Patent Application
20200209022
The present invention relates to a flow rate measurement device and a flow rate measurement method. More specifically, the invention relates to a flow rate measurement device including: a measuring instrument for measuring a differential pressure between a first pressure in a first pressure receiver of a tube body in which a fluid flows, and a second pressure in a second pressure receiver of the tube body; and a processor for calculating the flow rate of the fluid on the basis of a measured differential pressure. The invention also relates to a flow rate measurement method.
A conventional example of the flow rate measurement device as described above has been disclosed in Patent Document 1. This device measures the flow rate of a fluid by using a differential pressure between a static pressure that is measured in a static pressure measurement hole provided in a guide path, and a wake pressure that is measured in a wake pressure detection tube. Flow of a fluid includes a laminar flow region, a turbulent flow region, and a transition region therebetween where the laminar flow region changes to the turbulent flow region. However, in the differential pressure type flowmeter, calculation of a flow rate is not performed in consideration of the state of the fluid, and therefore, further improvement of measurement accuracy has been desired.
[PATENT DOCUMENT 1] Japanese Utility Model Registration No. 3200638
In view of the circumstances described above, an object of the present invention is to provide a flow rate measurement device and a flow rate measurement method which are able to accurately measure flow rates not only in the turbulent flow region but also in the laminar flow region and the transition region.
In order to achieve the above object, a flow rate measurement device according to the present invention is provided with: a measurement instrument configured to measure a differential pressure between a first pressure in a first pressure receiver of a tube body in which a fluid flows, and a second pressure in a second pressure receiver of the tube body; and a processor configured to calculate a flow rate of the fluid on the basis of a measured differential pressure. In this configuration, the processor includes: a parameter generator configured to obtain a reference differential pressure in a flow rate calculation formula according to expression 1 as follows, on the basis of a plurality of already-known flow rates and measured differential pressures corresponding to the respective already-known flow rates, and generate two parameter sets each including coefficients c1 to c3 of the flow rate calculation formula, with the reference differential pressure being a boundary; a differential pressure determination unit configured to select one of the two generated parameter sets by comparing a measured differential pressure of a fluid as a measurement target with the reference differential pressure; and a flow rate calculator configured to calculate a flow rate of the fluid by substituting the selected parameter set and the measured differential pressure into the flow rate calculation formula:
ΔP=c1×η×Q+c2×ρ×Q2+c3 (expression 1)
(Q: flow rate, ΔP: differential pressure, η coefficient of kinematic viscosity, ρ: density, c1 to c3: coefficients)
Meanwhile, the fluid flowing inside the tube body has: a laminar flow region in which viscous force is dominant and therefore the fluid flows smoothly and stably with no vortex; a turbulent flow region in which inertial force is dominant and therefore various vortexes are generated; and a transition region, between the laminar flow region and the turbulent flow region, in which viscous force and inertial force coexist. A differential pressure ΔP of this fluid is obtained by the flow rate calculation formula expressed by the quadratic function of the flow rate Q as shown in expression 1. Here, c1×η×Q is referred to as a viscosity term and c2×ρ×Q2 is referred to as an inertial term. The laminar flow region and the turbulent flow region have different states of flow of the fluid, and the dependences thereof on the viscosity term and the inertial term in the flow rate calculation formula are different from each other.
