Thermal mass flowmeter, design method for thermal mass flowmeter, and design program for thermal mass flowmeter

Abstract

To solve the problem in which: in a MEMS-type (or flow sensor-type) thermal mass flowmeter, in order for the flow velocity distribution at a location where a measurement sensor is installed to become uniform, a sufficiently long "entrance length" for developing a laminar boundary layer is required; if a sufficient "entrance length" cannot be secured, the output of the measurement sensor fluctuates, and stable flow rate measurement cannot be performed.SOLUTION: A design method for a thermal mass flowmeter according to the technology of the present disclosure is a design method for a thermal mass flowmeter including a thermal flow sensor (100), where, when a flow passage cross-sectional area S, a reference flow velocity Vm, an entrance length L, and a kinematic viscosity v of a fluid to be measured are given, one side of a rectangular pipe constituting an entrance section through which the fluid to be measured flows is obtained as a, which satisfies a conditional expression described later (formula (6)).SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to a thermal mass flow meter, a design method for a thermal mass flow meter, and a design program for a thermal mass flow meter. [Background technology]

[0002] In the technical field of measuring the flow rate of compressible fluids such as gases, mass flow meters that measure mass taking into account temperature and pressure are known. Mass flow meters can be broadly classified into Coriolis types and thermal types.

[0003] A thermal mass flow meter (sometimes simply referred to as a "thermal flow meter") measures the flow rate of a gas by utilizing the thermal diffusion property of the gas.

[0004] Thermal mass flow meters can be classified by the type of measurement sensor into capillary, hot wire, thermocouple, and MEMS (or flow sensor) types. Among the above, the MEMS type (or flow sensor type) thermal mass flow meter is a relatively new type. The MEMS type (or flow sensor type) thermal mass flow meter uses a MEMS type flow sensor formed from a silicon wafer as the measurement sensor. The MEMS type flow sensor is disclosed, for example, in Patent Document 1 (especially

[0030] -

[0032] , Figures 2-3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2023-168657 Summary of the Invention [Problem to be solved by the invention]

[0006] In a MEMS (or flow sensor) thermal mass flow meter, when the measurement target is a gas, a mesh or other component is inserted before the sensor to straighten the gas flow. However, when the measurement target is a liquid, inserting a mesh or other component before the sensor can cause problems such as the generation of bubbles or clogging. In a MEMS (or flow sensor) thermal mass flow meter, especially when the measurement target is a liquid, a sufficient "entrance section" is required to develop a laminar boundary layer in order to maintain a constant flow velocity distribution at the location where the measurement sensor is installed. If the boundary layer does not develop sufficiently, the output of the measurement sensor will fluctuate, making it impossible to perform stable liquid flow measurement.

[0007] In view of the above-mentioned problems, the present disclosure aims to provide a thermal mass flow meter, a design method for a thermal mass flow meter, and a design program for a thermal mass flow meter that ensures an "entrance section" of sufficient distance for developing a laminar boundary layer. [Means for solving the problem]

[0008] The thermal mass flow meter according to the disclosed technology is a thermal mass flow meter consisting of a piping section and a thermal flow sensor, in which the piping forming the flow path of the piping section has a rectangular cross-sectional shape consisting of a horizontal side a and a vertical side b of different lengths, and has an inlet section that forms a flow path with a constant flow velocity distribution along the fluid flow direction. [Effects of the Invention]

[0009] Since the design method for a thermal mass flow meter according to the disclosed technology has the above-mentioned technical features, a thermal mass flow meter based on this design method can ensure an "entrance section" of sufficient distance for developing a laminar boundary layer, the flow velocity distribution at the location where the thermal flow sensor 100 is installed is constant, and the output of the thermal mass flow meter does not fluctuate. [Brief explanation of the drawings]

