5a, 5b, 5c, 5d provided both in front of the measuring cell 30 and thereafter. In particular, this results in the formation of the standing wave required for the measurement with a maximum in the region of the measuring cell 30

ensured. In the resonator elements 5a, 5b, 5c, 5d is in the illustrated

Embodiment to radially symmetric, acoustically short resonators. The resonator elements 5a, 5b, 5c, 5d have a cylindrical shape with a circular or polygonal base. The base is arranged normal to the flow direction in the main channel 8. Acoustically short in the context of the present disclosure refers to the axial extent l _R of

radially symmetric resonator 5a, 5b, 5c, 5d and means that it is much shorter than the acoustic wavelength, in particular the resonant frequency.

The radii of the resonator elements 5a, 5b, 5c, 5d or the

circular / polygonal base are chosen so that at the selected resonance frequency of the photoacoustic cell maximum reflection of the (planar) acoustic waves in the main channel 8 to the resonator 5a, 5b, 5c, 5d occurs. As a result, on the one hand undesirable, from outside the

Measuring device 1 penetrating noise at the resonant frequency reflected maximum back to the outside and on the other hand in the excitation range of

(photoacoustic) resonator photoacoustically generated sound in the correct phase, i. resonant, reflected back in these.

The condition for maximum reflection (antiresonance) for an acoustically short cylinder is approximately through the zeros of the function

(2 y ^r o (fc r ₀ ) + y ₁ (fc r ₀ )); ₁ (fc r ₁ ) - (2; ₀ (fc r ₀ ) +; ₁ (fc r ₀ )) y ₁ (fc r ₁ ) ^ '

given. Here, j _v and Y _{v are} Bessel functions of the first and second order of the order v. r _t is the outer radius of the cylinder. r ₀ is the (equivalent) radius of the axially centered ports (here the main channel), k = 2π / λ is the magnitude of the acoustic wave vector.

Between the resonator elements 5a, 5b, 5c, 5d and the channel sections 2, 4 are the outer walls 8 of the channel sections, which are made thin and perforated. The perforation is formed by slot-shaped openings 6, the largest longitudinal extent of which runs normal to the flow direction 100 of the measurement fluid. The acoustic inertance of the slot-shaped openings 6 acts delaying the sound, whereby the outer radius of the radially symmetric resonator elements 5a, 5b, 5c, 5d must be chosen accordingly smaller. In addition, the openings 6 (see in particular Fig. 4, wherein for reasons of clarity, only three openings are denoted by the reference numeral "6"; the following explanations apply to all openings 6) shown inclined against the flow direction 100 of the measuring fluid For a given resonant frequency and effective length l _{eff of} the slit-shaped openings or slits 6, the acoustic characteristic of the resonator can be as independent as possible from the flow velocity, which means in other words that the openings 6 pass from outside to inside through the outer wall 8 of the channel sections 2, 4 are inclined with respect to the flow direction 100 of the measuring fluid at an acute angle α In principle, the angle α can be between 0 ° and 90 °, but is advantageously in a range between 25 ° and 65 °, preferably 45 °. 5 shows a section of a device according to the invention KISSING

Device 1 according to sectional view in Fig. 4, where by way of example with reference to an opening 6, the course of the angle α is shown. This course also applies to the other openings 6 in the illustrated embodiment of the device 1 according to the invention. Due to the inclined design of the openings 6 with respect to the flow direction 100, it is possible to reduce turbulences and stalls that would represent the measurement disturbing noise sources.

In particular, the acoustic resistance (loss or attenuation of the photoacoustic sound when positive, and unwanted sound generation when negative), as well as the reactance (delay of the photoacoustic sound, which would lead to a resonance frequency shift of the resonator) over a wide range of flow rate constant and kept minimal. The effective length l _{eff of} the slots 6 is considered to be the extent in a direction parallel to the flow direction 100 of the measurement fluid. In an advantageous

Embodiment, the effective length ff of the slots 6 is twice the thickness of the outer wall of the main channel 8.

An advantageous reduction of noise-generating turbulence and

Stalls can be prevented if the selection of the effective length l _{eff of} the slots 6 is based on the Strouhal number criterion. Since the Strouhal number is directly proportional to the effective length l _e ff of the slot-shaped opening 6, a shortening of the slots 6 in the flow direction 100 by a certain factor now achieves the additional effect that the flow rate changes

Change in the Strouhal number also decreases by this factor. This allows the Depending on the acoustic characteristic (resistance and reactance) of the flow velocity can be additionally limited or minimized.

The greater the effective length l _e tf, the higher the Strouhal number, but then it can also lead to the deposition of particles, water, etc. in the resonator elements 5a, 5b, 5c, 5d. In addition, _{leff is} less dependent on the Strouhal number of flow rate for smaller effective length.

In particular, in Fig. 6 it can be seen that each resonator element 5a, 5b, 5c, 5d on each of the four transverse sides of each three openings 6 are executed, which in

Flow direction 100 are arranged one after the other.

