WO2018141637A1

WO2018141637A1 - Flow sensor and air flow device with such flow sensor

Info

Publication number: WO2018141637A1
Application number: PCT/EP2018/051884
Authority: WO
Inventors: Frank Lehnert; Gaëtan MATTHEY; Urs Niederhauser
Original assignee: Belimo Holding Ag
Priority date: 2017-01-31
Filing date: 2018-01-25
Publication date: 2018-08-09

Abstract

The invention is directed to a flow sensor (10), said flow sensor (10) extending along a longitudinal axis (19) and having a sensing section (10a), which is intended to be subjected to a flow (F) perpendicular to said longitudinal axis for measuring the flow velocity of said flow (F), and which comprises a housing (11) extending along said longitudinal axis (19) and being configured to be passed by said flow (F) on opposite sides, whereby said sensing section (10a) comprises a recessed sensing area ( 10d) provided in said housing (11) with a sensor element for thermal anemometry being arranged in the center of said sensing area (10d), such that it is in thermal contact with said flow (F) passing said housing (11) on both said opposite sides. Measurement accuracy is improved, even under conditions of misalignment between plane of sensor element and flow direction, by said sensing section (10a) of said flow sensor (10) having an aerodynamic profile.

Description

FLOW SENSOR AND AIR FLOW DEVICE WITH SUCH FLOW SENSOR

BACKGROU ND OF THE INVENTION

The present invention relates to the technology of flow sensors. It refers to an air flow sensor according to the preamble of claim 1 . It further refers to an air flow device with such a flow sensor.

PRIOR ART

To measure the volume flow in systems conducting and/or distributing a flow of gas, especially air, or of a liquid, especially water, has in the past been object of various approaches using different technologies. According to document US 6,055 ,869 A, a fluid flow sensor uses lift forces exerted on a plate-like airfoil member to determine the velocity of a fluid flowing past the sensor. The member has a pair of spaced, low aspect ratio airfoil elements. A central portion of the airfoil member is coupled to a frame that positions the airfoil member in the fluid flow with an angle of attack with respect to the fluid flow direction. The flowing fluid gener- ates velocity related lift forces on the airfoil member, the magnitude of which decrease along the airfoil member from the upstream end to the downstream end. The lift forces so applied deflect the upstream airfoil element to a greater extent than the downstream airfoil element. Strain gauges are coupled to the airfoil elements to detect their deflection. The strain gauges are connected in a bridge configuration to provide a signal indicative of fluid velocity.

Document US 6,701 ,781 B 1 describes a mass air flow sensor including a housing, an air foil element, and at least one sensor element mounted on the surface of the air foil ele- ment, whereby disruption to the air flow is minimized and performance of the mass air flow sensor in lower air flows is improved.

Document US 8,864,370 B2 concerns a device for measuring at least one physical parameter of a fluid flow, in particular the total air temperature, comprising: a profiled body of elongate shape along a longitudinal axis and having at least two walls arranged contiguous to each other at an acute angle to form a wedge-shaped portion, said wedge- shaped portion extending in a direction parallel to the longitudinal axis of the profiled body; at least one sensing element (for measuring the physical parameter of the fluid flow, said sensing element being positioned in a window formed through the profiled body. Each of the walls forming the wedge-shaped portion comprises at least one notch forming a deflector angle relative to said wall so as to weaken the formation of ice on the wedge-shaped portion.

Document US 9,21 7,655 B2 A discloses sensor system for determining a parameter of a fluid medium, e.g., an intake air mass flowing through a channel, including at least one sensor chip situated in the channel for determining the parameter, which sensor chip is accommodated in a sensor carrier which (i) protrudes into the channel and (ii) has a leading edge situated transverse to the flow of the fluid medium. At least one vortex generator is provided, at least in the region of the leading edge, and configured for forming secondary flows in the flowing fluid medium in the region of the sensor carrier, for avoid- ing or reducing the entry of particles. The secondary flows extend in a plane essentially perpendicular to the main flow direction of the fluid medium, e.g. , facing away from the sensor area. In the HVAC field, one of the established technologies for measuring volume flows of air is thermal anemometry, wherein flow velocity of an air flow is measured by determining the cooling effect of the air flow on a heated sensing element.

