Flow rate measuring device

A flow rate measuring device includes a housing having a cross-section perpendicular to a Y-direction and taken at a position of an outlet of the housing. The cross-section includes a widest portion having a maximum width along a Z-direction. The cross-section has a width gradually decreasing from the widest portion toward an upstream side end and has a width gradually decreasing from the widest portion toward a downstream side end. A width of the housing at a middle position of the housing in an upstream area is defined as a width W1. A width at a middle position in a downstream area is defined as a width W2. A length of the housing in the upstream area is defined as a length L1. A length in the downstream area is defined as a length L2. W1>W2 and W1/L1>W2/L2.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2015-059215 filed on Mar. 23, 2015.

TECHNICAL FIELD

The present disclosure relates to a flow rate measuring device, which includes a bypass passage taking in a portion of an air flowing in a duct and measures a flow rate of the air by a flow rate sensor disposed in the bypass passage.

BACKGROUND

Conventionally, a device has been known as a flow rate measuring device that includes a housing defining a bypass passage therein to take in a portion of an air (main flow) flowing in a duct, and a flow rate sensor disposed in the bypass passage.

For example, Patent Document 1 (JP H8-297039 A) discloses a flow rate measuring device in which an outlet of a bypass passage is open at an outer wall of a housing, and the housing has an oval shape. An outline of the housing disclosed in Patent Document 1 includes a portion (widest portion), which is widest in the outline, at a middle of the housing in a direction of the main flow. The outline is symmetrical about the widest portion.

Since a rate of the main flow is accelerated in an upstream area of the widest portion, a separation of the air is unlikely to occur. Since a rate of the main flow is decreased in a downstream area of the widest portion, the separation of the air may be likely to occur. Hence, when both the upstream area and the downstream area where the likelihood of occurrence of the separation is different have a same shape, it may be difficult to reduce the separation in the downstream area.

Especially, when the outlet of the bypass passage is open at the outer wall of the housing, and when an eddy is generated due to the separation at a downstream area of an airflow near the outlet, the eddy may negatively affect a flow in the bypass passage, and whereby a detection accuracy of the flow rate sensor in the bypass passage may be deteriorated.

SUMMARY

It is an objective of the present disclosure to provide a flow rate measuring device that reduces a separation of a flow of an air flowing in a duct along a housing.

According to a first aspect of the present disclosure, a flow rate measuring device includes a housing disposed in a duct to protrude into an inside of the duct from an outside of the duct through which an air as a measuring target flows. The housing defines a bypass passage therein to take in a portion of the air flowing in the duct. An outlet of the bypass passage is open at an outer wall of the housing. The flow rate measuring device includes a flow rate sensor disposed in the bypass passage. A direction in which the air flows in the duct is defined as an X-direction, a direction in which the housing protrudes is defined as a Y-direction, and a direction perpendicular to both the X-direction and the Y-direction is defined as a Z-direction. The housing has a cross-section that is perpendicular to the Y-direction and that is taken at a position of the outlet. The cross-section includes a widest portion, between an upstream side end and a downstream side end, having a maximum width along the Z-direction. The cross-section has a width along the Z-direction gradually decreasing from the widest portion toward the upstream side end and has a width along the Z-direction gradually decreasing from the widest portion toward the downstream side end. A width of the housing along the Z-direction at a middle position of the housing between the widest portion and the upstream side end in the X-direction is defined as a width W1. A width of the housing along the Z-direction at a middle position of the housing between the widest portion and the downstream side end in the X-direction is defined as a width W2. A length of the housing along the X-direction between the widest portion and the upstream side end is defined as a length L1. A length of the housing along the X-direction between the widest portion and the downstream side end is defined as a length L2. W1>W2and W1/L1>W2/L2.

