CN114460980A - Flow detection device and gas mass flow controller - Google Patents
Flow detection device and gas mass flow controller Download PDFInfo
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- G05D7/00—Control of flow
- G05D7/06—Control of flow characterised by the use of electric means
- G05D7/0617—Control of flow characterised by the use of electric means specially adapted for fluid materials
- G05D7/0629—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means
- G05D7/0635—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means by action on throttling means
Abstract
The invention provides a flow detection device and a gas mass flow controller, wherein the device comprises a main channel, a bypass channel and an MEMS flow sensor, wherein the main channel is used for transmitting a gas to be detected, and a flow divider is arranged in the main channel; the two ends of the bypass channel are respectively communicated with the main channel at the two sides of the air inlet end and the air outlet end of the flow divider, and the bypass channel comprises a tapered section, a straight pipe section and a tapered section which are sequentially arranged along the gas transmission direction, wherein the hydraulic diameter of the straight pipe section meets the requirement that the gas flowing state through the straight pipe section is laminar flow; the MEMS flow sensor is arranged on the channel wall of the straight pipe section, and the detection surface of the MEMS flow sensor is exposed in the straight pipe section and used for detecting the gas flow in the straight pipe section.
Description
Technical Field
The invention relates to the technical field of flow detection, in particular to a flow detection device and a gas mass flow controller.
Background
Mass Flow Controllers (MFCs) are roughly classified into thermal type and pressure type according to the sensor signal detection principle. Thermal MFCs achieve flow control by detecting a heat variation signal caused when a fluid flows. The conventional thermal mass flow controller has slow response and relatively long response time, and cannot be applied to a process with high requirement on flow response time, for example, in some etching processes of semiconductors, a plurality of different gases need to be alternately used, the switching speed between the various gases is required to be very high, and the flow response is required to be fast.
With the development of MEMS (micro electro Mechanical Systems) chips, a thermal MEMS flow sensor is used to detect gas flow, and the thermal MEMS flow sensor can directly contact with a gas to be detected, has short response time, and can meet the requirement of high demand on flow response time.
The existing MEMS flow sensor is generally installed in a bypass channel communicating with a main channel, a part of gas in the main channel is introduced into the bypass channel through an orifice, and the MEMS flow sensor detects the flow rate of the gas introduced into the bypass channel. However, the following problems inevitably exist in the practical application of the flow rate detection structure:
firstly, because of the existence of the throttle hole, the main channel has a peak contraction port, and the channel wall of the main channel is not smooth, which easily causes the airflow to generate vortex after flowing corners at the front end and the rear end of the throttle hole, thereby possibly causing the inflow pressure or the outlet pressure fluctuation of the bypass channel, and further causing the flow entering the bypass channel to generate fluctuation, especially under the condition of high inflow pressure, the pressure fluctuation is very large, and thus the stable and accurate measurement of the flow is difficult to realize.
Secondly, because the structural design of the bypass channel is not reasonable, the flowing state of the gas entering the bypass channel is turbulent flow, and the flow rate are low, which not only causes the output signal intensity of the MEMS flow sensor to be low, but also causes the output signal intensity to be unstable, and the interference capability of the signal to resist noise is weak.
Disclosure of Invention
The invention aims to solve at least one of the technical problems in the prior art, and provides a flow detection device and a gas mass flow controller, which can not only stably and accurately detect the flow, but also improve the signal strength and stability output by an MEMS flow sensor and improve the interference capability of signal noise resistance.
The flow detection device comprises a main channel, a bypass channel and a MEMS flow sensor, wherein the main channel is used for transmitting a gas to be detected, and a flow divider is arranged in the main channel;
the two ends of the bypass channel are respectively communicated with the main channel at the two sides of the air inlet end and the air outlet end of the flow divider, and the bypass channel comprises a tapered section, a straight pipe section and a tapered section which are sequentially arranged along the gas transmission direction, wherein the hydraulic diameter of the straight pipe section meets the condition that the gas flowing through the straight pipe section is laminar;
the MEMS flow sensor is arranged on the channel wall of the straight pipe section, and the detection surface of the MEMS flow sensor is exposed in the straight pipe section and used for detecting the gas flow in the straight pipe section.