E.g., document EP 2 260 245 A1 discloses a device for measuring a volume flow in a ventilation pipe, which comprises a mounting that can be fixed in the ventilation pipe and a sensor element having a sensor surface, said element being disposed on the mounting and configured as a thermal anemometer. Upstream of the sensor element is a turbulence-generating element, for example in the form of a break-away edge, which is configured and disposed at a distance from the sensor surface such that highly turbulent flow is generated in the region of the sensor surface in a targeted manner. Downstream of the sensor surface is a flow element, which widens in the cross-section thereof in the flow direction, wherein starting from a height level of the sensor surface a height is reached that is greater than the height of the break-away edge opposite the sensor surface.

However, measuring devices, which use a sensing element that interacts with a fluid cir- culating around said element, are prone to measurement errors, which are induced by a misalignment between sensing element orientation and direction of the fluid flow.

SUMMARY OF THE INVENTION

It is an object of the invention, to provide a flow sensor with improved sensor accuracy, which has a reduced sensitivity to sensor misalignment with respect to the direction of fluid flow.

It is another object of the invention to propose an improved air flow device using such flow sensor. These objects are obtained by a flow sensor according to Claim 1 and an air flow device according to Claim 1 2.

The flow sensor according to the invention extends along a longitudinal axis and has a sensing section, which is intended to be subjected to a flow for measuring the flow ve- locity of said flow, and which comprises a housing extending along said longitudinal axis and being configured to be passed by said flow on opposite sides, whereby said sensing section comprises a recessed sensing area provided in said housing with a sensor element for thermal anemometry being arranged in the center of said sensing area, such that it is in thermal contact with said flow passing said housing on both said opposite sides. It is characterized in that said sensing section of said flow sensor has an aerodynamic profile.

The aerodynamic profile has the effect that the width of the flow sensor in flow direction can be substantially reduced compared to prior art sensors so that the sensor can be introduced into an air duct from outside the duct through a relatively small hole in the wall of the duct. Accordingly, mounting and /or exchanging the sensor in a duct is much easier and less time consuming.

According to an embodiment of the invention said aerodynamic profile has an upstream part and a downstream part, and the thickness perpendicular to the flow direction of the downstream part is smaller than the respective thickness of the upstream part. The front side of the upstream part should in general be formed, such that it generates and/or promotes turbulent or vortex flow at the site of the sensor element in order to enhance thermal coupling between the flow to be measured and the sensor element, even if there is some misalignment between the sensor and the direction of flow. Accordingly, said upstream part may have a flattened front side. This upstream part design, where the front side is oriented perpendicular to the flow direction, results in a substantially reduced sensitivity of the measuring accuracy to a misalignment.

Alternatively, said upstream part may have a rounded front side.

Another embodiment of the flow sensor according to the invention is characterized in that said recessed sensing area is bordered by a closed horseshoe-shaped rim with the front side of the horseshoe being its downstream side and the backside of the horseshoe being its upstream side.

Especially, said sensor element can be placed on a mounting plate, which extends in a middle plane of said aerodynamic profile and is exposed to said flow with its upper and lower sides throughout said recessed sensing area.

Said horseshoe-shaped rim may have a profile at said upstream side and said downstream side, which is symmetrical with respect to said mounting plate.

Said profile at said upstream side may be drop-shaped with its round side being the upstream side.

Alternatively, said profile at said upstream side may essentially have the form of an isosceles triangle with its base side being the upstream side.

In a further alternative said upstream part may have a bluff leading edge with sharp transitions to the longitudinal surfaces of said aerodynamic profile.

In all cases the aerodynamic profile at said downstream side may be oval. Furthermore, a cut-out may be provided in said mounting plate in the downstream part of said recessed sensing area , and said sensor element may be mounted on a tongue-like part of said mounting plate extending into said cut-out from the upstream side.

The air flow device according to the invention comprises a duct for conducting a flow of air and a flow sensor for measuring the velocity of said air flow according to the principles of thermal anemometry.

It is characterized in that said flow sensor is a flow sensor according to the invention, and that said flow sensor extends into the interior of said duct from outside said duct.

According to an embodiment of the inventive air flow device said longitudinal axis of said flow sensor may be oriented perpendicular to the flow direction of said air flow in said duct.