According to a second aspect of the present disclosure, a flow rate measuring device includes a housing disposed in a duct to extend in a direction inside the duct through which an air flows. The housing defines a bypass passage therein to take in a portion of the air flowing in the duct. An outlet of the bypass passage is open at an outer wall of the housing. A direction in which the air flows in the duct is defined as an X-direction, a direction in which the housing extends is defined as a Y-direction, and a direction perpendicular to both the X-direction and the Y-direction is defined as a Z-direction. The housing has a cross-section that is parallel to both the X-direction and the Z-direction and that is taken at a position of the outlet. The cross-section includes a widest portion, between an upstream side end and a downstream side end, having a maximum width along the Z-direction. The cross-section has a width along the Z-direction gradually decreasing from the widest portion toward the upstream side end and has a width along the Z-direction gradually decreasing from the widest portion toward the downstream side end. A width of the housing along the Z-direction at a middle position of the housing in the X-direction between the widest portion and the upstream side end is defined as a width W1. A width of the housing along the Z-direction at a middle position of the housing in the X-direction between the widest portion and the downstream side end is defined as a width W2. A length of the housing along the X-direction between the widest portion and the upstream side end is defined as a length L1. A length of the housing along the X-direction between the widest portion and the downstream side end is defined as a length L2. W1>W2and W1/L1>W2/L2.

According to these aspects, the housing has a downstream area downstream of the widest portion and an upstream area upstream of the widest portion, and a decrease rate of the housing in the width of the downstream area from the widest portion to the downstream side end is less than that of the upstream area from the widest portion to the upstream side end. Therefore, a flow rate of the air decreases, and a separation of the air unlikely occurs. In other words, generation of the separation of the air flowing through the duct along the housing can be suppressed.

DETAILED DESCRIPTION

A configuration of a flow rate measuring device1according to a first embodiment will be described referring toFIGS. 1 to 5. The flow rate measuring device1is an airflow meter, for example, that measures an amount of an intake air drawn into an engine for a vehicle. The flow rate measuring device1is attached to a duct D that defines an intake passage sending the intake air to the engine for a vehicle. An attachment hole Da is open at a wall of the duct D, and the flow rate measuring device1is inserted into the duct D through the attachment hole Da.

The flow rate measuring device1includes an engagement portion2, a housing3, a flow rate sensor4and so on.

The engagement portion2is engaged with the attachment hole Da and includes an outer peripheral surface facing an inner peripheral surface of the attachment hole Da. The outer peripheral surface includes a circumferential groove, as shown inFIG. 2. A gap between the inner peripheral surface of the attachment hole Da and the outer peripheral surface of the engagement portion2is sealed by an O-ring2adisposed in the circumferential groove, as shown inFIG. 1.

The housing3protrudes, into an inside of the duct D, in a direction (the radial direction of the duct D) approximately perpendicular to a flow direction (a direction of a main flow) of an air in the attachment hole Da. However, the direction, in which the housing3protrudes, may not be perpendicular to the flow direction. A portion of the flow rate measuring device1protruding from the engagement portion2to an outside of the duct D includes a connector6. Hereinafter, the direction of the main flow is referred to as an X-direction, a direction in which the housing3protrudes is referred to as a Y-direction, and a direction perpendicular to the X-direction and the Y-direction is referred to as a Z-direction.

The housing3protrudes from a portion of the duct D close to a wall of the duct D toward an axial center of the duct D, and the housing3defines a bypass passage8therein to take in a part of the air flowing in the duct D. The bypass passage8includes an inlet10through which a portion of the air flowing in the duct D flows, an inner passage11in which the air passing through the inlet10flows, and an outlet12that returns the air introduced into the bypass passage8to the air flowing in the duct D.

The inner passage11includes an intake passage13extending from the inlet10toward a downstream side of the duct D, and a circulation passage14that revolves a fluid (air) and guides the air to the outlet12.

The intake passage13branches into two branched passages. One branched passage is connected to the circulation passage14, and the other branched passage is connected to a dust discharge passage16. The dust discharge passage16is a passage for discharging a dust contained in the air introduced from the inlet10by allowing the dust to path through the housing3. A fluid, which has passed through the intake passage13and flows in the dust discharge passage16, flows approximately parallel to the main flow. A downstream end of the dust discharge passage16defines a dust discharge opening17.