Optionally, the MEMS flow sensor includes a substrate and a MEMS chip fixed on the substrate, wherein the substrate is disposed on an outer surface of a channel wall of the straight tube section; the MEMS chip is provided with the detection surface, and one end of the MEMS chip, which is provided with the detection surface, penetrates through the channel wall of the straight tube section and protrudes inwards relative to the inner surface of the channel wall of the straight tube section.
Optionally, the thickness of a portion of the detection surface protruding inward with respect to the inner surface of the channel wall of the straight tube section in a direction perpendicular to the axial direction of the straight tube section is 0.1mm or more and 0.2mm or less.
Optionally, the flow rate detection device further includes a pressing plate, and the pressing plate presses the substrate from outside of the channel wall of the straight tube section, and at least covers the entire substrate, so as to improve the compressive strength of the substrate.
Optionally, the MEMS chip includes an upstream cold stack, a heating resistor, and a downstream cold stack sequentially arranged along the gas transmission direction, wherein the upstream cold stack and the downstream cold stack are both configured to perform heat convection with the measured gas flowing through the upstream cold stack and the downstream cold stack successively; the heating resistor is used for generating heat through the loaded voltage so as to heat the measured gas.
Optionally, the hydraulic diameter of the straight pipe section satisfies: the Reynolds number is less than 2000, and the Knoop number is greater than a preset value, so that the signal intensity output by the MEMS flow sensor can be improved.
Optionally, the length of the upstream portion of the bypass channel located upstream of the MEMS flow sensor satisfies the following relationship:
L≥0.05·Re·dh
wherein L is the upstream portion length; re is Reynolds number; dhIs the hydraulic diameter of the straight pipe section.
Optionally, the bypass channel further comprises an upstream straight pipe section, the upstream straight pipe section is located upstream of the tapered section, and the sum of the lengths of the upstream straight pipe section and the tapered section is the length of the upstream part; and the upstream portion length is 2 times the length of the tapered section.
Optionally, the length of the divergent section is greater than or equal to 0.5 times the length of the convergent section.
Optionally, the maximum diameter of the tapered section and the maximum diameter of the diverging section are both equal to 2 times the hydraulic diameter of the straight pipe section.
Optionally, the shunt includes the shunt tubes cluster of constituteing by a plurality of shunt tubes, and is a plurality of the shunt tubes all follow the axial setting of main entrance, and length and diameter are all the same.
As another technical solution, the present invention further provides a gas mass flow controller, which includes a control unit, an analog-to-digital conversion unit, and a flow control valve, and the flow detection device provided by the present invention, wherein the MEMS flow sensor is configured to detect a gas flow in the straight pipe section, and send an analog signal of the gas flow to the analog-to-digital conversion unit;
the analog-to-digital conversion unit is used for converting the analog signal into a digital signal and sending the digital signal to the control unit;
the control unit is used for comparing the digital signal with the input set flow and sending a control signal to the flow control valve according to the comparison result;
the flow control valve is arranged on the main channel, is positioned at the downstream of the flow divider and is used for adjusting the gas flow output by the main channel according to the control signal.
The invention has the following beneficial effects:
compared with the flow dividing mode of a throttling hole in the prior art, the flow detecting device provided by the invention can avoid the inflow pressure or outlet pressure fluctuation of the bypass channel caused by the existence of a peak contraction opening and a structure with unsmooth channel wall in the main channel, thereby improving the stability of the gas flow flowing into the bypass channel; meanwhile, the bypass channel adopted by the invention comprises a tapered section, a straight pipe section and a tapered section which are sequentially arranged along the gas transmission direction, the hydraulic diameter of the straight pipe section meets the requirement that the gas flowing state passing through the straight pipe section is laminar, and the tapered section is used for avoiding unstable flow caused by vortex generation, flow diversion and the like of airflow; the gradually expanding section is used for improving the smoothness of the air flow flowing out of the straight pipe section, and therefore the gradually expanding section, the straight pipe section and the gradually expanding section can jointly play a role in improving the stability of the air flow flowing through the MEMS flow sensor, and therefore stable and accurate detection of the air flow can be achieved. In addition, the tapered section and the gradually expanding section can play a role in accelerating gas flow, and the gas flowing through the MEMS flow sensor can obtain higher flow and flow velocity by combining the design of the hydraulic diameter of the straight pipe section, so that the strength and stability of the signal output by the MEMS flow sensor can be improved, and the interference capability of signal noise resistance is improved.