[0010] [Figure 1]Fig. 1A is an external view of a thermal type flow sensor 100 according to embodiment 1. Fig. 1B is a cross-sectional view of the thermal type flow sensor 100 taken along line AA' in Fig. 1A. [Figure 2] 2 (FIGS. 2A and 2B) show the measurement principle of the thermal flow sensor 100, and both are cross-sectional views taken along the line AA'. FIG. 2A is an explanatory diagram showing isothermal lines when there is no flow. FIG. 2B is an explanatory diagram showing isothermal lines when there is flow. [Figure 3] FIG. 3 is a structural schematic diagram showing the structure of a direct measurement type thermal mass flowmeter. [Figure 4] FIG. 4 is a structural schematic diagram showing the structure of a bypass measurement type thermal mass flowmeter. [Figure 5] FIG. 5 is a bird's-eye view illustrating the design parameters of a thermal mass flowmeter. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiment 1 Fig. 1A is an external view of a thermal type flow sensor 100 according to embodiment 1. Fig. 1B is a cross-sectional view of the thermal type flow sensor 100 taken along line AA' in Fig. 1A. 1A and 1B, the thermal flow sensor 100 includes a silicon chip 110 having a cavity, an insulating film layer disposed on the silicon chip 110 so as to cover the cavity, a heater Rh disposed on the insulating film layer, an upstream temperature sensor Ru disposed upstream of the heater Rh, and a downstream temperature sensor Rd disposed downstream of the heater Rh. The thermal flow sensor 100 also includes an ambient temperature sensor Rr disposed further upstream than the upstream temperature sensor Ru. The thermal flow sensor 100 shown in FIGS. 1A and 1B is sometimes referred to as a "three-element type" because it uses three sensors: the upstream temperature sensor Ru, the downstream temperature sensor Rd, and the ambient temperature sensor Rr.

[0012] The part of the insulating film layer that covers the cavity forms a heat-insulating diaphragm. The heater Rh is located in the center of the insulating film layer that covers the cavity and heats the fluid flowing through the piping. The upstream temperature sensor Ru is used to detect the temperature of the fluid upstream of the heater Rh, and the downstream temperature sensor Rd is used to detect the temperature of the fluid downstream of the heater Rh.

[0013] 2 (FIGS. 2A and 2B) show the measurement principle of the thermal flow sensor 100, and both are cross-sectional views taken along the line AA'. FIG. 2A is an explanatory diagram showing isothermal lines when there is no flow. FIG. 2B is an explanatory diagram showing isothermal lines when there is flow.

[0014] As shown in Figure 2A, when the fluid in the pipe is stationary, the heat applied by the heater Rh diffuses symmetrically in the upstream and downstream directions. Therefore, the temperature measured by the upstream temperature sensor Ru is equal to the temperature measured by the downstream temperature sensor Rd. For example, if the upstream temperature sensor Ru and the downstream temperature sensor Rd are implemented using thermistors with the same characteristics, the electrical resistance of the upstream temperature sensor Ru and the downstream temperature sensor Rd will be equal when the fluid in the pipe is stationary.

[0015] In contrast, as shown in Figure 2B, when the fluid in the pipe flows from upstream (left side of Figure 2B) to downstream (right side of Figure 2B), the heat generated by the heater Rh is carried downstream (to the right side of Figure 2B). In this case, the temperature measured by the downstream temperature sensor Rd will be higher than the temperature measured by the upstream temperature sensor Ru. Therefore, when implemented using a thermistor, a difference will occur between the electrical resistance of the upstream temperature sensor Ru and the electrical resistance of the downstream temperature sensor Rd. The difference between the electrical resistance of the upstream temperature sensor Ru and the electrical resistance of the downstream temperature sensor Rd is correlated with the speed and flow rate of the fluid in the pipe. Therefore, the speed and flow rate of the fluid flowing through the pipe can be calculated from the difference between the electrical resistance of the upstream temperature sensor Ru and the electrical resistance of the downstream temperature sensor Rd.

[0016] Thermal mass flow meters can be broadly classified into direct measurement types and bypass measurement types depending on the measurement method.

[0017] Fig. 3 is a structural schematic diagram showing the structure of a direct measurement type thermal mass flow meter. As shown in Fig. 3, the direct measurement type thermal mass flow meter is designed to install a thermal flow sensor 100 near the wall surface inside a pipe. In Figure 3, the part labeled "flow straightening wire mesh" is used to straighten the flow when the object to be measured is a gas. As mentioned above, if the object to be measured is a liquid, placing a flow straightening part such as a mesh in front of the sensor can cause bubbles to form or lead to clogging.

[0018] FIG. 4 is a structural schematic diagram showing the structure of a bypass measurement type thermal mass flow meter. As shown in FIG. 4, a bypass measurement type thermal mass flow meter is provided with a bypass, and a thermal flow sensor 100 is installed in the bypass section. A bypass measurement type thermal mass flow meter is a so-called inferential measurement method in which the flow rate flowing through the main flow pipe (hereinafter referred to as the "main flow pipe") is estimated from the flow rate flowing through the bypass. As such, a bypass measurement type thermal mass flow meter cannot perform accurate measurement if the bypass ratio changes. Therefore, it is characterized by being vulnerable to pressure changes and drift.

