Thermal flow rate measuring device

A thermal type flow rate measuring device can certainly prevent adhesion of water droplet onto a sensor element and thus achieve high reliability. The thermal type flow rate measuring device includes an auxiliary passage defined within a main passage for introducing a part of fluid flowing through the main passage, a sensor disposed within the auxiliary passage for detecting flow rate of the fluid and capturing means formed on an inner periphery of the auxiliary passage for capturing liquid contained in the fluid and transferring the captured liquid.

BACKGROUND OF THE INVENTION

The present invention relates generally to a flow rate measuring device for detecting a flow rate of a fluid. More particularly, the invention relates to a thermal type flow rate measuring device which defines an auxiliary passage within a main passage flowing a fluid and measuring a flow rate of the fluid flowing through the auxiliary passage.

As prior art of a thermal type flow rate measuring device, there is a thermal type flow meter disclosed in Japanese Patent Application Laid-Open No. Heisei 9(1997)-304140. The disclosed thermal type flow meter is formed with an L-shaped auxiliary passage in a flow passage, and a sensor element for detecting flow rate is arranged within the auxiliary passage.

On the other hand, an air flow meter disclosed in Japanese Patent Application Laid-Open No. Heisei 9(1997)-287991 includes a reversed U-shaped auxiliary passage formed within an air passage. A sensor element for detecting flow rate is arranged in the vicinity of the outlet portion of the auxiliary passage.

In case of the air flow rate measuring device for measuring an intake air of an automotive vehicle, while traveling behind a vehicle splashing a large amount of water under rainy condition, while traveling in heavy rain, rain water may easily pass through an air cleaner to penetrate into an air intake passage in a form of fine mist.

It has been known to artisan in the art that the thermal type air flow rate measuring device inherently cause output error as adhering water droplet on the flow rate detecting sensor element and keeps output error until the adhered water droplet is evaporated completely. When error is output in out put of the air flow rate detecting sensor for automotive vehicle, it becomes difficult to obtain appropriate air/fuel ratio to cause difficulty in maintaining normal engine revolution.

In case of the technology disclosed in the foregoing Japanese Patent Application Laid-Open No. Heisei 9-304140, when moisture penetrates into the air intake passage, it may directly collide on the flow rate detecting sensor element to adhere thereon to cause difficulty in obtaining accurate output.

On the other hand, in case of the technology disclosed in Japanese Patent Application Laid-Open No. Heisei 9-287991, since the flow rate detecting sensor element is disposed in the reversed U-shaped passage. Therefore, possibility that the water droplet directly contact with the flow rate detecting sensor element to adhere thereon, is quite low.

However, even with the U-shaped auxiliary passage structure, it is not possible to completely avoid adhesion of water droplet on the sensor element. Namely, when moisture penetrates into the auxiliary passage, a part of mixture may adhere on the peripheral wall of the auxiliary passage initially in a form of small water droplet. Then, small water droplets are coupled to form large water droplet.

Since large water droplet may be easily brown by air flow, the water droplet thus blown may adhere on the sensor element.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a thermal type flow rate measuring device which can certainly prevent adhesion of water droplet onto a sensor element and thus achieve high reliability.

According to the first aspect of the present invention, a thermal type flow rate measuring device comprises:an auxiliary passage defined within a main passage for introducing a part of fluid flowing through the main passage;a sensor disposed within the auxiliary passage for detecting flow rate of the fluid; andcapturing means formed on an inner periphery of the auxiliary passage for capturing liquid contained in the fluid and transferring the captured liquid.

In the preferred construction, a thermal type flow rate measuring device further comprises discharging means for discharging the liquid captured by the capturing means out of the auxiliary passage.

The capturing means or the capturing means and the discharge means may be formed with one of a plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges, the captured liquid may be transferred along the one of a plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges,

A plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges may extend from upstream side to downstream side in the auxiliary passage in a manner directed from portions on the inner periphery of the auxiliary passage where liquid passing through the auxiliary passage is hardly concentrated to portions on the inner periphery of the auxiliary passage where liquid passing through the auxiliary passage is easily concentrated.

In the alternative, a plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges extends from upstream side to downstream side in the auxiliary passage in a manner directed from portions of the inner periphery of the auxiliary passage where centrifugal to be applied may be small to portions of the inner periphery of the auxiliary passage where centrifugal force to be applied is large.

A plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges may extend oblique relative to flow direction of the fluid.

The capturing means and/or the discharge means may have hydrophilic film formed on the inner periphery of the auxiliary passage so that when the liquid adheres on the inner periphery of the auxiliary passage, a contact angle is less than or equal to 30°.

The capturing means and the discharge means may be a plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges, and another groove coupled with a plurality of grooves, a plurality of beam-like ridges and a combination of a plurality of grooves and a plurality of beam-like ridges, extends from a portion of the inner periphery of the auxiliary passage in the vicinity of an inlet portion of the auxiliary passage and a portion of the inner periphery of the auxiliary passage in the vicinity of an outlet portion of the auxiliary passage,

The capturing means may be a plurality of grooves formed on the inner periphery of the auxiliary passage, and the inner periphery of the auxiliary passage of the portions other than those formed the plurality of grooves are provided water repellent characteristics so that when the liquid adheres on the inner periphery of the auxiliary passage, its contact angle thereof becomes greater than or equal to about 90°.

The capturing means may be a plurality of grooves formed in the inner periphery of the auxiliary passage, and the discharge means is through holes formed on at downstream side of the plurality of grooves in communication with the grooves.

According to the second aspect of the present invention, a thermal type flow rate measuring device comprises:a sensor provided within a main passage through which a fluid flows, for measuring flow rate of the fluid in the main passage;the main passage being curved in the vicinity of an upstream side of the sensor; andcapturing means formed on an inner periphery of the main passage for capturing a liquid carried by the fluid.

According to the third aspect of the invention, a thermal type flow rate measuring device in a vehicle control system including a thermal type flow rate control device, a control unit for controlling a vehicle on the basis of the condition of vehicle detected by the thermal type flow rate measuring sensor, the thermal type flow rate measuring device including an auxiliary passage introducing a part of fluid flowing through a main passage, and a sensor for detecting flow rate of the fluid, for detecting flow rate of the liquid,a liquid capturing means and/or discharging means being provided on an inner periphery of the auxiliary passage.

As set forth above, the grooves and/or the beam-like ridge captures the liquid, such as water droplet to prevent confluence of liquid for growth to increase size of the water droplet and whereby to prevent splashing as blown by flow of the fluid.

Also, with suppressing occurrence of splashing of the liquid, the penetrating liquid can be efficiently discharged out of auxiliary passage. Thus adhering of liquid on the flow rate detecting sensor can be successfully prevented.

A plurality of grooves and/or beam-like ridges are formed on the inner periphery of the auxiliary passage, and, the grooves and/or beam-like ridges are coupled with another groove which extends from the portion in the vicinity of the inlet portion and continuously to be discharged out of the auxiliary passage. By this, adhering of the liquid on the mass production sensor can be prevented.

A plurality of grooves are formed in the inner periphery and the inner peripheral surface of the auxiliary passage other than the portion of the grooves is formed with a water repellent film for capturing in the grooves before growth of liquid through binding. Thus, the liquid adhering on the inner periphery of the auxiliary passage can be transferred easily so that they may be captured before growth of water droplet.

By this, splashing of the liquid can be prevented to avoid adhesion of the water droplet on the flow rate detecting sensor.

The grooves are formed in the peripheral wall of the auxiliary passage, and through openings are formed in lower side of the grooves for discharging captured liquid through the through holes. By this, splashing of liquid can be prevented and also retention of the liquid in the auxiliary passage is prevented.

The main passage is curved at upstream side in the vicinity of the portion where the auxiliary passage is mounted, and on the inner periphery of the curved main passage, the liquid capturing means is formed. For example, on the inner periphery of the curved or bent portion, a plurality of grooves or beam-like ridges are formed to prevent the liquid from splashing of the liquid.

By this, splashing of liquid can be prevented, amount of liquid to penetrate into the auxiliary passage can be reduced to lower probability of adhesion of liquid on the flow rate detecting sensor can be reduced.

With the construction set forth above, liquid may not adhere on the flow rate detecting sensor to permit stable output to realize highly reliable flow meter.