The gas mass flow controller provided by the invention can stably and accurately detect the flow, can improve the strength and stability of the signal output by the MEMS flow sensor and improve the interference capability of the signal against noise by adopting the flow detection device provided by the invention.
Drawings
Fig. 1 is a structural diagram of a flow rate detection device according to an embodiment of the present invention;
FIG. 2 is an enlarged view of the structure of the region I in FIG. 1;
FIG. 3 is a block diagram of a MEMS chip;
FIG. 4 is an enlarged view of the area I in FIG. 1;
fig. 5 is a block diagram of a gas mass flow controller according to an embodiment of the present invention.
Detailed Description
In order to make those skilled in the art better understand the technical solution of the present invention, the flow rate detection device and the gas mass flow controller provided by the present invention are described in detail below with reference to the accompanying drawings.
Referring to fig. 1 and fig. 2 together, a flow detection device according to an embodiment of the present invention includes a main channel 1, a bypass channel 2, and an MEMS flow sensor 4, wherein the main channel 1 is used for transmitting a gas to be detected, and a flow divider 3 is disposed in the main channel 1; both ends of the bypass channel 2 are respectively communicated with the main channel 1 at both sides of the air inlet end and the air outlet end of the flow divider 3. The flow divider 3 is used to divide a part of the flow of the gas to be measured in the main channel 1 into the bypass channel 2. A part of the gas to be measured flowing in from the gas inlet of the main channel 1 flows into the bypass channel 2 under the splitting action of the splitter 3, and the rest of the gas to be measured continues to flow along the main channel 1 through the splitter 3 and then flows out from the gas outlet of the main channel 1.
Through set up shunt 3 in main entrance 1, need not to set up the orifice in main entrance 1 to make the channel wall of main entrance 1 can not produce peak and the structure of not smooth, thereby can avoid bypass entrance 2's incoming flow pressure or outlet pressure to produce undulant, thereby can improve the stability of the gas flow who flows in bypass entrance 2, and then help carrying out stability, accurate detection to the flow.
Further, as shown in fig. 2, the bypass passage 2 includes a tapered section 21, a straight pipe section 22, and a divergent section 23, which are arranged in this order in the gas transport direction (the direction of the arrow in fig. 2), wherein the hydraulic diameter d of the straight pipe section 22hIt suffices to make the gas flow state through the straight tube section 22 laminar. Laminar flow is a state of flow of a fluid in which the fluid flows smoothly and straightly in directions parallel to the axial direction of a pipe. Hydraulic diameter (hydraulic diameter) dhIs four times the ratio of the flow cross-sectional area to the circumference of the bypass channel 2, and for a circular channel, the diameter is the hydraulic diameter dh。
The straight pipe section 22 is constructed such that the flow of gas through the straight pipe section 22 is laminarThe tapered section 21 is used to avoid flow instability caused by vortex generation, flow splitting and the like of the airflow; the gradually expanding section 23 is used for improving the smoothness of the airflow flowing out of the straight pipe section 22, so that the gradually expanding section 21, the straight pipe section 22 and the gradually expanding section 23 can jointly play a role in improving the stability of the flow of the gas flowing through the MEMS flow sensor 4, and therefore stable and accurate detection of the flow can be achieved. In addition, the tapered section 21 and the diverging section 23 can also play a role of accelerating gas flow, and the hydraulic diameter d of the straight pipe section 22 is combinedhThe design of (3) can make the gas flowing through the MEMS flow sensor 4 obtain higher flow and flow velocity, thereby improving the signal strength and stability output by the MEMS flow sensor 4 and improving the interference capability of signal noise resistance.
In addition, the bypass passage 2 further includes an upstream vertical section 24 and a downstream vertical section 25, wherein one end of the upstream vertical section 24 communicates with the main passage 1 on the side of the air intake end of the flow divider 3, and the other end communicates with an upstream straight section 26 located upstream of the tapered section 21; one end of the downstream vertical section 25 communicates with the main channel 1 on the side of the outlet end of the flow divider 3, and the other end communicates with a downstream straight section 27 located downstream of the above-mentioned divergent section 23.
