Optimization method of control elements based on bistable slit flow switching

Abstract

The present invention relates to a control element optimization method based on bistable slit flow switching, comprising the following steps: determining the slit width and obtaining the flow boundary layer displacement thickness based on the flow velocity of the fluid when it reaches the slit, the distance of the fluid from the slit inlet, and the fluid's kinematic viscosity coefficient; when the fluid displacement in the flow boundary layer reaches a maximum and the displacement thickness no longer changes, the flow field develops from the inlet section to the fully developed section, the length of the inlet section being the shortest heating distance of the slit of the control element; determining the flow splitting distance and flow splitting angle of the control element based on the angle formed between the central streamline of the fluid and the wall when attached to the wall, the wall attachment angle, the slit width, and the width of the outflow channel. Bistable slit flow switching control is achieved using temperature, and the structure of the control element is obtained based on fluid parameters, adapting to small-scale devices and enriching existing control methods.

Description

Technical Field

[0001] The present invention relates to the field of energy and power technology, and in particular to a control element optimization method based on bistable slit flow switching. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Bistable slit flow occurs when a fluid flows through a confined space. Due to the Coanda effect, the exit flow deviates to one wall and stably attaches to it. When stimulated by a certain mechanism, the flow switches to the other wall and reattaches. This "attachment-switching-reattachment" pattern gives bistable slit flow properties similar to those of switching and oscillating circuits. Therefore, it is often used to manufacture devices such as switching vortex valves and flow oscillators, and has broad applications in flow separation, drag and noise reduction, combustion control, and sensing equipment.

[0004] Based on the characteristics of bistable slit flow, the main methods to achieve flow attachment switching are divided into passive control and active control.

[0005] Passive control relies on system structure processing to achieve switching control. It does not require external energy and is mostly back-pressure switching. Its advantages are no moving parts and low energy consumption, but the oscillation frequency cannot be decoupled from the flow rate, reducing accuracy. Active control, on the other hand, controls the fluid by introducing external signals such as electrical signals, acoustic signals, plasma signals, and piezoelectric signals. Most of these signals act on the nozzle outlet with lower pressure. Although they require additional energy, they have high precision and flexibility and are widely used in the design of fluid oscillators and fluid valves.

[0006] The above control methods are often targeted at large-scale devices. For micro-scale devices, the current control methods are not ideal and cannot reliably meet the needs. Summary of the Invention

[0007] In order to solve the technical problems existing in the above-mentioned background technology, the present invention provides a control element optimization method based on bistable slit flow switching, which uses temperature to realize bistable slit flow switching control, and obtains the structure of the control element according to the fluid parameters, which is suitable for small-scale devices and enriches the original control means, so that the bistable slit flow is not only applied in the field of flow control, but also expands its application prospects in the field of thermal science.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A first aspect of the present invention provides a control element optimization method based on bistable slit flow switching, comprising the following steps:

[0010] Determine the slit width, and obtain the displacement thickness of the flow boundary layer based on the flow velocity of the fluid when it reaches the slit, the distance of the fluid from the slit entrance, and the kinematic viscosity coefficient of the fluid;

[0011] When the displacement of the fluid in the flow boundary layer reaches the maximum and the displacement thickness no longer changes, the flow field develops from the inlet section to the fully developed section. The length of the inlet section is the shortest heating distance of the slit of the control element.

[0012] The splitter distance and splitter angle of the control element are determined according to the angle formed by the central streamline of the fluid and the wall surface when attached to the wall, the wall attachment angle, the slit width and the width of the outflow channel.

[0013] The kinematic viscosity coefficient of a fluid is the ratio of the fluid's dynamic viscosity to the fluid's density, and the fluid's dynamic viscosity changes with temperature.

[0014] According to the velocity distribution obtained from the flow equation of the fluid in the slit, the state where the displacement of the fluid in the flow boundary layer reaches the maximum and the displacement thickness no longer changes is determined.

[0015] Velocity distribution. As shown in the following formula:

[0016]

[0017] where μ(T) is the local fluid dynamic viscosity at x, μ(T1) and μ(T2) are the dynamic viscosities of the two walls at temperatures T1 and T2, respectively; h is the distance from the center of the slit to the wall, and the slit width is 2h; dp / dx is the pressure gradient along the x direction, and the flow in the surface slit is pressure-driven.