The circulation passage14guides the fluid flowing from the intake passage13toward one side in the Y-direction (a radially outside of the duct D, toward the engagement portion2), and then guides the fluid toward an upstream side of the main flow, i.e. an opposite direction in which the air flowing in the intake passage13. Subsequently, the circulation passage14guides the fluid toward the other side in the Y-direction, and then guides the fluid toward a downstream side of the main flow and guides the fluid to the outlet12.

The outlet12is open on an outer wall3aof the housing3. Therefore, a discharge of the fluid from the outlet12is promoted due to a Venturi effect by using the main flow passing in front of the outlet12.

The flow rate sensor4outputs an electric signal (e.g. voltage signal) according to a flow rate of the air flowing in the bypass passage8. Specifically, the flow rate sensor4includes a heating element and a thermosensitive element, which are formed of thin film resistors, on a membrane disposed on a semiconductor substrate. The elements are connected to a substrate housed within a circuit module. The flow rate sensor4is housed in a portion of the circulation passage14in which the fluid flows in a direction opposite to the direction of the main flow. The flow rate sensor4disposed in the circulation passage14detects the flow rate.

FIG. 5shows a cross-section of the housing3perpendicular to the Y-direction taken at a position of the outlet12. In this embodiment, the cross-section is parallel to both the X-direction and the Z-direction. Any cross-sections at any positions within a range in the Y-direction where the outlet12exists have the same shape as described below.

In the cross-section, an end portion on an upstream side in the X-direction is defined as an upstream side end20, and an end portion on a downstream side is defined as a downstream side end21. In the present embodiment, the upstream side end20and the downstream side end21are on an imaginary line Xa parallel to the X-direction. One side of the cross-section in the Z-direction with respect to the imaginary line Xa has a shape symmetrical to that of the other side. The outlet12is open on both the one side and the other side of the outer wall3ain the Z-direction. The main flow flows along the outer wall3a.

The cross-section includes a widest portion22between the upstream side end20and the downstream side end21. The widest portion22has a maximum width along the Z-direction. A width of the cross-section along the Z-direction gradually decreases from the widest portion22toward the upstream side end20. Also, the width of the cross-section along the Z-direction gradually decreases from the widest portion22toward the downstream side end21. Therefore, a projected area of a portion of the housing3including the outlet12and projected in the direction of the main flow gradually increases from the upstream side end20toward the widest portion22, and the projected area decreases from the widest portion22toward the downstream side end21.

In the present embodiment, an outline of the cross-section are formed of a plurality of curves that are continuously connected to each other and are convex radially outward of the housing3. In other words, both the outer wall3aon the one side of the housing3in the Z-direction and the outer wall3aon the other side of the housing3in the Z-direction are defined by the plurality of continuous curves and each of the plurality of continuous curves is gently inclined with respect to the imaginary line Xa.

A width of the housing3along the Z-direction, at a middle position in the X-direction between the widest portion22and the upstream side end20, is defined as a width W1. A width of the housing3along the Z-direction of the housing3, at a middle position in the X-direction between the widest portion22and the downstream side end21, is defined as a width W2. A length of the housing3along the X-direction from the widest portion22to the upstream side end20is defined as a length L1. A length of the housing3along the X-direction from the widest portion22to the downstream side end21is defined as a length L2. Then, W1>W2and W1/L1>W2/L2.

An upstream end12aof the outlet12is positioned upstream of the widest portion22in the X-direction. In the present embodiment, the outlet12is open in a particular region of the housing3from a portion upstream of the widest portion22to a portion downstream of the widest portion22.

As shown inFIG. 5, an imaginary line representing a position of the widest portion22in the X-direction is defined as an imaginary line Z0. If the widest portion22extends in a predetermined area along the X-direction, the imaginary line Z0would be a line passing a middle position of the predetermined area in the X-direction.

Here, an area of the cross-section upstream of the imaginary line Z0is referred to as an upstream area. An imaginary line extending along the Z-direction and representing a middle position of the upstream area in the X-direction is referred to as an imaginary line Z1.