In the bypass passage 2, any one of the tapered section 21, the straight tube section 22, the gradually expanding section 23, the upstream vertical section 24, the downstream vertical section 25, the upstream straight tube section 26, and the downstream straight tube section 27 may have a circular cross-sectional shape or any other shape than a circular cross-sectional shape, and the embodiment of the present invention is not particularly limited thereto.
The MEMS flow sensor 4 is disposed on a channel wall of the straight tube section 22, and a detection surface a of the MEMS flow sensor 4 is exposed in the straight tube section 22 for directly contacting with a gas in the straight tube section 22 to detect a gas flow. The MEMS flow sensor has short response time and can meet the requirement of high flow response time.
In some alternative embodiments, as shown in fig. 2, the MEMS flow sensor 4 comprises a substrate 41 and a MEMS (micro electro mechanical systems) chip 42 fixed on the substrate 41, wherein the substrate 41 is disposed on the outer surface of the channel wall of the straight tube section 22; the MEMS chip 42 has, for example, the detection surface a described above, and one end of the MEMS chip 42 having the detection surface a penetrates the channel wall of the straight tube section 22 and protrudes inward with respect to the inner surface of the channel wall of the straight tube section 22. Specifically, an opening penetrating through the thickness of the straight tube section 22 is provided on the channel wall, the MEMS chip 42 is inserted into the opening, and a part of the MEMS chip 42 protrudes inward relative to the inner surface of the channel wall of the straight tube section 22, so that the detection surface a can be exposed in the straight tube section 22, and the gas to be detected in the straight tube section 22 can contact with the detection surface a and perform heat convection when flowing through the MEMS chip 42, thereby detecting the gas flow rate. Preferably, in order to enhance the effect of convective heat transfer, as shown in fig. 2, the thickness H of the detection face a in a direction perpendicular to the axial direction of the straight tube section 22 is 0.1mm or more and 0.2mm or less with respect to a portion of the inner surface of the channel wall of the straight tube section 22 that protrudes inward.
In some optional embodiments, as shown in fig. 2, the flow rate detection device further comprises a pressing plate 5, wherein the pressing plate 5 presses the substrate 41 from the outside of the channel wall of the straight tube section 22, and at least covers the whole substrate 41, so as to improve the compressive strength of the substrate 41. Specifically, the peripheral edges of the base plate 41 are superposed on the outer surfaces of the channel walls of the straight tube section 22 and are fixed and connected with the channel walls of the straight tube section 22 in a sealing manner, for example, by bonding with a sealant. The pressing plate 5 covers the entire substrate 41 and presses the substrate 41 to improve the compressive strength of the substrate 41 and prevent the substrate 41 from being damaged by pressure. Further, the MEMS chip 42 is, for example, a minute unit having a size of less than 2mm × 0.5mm × 1mm, which is fixed on the substrate 41, and is located on the side of the substrate 41 opposite to the channel wall of the straight tube section 22.
It should be noted that the fixing manner of the MEMS flow sensor 4 disposed on the channel wall of the straight tube section 22 is not limited to this, and in practical application, any other fixing manner may be adopted as long as the detection surface a can be exposed in the straight tube section 22 and can perform convection heat exchange with the gas flowing through, and the embodiment of the present invention is not limited to this.
In some optional embodiments, the MEMS flow sensor 4 is, for example, a thermal differential MEMS flow sensor, specifically, as shown in fig. 3, the MEMS chip 42 includes an upstream cold stack 421, a heating resistor 422, and a downstream cold stack 423 that are sequentially arranged along a gas transmission direction (arrow direction in fig. 2), where the upstream cold stack 421 and the downstream cold stack 423 are both temperature sensing elements, and are symmetrically distributed on both upstream and downstream sides of the heating resistor 422, and are both used for performing heat convection with a measured gas that flows through the upstream cold stack 421 and the downstream cold stack 423 successively; the heating resistor 422 (resistance value R) is a heating element for heating the gas to be measured by applying a voltage (power supply voltage V) thereto to generate heat, and the surface temperature (e.g., 200 ℃) of the heating resistor 422 is higher than the initial temperature (less than 60 ℃) of the gas when the voltage is applied thereto. When no gas flows, the surface temperature of the contact surface A is normally distributed by taking the heating resistor 422 as a center, and the temperature of the upstream cold stack 421 is the same as that of the downstream cold stack 423 at the moment, and the upstream cold stack 421 and the downstream cold stack 423 have the same electric signal; when gas flows (the flow velocity is v), gas molecules sequentially flow through the upstream cold stack 421 and the downstream cold stack 423 to perform heat convection between the upstream cold stack 421 and the downstream cold stack 423, so that a heat transfer phenomenon occurs, a temperature difference is generated between the upstream cold stack 421 and the downstream cold stack 423, the surface temperature distribution of the contact surface a is deviated, at the moment, the electric signals of the upstream cold stack 421 and the downstream cold stack 423 are different accordingly, namely, the voltage U between the upstream cold stack 421 and the downstream cold stack 423 is not zero, and the gas flow can be calculated by detecting the voltage U.
