Abstract:
A system for measuring air flow rate in a duct includes an air flow sensor and an air flow conditioning device for collecting and accelerating a portion of the total air stream to the sensor, which may be means as known in the art. The conditioning device includes two opposed converging walls defining a nozzle, preferably a venturi, the sensor being disposed in the throat of the nozzle. The surfaces of the venturi walls are textured to trip the boundary layer near the wall surface into turbulence to maintain attachment of flowing air to the walls even when the angle of attack of the air is significantly non-axial. The wall texturing may be random or may be an organized pattern. A further embodiment includes a second flow conditioner/sensor in parallel with the first whereby an averaged flow measurement is taken for a more accurate reading.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to systems for measuring the flow velocity, flow volume, or mass flow rate of a gas through a conduit; more particularly, to such systems having convergent walls for forming a laminar flow of gas past a sensor; and most particularly, to such a system having a textured wall surface for creating turbulent flow in the wall boundary layer in the wall-convergence zone to maintain wall contact with gas flows approaching from off-axis directions.  
       BACKGROUND OF THE INVENTION  
       [0002]     Air flow measurement systems incorporating any of various types of sensor elements are well known, for example, a heated wire whose temperature is an inverse function of the velocity of air flowing past the wire. In most air flow measurement devices, the sensor interacts with only an extremely small portion of the total flow field, typically central to the field. It is a well known problem in the design and use of air flow measurement devices that changes in upstream ducting configuration, such as manipulation of an upstream throttle valve, or using a given device in a differently-configured duct or manifold, can alter the flow field around the sensor and thus alter the sensor output non-linearly, i.e., the output signal will contain a degree of error. Therefore, it is known in the art of air flow measurement to provide an air flow conditioning device ahead of the sensor to reduce the degree of error in the sensor output over a desired range of flow measurements.  
         [0003]     Many types of air flow conditioning devices exist. Straight sections of pipe tend to allow the flow perturbations to equalize and subside before reaching the sensor. Although simple in concept, the allotted packaging space may prevent the installation of a pipe section of sufficient length; typically, a straight pipe having a length of at least ten diameters should precede the sensor. Tube bundles and honeycomb-shaped bundles can effectively eliminate cross-flow conditions, but they require a means of locating and securing the conditioning device in the upstream duct, and they introduce additional restrictions to flow through greatly increased surface area. These considerations also apply to screens, fins, grids, and various porous media intended for eliminating cross-flow conditions.  
         [0004]     Another technique to reduce the sensitivity to upstream flow field variations is to place the sensor at the throat of a nozzle, or venturi. Since the entrance to the venturi covers a larger cross-sectional area of the flow field than does the throat, the venturi effectively samples more of the flow field. It also accelerates the flow of gathered air past the sensor, thereby increasing the gain of the sensor.  
         [0005]     As air moves over the surface of the venturi, a boundary layer forms on the walls of the venturi in which the air velocity is zero at the venturi surface and is at the velocity of the main flow at the top of the boundary layer. Beginning at the leading edge of the solid surface, the boundary layer increases in thickness in the direction of air flow. As the boundary layer grows, it acts to reduce the throat area of the venturi, which further accelerates the flow past the sensor.  
         [0006]     As much flexibility as possible in the mounting and positioning of air flow measurement systems is preferred. Devices with fewer requirements as to upstream ducting configurations, while still providing the desired measurement accuracy, will have a significant advantage over competing devices having less latitude. Having fewer restrictions as to the upstream ducting configuration means that the flow field presented to the air flow measurement device can exhibit greater variation.  
         [0007]     In prior art venturi nozzles, when the flow approaches the surface at a severe off-axis angle such as that caused by a sharp bend in the upstream ducting, the thickness of the boundary layer along the nozzle wall can change abruptly. Such changes result from the transition from laminar to turbulent flow, or even separation of the boundary layer from the surface if the angle of attack is great enough. The flow separates from the nozzle wall at different locations, depending upon the shape and location of the upstream ducting. The thus-modified boundary layer changes the effective area of the nozzle throat, causing the sensor to produce a signal value different than if the same bulk flow rate had a straight-on approach to the venturi.  
         [0008]     The location of the separation point depends upon many factors, including flow rate, angle of attack of the air onto the wall, and laminar or turbulent flow. All of these conditions impact the effective throat area of the venturi; hence, the velocity over the sensor will vary with changing upstream conditions even if the actual mass flow rate remains constant, thus generating an erroneous signal.  