[0018] When the flow field enters the fully developed stage, the position where the maximum flow velocity is offset along the y direction is determined based on the maximum flow velocity determined at the confluence of the fluid and the mainstream and the change in the displacement thickness of the flow boundary layer. The y direction is perpendicular to the flow direction of the fluid in the slit.

[0019] The shortest heating distance of the slit is determined according to the displacement thickness of the flow boundary layer and the position of the maximum flow velocity offset along the y direction.

[0020] The potential difference height of the control element is zero, and the fluid flows out of the slit without forming a jet impact.

[0021] The central streamline of the fluid forms an angle θ with the wall when it is attached to the wall. The wall attachment angle α, the slit width 2h and the deflection radius R satisfy the following formula:

[0022]

[0023] The angle range of the wall attachment angle α is 7° to 55°, and the splitting angle is 2α.

[0024] The value of the deflection radius R is determined according to the selected wall attachment inclination angle.

[0025] The splitter distance H is the distance from the splitter top to the slit outlet, as shown in the following formula:

[0026]

[0027] Where L is the width of the outflow channel, R is the deflection radius, α is the wall attachment angle, and θ is the angle between the central streamline of the fluid and the wall when it is attached to the wall.

[0028] Compared with the existing technology, one or more of the above technical solutions have the following beneficial effects:

[0029] 1. The optimized control element can use temperature to respond to the switching control of the fluid deflection direction, realizing a temperature-controlled switch function based on bistable slit flow, and this function is designed for devices at a microscale.

[0030] 2. While adapting to small-scale devices, it enriches the original control methods, so that the bistable slit flow is not only used in the field of flow control, but also expands its application prospects in the field of thermal science. Based on the temperature response mechanism and flow deflection characteristics, it can be used in the design and processing of thermal control devices, thermal sensor devices, thermal detection devices and thermal logic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0032] Figure 1 is a schematic diagram of a diverter valve model based on passive control of bistable slit flow provided by one or more embodiments of the present invention;

[0033] Figure 2 Schematic diagram of a diverter valve model based on active control (acoustic control) of bistable slit flow provided by one or more embodiments of the present invention;

[0034] Figure 3 is a schematic diagram of velocity distribution between parallel plates without external influences provided by one or more embodiments of the present invention;

[0035] Figure 4 Schematic diagram of velocity distribution between parallel plates when the upper wall temperature is high and the lower wall temperature is low, provided by one or more embodiments of the present invention;

[0036] FIG5(a) and FIG5(b) are both schematic diagrams of the concept of a temperature-controlled switch based on bistable slit flow according to one or more embodiments of the present invention;

[0037] Figure 6 This is a schematic diagram of the outlet structure of a common jet wall attachment element provided by one or more embodiments of the present invention;

[0038] Figure 7 Schematic diagram of the structure of a temperature-controlled switch based on bistable slit flow according to one or more embodiments of the present invention;

[0039] Figure 8 This is a schematic diagram illustrating the structural dimensions of a temperature-controlled switch based on bistable slit flow according to one or more embodiments of the present invention;

[0040] Figure 9 The outlet flow rate change and flow ratio under different slit wall temperature differences provided by one or more embodiments of the present invention;

[0041] Figure 10 This is the process of switching the bistable slit jet from the wall-attached flow on one side to the wall-attached flow on the other side (T represents a switching cycle) provided by one or more embodiments of the present invention. DETAILED DESCRIPTION

[0042] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0043] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0044] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0045] As described in the background art, for bistable slit flow, there are two control methods for achieving flow attachment switching: active and passive.

[0046] Passive control, such as Figure 1 Figure 2 shows a diverter valve model based on passively controlled bistable slit flow. Switching control achieved through system structure processing requires no external energy and is mostly backpressure switching. This has the advantages of no moving parts and low energy consumption, but the oscillation frequency cannot be decoupled from the flow rate, reducing accuracy.

[0047] Active control, such as Figure 2Figure 2 shows a model of a diverter valve based on active (acoustically controlled) bistable slit flow. These control the flow by introducing external signals, such as electrical, acoustic, plasma, and piezoelectric signals. These methods mostly act on the nozzle outlet at lower pressures. While they consume additional energy, they offer high precision and flexibility, making them widely used in the design of fluid oscillators and fluid valves.