The types of the gases are different, the convection heat exchange parameters of the gases are likely to be different, and the flow and the type of the gases passing through the MEMS chip 42 can be determined according to the voltage U, the initial temperature of the measured gases, and the like. For example: argon (Ar) has a specific heat capacity Cp of 0.519J/g/K, a thermal conductivity lambda of 0.0174W/m/K, and nitrogen (N)2) The specific heat capacity Cp is 1.04J/g/K, the heat conduction coefficient lambda is 0.0241W/m/K, and K is the Kelvin temperature. Obviously, the same mass of argon and nitrogen, the temperature is increased or decreased by 1 degree, and the absorbed or released heat is different, and the gas species can be judged by the difference.
In some alternative embodiments, the stability of the gas flow through the straight tube section 22 is better, since the smaller the reynolds number, while the convective heat transfer between the gas under test and the contact surface a is stronger, since the higher the knoop number,the higher the convective heat transfer power exchanged from the heating resistor 422 of the MEMS chip 42 to the gas to be measured, the higher the thermal power transferred to the downstream cold stack 423 through the gas to be measured, and the stronger the signal strength output by the MEMS flow sensor 4, and based on this, the hydraulic diameter d of the straight pipe section 22 can be designed according to the reynolds number and the knoop numberhThus, the flow state of the gas passing through the straight pipe section 22 can be made laminar, and the signal strength output by the MEMS flow sensor 4 can be improved.
The Reynolds number is a dimensionless number that can be used to characterize the flow of a fluid, and can be used to distinguish whether the flow of the fluid is laminar or turbulent. For fluid flowing in the pipeline, the flow with the Reynolds number smaller than 2300 is laminar flow, the Reynolds number equal to 2300-4000 is in a transition state, and the flow with the Reynolds number larger than 4000 is turbulent flow.
The Nusselt number is the ratio of convective heat to conductive heat transfer across a boundary (surface) of a fluid during heat transfer across the boundary.
Specifically, in order to improve the signal strength output from the MEMS flow sensor 4 while making the gas flowing state through the straight tube section 22 laminar, the hydraulic diameter d of the straight tube section 22hSatisfies the following conditions: the Reynolds number is less than 2000, and the Nurseel number is greater than a preset value, and the preset value is set to satisfy: the signal strength output by the MEMS flow sensor 4 can be improved so that the signal anti-noise interference capability of the MEMS flow sensor 4 meets the requirements. Preferably, the reynolds number is less than 200.
According to Reynolds number Re and hydraulic diameter dhThe following relationships:
wherein, is
The mass flow of the gas to be detected is g/min; mu is the kinetic viscosity of the measured gas.
It is possible to deduce the hydraulic alignment of the straight pipe section 22Diameter dhThe value range of (A) satisfies the following relational expression:
at the design of the hydraulic diameter d of the straight pipe section 22hIn a particular process, a suitable reynolds number Re, for example 200, may be selected first to provide laminar gas flow conditions through the straight tube section 22, and then the hydraulic diameter d may be used according to the reynolds number selectedhThe hydraulic diameter d is obtained by calculating the above relationhThe value range of (a). Then, a proper hydraulic diameter d can be selected from the value range through an experiment or simulation mode and the likehPassing through the hydraulic diameter dhThe calculated nussel number is equal to a preset value which can achieve the purpose of increasing the signal strength output by the MEMS flow sensor 4, thereby achieving the hydraulic diameter d of the straight pipe section 22hSetting of (4).
The nussel number can be obtained by calculation using the following relation:
where L is the upstream portion length of the bypass channel 2 upstream of the MEMS flow sensor 4 (i.e., contact surface a), which is the sum of the lengths of the tapered section 21 and the upstream straight tube section 26 described above. Pr is the prandtl number, which represents the number of dimensionless combinations of energy and momentum transfer processes in the fluid that affect each other, indicating the relationship of the temperature boundary layer and the flow boundary layer.