An area of the cross-section downstream of the imaginary line Z0is referred to as a downstream area. An imaginary line extending along the Z-direction and representing a middle position of the downstream area in the X-direction is referred to as an imaginary line Z2.

A point at which the imaginary line Z1intersects with the outer wall3aof the housing3on the one side in the Z-direction is defined as an intersection Q1. A point at which the imaginary line Z1intersects with the outer wall3aof the housing3on the other side in the Z-direction is defined as an intersection Q2. A point at which the imaginary line Z2intersects with the outer wall3aof the housing3on the one side in the Z-direction is defined as an intersection Q3. A point at which the imaginary line Z2intersects with the outer wall3aof the housing3on the one side in the Z-direction is defined as an intersection Q4.

Since W1/(L1/2) represents an average gradient of the outer wall3abetween the upstream side end20and the intersection Q1or the intersection Q2, W1/L1represents a degree of a gradient (increase rate of width) of the outer wall3abetween the upstream side end20and the intersection Q1or the intersection Q2. Since W2/(L2/2) represents an average gradient of the outer wall3abetween the downstream side end21and the intersection Q3or the intersection Q4, W2/L2represents a degree of a gradient (increase rate of width) of the outer wall3abetween the downstream side end21and the intersection Q3or the intersection Q4.

In the present embodiment, since W1/L1>W2/L2, the increase rate of width in the downstream area is smaller than the increase rate of width in the upstream area. The cross-section has a teardrop shape in which the downstream side end21is thinner than the upstream side end20.

In other words, a point angle θ2in the downstream area is smaller than a point angle θ1in the upstream area. The point angle θ1is an angle between a line from the intersection Q1to the upstream side end20and a line from the intersection Q2to the upstream side end20. The point angle θ2is an angle between a line from the intersection Q3to the downstream side end21and a line from the intersection Q4to the downstream side end21.

In the present embodiment, the length L2is longer than the length L1. In other words, a dimension of the downstream area along the X-direction is larger than a dimension of the upstream area along the X-direction.

In the present embodiment, the upstream end12aof the outlet12is located upstream of the widest portion22in the X-direction. In other words, a position of the upstream end12ais located upstream of the imaginary line Z0.

In the flow rate measuring device1of the present embodiment, the cross-section of the housing3perpendicular to the Y-direction taken at a position of the outlet12satisfies relationships, W1>W2and W1/L1>W2/L2. Therefore, the projected area of the housing3downstream of the widest portion22projected in the direction of the main flow changes gently compared to the projected area on the upstream side. Since an increase rate of a gradient, with respect to the direction of the main flow, of the outer wall3aof the downstream area from the widest portion22toward the downstream side end21can be gentle as compared to an increase rate of a gradient of the upstream area from the widest portion22toward the upstream side end20, a separation of a fluid can become unlikely to occur even when a flow rate of the fluid decreases.

As shown inFIG. 6, in the present embodiment, the main flow is likely to flow along the housing3, especially in the downstream area, as compared to a comparative example, and thus generation of the separation of the fluid can be suppressed. Accordingly, occurrence of an eddy in the downstream area can be limited.

In the comparative example, although an acceleration effect for air can be obtained at the upstream area, a gradient in the downstream area is similar to a gradient in the upstream area. Therefore, a change of a projected area of the housing3in the downstream area projected in the direction of the main flow may be greater than that of the present embodiment. In other words, a decrease rate in the width of the downstream area from the widest portion22toward the downstream side end21according to the comparative example is greater than that of the present embodiment. Accordingly, since a rate of the main flow flowing along the housing3in the downstream area of the comparative example is slow. Moreover, the change of the projected area in the downstream area may be bigger. Therefore, a separation of a fluid may be likely to occur.

Since the occurrence of the eddy due to separation can be limited in the present embodiment, the eddy does not affect the flow in the bypass passage8, and thus a detection accuracy of the flow rate sensor4can be increased.

In the present embodiment, L1and L2are set as L2>L1. Accordingly, the gradient, with respect to the main flow, of the outer wall3ain the downstream area can be gentle easily.