According to the above configuration, the reference differential pressure in the flow rate calculation formula according to expression 1 is obtained based on the plurality of already-known flow rates and the measured differential pressures corresponding to the respective already-known flow rates, and the two parameter sets each including the coefficients c1 to c3 of the flow rate calculation formula are generated with the reference differential pressure being the boundary. Thus, with the reference pressure being the boundary, the flow rate calculation formula is divided into a range where laminar flow separation hardly occurs (where the flow rate is small) and a range where laminar flow separation occurs (where the flow rate is large), and the coefficients of the flow rate calculation formula are generated in each range. Therefore, the parameter sets as the coefficients can be generated according to the state of flow of the fluid in the vicinity of the outer peripheral surface of a columnar member. Then, one of the two parameter sets generated is selected by comparing the measured differential pressure of the fluid as the measurement target with the reference differential pressure, and the flow rate of the fluid is calculated by substituting the selected parameter set and the measured differential pressure into the flow rate calculation formula. Therefore, the flow rate can be calculated based on the parameters according to the state of the fluid as the measurement target, whereby calculation accuracy is improved. Moreover, since different parameters are generated with the reference differential pressure as the boundary, calculation accuracy in a region where the flow rate is small so that the transition region and the laminar flow region are included is also improved. Therefore, highly accurate measurement of flow rates is achieved not only in the turbulent flow region but also in the laminar flow region and the transition region.
In the above configuration, the tube body includes, inside thereof, the columnar member extending in a direction perpendicular to a flow direction of the fluid. Preferably, the first pressure receiver is a first measurement hole provided on an upstream side, in the flow direction, of the columnar member, and the second pressure receiver is a second measurement hole provided on a downstream side, in the flow direction, of the columnar member. Preferably, the columnar member has a streamline shape that is line symmetric with respect to a first plane that is perpendicular to a center axis of the tube body and passes the center of a length, along the flow direction, of the columnar member. Preferably, the first measurement hole and the second measurement hole are arranged in symmetry with respect to the first plane. Since the columnar member provided in the tube body so as to extend in the direction perpendicular to the flow direction of the fluid has the streamline shape that is line symmetric with respect to the first plane that passes the center of the length along the flow direction, the fluid that flows along the streamline shape causes a laminar flow and a turbulent flow. In the columnar member, the first measurement hole as the first pressure receiver is provided on the upstream side in the flow direction and the second measurement hole as the second pressure receiver is provided on the downstream side in the flow direction such that these holes are line symmetric with respect to the first plane. Since this configuration allows the columnar member to have bidirectionality with respect to the flow direction of the fluid, the parameters need not be reset in accordance with the flow direction. Therefore, even when the flow direction of the fluid is reversed, distribution of changes (changes in speed, pressure, etc.) of the fluid due to the columnar member also becomes symmetric with respect to the first plane, whereby the flow rate can be measured by using the same parameter set regardless of the flow direction of the fluid.
The columnar member is preferably line symmetric with respect to a second plane that includes a center axis of the tube body and is parallel to the direction along which the columnar member extends. Thus, the pressure applied to the columnar member from the fluid is substantially symmetrically uniform with respect to the second plane, whereby the measurement accuracy is improved, and the durability of the columnar member is also improved.
Preferably, the columnar member, which is projected on a third plane that is perpendicular to the first plane and the second plane, has a shape with an outer peripheral surface that is defined by expression 2 and expression 3 as follows:
L=2(K+r) (expression 2)
d=2(K(1/cos θ−tan θ)+r) (expression 3)
(O: origin point, d: width of the columnar member, L: length of the columnar member in the flow direction, r: radius of an arc centered around point a that is ±K away from the origin point O in the flow direction, θ: center angle of the arc/2)
According to the above expressions 2 and 3, the width d of the columnar member in the tube diameter direction depends on the center angle of the arc. By adjusting the length of the columnar member in the flow direction and the center angle of the arc, the columnar member is allowed to have a shape that can control pressure loss and occurrence of turbulent flow and can improve measurement accuracy.
The length of the columnar member in the direction along which the columnar member extends in the tube body is preferably smaller than the diameter of the tube body. Thus, a gap, through which the fluid can flow down, is formed between the tube wall and an end portion of the columnar member in the extending direction, whereby the ratio of the columnar member to the cross section of the tube body perpendicular with respect to the flow direction of the fluid is reduced. Therefore, pressure loss of the fluid due to the columnar member can be reduced, and measurement accuracy can be ensured.