According to another embodiment of the inventive air flow device said flow sensor ex^¬ tends into said duct, such that said recessed sensing area is located in the middle of said duct. BRI EF DESCRI PTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

Fig. 1 shows an embodiment of an air flow device according to the invention ; shows part of an embodiment of a flow sensor according to the invention ; Fig . 3 shows a side view (a ) of the flow sensor of Fig. 2 and different cross sections

(b) of said flow sensor;

Fig. 4 shows flow patterns around the flow sensor of Fig . 2 in various situations;

Fig . 5 shows flow patterns around a flow sensor according to another embodiment of the invention with a different upstream side of its rim in various situations;

Fig. 6 is a diagram showing sensor accuracy of a prior art flow sensor and flow sensors according to the invention with structures shown in Fig. 4 and 5 ; and

Fig. 7 shows cross sections of the flow sensor according to the embodiment of

Fig .5. DETAI LED DESCRI PTION OF DIFFERENT EMBODI M ENTS OF THE I NVENTION

Fig. 1 shows an embodiment of an air flow device according to the invention. The air flow device 1 00 of Fig . 1 comprises a duct 1 3 , which conducts, e.g. as part of an HVAC system, a flow F of air. A flow sensor 1 0 extends into the interior of said duct 1 3 from out^¬ side said duct 1 3. Flow sensor 1 0 extends along a longitudinal axis 1 9 and has a sensing section 1 0a, which is subjected to air flow F in order to measure the flow velocity of flow F. The flow direction of flow F is thereby essentially perpendicular to longitudinal axis 1 9. Flow sensor 1 0 is mounted with a mounting section 1 0c on duct 1 3 by means of a fixture 1 8 at the outside of duct 1 3. From said mounting section 1 0c a sensor cable 1 2 runs to a measuring unit not shown . Flow sensor 1 0 comprises a housing 1 1 , which is subdivided into sensing section 1 0a and an electronics section 1 0b. Electronics section 1 0b houses an electronic circuit ( 1 5 in Fig . 3 ), which supplies heating power to a sensor element ( 1 4 in Fig. 2) and analyzes the response of said sensor element to air flow F in accordance with the principles of thermal anemometry. Sensing section 1 0a comprises a recessed sensing area 1 0d provided in housing 1 1 with said sensor element 1 4 for thermal anemometry. Sensor element 1 4 is arranged in the center of sensing area 1 0d, such that it is in thermal contact with air flow F passing housing 1 1 on opposite sides. Flow sensor 1 0 preferably extends into duct 1 3 , such that said recessed sensing area 1 0d is located in the middle of duct 1 3.

Sensing section 1 0a of flow sensor 1 0 has an aerodynamic profile ( 1 6 in Fig. 3 (b) ), and extends along longitudinal axis 1 9. It is this aerodynamic profile that allows to keep the width S (see Fig. 3 ) of the sensor small (for example smaller than 40mm) so that in can be easily inserted into an air duct 1 3 through a small hole in the duct wall.

Recessed sensing area 1 0d is bordered by a closed horseshoe-shaped rim 20 the bulged front side of which being its downstream side (downstream part 20b) and the backside of the horseshoe being its upstream side (upstream part 20a or 20a', respectively;see Figs. 2-5 ). In the center of recessed sensing area 1 0d flow sensor 1 4 is placed on a flat mounting plate 1 7, which extends in a middle plane (22 in Fig. 4(b) ) of aerodynamic profile 1 6 and is exposed to flow F with its upper and lower sides throughout recessed sensing area 1 0d. Horseshoe-shaped rim 20 has a profile at its upstream side and downstream side, which is symmetrical with respect to mounting plate 1 7. According to one embodiment shown in Fig. 4 the aerodynamic profile at the upstream side is drop-shaped with its round side being the upstream side (see also Fig. 3 ). According to another embodiment shown in Fig. 5 the aerodynamic profile at the upstream side is essentially that of an isosceles triangle with its base side being the upstream side. The edges of the triangle are slightly rounded.

In both cases the aerodynamic profile at the downstream side is preferably oval. In both cases the aerodynamic profile at upstream parts 20a or 20a', respectively, has a first thickness D 1 perpendicular to the plane of mounting plate 1 7, and the aerodynamic profile at downstream parts 20b has a second thickness D2 perpendicular to the plane of mounting plate 1 7, whereby said first thickness D 1 is larger than said second thickness D2 (D 1 > D2 ). As can be clearly seen in Fig. 3 (a), a cut-out 1 7b is provided in mounting plate 1 7 in the downstream part of recessed sensing area 1 0d. Sensor element 14 is mounted on a tongue-like part 1 7a of mounting plate 1 7, which extends into cut-out 1 7b from the upstream side.

The design of flow sensor 1 0 shown in Fig. 3 differs in various aspects from the prior art sensor design known from document EP 2 260 245 A1 mentioned above:

On one hand, the width of the prior art sensor in flow direction cannot be shortened, because this would result in an uncontrolled flow.