Since the power applied to the heating resistor 422 of the MEMS chip 42 is a constant power and is supplied by a constant voltage source of an external circuit, and the change of the heating resistor 422 with temperature is very small, the resistance R of the heating resistor 422 is approximately considered as a constant, and the position of the contact surface a corresponding to the heating resistor 422 is approximately a constant temperature surface. Based on this, the prandtl number Pr is:
wherein Cp is the voltage specific heat capacity of the fluid; mu is the dynamic viscosity of the gas to be detected; and lambda is the heat conduction coefficient of the measured gas.
In some alternative embodiments, as shown in fig. 4, the upstream portion length L satisfies the following relationship:
L≥0.05·Re·dh
by selecting a proper value from the value range of the upstream part length L, the detected gas can have a sufficiently long path before reaching the straight pipe section 22 to fully develop, so that a fully developed laminar flow state is obtained when reaching the straight pipe section 22, and the accuracy of flow detection is further improved. Preferably, the upstream portion length L is 2 times the length of the tapered section 21.
Specifically, an upstream straight tube section 26 (diameter d) for extending the flow path of the gas to be measured before reaching the straight tube section 22 is provided upstream of the tapered section 21, that is, the upstream portion length L is equal to the length L of the tapered section 211And the length of the upstream straight tube section 26. However, the embodiment of the present invention is not limited to this, and in practical applications, the straight upstream pipe section 26 may not be provided, but only the tapered section 21 may be provided, in which case, the length of the tapered section 21 is the upstream partial length L.
In some alternative embodiments, as shown in FIG. 4, the length L of the diverging section 232Length L of tapered section 21 or more10.5 times of the total weight of the powder. This helps to promote a smoother flow of the air stream at the outlet of the straight tube section 22.
In some alternative embodiments, as shown in fig. 4, the maximum diameter of the tapered section 21 and the maximum diameter of the diverging section 23 are both equal to the hydraulic diameter d of the straight section 22h2 times of the total weight of the powder. The maximum diameter of the tapered section 21 and the maximum diameter of the diverging section 23 are equal to the diameter d of the upstream straight section 26, for example. This helps to make the inner surface of the wall of the entire bypass channel 2 as smooth as possible at the connection of the different tube sections, while at the same time allowing the gas flow to be kept atThe flow at the outlet of the straight pipe section 22 is smoother.
It should be noted that smooth transition treatments can be performed between the tapered section 21 and the straight pipe section 22, and between the tapered section 23 and the straight pipe section 22, so as to improve the smoothness of the inner surface of the channel wall, and further improve the stability of the gas flow.
In the above design of the bypass passage 2, the tapered section 21, the straight pipe section 22 and the diverging section 23 can collectively function to improve the stability of the gas flow passing through the MEMS flow sensor 4, so that stable and accurate detection of the flow can be realized. In addition, the tapered section 21 and the diverging section 23 can also play a role of improving the flow of the accelerated gas, and the hydraulic diameter d of the straight pipe section 22 is combinedhThe design of (3) can make the gas flowing through the MEMS flow sensor 4 obtain higher flow and flow velocity, thereby improving the signal strength and stability output by the MEMS flow sensor 4 and improving the interference capability of signal noise resistance.
In order to ensure that the gas flow rate flowing into the bypass channel 2 is always constant, so as to further improve the stability of the incoming flow pressure of the bypass channel 2 and the stability of the gas flow rate, in some alternative embodiments, the flow divider 3 includes a flow dividing pipe bundle composed of a plurality of flow dividing pipes, and the plurality of flow dividing pipes are all arranged along the axial direction of the main channel 2 and have the same length and diameter.
The number of the flow dividing pipes in the flow divider 3 is different, and the gas flow rate range (i.e., the range of the flow rate detecting means) passing through the flow divider 3 is also different, regardless of the number of shunt tubes, however, since the length and diameter of the individual shunt tubes are fixed, the pressure drop across it is also fixed, and the two ends of the bypass channel 2 communicate with the main channel 1 at the two sides of the inlet end and the outlet end of the flow divider 3, respectively, which makes the pressure drop across the bypass channel 2 also fixed, so that the gas flow into the bypass channel 2 is always constant, i.e. the pressure drop across the bypass channel 2 is determined by the pressure drop across the individual shunt tubes, therefore, the flow divider 3 can always keep the gas flow rate flowing into the bypass passage 2 constant regardless of the variation of the gas flow rate range of the flow divider 3 by adopting the above-described structure.