         [0009]     In the patent literature, U.S. patent Publication No. US 2001/0037678 discloses a conditioning device having an inlet-side bypass passage containing a venturi-like throttle unit leading to the sensor, whereby the sensor is purported to be isolated from turbulence in the main air flow field. A potential shortcoming of this device is the assumption that flow through the bypass is a constant percentage of the total flow at all flow rates.  
         [0010]     U.S. patent application Publication No. US 2004/0055570 discloses a conditioning device which essentially divides the air flow approaching a sensor into concentrically central and outer regions, and diverts the outer regions around the sensor, on the assumption that the central region is more laminar and that variations in upstream duct configurations will result in variations in turbulence principally in the outer regions which bypass the sensor.  
         [0011]     U.S. Pat. No. 6,267,006 discloses an upstream flow conditioner comprising a conical section leading to a mass flow sensor and having radial fin-like flow conditioning elements (FCEs) which purportedly shape the airflow pattern presented to the mass flow sensor to provide an airflow of uniform velocity with a low magnitude of turbulence fluctuations.  
         [0012]     What is needed in the art is an improved air flow measurement device which exhibits a reduced sensitivity to changes in the upstream air flow field.  
         [0013]     It is a principal object of the present invention to provide an air flow measurement device which exhibits reduced sensitivity to upstream variations in air flow turbulence.  
       SUMMARY OF THE INVENTION  
       [0014]     Briefly described, an air flow measurement system for measuring air flow rate in a duct includes an air flow sensor and an air flow conditioning device for collecting and accelerating toward the sensor a portion of the total air stream flowing through the duct. The sensor may be a conventional hot wire or other means as is known in the art. The conditioning device includes at least two opposed converging walls defining a nozzle, preferably a venturi, the sensor being disposed in the throat of the nozzle. The surfaces of the venturi walls are textured to create local cells of turbulence in the air boundary layer near the wall surface. Tripping the boundary layer flow into turbulence energizes the boundary layer and aids in maintaining attachment of flowing air to the walls even when the angle of attack of the air is significantly non-axial. The wall texturing may be random or may be an organized pattern. A currently preferred tripping pattern is a dimpled array similar to the surface of a golf ball. Keeping the boundary layer attached to the surface results in an effective throat area that does not change significantly with changes in the upstream flow field and angle of attack of incoming air to the air flow conditioner. Hence the flow profile through the throat and over the sensor remains similar to the profile produced with parallel walls and results in reduced sensitivity to variations in the upstream duct configurations.  
         [0015]     A currently preferred embodiment comprises two venturis and sensors disposed in parallel, providing a two-sensor average output signal.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0017]      FIG. 1  is a cutaway isometric view of a prior art air flow rate sensor suspended centrally in a cylindrical duct;  
         [0018]      FIG. 2  is an elevational cross-sectional view of a prior art venturi air flow conditioning device disposed in a duct upstream of an air flow rate sensor, showing substantially laminar air flow axial of the duct;  
         [0019]      FIG. 3  is a schematic cross-sectional view of an exemplary prior art venturi air flow conditioning device, showing a wall boundary air layer and a flow separation region;  
         [0020]      FIG. 4  is a view like that shown in  FIG. 3 , showing the effect of upstream turbulence and non-axial impingement in the asymmetric build-up of the boundary layer and consequent reduction in throat cross-sectional area;  
         [0021]      FIG. 5  is an isometric view of a first embodiment of an air flow conditioning device in accordance with the invention, showing a textured (dimpled) surface of the venturi walls;  
         [0022]      FIG. 6  is a cross-sectional view of the device shown in  FIG. 5 , showing air flow patterns when subjected to the conditions shown in  FIG. 3 ;  
         [0023]      FIGS. 7 through 16  are views of various exemplary patterns for texturing of the venturi walls in accordance with the invention;  
         [0024]      FIG. 17  is an isometric view of a second embodiment of an air flow conditioning device in accordance with the invention, showing two venturis in parallel;  
         [0025]      FIG. 18  is a graph showing mass flow deviation (error) as a function of air flow rate and angular orientation of an entrance elbow duct to a prior art air flow measurement apparatus;  
         [0026]      FIG. 19  is a graph showing noise levels in the average flow signals shown in  FIG. 18 ;  
         [0027]      FIG. 20  is a graph like that shown in  FIG. 18 , showing results for an air flow measurement device in accordance with the invention when employing the air flow conditioner shown in  FIG. 17 ; and  
         [0028]      FIG. 21  is a graph showing noise levels in the average flow signals shown in  FIG. 20 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Referring to  FIG. 1 , a first prior art air flow measurement system  10  includes a flow sensor  12  disposed in the interior of a cylindrical duct  16  for conveying air and attached by supports  18  to the interior wall  20  of duct  16 . System  10  includes no upstream air flow conditioning means; thus sensor  12  samples only a tiny region of the cross-sectional area of duct  16  and is therefore highly sensitive to turbulent flow around the sensor, as may occur due to turbulence-generating changes in the upstream ducting (not shown in  FIG. 1 ). Even if total flow rate remains fixed, any changes to the upstream ducting configuration can alter the flow field around the sensor and cause the sensor output to change. The air flow signal will then contain a degree of error.  