The upstream end12aof the outlet12is located upstream of the widest portion22in the X-direction, and a rate of the main flow in the upstream area flowing along the outer wall3ais accelerated and flows fast. Accordingly, since the upstream end12aof the outlet12is included in the upstream area, a Venturi effect can be used effectively.

A portion of the outer wall3a close to the outlet12is under a circumstance where the separation of the fluid is likely to occur due to a junction of a flow from the outlet12and the main flow. However, since the outlet12is positioned in the upstream area, in which the rate of the main flow is high and the separation is unlikely to occur, occurrence of the separation due to the junction of the flows can be suppressed.

Points of a second embodiment different from a first embodiment will be mainly described referring toFIGS. 7 and 8. Same reference numerals as the first embodiment represent the same elements, and preceding descriptions will be referred. In the present embodiment, an outline of a cross-section of a housing3perpendicular to a Y-direction is formed of a plurality of continuous straight lines. In other words, both an outer wall3aof the housing3, along which a main flow flows on one side in a Z-direction, and an outer wall3a, along which the main flow flows on the other side in the Z-direction, are defined by the plurality of lines inclined with respect to an imaginary line Xa.

The outline of the cross-section in an upstream area is formed of a straight line31and a straight line32. The straight line31extends from an upstream side end20and is inclined with respect to the imaginary line Xa at an incline angle d1. The straight line32is connected to an end of the straight line31opposite from the upstream side end20and inclined with respect to the imaginary line Xa at an incline angle d2. The incline angle d1is larger than the incline angle d2. The outline of the cross-section in a downstream area is formed of a straight line33and a straight line34. The straight line33extends from a downstream side end21and is inclined with respect to the imaginary line Xa at an incline angle d3. The straight line34is connected to an end of the straight line33opposite from the downstream side end21and inclined with respect to the imaginary line Xa at an incline angle d4. The incline angle d3is larger than the incline angle d4.

A position where the straight line34and the straight line32intersect with each other is a widest portion22. The incline angles of the straight lines decrease with a decrease in a distance from the widest portion22. That is, the incline angles d1, d1of the straight lines31,31are greater than the incline angles d2, d2of the straight lines32,32. Similarly, the incline angles d3, d3of the straight lines33,33are greater than the incline angles d4, d4of the straight lines34,34.

Also in the present embodiment, as with the first embodiment, W1>W2and W1/L1>W2/L2. In the cross-section, a distance L2in an X-direction between the downstream side end21and the widest portion22is larger than a distance L1in the X-direction between the upstream side end20and the widest portion22. An upstream end12aof an outlet12is located upstream of the widest portion22in the X-direction. Accordingly, the same effects as the first embodiment can be obtained.

A corner portion at which the straight lines intersects with each other may be chamfered or have a round shape. A corner of the upstream side end20and a corner of the downstream side end21may be cut off so that the housing3includes a plane35perpendicular to the X-direction.

In the above-described embodiments, the bypass passage8is defined by the inner passage11including the circulation passage14, but the bypass passage8is not limited to this. The bypass passage8may be provided along the direction of the main flow. The bypass passage8may guide the air, which is introduced from the inlet10, along a normal flow in the direction of the main flow, and subsequently, the bypass passage8discharges the air from the outlet12.

In the above-described embodiments, the cross-section perpendicular to the Y-direction taken at the position of the outlet12is symmetrical about the imaginary line Xa, but the cross-section may not be symmetrical. However, the cross-section is preferred to be symmetrical in terms of the limitation of occurrence of the separation and the eddy. It is because the symmetrical cross-section may limit an occurrence of differences of flow rate and pressure, and the flow around the housing3may become stable.

In the above-described embodiments, the length L2is larger than the length L1, but the length L2may be equal to the length L1.

In the above-described embodiments, the housing3of the flow rate measuring device1protrudes into the inside of the duct D from the outside of the duct D through the attachment hole Da. However, the flow rate measuring device1may be attached to an inner wall of the duct D. In this case, the attachment hole Da may be eliminated.

Additional advantages and modifications will readily occur to those skilled in the art. The disclosure in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described.