Specifically, the flow rate of the gas flowing into the bypass passage 2 satisfies the following relational expression:
QS＝C·dpb·f(ds,Ls,mu,T)
where ds is the characteristic diameter of the bypass channel 2, including the diameters of the upstream 26, tapered 21, 22, tapered 23 and downstream 27 straight tube sections described above; ls is the characteristic length of the bypass channel 2, i.e. the total length between the two ends of the bypass channel 2 in fig. 1; mu is the viscosity of the gas; t is the gas temperature; dpb is the pressure drop of splitter 3, and dpb is P1-P2; c is a constant.
The flow divider 3 is installed in the main passage 1 with its front end located at a distance (e.g., 3mm) downstream of the inlet centerline of the bypass passage 2 and its rear end located at a distance (e.g., 3mm) upstream of the outlet centerline of the bypass passage 2. The pressure drop of the above-mentioned flow divider 3 satisfies the following relation:
wherein Q isbFor the flow of each shunt, LbIs the length of the shunt tube, dbThe diameter of the shunt.
Due to the length L of the shunt tubes in the shunt 3bAnd diameter dbIs fixed, which makes the pressure drop across the shunt tube fixed, so that the pressure drop (P1-P2) across the bypass channel 2 can be always kept the same, and thus the gas flow into the bypass channel 2 can be realized without changing with the span change of the flow control device. In addition, different flow rates can be obtained by changing the number of the shunt pipes, but the pressure drop at the two ends of the shunt pipes is always unchanged.
For example, if one shunt tube can pass gas with a flow rate of 20sccm and the number of the shunt tubes is 5, the range of the flow rate measuring device is 100sccm (standard milliliter per minute), and if the number of the shunt tubes is increased to 10, the flow rate measuring device will increase the flow rate of the gas passing through the shunt tubes to 10Is 200sccm, but because of the length L of each shunt tubebAnd diameter dbIs fixed, and the pressure drop at the two ends of the shunt pipe is always unchanged.
It should be noted that the shunt 3 in the embodiment of the present invention is not limited to use of a circular shunt tube, and in practical applications, any other shunt tube with any shape, such as a circular ring or a cone, may also be used. Alternatively, an integral structure with a plurality of channels such as circular holes, circular ring holes or conical holes may be adopted.
It should be noted that the embodiment of the present invention is not limited to the structure using the above-mentioned flow divider, and in practical applications, any other flow divider may be used as long as it can ensure that the pressure drop across the flow rate detection device is always constant when the measurement range of the flow rate detection device is changed.
In addition, the flow rate detection device provided by the embodiment of the invention can be used as a flow meter for detecting the flow rate of the gas to be detected.
As another technical solution, as shown in fig. 5, an embodiment of the present invention further provides a gas mass flow controller, which includes a control unit 6, an analog-to-digital conversion unit 5, and a flow control valve 7, and the above flow detection device provided in the embodiment of the present invention, wherein, as shown in fig. 1, the MEMS flow sensor 4 is configured to detect a gas flow in the straight pipe section 22, and send an analog signal of the gas flow to the analog-to-digital conversion unit 5; the analog-to-digital conversion unit 5 is used for converting the analog signal into a digital signal and sending the digital signal to the control unit 6; the control unit 6 is used for comparing the digital signal with an input set flow and sending a control signal to the flow control valve 7 according to a comparison result; the flow control valve 7 is provided in the main channel 1 downstream of the flow divider 3, and adjusts the flow rate of the gas output from the main channel 1 according to the control signal so that the flow rate of the gas can be matched with the set flow rate. The flow control valve 7 may be, for example, a solenoid valve, a piezoelectric valve, or another valve capable of automatic control.
According to the gas mass flow controller provided by the embodiment of the invention, by adopting the flow detection device provided by the embodiment of the invention, not only can the flow be stably and accurately detected, but also the signal strength and stability output by the MEMS flow sensor can be improved, and the interference capability of signal noise resistance is improved.