[0048] Current control methods are all based on large-scale devices and are not designed for small-scale devices. However, at a microscale, temperature has a significant impact on flow. Therefore, the following embodiment provides a control element optimization method based on bistable slit flow switching, and designs a temperature-controlled switch using temperature as a trigger mechanism to obtain thermal devices, temperature sensors, temperature detectors, temperature control logic systems, and other devices that can dynamically respond to and regulate heat flow according to fluctuations in the external environment temperature.

[0049] Example 1:

[0050] The control element optimization method based on bistable slit flow switching includes the following steps:

[0051] Determine the slit width, and obtain the displacement thickness of the flow boundary layer based on the flow velocity of the fluid when it reaches the slit, the distance of the fluid from the slit entrance, and the kinematic viscosity coefficient of the fluid;

[0052] When the displacement of the fluid in the flow boundary layer reaches the maximum and the displacement thickness no longer changes, the flow field develops from the inlet section to the fully developed section. The length of the inlet section is the shortest heating distance of the slit of the control element.

[0053] The splitter distance and splitter angle of the control element are determined according to the angle formed by the central streamline of the fluid and the wall surface when attached to the wall, the wall attachment angle, the slit width and the width of the outflow channel.

[0054] Specifically:

[0055] 1 Concept:

[0056] like Figure 3 As shown in the figure, when an incompressible fluid flows between parallel plates, a boundary layer forms near the plate walls due to viscosity. When the two plates are relatively stationary and the wall temperature is the same as the fluid, the boundary layer thickness at both walls is the same, and the flow velocity between the plates follows a symmetrical parabolic distribution along the height of the gap.

[0057] like Figure 4 As shown in the figure, when the temperature of the upper wall is higher than that of the inlet fluid, a temperature difference forms between the two walls. Since the fluid viscosity decreases with increasing temperature, the thickness of the flow boundary layer at the high-temperature wall becomes thinner, resulting in an asymmetric distribution of the pressure drop gradient at the two walls, causing the maximum flow velocity (i.e., the highest point of the parabola) to shift toward the high-temperature wall.

[0058] like Figure 5(a)-Figure 5(b) As shown in the figure, under the influence of different temperatures on the upper and lower walls, the fluid flow in the slit exhibits an asymmetric distribution. Due to the Coanda effect, the flow deflects to one side at the slit exit. When the high-temperature wall switches from the upper wall to the lower wall, the flow deflects to the other side at the slit exit, thus achieving bidirectional temperature control of the slit flow deflection and forming a temperature-controlled switch based on bistable slit flow.

[0059] 2Structural design:

[0060] In order to maximize the temperature regulation effect on the bistable slit flow, while ensuring continuous flow, this embodiment takes a slit width of 1 mm as an example to illustrate the subsequent solution.

[0061] (1) Design of slit heating length

[0062] According to boundary layer theory, the displacement thickness of the boundary layer of two-dimensional flow is:

[0063]

[0064] Where δ is the displacement thickness of the flow boundary layer, m; K is the coefficient; U is the incoming flow velocity, m / s; ν is the kinematic viscosity coefficient, ν = μ(T) / ρ, where μ(T) is the dynamic viscosity, which changes with temperature, μ(T) = 1.93219×10 -7 T 2 -1.3796×10 -4 T + 0.02494, Pa·s; ρ is density, kg / m 3 ; x is the distance from the entrance, m. Considering the entrance section effect, K is taken as 1.385, and formula (2-1) is written as:

[0065]

[0066] At the same time, according to the Navier-Stokes equations, the fluid flow equation in the gap can be simplified as:

[0067]

[0068] According to the boundary conditions, the velocity distribution is obtained through calculation and deduction:

[0069]

[0070] Where μ(T) is the local fluid dynamic viscosity at x, μ(T1) and μ(T2) are the dynamic viscosities of the two walls at temperatures T1 and T2, respectively; h is the distance from the center of the slit to the wall, and the slit width is 2h; dp / dx is the pressure gradient along the x-direction; the flow within the surface slit is pressure-driven. Once the velocity boundary layer reaches its maximum displacement thickness, the boundary layer thickness ceases to change, and the flow field progresses from the inlet section to the fully developed section. Therefore, the minimum heating distance is the entire inlet section length.