On the other hand, even when contaminant of liquid containing oil droplet, such as engine oil contained in blow-by gas, adhesion of the liquid can be successfully prevented and whereby to prevent variation of characteristics of the sensor due to clear measurement. Also, variation of characteristics of the scanning element is not splashed to prevent penetration can be prevented to whereby prevent contamination by oil to cause variation of oil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a partial section of the first embodiment of a thermal type flow rate measuring device according to the present invention.

InFIG. 1, a thermal type flow rate measuring device1includes an auxiliary passage6, through which a part of fluid flowing through a main passage31. A flow rate measuring sensor element4is disposed within the auxiliary passage6for detecting flow rate. The thermal type flow rate measuring device1also includes a control circuit32for driving the flow rate measuring sensor element4and externally outputting a flow rate signal, and a housing34for receiving the control circuit32.

On the inner periphery of the auxiliary passage6in the vicinity of an inlet portion7through which fluid is introduced into the auxiliary passage, a plurality of grooves2or beam-like ridges3extending oblique in relation to a flow axis10of the fluid at an angle greater than 0° and smaller than 90° in clockwise direction (tilted from the inner side surface6b to the outer side surface6a of the auxiliary passage6). The reference numeral5denotes a temperature measuring resistor,33denotes a cover.

FIG. 2is a section of the auxiliary passage6ofFIG. 1, as taken along line A-A. On example of a shape of the groove2. The auxiliary passage6is formed with a material, such as PBT resin containing 30% of glass fiber. A thickness of the peripheral wall of the auxiliary passage6is about 2 mm, a width of the groove2is in a range about 0.3 mm to about 2 mm, and a depth of the groove2is in a range about 0.2 mm to about 1 mm.

It should be noted that a depth of the groove2is not particularly limited. When a thickness of the peripheral wall of the auxiliary passage6is large, the grooves2may be further deep.

On the other hand,FIG. 3is section of the auxiliary passage6ofFIG. 1as taken along line A-A similarly toFIG. 2. One example of the cross-sectional shape of the beam-like ridge3is shown inFIG. 3. The thickness of the peripheral wall of the auxiliary passage6is about 2 mm, a width of the beam-like ridge3is in a range about 0.3 mm to about 2 mm, and height of the beam-like ridge3is about 0.2 mm to 1 mm.

It should be noted that when the height of the beam-like ridge3is excessively high, it may interfere flow of the fluid. Therefore, the height of the beam-like ridge3cannot be made large in unlimited manner. The width L of the normally designed auxiliary passage6is about 10 mm. In such case, the height of the beam-like ridge3is about 1 mm at most.

Accordingly, when the width of the auxiliary passage6is wider, the height of the beam-like ridge3can be greater than or equal to 1 mm.

As set forth above, by forming the grooves2or the beam-like ridges3within the auxiliary passage6, in which the flow rate measuring sensor element4is arranged, adhesion of water droplet21on the flow rate measuring sensor element4can be successfully prevented.

Mechanism of prevention of adhesion of water droplet will be discussed with reference toFIGS. 4,5and6.

FIGS. 4(a),4(b) and4(c) are illustrations showing a manner of primary movement of a water droplet21when the grooves2or the beam-like ridges3in the auxiliary passage6, andFIGS. 5(a),5(b) and5(c) are illustrations showing a manner of movement of a water droplet21adhering on the inner periphery of the auxiliary passage6of the construction shown inFIG. 4. On the other hand,FIGS. 6(a),6(b) and6(c) are illustrations showing a manner of movement of a water droplet21adhering on the inner periphery of the auxiliary passage6where a plurality of grooves2or the beam-like ridges3are formed in the auxiliary passage6.

It should be noted that inFIGS. 4(a) to 6(c), the flow rate measuring sensor element4and temperature measuring resistor5arranged within the auxiliary passage6is eliminated from illustration. However, the flow rate measuring sensor element4and the temperature measuring resistor5are indeed arranged within the auxiliary passage6similar toFIG. 1.

When water droplet21penetrates into the auxiliary passage6, as shown inFIG. 4(a), most of water droplet21collides on curved portion on the outer side of the inner periphery6a of the auxiliary passage6bent in generally U-shaped configuration to adhere on the outer side inner periphery. Then, as shown inFIG. 4(b), the water droplet21adhering on the inner periphery of the auxiliary passage6flows along the periphery to move toward an outlet8. At this time, centrifugal force generated by flow10of the fluid acts on the water droplet21to peel off the periphery.