         [0030]     Referring to  FIG. 2 , a second prior art air flow measurement system  10 ′ includes a flow sensor  12  but also includes a venturi nozzle air flow conditioning device  22  disposed in duct  16  upstream of sensor  12 . Device  22  includes opposed members  24  having walls  26  converging in the direction of air flow. A portion  30  of air  28  entering duct  16  is gathered by device  22  and funneled toward sensor  12  disposed at the exit to nozzle throat  32 . System  10 ′ thus samples and averages a much larger percentage of the flow cross-section than does system  10 . Further, device  22  accelerates the air toward sensor  12 , thus increasing the gain of the sensor.  
         [0031]     Referring to  FIG. 3 , when air flows through a venturi  34 , a boundary layer  36  is formed along walls  26  beginning at a point  38  and extending into and through throat  32  to a flow separation region  40  beginning at a point  42 . As discussed above, the shape and thickness of boundary layer  36  is a function of wall shape, flow velocity, and attack angle (direction of entering flow with respect to the wall surface). When the flow is essentially axial, as shown in  FIGS. 2 and 3 , boundary layers  36  are substantially identical on upper and lower wall surfaces  26 .  
         [0032]     Referring to  FIG. 4 , duct  16  is shown as having a 90° elbow portion  44  upstream of system  10 ′. Air portion  30  entering elbow  44  must turn  46  through a 90° angle. Severe turbulence is introduced into the air flow by such turning because air  48  flowing along the outside of the elbow flow path must travel a much longer path than air  50  flowing along the inside, resulting in generally non-axial flow of air  52  approaching device  10 ′. Because of a high attack angle on surface  26   a  and a correspondingly low attack angle on surface  26   b,  defining an asymmetric attack on both surfaces  26   a,   26   b,  a thin boundary layer  36   a  is formed along surface  26   a  and a thick boundary layer  36   b  is formed along surface  26   b.  The result is that the diameter  54  of throat  32  is functionally reduced, thus reducing the volume of air flowing through throat  32  and hence altering the signal output of sensor  12 .  
         [0033]     Referring to  FIGS. 5 and 6 , an improved airflow measurement system  100  in accordance with the invention comprises a duct  116 , a sensor  12 , and a first embodiment  122  of an improved air flow conditioning device, also referred to herein as an air flow conditioner. Device  122  comprises first and second spaced apart members  124   a,   124   b  suspended within duct  116  by supports  118 . Duct  116  may be, for example, an intake air manifold of an internal combustion engine  119 . Members  124   a,   124   b  include opposed surfaces  126   a,   126   b  which are convergent in the direction of air flow through duct  116  to define a throat  132  for receiving sensor  12 . Convergent surfaces  126   a,   126   b  may be planar (not shown) or preferably curved, as shown in  FIGS. 5 and 6 . Preferably, device  122  is a venturi.  
         [0034]     The converging regions of surfaces  126   a,   126   b  are provided with a texture which may comprise a plurality of features  170  arranged in a pattern ( FIG. 7 ), the features being separated by non-featured areas  172  of surfaces  126   a,   126   b.  In a currently preferred embodiment, the features comprise shallow depressions, as shown in  FIGS. 5 and 6 , similar to the well-known dimples on a golf ball; alternatively, features  127  may comprise bumps (not shown) also separated by non-featured areas. In currently preferred embodiments, the rows of features are staggered to the left and right in the direction of air flow.  
         [0035]     The following explanation, while currently believed by the inventors to be correct, should not be relied upon for patentability of the invention. It is believed that features  170  “trip” the flow in the boundary layer  136   a,   136   b  from laminar to turbulent, forming eddies  171  along the surface and causing the boundary layer to remain attached along the entire surface of the nozzle into the throat. The boundary layer does not build up, as in the prior art, and does not variably occlude the nozzle throat. The flow  174  through the nozzle throat  132  remains almost parallel to the nozzle axis  176  despite the extreme non-axial angle of attack of the incoming air flow.  
         [0036]     The currently-preferred pattern used to trip the flow to turbulent employs round dimples in a staggered pattern. Many other shapes and patterns will also accomplish the same effect.  