[0071] When the flow field enters the fully developed stage, affected by the viscous shear force of the two walls, the maximum flow velocity appears at the confluence of the fluid and the mainstream.

[0072]

[0073] Right now:

[0074]

[0075] When the temperature of the low-temperature wall is the same as the incoming flow temperature, equation (2-6) can be simplified to:

[0076]

[0077] Because the position of the maximum velocity shifted in the y direction is caused by the change in the displacement thickness of the flow boundary layer, so:

[0078]

[0079] Substituting equations (2.2) and (2.7) into the equations, and while meeting the requirements of the large temperature difference control experiment, the size of the slit heating length is obtained through calculation and rounding. In this embodiment, the slit heating length is 20 mm.

[0080] (2) Export structure design

[0081] like Figure 6 As shown in the figure, the common outlet structures and structural parameters of flow attachment elements include: slit-slit width (2h), level difference-level difference height (s), splitter-split distance (H)-split angle (α), and wall attachment surface-wall inclination angle (β).

[0082] Due to the potential difference s, the fluid flowing out of the slit will experience a change in the cross-sectional area of ​​the flow channel, resulting in local pressure loss. Local pressure loss h m for:

[0083]

[0084] Where A1 and A2 are the cross-sectional areas within the slit and at the slit outlet after the sudden expansion, respectively, and U1 and U2 are the average flow velocities within the slit and at the slit outlet after the sudden expansion, respectively. The potential difference s is related to the sudden expansion cross-sectional area A2. According to Equation (2-9), a larger potential difference results in greater local pressure loss and a greater distance between the wall attachment point and the outlet, which is unfavorable for wall attachment flow.

[0085] In order to enhance the wall-coanda effect and avoid jet impact, this embodiment proposes a structure with negligible potential difference that enhances the wall-coanda flow.

[0086] (3) Design of split distance H

[0087] The splitter distance H is the distance from the splitter tip to the slit outlet. If the splitter distance is too short, the splitter will not be complete and the flow will be affected by the wall. If the splitter distance is too long, it will easily cause flow oscillation under the condition of flow entrainment near the two walls and cannot form a stable wall flow. In order to meet the requirements of stable wall attachment and allow the fluid to enter the side channel to the greatest extent, the splitter distance is designed as follows, and the relevant structural parameters are as follows Figure 7 shown.

[0088] The dotted line in the figure is the center streamline of the fluid, which forms an angle θ with the wall when attached to the wall. The angle is affected by the flow velocity at the slit outlet, the slit width, and the pressure inside and outside the slit. From the geometric structure relationship in the figure, it can be seen that:

[0089]

[0090] According to the Coanda effect, the wall attachment angle α is commonly used in the range of 7° to 55°, with 12° being used here. Assuming the angle θ between the center streamline and the wall is 30°, the deflection radius R is 4.36 mm according to equation (2-10). Therefore, based on the geometric structure, the calculation formula for the split wedge distance H is as follows:

[0091]

[0092] Where L is the width of the outlet channel, here L = 4h = 2mm. According to the above formula, H≈6.93mm. Due to the influence of the temperature applied in the slit on the outlet flow rate, the contact angle θ will vary within a certain range. The larger the contact angle, the shorter the splitter distance can be, and the smaller the contact angle, the larger the splitter distance. At the same time, considering that the volume expansion will occur after the slit flows out, H is selected as 8mm here. The splitter angle 2α is 24°, such as Figure 8 shown.

[0093] 3Device performance:

[0094] (1) Temperature-responsive flow deflection control performance

[0095] The device obtained according to the above structural design has temperature-responsive flow deflection control performance and can realize a temperature-controlled switch function based on bistable slit flow.

[0096] The two-dimensional numerical simulation method was used to test the flow deflection of the two walls of the slit section of the device under different temperature differences when the flow velocity at the slit inlet was 0.08m / s. Figure 9 As shown in the figure, test results show that as the temperature difference between the two slit walls increases, the difference in flow rates at the two outlets gradually increases, indicating that temperature can affect flow deflection. When both walls maintain the same temperature of 20°C, the ratio of the two outlet flow rates is 2.13. However, when the temperature of one wall rises to 60°C, the ratio of the flow rate flowing along the high-temperature wall to the flow rate flowing along the low-temperature side reaches 50.95, approximately 24 times the value when there is no temperature difference.