As shown inFIG. 4(c), the water droplet21is discharged out of the auxiliary passage6through the outlet portion8. Therefore, water droplet21will not adhere on the flow rate measuring sensor element4.

As set forth above, by employing U-shaped construction of the auxiliary passage, adhesion of water droplet21on the flow rate measuring sensor element4can be prevented in certain extent.

However, in practice, when the output of the flow rate measuring sensor element4is monitored with making mist form moisture penetrates into the main passage31from upstream, output fluctuates frequently. Thus it can be appreciated that simply U-shaped auxiliary passage is insufficient to prevent adhesion of moisture on the flow rate measuring sensor element4.

Therefore, by preparing transparent resin auxiliary passage6, movement of water droplet in the auxiliary passage when the water droplet21is sprayed is observed. As a result, as shown inFIGS. 5(a),5(b) and5(c), it can be appreciated that water droplet21adheres on the flow rate measuring sensor element4.

Namely, as shown inFIG. 5(a), a part of the water droplet21passing through the auxiliary passage6adheres inner peripheral surface (side surface) in the vicinity of the inlet portion7of the auxiliary passage6between outer side surface6a and inner side surface6b. Then, adhered water droplet21moves along the side surface as pushed by flow10of the fluid. At this time, size of the water droplet21is less than or equal to 1 mm in diameter.

While moving along the side surface of the auxiliary passage6, a plurality of water droplets are aggregated to form greater water droplet21. Size of the water droplet21thus formed becomes about 3 mm in diameter at most, as shown inFIG. 5(b).

As shown inFIG. 5(c), grown water droplet21is blown off as pushed by the flow10of the fluid. Then, a part of the blown water droplet adheres on the flow rate measuring sensor element4.

On the other hand, in case of the embodiment shown inFIGS. 6(a),6(b) and6(c), a plurality of grooves2or a plurality of beam-form ridges3are formed on the side surface of the auxiliary passage6. A plurality of grooves2or a plurality of beam-form ridges3have structure directed to directions along which large centrifugal force act in the auxiliary passage6by tilting at an angle about 45° with respect to flow axis10of the fluid.

As shown inFIG. 6(a), the water droplet21adheres on the side surface of the auxiliary passage6similarly toFIG. 5(a). However, as shown inFIG. 6(b), adhered water droplet21contact with a plurality of grooves2or a plurality of beam-like ridges3and is captured in the grooves2or the beam-like ridges3by surface tension and is moved along the grooves2or the beam-like ridges3to the side wall or the outer side periphery6a on the side where large centrifugal force generated by flow10of the fluid in the auxiliary passage6.

Then, as shown inFIG. 6(c), the water droplet21reaching to the outer side periphery6a is subject to centrifugal force generated by flow of the fluid to move toward the outlet portion8along the outer side peripheral6a. Therefore, water droplet21does not peel off the outer side periphery6a.

As set forth above, by forming a plurality of grooves2or a plurality of beam-like ridges3(capturing means, discharging means) within the auxiliary passage6, the water droplet21is captured and moved along the outer side periphery6a, on which large centrifugal force acts, to be discharged from the outlet portion8(discharge means) to avoid adhesion of water droplet21on the flow rate measuring sensor element4. Thus, highly reliable thermal type flow rate measuring device can be realized.

It should be noted that a plurality of grooves or a plurality of ridges may also be formed to tilt toward the inner peripheral surface where the water droplets concentrate in the auxiliary passage as actually flowing the water droplets. Then, comparable effect may be obtained.

On the other hand, with applying the shown construction, even when liquid state contaminant containing oil droplet as engine oil contained in blow-by gas, other than water droplet21, adhesion of oil droplet on the flow rate measuring sensor element4can be successfully prevented. Therefore, degradation of variation of characteristics due to oil contamination can be prevented. In this specification, water droplet, oil droplet or the like may generally defined as liquid body.