It will be understood that the above embodiments are merely exemplary embodiments taken to illustrate the principles of the present invention, which is not limited thereto. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit and substance of the invention, and these modifications and improvements are also considered to be within the scope of the invention.
Claims (12)
1. The flow detection device is characterized by comprising a main channel, a bypass channel and an MEMS flow sensor, wherein the main channel is used for transmitting a gas to be detected, and a flow divider is arranged in the main channel;
the two ends of the bypass channel are respectively communicated with the main channel at the two sides of the air inlet end and the air outlet end of the flow divider, and the bypass channel comprises a tapered section, a straight pipe section and a tapered section which are sequentially arranged along the gas transmission direction, wherein the hydraulic diameter of the straight pipe section meets the condition that the gas flowing through the straight pipe section is laminar;
the MEMS flow sensor is arranged on the channel wall of the straight pipe section, and the detection surface of the MEMS flow sensor is exposed in the straight pipe section and used for detecting the gas flow in the straight pipe section.
2. The flow rate detection device according to claim 1, wherein the MEMS flow sensor includes a substrate and a MEMS chip fixed on the substrate, wherein the substrate is provided on an outer surface of a channel wall of the straight tube section; the MEMS chip is provided with the detection surface, and one end of the MEMS chip, which is provided with the detection surface, penetrates through the channel wall of the straight tube section and protrudes inwards relative to the inner surface of the channel wall of the straight tube section.
3. The flow rate detecting device according to claim 2, wherein a thickness of the detecting surface in a direction perpendicular to an axial direction of the straight tube section is 0.1mm or more and 0.2mm or less with respect to a portion of the straight tube section which protrudes inward from the inner surface of the channel wall.
4. The flow rate detecting device according to claim 2, further comprising a pressing plate that presses the base plate from outside the channel wall of the straight tube section and covers at least the entire base plate for improving the compressive strength of the base plate.
5. The flow detection device according to claim 2, wherein the MEMS chip comprises an upstream cold stack, a heating resistor, and a downstream cold stack sequentially arranged along the gas transmission direction, wherein both the upstream cold stack and the downstream cold stack are configured to perform heat convection with the detected gas flowing through them in sequence; the heating resistor is used for generating heat through the loaded voltage so as to heat the measured gas.
6. A flow sensing device according to any one of claims 1 to 5 wherein the hydraulic diameter of the straight tube section is such that: the Reynolds number is less than 2000, and the Knoop number is greater than a preset value, so that the signal intensity output by the MEMS flow sensor can be improved.
7. The flow sensing device of claim 6, wherein an upstream portion of the bypass channel upstream of the MEMS flow sensor has a length that satisfies the relationship:
L≥0.05·Re·dh
wherein L is the upstream portion length; re is Reynolds number; d is a radical ofhIs the hydraulic diameter of the straight pipe section.
8. The flow sensing device according to claim 7, wherein the bypass passage further comprises an upstream straight tube section located upstream of the tapered section, and a sum of lengths of the upstream straight tube section and the tapered section is the upstream portion length; and the upstream portion length is 2 times the length of the tapered section.
9. The flow rate detecting device according to claim 7 or 8, wherein the length of the divergent section is equal to or greater than 0.5 times the length of the convergent section.
10. A flow sensing device according to any one of claims 1 to 5, wherein the maximum diameter of the tapered section and the maximum diameter of the diverging section are each equal to 2 times the hydraulic diameter of the straight tube section.
11. A flow sensing device according to claim 1, wherein the diverter includes a bundle of multiple shunt tubes, each of the multiple shunt tubes being arranged axially of the main channel and having the same length and diameter.
12. A gas mass flow controller comprising a control unit, an analog-to-digital conversion unit and a flow control valve, and the flow detection device of any one of claims 1 to 11, wherein the MEMS flow sensor is configured to detect a gas flow in the straight pipe section and send an analog signal of the gas flow to the analog-to-digital conversion unit;
the analog-to-digital conversion unit is used for converting the analog signal into a digital signal and sending the digital signal to the control unit;
the control unit is used for comparing the digital signal with the input set flow and sending a control signal to the flow control valve according to the comparison result;
the flow control valve is arranged on the main channel, is positioned at the downstream of the flow divider and is used for adjusting the gas flow output by the main channel according to the control signal.
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