By the use of perforated channel section walls 7, ie by slit-shaped, inclined openings 6 in the walls of the inlet 2 and outlet channel section 4 being designed as a transition to surrounding resonator elements 5a, 5b, 5c, 5d, photoacoustic measurements of an analyte in a measuring fluid become unusual high flow rates of up to 25l / min allows.

More specifically, perforated channel section walls 7, ie walls with inclined slot-shaped openings 6, are provided between the measuring fluid-containing channel and acoustic filters in the form of resonator elements 5a, 5b, 5c, 5d and other connection and crossing points along the fluid path through the measuring device 1 , The slot-shaped openings 6 are inclined counter to the flow direction 100 of the measuring fluid. This embodiment allows a directed flow of the measuring fluid through the main channel 8 (ie the inlet 2 and discharge channel 4 and the interposed photoacoustic measuring cell 30), wherein a flow separation, the occurrence of accompanying turbulence, flow-induced pulsations and speed-dependent acoustic resistance and Reaction effects are reduced. The embodiment of the device 1 according to the invention also reduces the occurrence of flow noise, ie acoustic noise resulting from the high flow velocities

Noise of the measurement.

The device 1 allows the most trouble-free particle measurement, especially of "hydrometeors", ie liquid, in particular water droplets or ice particles with diameters of a few to a few hundred micrometers.The particles ideally pass through the measuring range 3 without it Interactions with the channel walls 7 of the surrounding main channel 8 or its channel sections 2, 4 comes. This makes it possible to measure and distinguish the concentrations of the different states of matter. For this purpose, a plurality of laser sources 13 with different wavelengths can be provided in the measuring cell 30 in order to profit from the individual absorption cross sections of the three states of aggregation at different wavelengths.

Both an "in-situ" measurement in fluid streams is possible, which allows a lower response time and adsorption or interaction length compared to an extractive sampling, that is, where a partial or total current is drawn via a sampling device an element 16, which may be either a removal device described above, which projects into a fluid line - eg an exhaust pipe of an internal combustion engine or a supply or discharge pipe to a fuel cell or an electrolyzer - or directly represents such a fluid line 1 or its measuring cell 30 also a

Conditioning means may be provided to condition the measurement fluid (e.g., temperature, water content or proportion of other reagents).

According to the invention, radially symmetrical, acoustically short resonator elements 5a, 5b, 5c, 5d are provided on the main channel 8, which are connected to the main channel 8 via slot-shaped openings 6 in the channel section walls 7. The slot-shaped openings 6 are oriented normal to the flow direction 100 in the main channel 8, their extension parallel to the flow direction 100 is thus significantly less than their extent normal. The effective length ff of the slot-shaped openings 6 is chosen to be much smaller than the axial one

Extension l _{R of} the radially symmetric resonator elements 5a, 5b, 5c, 5d. This can reduce geometric sources of flow noise. In addition, the openings 6 are designed inclined against the flow direction 100 of the measuring fluid.

Claims

claims

1. Device (1) for photoacoustic measurement of the concentration of

Particles, in particular water in various states of aggregation, in a measuring fluid, with a supply duct section (2) for the measuring fluid, a measuring section (3) with a photoacoustic measuring cell (30) and a discharge channel section (4) for the measuring fluid, characterized

characterized in that in the region of the feed channel section (2) and / or in the region of the discharge channel section (4) at least one to Zuleitungs- (2) and / or discharge channel section (4) concentric resonator element (5a, 5b, 5c, 5d) is provided with the supply line (2) and the discharge channel section (4) via at least one opening (6) in an outer wall (7) of the supply line (2) and / or

Ableitungskanalabschnitts (4) is fluidly connected.

2. Device (1) according to claim 1, characterized in that the

at least one opening (6) is slot-shaped, wherein

preferably the width of the opening (6) in the direction normal to

Flow direction (100) of the measurement fluid is greater than the length of the opening (6) in the direction parallel to the flow direction (100).

3. Device (1) according to claim 1 or 2, characterized in that the longitudinal axis (60) of the at least one opening (6) at an acute angle (a), preferably between 25 ° and 65 ° m against the longitudinal axis (200). the supply (2) and / or discharge channel section (4) is inclined.

4. Device (1) according to one of claims 1 to 3, characterized

in that the supply line (2) and / or

Ableitungskanalabschnitt (4) have a quadrangular cross-section.

5. Device (1) according to one of claims 1 to 4, characterized

in that at least three openings (6) are provided per resonator element (5a, 5b, 5c, 5d).

6. Device (1) according to claim 4 or 5, characterized in that on each of the four transverse sides of the supply line (2) and / or Ableitungskanalabschnitts (4) per resonator element (5a, 5b, 5c, 5d) at least one, preferably three openings (6) are arranged.

Use of a device (1) according to any one of claims 1 to 6 water concentration measurement in fuel cell and / or Elektrolyseuranordnungen.