It should be noted that a plurality of grooves2or a plurality of beam-like ridges3may be a combination of a plurality of grooves2and a plurality of beam-like ridges3as shown inFIG. 7to achieve comparable effect. Therefore, any construction similar to or equivalent of the grooves2and the beam-like ridges3or combination thereof may be encompassed within the scope of the inventive idea of the present invention.

On the other hand, while the grooves2and the beam-like ridges3in the shown embodiment are illustrated in straight, it can be curved shape.

Also, in the shown embodiment, while the grooves2and the beam-like ridges3are in continuous straight shape, the grooves and the beam-like ridges are separated shape as shown by dotted line.

Furthermore, unless limitation in manufacturing, the grooves2and/or the beam-like ridges3may be formed on the entire inner periphery of the auxiliary passage.

Next, second embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIG. 8.

The construction of the auxiliary passage6as shown inFIGS. 1,4,5and6, all of the auxiliary passages are formed into U-shaped configuration. However, in the second embodiment of the present invention, the auxiliary passage6may be formed into a spiral shape and the flow rate measuring sensor element is the flow rate measuring sensor element4shown inFIG. 8.

On upstream sides position of the flow rate measuring sensor element4, a plurality of grooves2or a plurality of beam-like ridges3moderately tilted relative to flow10of the fluid may be formed. Then these grooves2or the beam-like ridges3may be formed to extend to the outer periphery portion where large centrifugal force is applied by flow10of the fluid.

Water droplets21adhering on the auxiliary passage is trapped by the grooves2or the beam-like ridges3and discharged out of the auxiliary passage6through the outlet portion8. Advantage point of the spiral shape of the auxiliary passage6is that since no sharply curved portion is present, the water droplets21trapped by the grooves2or the beam-like ridges3can be easily moved to the outside portion, and the water droplet21is easily moved to the outlet portion8via the outside portion.

By this, drainage performance can be improved to enhance water droplet adhesion preventing effect.

As set forth above, with the second embodiment of the present invention, similar effect to the first embodiment can be obtained. Furthermore, since the auxiliary passage6is formed into spiral shape, the water droplets21trapped by the grooves2or the beam-like ridges3can be easily moved to the outside portion, and the water droplet21is easily moved to the outlet portion8via the outside portion. Also, drainage performance can be improved to enhance water droplet adhesion preventing effect.

Next, the third embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIG. 9.

In the embodiment shown inFIG. 9, axially extending grooves2with branched grooves2a are formed in the U-shaped auxiliary passage6. Respective branched grooves2a are connected to the axially extending groove2. The axially extending groove2may be continuous from a portion in the vicinity of the inlet portion7and a portion in the vicinity of the outlet portion8.

It should be noted that, inFIG. 9, the flow rate measuring sensor element4arranged within the auxiliary passage6is eliminated from illustration but is indeed arranged in similar manner as illustrated inFIG. 1.

In the construction of the third embodiment of the present invention, the water droplet21captured in the grooves2and2a is guided by the grooves2and2a up to the position in the vicinity of the outlet portion8and is discharged out of the auxiliary passage6. With such construction, capturing means and discharging means of water droplet21can be formed by the grooves2and2a.

Even with the shown third embodiment, comparable effect as the first embodiment can be achieved.

It should be noted that, while not illustrated, similar effect can be obtained even with the construction, in which the beam-like ridges3are coupled with the axially extending groove2in place of the branched grooves2a. On the other hand, in the shown embodiment, the grooves2and2a are connected for integration. However, each individual branched groove2a is equivalent to the tilted groove2in the former embodiment. Therefore, even the groove structure shown inFIG. 9may be included in the scope of “a plurality of grooves” as defined in the former embodiment.

Next, the fourth embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIG. 10.

In the embodiment shown inFIG. 10, a single bent groove2in zigzag pattern is formed on the inner periphery of the U-shaped auxiliary passage6. On the general inner periphery other than the groove2, water repellent film22formed with water repellent cyclic silicon polymer, fluorine type compound or the like, is formed.

It should be noted that “water repellent” referred herein means that normal resin material has contact angle with water in a range of about 70° to about 90°, whereas the water repellent resin has contact angle with water in a range greater than or equal to 90°.

By forming water repellent film22on the inner periphery of the auxiliary passage6, water droplet21adhering on the inner periphery of the auxiliary passage6is easily moved to be captured by the grooves2before aggregated to grow into greater size of water droplet.