The first measurement hole and the second measurement hole are preferably positioned on the center axis of the tube body. Thus, the differential pressure is measured at the center position where the pressure due to the fluid flowing in the tube body is highest, whereby measurement accuracy of the flow rate is improved.
The columnar member is solid, and tubular communication paths communicating with the measurement instrument may be formed in the first measurement hole and the second measurement hole, respectively. Since the columnar member is solid, measurement accuracy can be ensured while ensuring strength (resistance) against the fluid.
In any one of the above configurations, the fluid is preferably breathing air. Since the columnar member has bidirectionality as described above, the flow rate measurement device of the present invention can be implemented as a breath measurement device, for example. Alternatively, the fluid may be medical gas.
The flow rate measurement device may further include a columnar member extending in a direction perpendicular to the flow direction of the fluid. The first pressure receiver may be a first opening provided through the tube wall of the tube body. The columnar member may be positioned on the downstream side in the flow direction relative to the first opening. The second pressure receiver may be a second opening provided on the downstream side of the columnar member.
The tube body may have, on the tube wall, an orifice that reduces the tube path. The first pressure receiver may be a first opening provided in the tube wall on the upstream side in the flow direction of the fluid relative to the orifice. The second pressure receiver may be a second opening provided in the tube wall on the downstream side in the flow direction of the fluid relative to the orifice.
In order to achieve the above object, a flow rate measurement method according to the present invention is a method for measuring a differential pressure between a first pressure in a first pressure receiver of a tube body in which a fluid flows and a second pressure in a second pressure receiver of the tube body, and calculating a flow rate of the fluid on the basis of a measured differential pressure. The method includes: causing a fluid to flow in the tube body at a plurality of already-known flow rates; obtaining a reference differential pressure in a flow rate calculation formula according to expression 4 as follows, on the basis of the plurality of already-known flow rates and measured differential pressures corresponding to the respective already-known flow rates, and generating two parameter sets each including coefficients c1 to c3 of the flow rate calculation formula, with the reference differential pressure being a boundary; causing a fluid as a measurement target to flow in the tube body; selecting one of the two generated parameter sets by comparing a measured differential pressure of the fluid as the measurement target with the reference differential pressure; and calculating a flow rate of the fluid by substituting the selected parameter set and the measured differential pressure into the flow rate calculation formula:
ΔP=c1×η×Q+c2×ρ×Q2+c3 (expression 4)
(Q: flow rate, ΔP: differential pressure, η coefficient of kinematic viscosity, ρ: density, c1 to c3: coefficients)
The features of the flow rate measurement device and the flow rate measurement method according to the present invention realize highly accurate measurement of flow rates not only in the turbulent flow region but also in the laminar flow region and the transition region.
Other objects, configurations, and effects of the present invention will become apparent from the following description of embodiments of the present invention.
Next, the present invention will be described in more detail with reference to
As shown in
As shown in
As shown in
As shown in
The tube body 4 is a circular tube mounted to a casing 20 so as to penetrate the casing 20 as shown in
In the present embodiment, as shown in
In addition, since the first and second pressure receivers 51a and 51b for measuring the differential pressure ΔP are provided in the columnar member 5, it is not necessary to provide a static pressure hole for measuring a static pressure in the tube wall 41 of the tube body 4, in contrast to, for example, a conventional orifice flowmeter provided with a restrictor (orifice) or the back-pressure and differential-pressure type flowmeter disclosed in Patent Document 1 described above. Thus, the size of a straight tube portion, which is required to be provided before and after the pressure receiver 51 for measuring the differential pressure ΔP, can be made equal to about a diameter D of the tube body 4 (it is about 15 times the diameter D in the orifice flowmeter, and about 4 times the diameter D in the back-pressure and differential-pressure flowmeter). Therefore, significant size-reduction and weight-reduction can be achieved as compared to the conventional flow rate measurement devices. In the example of the present embodiment, the overall length of the tube body 4 can be reduced, thereby making the measuring instrument 2 compact.