[0019] Design considerations such as whether to use a direct measurement method or a bypass measurement method, and even what dimensions of pipe to use, are very important in achieving stable and accurate measurements. For example, in order to maintain a constant flow velocity distribution at the location where the thermal flow sensor 100 is installed, an "inlet section" of sufficient distance is required to develop a laminar boundary layer. If an "inlet section" of sufficient distance cannot be secured, the output of the thermal flow sensor 100 will fluctuate, making it impossible to perform stable flow measurement.

[0020] Fig. 5 is a bird's-eye view illustrating the design parameters of a thermal mass flow meter. The design parameters illustrated in Fig. 5 are the length of the horizontal side in the cross section of the pipe (hereinafter referred to as "cross-sectional horizontal side a"), the length of the vertical side in the cross section of the pipe (hereinafter referred to as "cross-sectional vertical side b"), and the distance of the straight section (hereinafter referred to as "run-up section") up to the location where the thermal flow sensor 100 is installed (hereinafter referred to as "run-up distance L").

[0021] To determine the inlet distance L required for a stable flow velocity distribution, it is necessary to calculate the Reynolds number Re. The pipe shown in Figure 5 is a non-circular pipe. In the case of a pipe with a rectangular cross section (called a "rectangular pipe") as shown in Figure 5, the diameter of the equivalent circular pipe, i.e., the hydraulic diameter D, is used to calculate the Reynolds number Re. H (Hydraulic diameter) is required. For rectangular pipes, the hydraulic diameter D H Specifically, is given by the following formula: TIFF2025187472000002.tif13166However, formula (1) assumes that all four sides are closed and all are wetted verandas.

[0022] Reynolds number R e is the hydraulic diameter D H is given by the following formula: TIFF2025187472000003.tif12166Here, on the right side of equation (2), V m is the flow velocity and v is the dynamic viscosity.

[0023] Based on experience from experiments and implementation, the minimum required run-up distance L min is the Reynolds number R e , and hydraulic diameter D H It can be expressed by the following mathematical model, which is proportional to TIFF2025187472000004.tif9166

[0024] In some cases, the flow path cross-sectional area S is specified in advance at the site where the thermal mass flowmeter is used. In consideration of this, Equation (1) can be transformed as follows using the cross-sectional side a and the flow path cross-sectional area S: TIFF2025187472000005.tif15166

[0025] Substituting formulas (2) and (4) into formula (3), the minimum run-up distance L min is given by the following formula: TIFF2025187472000006.tif33166

[0026] As mentioned above, in the field where a thermal mass flowmeter is used, the flow path cross-sectional area S may be specified in advance, and the face-to-face dimension of the front and rear piping (i.e., the run-up distance L) may also be specified. In this case, the design parameter is the cross-sectional side a, and the design comes down to the question of what dimension the cross-sectional side a should be. The formula (5) can be transformed with respect to the cross-sectional side a to be given by the following formula: TIFF2025187472000007.tif55166 The above formula (6) is one of the main conditional formulas related to the design method of the disclosed technology.

[0027] <Numerical example> φ4 [mm] (cross-sectional area 12.56 [mm 2 ]) inside the pipe, water (v=0.893[mm 2 Suppose we are given a situation in which we want to measure the flow rate of a given volume ([ / s]). First, the reference flow velocity (V m ) can be calculated as follows, paying attention to the units: TIFF2025187472000008.tif32166However, in equation (17), for simplicity, the flow sectional area S of the flow piping is set to 12 [mm 2 ]

[0028] In this numerical example, when the run-up distance L is given as 10 cm, the right-hand side of the design conditional expression (6) becomes the following value, taking into consideration the units. TIFF2025187472000009.tif25166Here, the left side of equation (6) is D H -2 In view of this, the unit of the value calculated by formula (8) is [mm ―2 ] is correct.