With such construction, superior water droplet21capturing ability can be obtained in the condition where flow velocity of the fluid is low.

As set forth above, even in the fourth embodiment of the present invention, similar effect as the first embodiment can be obtained.

It should be noted that, in the shown embodiment, single bent groove2is employed, However, the single bent groove of zigzag pattern may achieve substantially the same function as the former embodiment, Therefore, single groove as bent in the shown pattern or the like may fall within the scope defined by the wording “plurality of grooves”.

Next, the fifth embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIG. 11.

The fifth embodiment shown inFIG. 11is formed with a plurality of tilted grooves2oblique relative to the flow axis of the fluid, on the inner periphery of the U-shaped auxiliary passage6. Also, on downstream side ends of the grooves2, water drainage through holes9are formed.

It should be noted that, inFIG. 11, the flow rate measuring sensor element4arranged within the auxiliary passage6is eliminated from illustration but is indeed arranged in the similar manner as the former embodiment.

In the construction of the fifth embodiment of the present invention, the capturing means of water droplet21is the grooves2, and discharging means is the through holes9formed at the downstream portion of the grooves. With the construction where a plurality of water droplet discharging through holes9, even when large amount of water droplets penetrate into the auxiliary passage6, water droplets21may be efficiently discharged.

On the other hand, by forming the through holes9in the inner periphery of the auxiliary passage6, flow velocity of the fluid in the auxiliary passage6can be improved.

Namely, when the through holes are not formed in the inner periphery of the auxiliary passage6, while the fluid flow velocity can be high at the center portion of the auxiliary passage6, the fluid may be retained in the vicinity of the inner periphery. Therefore, as considered the whole auxiliary passage6, fluid flow velocity becomes lower than flow velocity of the fluid in the main passage.

In contrast to this, when the through holes are formed in the inner periphery of the auxiliary passage6, since the through holes are communicated with the main passage, portion retaining the fluid can be reduced to improve flow velocity of the fluid in the auxiliary passage6.

It should be noted that even when the grooves and the beam-like ridges are formed in the inner periphery of the auxiliary passage6, portion retaining the fluid can be reduced to improve flow velocity of the fluid in the auxiliary passage6.

Next, the sixth embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIGS. 12 and 13.

FIG. 12is a partial section of the sixth embodiment of the thermal type flow rate measuring device1according to the present invention, andFIG. 13is a section taken along line A-A ofFIG. 12.

As shown inFIG. 12, the flow rate measuring sensor element4is disposed within a straight pipe form auxiliary passage6, and a plurality of grooves2tilted relative to flow of the air are formed in the inner periphery of the auxiliary passage6. As shown inFIG. 13, the straight pipe form auxiliary passage6has tilted structure relative to the flow direction of the air, and the water droplets21or the like do not directly collide on the flow rate measuring sensor element4.

However, even with such construction, if the grooves2are not formed on the inner periphery of the auxiliary passage6, the water droplet21may adhere on the flow rate measuring sensor element4in the mechanism discussed in connection withFIG. 5.

Advantage of the sixth embodiment of the thermal type flow rate measurement device according to the present invention can prevent adhesion of the water droplets even in simple straight pipe form auxiliary passage6. It should be noted that comparable effect may be achieved even with the beam-like ridges3or combination of the grooves and ridges, in place of the grooves2.

Next, the seventh embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIG. 14.

As shown inFIG. 14, an upstream side end of the main passage31is connected to a bent upstream side duct41, and a downstream side end of the main passage31is connected to a straight duct42. Within the main passage31, the straight pipe form auxiliary passage6is disposed. On the inner periphery of the auxiliary passage6, the grooves2are formed. While not clearly illustrated, these grooves2are titled relative to the flow direction of the fluid as illustrated inFIG. 12.

By bent upstream side duct41, flow11of water droplets passing through the main passage31and the auxiliary passage6is bent by inertia effect. Therefore, even with the straight pipe form auxiliary passage6, most of the water droplets21may not collide on the flow rate measuring sensor element4.

However, because of the amount of water droplets21adhering on the side wall of the auxiliary passage6, adhesion of water droplets on the flow rate measuring sensor element4cannot be avoided satisfactorily without use the present invention, in which the grooves2are formed on the inner periphery.