         [0037]     Referring to  FIGS. 7 through 16 , exemplary patterns in accordance with the invention are, respectively, staggered circles  180  ( FIG. 7 ); aligned squares  182  ( FIG. 8 ); staggered squares  184  ( FIG. 9 ); hexagons  186  ( FIG. 10 ); triangles  188  ( FIG. 11 ); crosses  190  ( FIG. 12 ); horizontal bars  192  ( FIG. 13 ); arbitrary shapes  194  ( FIG. 14 ); NACA duct shapes  196  ( FIG. 15 ); and vertical bars  198  ( FIG. 16 ). These shapes represent a small set of the many possible means for tripping the boundary layer flow to turbulent. The flow trips may include depressions and/or protrusions in/from the converging surfaces. The shapes may have any arbitrary outline. The pattern may have staggered, aligned, or random distribution. The pattern may be ordered or random (chaotic). The edges of the shapes may have sharp, square, rounded, or chamfered profiles. The cross-sections of the features may have flat, rounded, pointed or other profiles. Any form of surface roughness that trips the boundary layer flow into turbulence is comprehended by the invention.  
         [0038]     Referring to  FIG. 17 , an improved air flow measurement system  200  in accordance with the invention comprises a duct  216 , first and second sensors  12   a,   12   b,  and a second embodiment  222  of an improved air flow conditioner. Device  222  comprises first and second nozzles  217   a,   217   b,  each comprising first and second spaced apart members  224   a,   224   b  suspended within duct  216  by supports  218 . Nozzles  217   a,   217   b  are disposed in parallel within duct  216 ; therefore, system  200  samples more of the total air flow than does system  100 . Sensors  12   a,   12   b  provide independent signals  260   a,   260   b  which are averaged by a signal averaging mechanism  262  known in the art to further smooth the output signal over that provided by a single sensor as in embodiment  100  shown in  FIG. 5 . Members  224   a,   224   b  include opposed surfaces  226   a,   226   b  which are convergent in the direction of air flow through duct  216  to define throats  232   a,   232   b  for receiving sensors  12   a,   12   b.  Convergent surfaces  226   a,   226   b  may be planar (not shown) or preferably curved, as shown in  FIGS. 5 and 6 . Preferably, nozzles  217   a,   217   b  are venturis.  
         [0039]     Referring to  FIGS. 18 through 21 , a performance comparison is shown between a prior art air flow measurement system in use currently in vehicles and an air flow measurement system in accordance with system  200 .  
         [0040]     Each system was tested by mounting the air flow conditioner and associated sensor in a test bed comprising a cylindrical duct, the duct being preceeded by a 90° elbow, generally as shown in  FIG. 4 , to generate substantial turbulence in the air flow through the duct. Further, the elbow was rotatably attached to the cylindrical duct such that air flow signals could be generated by the sensor at a plurality of angular positions of the elbow with respect to the duct. In this test, eight angular positions were used at 45° increments of rotation of the elbow around the axis of the duct; the positions are identified in  FIGS. 18 through 21  as A through H. Further, the mass air flow through the test bed was varied incrementally at each angular position between 2 and 340 grams per second (g/sec).  
         [0041]     Referring to  FIGS. 18 and 19 , it is seen that the prior art flow sensor was very sensitive to relative angle of the elbow to the duct. Positions A,B,C,H generally gave negative flow deviations from zero line  300  of up to about 50%, whereas positions D,E,F,G generally gave positive flow deviations of up to about 50% ( FIG. 18 ). The deviations became only marginally less as flow rate increased. Further, the percent noise in the signal was extremely high ( FIG. 19 ), being greater than 100% for a number of flows at elbow position A, and being generally between about 10% and 70% for the rest of the determinations. Clearly, this device is highly sensitive to turbulence in the air flow and epitomizes the problem that the present invention is intended to minimize.  
         [0042]     Referring to  FIGS. 20 and 21 , under the identical test conditions, embodiment  200  in accordance with the invention showed a dramatic improvement. All flow deviations ( FIG. 20 ) from zero line  300  were within ±20%, and at flow rates above 40 g/sec all deviations were well within ±10% with almost no systematic sensitivity to elbow position. Further, the signal noise was generally less than about 25% and at high flow rates was less than 10% ( FIG. 21 ).  
         [0043]     In a practical use application, the greater accuracy is, of course, higly desirable, but also important is the very large reduction in signal noise. A noisy signal will require a great many more sequential readings to determine a true average; thus, a low signal noise device can respond much more rapidly in providing accurate air flow signals to a user system.  
         [0044]     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.