Furthermore, regarding the columnar member 5, as shown in
As shown in
Further, regarding the columnar member 5 having the streamline shape according to the present embodiment, the shape thereof, which is projected onto a third plane S3 perpendicular to the first plane 51 and the second plane S2, has an outer peripheral surface 50a that is defined by the aforementioned expressions 2 and 3. It is assumed that the origin point O is the center of the columnar member 5. The “streamline shape” means that the shape of the columnar member 5 projected onto the third plane S3 is a curve that is smoothly continuous along the flow direction X. In the present embodiment, as shown in
Here, in the flow rate calculation formula according to the above expression 1, the term c1×η×Q is referred to as a viscosity term, and the term c2×ρ×Q2 is referred to as an inertial term. If the temperature and the pressure of the fluid are substantially constant, the kinematic viscosity coefficient 11 and the density p have constant values in the fluid, so that the relationship in magnitude between the viscous force and the inertial force is the relationship in magnitude between c1 and c2. The parameter set B generated by the parameter generator 34 is a combination of coefficients c1, c2, and c3 in the respective terms of the flow rate calculation formula.
In measuring the differential pressure ΔP, the fluid collides with the columnar member 5 and changes its flow. At this time, depending on the shape of the columnar member 5, the parameters in the viscosity term and the inertial term vary. Specifically, when the width d of the columnar member 5 is increased like a circular shape, the inertial force is increased. On the other hand, when the length L of the columnar member 5 in the down-flow direction is increased like an oval shape, the viscous force is increased. However, if the length L is too long, the columnar member 5 enters a turbulent boundary layer and is affected by transient characteristics of vortex.
Since the viscosity term, which is the first-order term with the flow rate Q being a variable in the flow rate calculation formula according to expression 1, is proportional to the length L of the columnar member 5 in the flow direction X, a gradient N of the flow rate calculation formula satisfies N=2×c2×ρ×Q+c1′×η×L. Therefore, in particular, when the flow rate Q is small (when the differential pressure ΔP is small), the gradient N can be approximated to N=c1′×η×L, and thus the length L becomes dominant in the gradient N of the flow rate calculation formula. Therefore, the gradient N increases with an increase in the length L, and the calculation accuracy of the flow rate Q is improved when the differential pressure ΔP is small.
Meanwhile, the inertial term, which is the second-order term in the flow rate calculation formula, is proportional to the cross-sectional area of the columnar member 5 in the flow direction X. Therefore, when the width d of the columnar member 5 is reduced, the length L becomes more dominant in the gradient N of the flow rate calculation formula. In addition, when the width d of the columnar member is reduced, the area occupied by the columnar member 5 in the cross section perpendicular to the flow direction X is small, whereby pressure loss can be reduced.
Meanwhile, in the vicinity of the outer peripheral surface 50a of the columnar member 5, a boundary layer of the fluid is formed along the flow direction X. This boundary layer is a laminar boundary layer caused by laminar flow when the flow rate Q of the fluid is small, and is a turbulent boundary layer caused by turbulent flow when the flow rate Q is large. Furthermore, in the laminar boundary layer, as shown in
Next, the procedure of measuring a flow rate Q of a fluid by using the flow rate measurement device 1 according to the present embodiment will be described.
First, a fluid is caused to flow in the tube body 4 at already-known different flow rates Q, and differential pressures ΔP corresponding to the flow rates Q are measured multiple times by the measuring instrument 3 to obtain a whole data group A of the differential pressures ΔP corresponding to the flow rates Q.