[0029] Finally, the range of the cross-sectional horizontal side a that satisfies formula (6) is determined as follows: TIFF2025187472000010.tif21166In the design stage, the range of the cross-sectional side a that satisfies the conditional expression does not necessarily have to be determined analytically; it can be determined numerically.The product of the boundary values ​​of the range of the cross-sectional side a given by equation (9) is 1.259 x 9.533 = 12.0 mm 2 ], which is equal to the flow path cross-sectional area S. This can also be said to represent the symmetry between the horizontal side a of the cross section and the vertical side b of the cross section, which is seen when the flow path cross-sectional area S is fixed. Examples of combinations of the cross-sectional horizontal direction side a and the cross-sectional vertical direction side b that satisfy the formula (9) are as follows: TIFF2025187472000011.tif45166

[0030] One aspect of a thermal mass flow meter according to the disclosed technology is a thermal mass flow meter comprising a piping section and a thermal flow sensor, in which the piping forming the flow path of the piping section has a rectangular cross-sectional shape consisting of a horizontal side a and a vertical side b of different lengths, and has an inlet section forming a flow path in which the flow velocity distribution is constant along the fluid flow direction. Also, in the thermal flow sensor according to the disclosed technology, the sensor element (heater and temperature sensor) is installed on the back side of the fluid contact surface of the piping. The thermal mass flow meter according to the disclosed technology has the above-mentioned technical features, and therefore has the advantage that an "intake section" of sufficient distance can be secured to develop a laminar boundary layer, the flow velocity distribution at the location where the thermal flow sensor 100 is installed is constant, and the output of the thermal mass flow meter does not fluctuate.

[0031] One aspect of the design method for a thermal mass flow meter according to the present disclosure is to determine a flow path cross-sectional area S and a reference flow velocity V m When the inlet distance L and the dynamic viscosity coefficient v of the fluid to be measured are given, one side of the rectangular pipe that constitutes the inlet section through which the fluid to be measured flows is found to be a that satisfies the aforementioned conditional equation, Equation (6). Since the design method for a thermal mass flow meter according to the disclosed technology has the above-mentioned technical features, a thermal mass flow meter based on this design method can ensure an "entrance section" of sufficient distance for developing a laminar boundary layer, the flow velocity distribution at the location where the thermal flow sensor 100 is installed is constant, and the output of the thermal mass flow meter does not fluctuate.

[0032] The present disclosure may be realized as a design program for a thermal mass flow meter. For example, one aspect of the design program for a thermal mass flow meter according to the present disclosure may be a program for designing a flow path cross-sectional area S and a reference flow velocity V m The method may be configured to have a computer execute the steps of: acquiring the approach distance L and the dynamic viscosity coefficient v of the fluid to be measured; and calculating the range of a that satisfies the aforementioned conditional formula (6) for one side of a rectangular pipe that constitutes the approach section through which the fluid to be measured flows. The design program for a thermal mass flow meter according to the disclosed technique has the above-described technical features, and therefore, a design support device for a thermal mass flow meter can be realized using a general-purpose computer. [Industrial Applicability]

[0033] The disclosed technique can be applied to the design of thermal mass flow meters, particularly to the design of MEMS-type (flow sensor-type) thermal mass flow meters, and has industrial applicability. [Explanation of symbols]

[0034] 100 Thermal flow sensor, 110 Silicon chip, Rh Heater, Rr Ambient temperature sensor, Rd Downstream temperature sensor, Ru Upstream temperature sensor.

Claims

1. In a thermal mass flow meter comprising a piping section and a thermal flow sensor, The piping forming the flow path of the piping section includes: The cross-sectional shape is a rectangle having a cross-sectional horizontal side a and a cross-sectional vertical side b of different lengths, The flow path has an entrance section that forms a flow path in which the flow velocity distribution is constant along the flow direction of the fluid. Thermal mass flow meter.

2. Flow path cross-sectional area S and reference flow velocity V ｍ When the inlet distance L and the dynamic viscosity coefficient v of the fluid to be measured are given, the length of one side of the rectangular pipe constituting the inlet section through which the fluid to be measured flows is expressed as follows: is a that satisfies The thermal mass flow meter according to claim 1 .

3. The thermal flow sensor is disposed on the longer side of the cross-sectional horizontal side a and the cross-sectional vertical side b. The thermal mass flow meter according to claim 1 .

4. A method for designing a thermal mass flow meter including a thermal flow sensor, comprising: Flow path cross-sectional area S and reference flow velocity V ｍ When the inlet distance L and the dynamic viscosity coefficient v of the fluid to be measured are given, the length of one side of the rectangular pipe constituting the inlet section through which the fluid to be measured flows is expressed by the conditional expression We find a that satisfies How to design a thermal mass flow meter.

5. A design program for a thermal mass flowmeter for causing a computer to execute the method according to claim 4.

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