[0097] (2) Temperature-responsive flow attachment switching performance

[0098] In addition to the flow deflection regulation function, the flow attachment switch of the slit outflow can also be realized by temperature, thus forming a temperature control switch based on bistable flow. Figure 10 As shown in the figure, numerical simulation verifies that when the high-temperature wall surface (60℃) is exchanged with the low-temperature wall surface (20℃), the slit outflow can switch from stable wall-attached flow on one side to stable wall-attached flow on the other side, and the entire flow switching time is 4.54s.

[0099] Numerical simulations have demonstrated that bistable slit flow exhibits temperature-responsive characteristics. The temperature difference between the two slit walls can be used to control the deflection and wall adhesion of the slit outflow. Analogous to an electrical switch, this method enables continuous flow regulation under temperature control and enables both opening and closing of a single-sided flow path. Therefore, it can be used to design temperature-controlled switches based on bistable slit flow.

[0100] The above process provides the structure of the control element required to achieve bistable slit flow switching using temperature. This is suitable for small-scale devices and enriches existing control methods. This allows bistable slit flow to be applied not only in the field of flow control but also in the field of thermal science. This temperature response mechanism and flow deflection characteristics make it suitable for the design and fabrication of thermal control devices, thermal sensors, thermal detection devices, and thermal logic devices.

[0101] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A control element optimization method based on bistable slit flow switching, characterized in that: The following steps are involved: Determine the slit width, and obtain the displacement thickness of the flow boundary layer based on the flow velocity of the fluid when it reaches the slit, the distance of the fluid from the slit entrance, and the kinematic viscosity coefficient of the fluid; When the displacement of the fluid in the flow boundary layer reaches the maximum and the displacement thickness no longer changes, the flow field develops from the inlet section to the fully developed section. The length of the inlet section is the shortest heating distance of the slit of the control element. The splitter distance and splitter angle of the control element are determined based on the angle formed between the central streamline of the fluid and the wall when it is attached to the wall, the wall attachment angle, the slit width and the width of the outflow channel; The velocity distribution obtained from the flow equation of the fluid in the slit is used to determine the state where the displacement of the fluid in the flow boundary layer reaches the maximum and the displacement thickness no longer changes. The velocity distribution is shown in the following formula: ； Where, yes The local fluid dynamic viscosity at and The temperatures are and The dynamic viscosity of the two walls; is the distance from the center of the slit to the wall, and the slit width is 2 ; For the The pressure gradient in the direction of the flow is driven by pressure.

2. The control element optimization method based on bistable slit flow switching according to claim 1, characterized in that: The kinematic viscosity coefficient of the fluid is the ratio of the fluid dynamic viscosity to the fluid density, and the fluid dynamic viscosity changes with temperature.

3. The control element optimization method based on bistable slit flow switching according to claim 1, characterized in that: When the flow field enters the fully developed stage, the position where the maximum flow velocity is offset along the y direction is determined based on the maximum flow velocity determined at the confluence of the fluid and the mainstream and the change in the displacement thickness of the flow boundary layer. The y direction is perpendicular to the flow direction of the fluid in the slit.

4. The control element optimization method based on bistable slit flow switching according to claim 1, characterized in that: The shortest heating distance of the slit is determined according to the displacement thickness of the flow boundary layer and the position of the maximum flow velocity offset along the y direction.

5. The control element optimization method based on bistable slit flow switching according to claim 1, characterized in that: The potential difference height of the control element is zero, and the fluid flows out of the slit without forming a jet impact.

6. The control element optimization method based on bistable slit flow switching according to claim 1, characterized in that: The central streamline of the fluid forms an angle with the wall when it is attached to the wall , wall inclination angle , slit width 2 and deflection radius Satisfy the following formula: 。 7. The control element optimization method based on bistable slit flow switching according to claim 6, characterized in that: Coanda inclination angle The angle range is 7°~55°, and the split angle is 2 , the deflection radius is obtained according to the selected wall inclination angle R The numerical value of .

8. The control element optimization method based on bistable slit flow switching according to claim 1, characterized in that: The splitter distance H is the distance from the splitter top to the slit outlet, as shown in the following formula: ； Where, L is the width of the outflow channel, is the deflection radius, for Wall inclination angle, The angle formed by the central streamline of the fluid and the wall when it attaches to the wall.

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