Next, for example, a provisional reference flow rate Q″ and a provisional reference differential pressure ΔP″ corresponding thereto are determined. Then, data, of the whole data group A, which are positioned in a range smaller than a reference flow rate Q′ and a reference differential pressure ΔP′ are regarded as a small data group A1, and the other data are regarded as a large data group A2. Then, as shown in
After generation of the parameter sets B1 and B2, measurement of actual breathing air is performed as follows. A subject sends expired air from a first end portion 4a of the tube body 4 to a second end portion 4b of the tube body 4. At this time, the pressure sensor 21 measures a differential pressure ΔP of the expired air at the first and second measurement holes 51a and 51b. The measured differential pressure ΔP is transmitted and received by the first communication unit 23 and the second communication unit 31, and is input to the data processor 32. Then, the differential pressure determination unit 35 compares the magnitudes of the reference differential pressure ΔP′ generated by the parameter generator 34 and the measured differential pressure ΔP. The differential pressure determination unit 35 selects the parameter set B1 when the differential pressure ΔP is smaller than the reference differential pressure ΔP′, and selects the parameter set B2 when the differential pressure ΔP is larger than the reference differential pressure ΔP′.
Then, the flow rate calculator 35 substitutes the selected parameter set B and the measured differential pressure ΔP into the flow rate calculation formula to calculate a flow rate Q. The calculated flow rate Q is stored in the storage unit 37 and displayed on the display 33. Since the columnar member 5 of the flow rate measurement device 1 has bidirectionality, inspired air can also be measured as well as the expired air. That is, both the expired air and the inspired air can be measured by the single flow rate measurement device 1 without changing the settings and the like. Note that the flow rate Q may be displayed in any mode on the display 33. For example, the flow rate Q may be displayed as a time course graph having a horizontal axis representing time and a vertical axis representing the flow rate Q, or as an integrated flow rate from start of measurement.
For the purpose of confirming the effectiveness of the present invention, the inventors have compared a case where a differential pressure ΔP was calculated with only one parameter set B by using the whole data group A and a case where a differential pressure ΔP was calculated by using two parameter sets B1 and B2. The result is shown in
As shown in
On the other hand, in the case where the parameter set to be applied to the flow rate calculation formula is selectable from the parameter sets B1 and B2 with the reference differential pressure ΔP′ as the boundary, as shown in
Finally, possibilities of other embodiments of the present invention will be described. The same members as those of the above embodiment are denoted by the same reference numerals.
In the above embodiment, the leading end 53 of the columnar member 5 abuts on the tube inner surface 41a (contacts the tube inner surface 41a or is adhered to the tube inner surface 41a). However, the columnar member 5 is not limited thereto, and the leading end 53 need not abut on the tube inner surface 41a. A length L2 of the columnar member 5 in the extending direction Y in the intra-tube space 40 may be smaller than the diameter D of the tube body 4. For example, as shown in
In the above embodiment, the first measurement hole 51′a as the first pressure receiver is provided on the upstream side US of the columnar member 5 while the second measurement hole 51′b as the second pressure receiver is provided on the downstream side DS of the columnar member 5, and the differential pressure ΔP between the first and second measurement holes 51′a and 51′b is measured. However, each of flowmeters shown in
The flowmeter shown in
Meanwhile, in the flowmeter shown in
Furthermore, in the aforementioned embodiment, the measuring instrument 2 and the processor 3 are provided with the respective communication units so as to be configured as separated units. However, the measuring instrument 2 and the processor 3 may be integrated with each other. For example, a data processor 32, a display 33, and the like can be provided in the measuring instrument 2 itself.
In the aforementioned embodiment, breathing air is described as an example of a fluid. However, the fluid is not limited to breathing air, and may be, for example, medical gas such as oxygen, for a patient, which is equipped on the wall of a hospital room. Of course, the aforementioned embodiment is applicable not only to medical gases but also to other gases and liquids. Furthermore, in a case where the flow rate of a fluid that flows in an already-existing tube is measured, a columnar member included in a measurement device may be inserted into a hole opened through the tube, or the already-existing tube may be cut out and a tube equipped with a columnar member may be connected.
The present invention can be used as a flow rate measurement device and a flow rate measurement method for measuring not only the flow rate (vital capacity) associated with breathing but also the flow rates of various gases and liquids.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/012821 | 3/28/2018 | WO | 00 |