On the other hand, as the prior art sensor is mounted with a damper, it will always be aligned in parallel with flow direction, so that measurement accuracy is not influenced by a misalignment with respect to flow direction. However, if the prior art sensor is shortened and without a damper it will not necessarily be centered. By mounting the sensor inside the duct by a pipe fitter (fixture 1 8 in Fig. 1 ) such sensor is not always in parallel with the flow direction.

If the flow direction and the sensor plane are out of alignment by an angle a of, for ex- 5 ample, 6° the measuring accuracy will be bad with the prior art design.

With prior art design flow changes between more or less laminar and turbulent.

To improve the air flow measurement the air flow pattern should be in all conditions similar.

Further, if the flow is of high speed, it is turbulent anyway. To make the measurement i o independent of flow conditions, the best solution is to have the airflow always turbulent around the sensor element and sensor.

Figs. 4 and 5 show two airfoil designs according to different embodiments of the invention and the resulting air flow patterns around the aerodynamic profile 1 6 and 1 6', respectively, for different flow and alignment conditions, whereby Fig. 4(a) and 5 (a) refer

1 5 to low flow speed with perfect alignment between flow direction and plane of sensor element 1 4, Fig. 4(b) and 5(b) refer to high flow speed with perfect alignment between flow direction and plane of sensor element 1 4, Fig. 4(c) and 5(c) refer to low flow speed with a misalignment between flow direction and plane of sensor element 1 4 of a=6°, and Fig. 4(d) and 5(d) refer to high flow speed with a misalignment between flow direc- 0 tion and plane of sensor element 1 4 of a=6°. The width S and S' in both cases can be < 40mm. Fig. 6 is a diagram showing sensor accuracy (measured air flow versus reference air flow) of a prior art flow sensor and flow sensors according to the invention with structures shown in Fig. 4 and 5. Curve R is the reference airflow in the middle of the dotted tolerance curves T. Curve A represents the accuracy of the prior art design at a 6° misalign- ment. Curve B represents the accuracy of the design according to Fig. 5 at a 6° misalignment. Curve C represents the accuracy of the design according to Fig. 5 at perfect alignment.

The design according to Fig. 4, which is already a substantial improvement over the prior art design, comprises a rim with a profile at upstream part 20a, which is drop-shaped with its rounded side being the upstream side, and a profile at downstream part 20b that is oval.

The optimized design according to Fig. 5 comprises a rim with a profile at upstream part 20a', which has essentially the form of an isosceles triangle with its base side or flattened side being the upstream side, and a profile at downstream part 20b that is oval. With the design according to Fig. 4 the flow is just more homogeneous. However, as shown in Fig. 4(c), the flow may be laminar on one side at low flow speed and with a 6° misalignment.

With the isosceles triangle profile of the design according to Fig. 5 the flow it is under all conditions very turbulent around the sensor element 1 4 with vortices V at every place. The accuracy is very good at straight and 6° flow (see Curves B and C in Fig. 6).

With the new sensor design the width S can be < 40mm and the flow sensor can be put from outside in a hole of the duct, and the measurement is still exact. With the new flow sensor one can have a robust measurement of air flow. The measurement is independent of the duct configuration, i.e. whether there is a straight pipe in front of the sensor or an elbow pipe. The measurement is also insensitive against dust and pollution inside the duct. The optimized airfoil profile in front of the sensor element 1 4 is used to shorten the width S of the housing 1 1 . The profile may be drop-like (Fig. 4). The profile can also have the form of an isosceles triangle or a concave shape at its front side with a bluff leading edge with sharp transitions to the longitudinal surfaces of the aerodynamic profile to generate and improve turbulence, i.e. generate vortices V and to avoid laminar flow in the area of the sensor element.

Fig. 7 shows cross sections of the flow sensor according to the embodiment of Fig.5. The cross sections are taken at positions C and B of the flow sensor, in accordance with the positions C and B for the embodiment as shown in Fig.3. The lower cross section is taken at position B in the recessed sensing area of the flow sensor and shows the upstream part 20a' of the flow sensor with a flattened front side 21 ' and a profile which has essentially the form of an isosceles triangle with its base side being the upstream side, as shown also in Fig.5. The edges of the triangle are slightly rounded. The sensor 1 4 is placed on the mounting plate 1 7, which extends in a middle plane of the aerodynamic profile 1 6'. The downstream part 20b has a profile that is oval. The first thickness D 1 ' of the aerodynamic profile at the upstream part 20a' and the second thickness D2' of the aerodynamic profile at the downstream part 20b satisfy the relationship D 1 ' > D2'. The upper cross section taken at C shows the electronics section 1 0b'. For better visibility, any electronic circuit components have been omitted in the Figure. The flattened front side 21 ' can also be recognized in the upper cross section. LIST OF REFERENCE NUMERALS