It should be noted that a comparable effect may be achieved even with the beam-like ridges3or a combination of the grooves and ridges, in place of the grooves2. In another alternative, when water droplets21splash on the inner periphery of the auxiliary passage6to splash the water droplets as a reaction, it would be effective to form a hydrophilic film having hydrophilic property on the inner periphery.

It should be noted that “hydrophilic” meant herein is to have contact angle between the hydrophilic film and water less than or equal to about 30°.

After experiments using surface active agent or the like, higher hydrophilic property may result in uniformity in wetting the inner periphery of the auxiliary passage6to reduce possibility of splashing of water droplets21. In this case, even without the grooves2or beam-like ridges3, sufficient water droplets21adhesion preventing effect can be achieved.

Next, the eighth embodiment of the thermal type flow rate measuring device according to the present invention will be discussed with reference toFIG. 15.

As shown inFIG. 15, the main passages31and31a is bent at a position in the vicinity of a site where the auxiliary passage6is mounted. On the inner periphery of the bent main passage31a, a plurality of grooves2or a plurality of beam-like ridges3are formed. Water droplets penetrating into the main passage31a adhere on the inner periphery in the vicinity of a bent portion of the main passage31a by inertia effect.

Then, adhered water droplet is captured by a plurality of grooves or a plurality of beam-like ridges to prevent growth of water droplet to reduce splashing of water droplet to penetrate into the auxiliary passage6.

It should be noted that, in the eighth embodiment of the present invention, discussion has been given in terms of the thermal type flow meter provided with the auxiliary passage6, prevention of adhering of water droplet on the thermal type flow rate measuring device1even without providing the auxiliary passage can be achieved by taking measure for preventing splashing of water droplets in the main passage. Therefore, it becomes unnecessary to form the auxiliary passage to contribute for lowering of cost of the thermal type flow rate measuring device.

On the other hand, in case of the vehicle having air intake, into which water droplets easily penetrate, the measure for preventing splashing of water droplets is taken even for the inner periphery of the auxiliary passage, further improvement can be achieved.

It should be noted that comparable effect may be expected even by forming the hydrophilic film on the inner periphery of the bent portion in place of the plurality of grooves or a plurality of beam-like ridges.

FIG. 16is a section showing an example of the case where the thermal type flow rate measuring device according to the present invention is applied for measurement of air flow rate in an internal combustion engine, particularly gasoline engine.

InFIG. 16, an air cleaner102, a body105, a duct106, a throttle angle sensor107, an idle air control valve108, a throttle body109are assembled with an intake manifold110for integration to form an air intake passage.

An intake air101to the engine is detected flow rate by the thermal type flow rate sensor1according to the present invention while passing through the air intake passage or through a bypass passage. The detected flow rate signal is input to a control unit111in a form of voltage, frequency or the like to be used for controlling combustion portion and sub-system thereof constituted of an injector112, a revolution speed sensor113, an engine cylinder114, an exhaust manifold115, a gas116, an oxygen concentration sensor117.

By applying the thermal type flow rate measuring device according to the present invention for controlling the combustion portion and sub-system, adhesion of water droplets, oil droplets and so forth on the sensor element can be prevented to permit accurate measurement of intake air flow rate to improve precision of engine control.

It should be noted that while not illustrated, since diesel engine has substantially the same basic construction, the present invention is applicable.

On the other hand, the present invention is applicable for measurement of not only air flow rate but also flow rate of other fluid. For example, the present invention is applicable even for measurement of flow rate of hydrogen gas of the fuel cell. As applied to measurement of hydrogen gas, gas with 100% humidity is used to easy cause water droplet. Therefore, by applying the present invention, precision in measurement can be improved. Furthermore, the present invention is applicable for measurement of flow rate of propane gas.

Since the present invention is constructed by forming a capturing means for a liquid, such as water droplet, oil droplet or the like in the auxiliary passage or the main passage, and by discharging the captured liquid via a passage separated from the flow rate detecting sensor element, adhesion of the liquid, such as water droplet or the like on the flow rate detecting sensor element is successfully prevented to provide thermal type flow rate measuring device with high reliability.

Although the present invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omission and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalent thereof with respect to the feature set out in the appended claims.