10 flow sensor

10a sensing section

10b electronics section

10c mounting section

10d sensing area

11 housing

12 sensor cable

13 duct

14 sensor element

15 electronic circuit

16,16' aerodynamic profile

17 mounting plate

17a part (tongue-like)

17b cut-out

18 fixture

19 axis (longitudinal)

20 rim (horseshoe-shaped)

20a, 20a' upstream part

20b downstream part

21 front side (rounded)

21' front side (flattened)

22 middle plane

100 air flow device

D1.D2 thickness F flow (e.g. air)

S,S' width (airfoil)

V vortex

a angle (misalignment)

Claims

What we claim is:

1. Flow sensor (10), said flow sensor (10) extending along a longitudinal axis (19) and having a sensing section (10a), which is intended to be subjected to a flow (F) for measuring the flow velocity of said flow (F), and which comprises a housing (11) extending along said longitudinal axis ( 19) and being configured to be passed by said flow (F) on opposite sides, whereby said sensing section ( 10a) comprises a recessed sensing area ( 10d) provided in said housing (11) with a sensor element (14) for thermal anemometry being arranged in the center of said sensing area

( 10d), such that it is in thermal contact with said flow (F) passing said housing (11) on both said opposite sides, characterized in that said sensing section (10a) of said flow sensor (10) has an aerodynamic profile (16, 16').

2. Flow sensor as claimed in Claim 1 , characterized in that said aerodynamic profile (16, 16') has an upstream part (20a, 20a') and a downstream part (20b), and that the thickness (D2) perpendicular to the flow direction of the downstream part (20b) is smaller than the respective thickness (D1) of the upstream part (20a, 20a').

3. Flow sensor as claimed in Claim 2, characterized in that said upstream part (20a') has a flattened front side (21').

4. Flow sensor as claimed in Claim 2, characterized in that said upstream part has a bluff leading edge with sharp transitions to the longitudinal surfaces of said aerodynamic profile.

5. Flow sensor as claimed in Claim 2, characterized in that said upstream part (20a ) has a rounded front side (21 ).

6. Flow sensor as claimed in one of the Claims 1 to 5, characterized in that said recessed sensing area ( 1 0d) is bordered by a closed horseshoe-shaped rim (20) with

5 the front side of the horseshoe being its downstream side and the backside of the horseshoe being its upstream side.

7. Flow sensor as claimed in Claim 6, characterized in that said sensor element ( 1 4) is placed on a mounting plate ( 1 7), which extends in a middle plane (22 ) of said aerodynamic profile ( 1 6, 1 6') and is exposed to said flow (F) with its upper and lower o sides throughout said recessed sensing area ( 1 0d).

8. Flow sensor as claimed in Claim 6, characterized in that said horseshoe-shaped rim (20) has a profile at said upstream side and said downstream side, which is symmetrical with respect to said mounting plate ( 1 7 ).

9. Flow sensor as claimed in Claim 8, characterized in that said profile at said up¬5 stream side is drop-shaped with its round side being the upstream side.

10. Flow sensor as claimed in Claim 8, characterized in that said profile at said upstream side has essentially the form of an isosceles triangle with its base side being the upstream side.

1 1. Flow sensor as claimed in Claim 9 or 1 0, characterized in that said profile at said0 downstream side is oval.

12. Flow sensor as claimed in Claim 6, characterized in that a cut-out ( 17b) is provided in said mounting plate (17) in the downstream part of said recessed sensing area ( 10d), and that said sensor element (14) is mounted on a tongue-like part (17a) of said mounting plate (17) extending into said cut-out (17b) from the upstream side.

13. Airflow device (100), comprising a duct ( 13) for conducting a flow (F) of air and a flow sensor (10) for measuring the velocity of said air flow according to the principles of thermal anemometry, characterized in that said flow sensor (10) is a flow sensor according to one of the Claims 1 to 12, and that said flow sensor (10) ex- tends into the interior of said duct (13) from outside said duct (13).

14. Air flow device as claimed in Claim 13, characterized in that said longitudinal axis (19) of said flow sensor (10) is oriented perpendicular to the flow direction of said airflow (F) in said duct (13).

15. Air flow device as claimed in Claim 13, characterized in that said flow sensor ( 10) extends into said duct (13), such that said recessed sensing area (1 Od) is located in the middle of said duct (13).