Control element optimization method based on bistable slit flow switching

Abstract

The invention relates to a control element optimization method based on bistable slit flow switching, which comprises the following steps of: determining the width of a slit, and obtaining the displacement thickness of a flow boundary layer according to the flow velocity when fluid reaches the slit, the distance between the fluid and an inlet of the slit 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 does not change any more, the flow field develops to the full development section from the inlet section, and the length of the inlet section is the shortest heating distance of the slit of the control element; and according to an included angle formed between the central streamline of the fluid and the wall surface during wall attachment, a wall attachment inclination angle, the width of the slit and the width of the outflow channel, determining the shunting split distance and the shunting split opening angle of the control element. Bistable slit flow switching control is achieved through temperature, the structure of a control element is obtained according to fluid parameters, the device adapts to small-scale devices, and original regulation and control means are enriched.

Description

Technical Field

[0001] The 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 refers to the situation where, when a fluid flows in a narrow space, under the action of the Coanda effect, the outlet flow tends to stably attach to one wall, and under the stimulation of a certain mechanism, switches to the other wall and reattaches. This "attachment-switching-reattachment" mode gives the bistable slit flow the performance of analog circuit switches and oscillation circuits. Therefore, it is often used to prepare devices such as switch vortex valves and flow oscillators, and can be widely used in the fields of flow separation, drag and noise reduction, combustion control, and sensor 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 is a switching control realized by system structure processing. It does not require external energy. Most of them are back pressure switching. Its advantages are no moving parts and low energy consumption, but the oscillation frequency cannot be decoupled from the flow rate, which reduces the accuracy. Active control controls the fluid by introducing external signals, such as electrical signals, acoustic signals, plasma signals, and piezoelectric signals. Most of them act on the nozzle outlet with lower pressure. Although they need to consume additional energy, they have high accuracy and strong flexibility. They 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 are difficult to 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 solution:

[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 according to 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, and 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 between 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 outlet channel.

[0013] The kinematic viscosity coefficient of a fluid is the ratio of the fluid's dynamic viscosity to the fluid's density. 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 two wall dynamic viscosities 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 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 inclination 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 outlet 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 prior art, 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 the 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 in the specification, 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 is a schematic diagram of a diverter valve model based on active control (sound 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 influence provided by one or more embodiments of the present invention;

[0035] Figure 4 It is a 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] FIG. 5( a ) and FIG. 5( b ) are both conceptual schematic diagrams of a temperature-controlled switch based on bistable slit flow provided by one or more embodiments of the present invention;

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

[0038] Figure 7 is a schematic diagram of a temperature control switch structure based on bistable slit flow provided by one or more embodiments of the present invention;

[0039] Figure 8 is a schematic diagram of an example of the structural dimensions of a temperature-controlled switch based on bistable slit flow provided by one or more embodiments of the present invention;

[0040] Fig. 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] Fig.10 It is a process in which a bistable slit jet is switched from a wall-attached flow on one side to a 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 in conjunction with the accompanying drawings and embodiments.

[0043] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those 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 exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates 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 switching the flow attachment to the wall: active and passive.

[0046] Passive control, such as Figure 1 The figure shows a diverter valve model based on passive control of bistable slit flow. The switching control realized by system structure processing does not require external energy, and most of them are back pressure switching. Its advantages are no moving parts and low energy consumption, but the oscillation frequency cannot be decoupled from the flow, which reduces the accuracy.

[0047] Active control, such as Figure 2As shown in the figure, the diverter valve model based on active control (acoustic control) bistable slit flow. By introducing external signals to control the fluid, such as electrical signals, acoustic signals, plasma signals and piezoelectric signals, most of them act on the nozzle outlet with lower pressure. Although they need to consume additional energy, they have high precision and strong flexibility and are 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. At a microscale, temperature has a significant effect on flow. Therefore, the following embodiments provide a control element optimization method based on bistable slit flow switching, and design 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 following external environmental temperature fluctuations.

[0049] Embodiment 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 according to 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, and 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 between 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 outlet channel.

[0054] Specific:

[0055] 1 Concept:

[0056] like Figure 3 As shown in the figure, when an incompressible fluid flows between parallel plates, a flow boundary layer will be formed near the wall of the plate due to the influence of viscosity. When the two plates are relatively still and the wall temperature is the same as that of the fluid, the flow boundary layer thickness at the two walls is the same, and the flow velocity of the fluid between the plates is distributed in a symmetrical parabola along the gap height.

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

[0058] like Figure 5(a)-Figure 5(b) As shown in the figure, under the influence of different temperatures of the upper and lower walls, the fluid flow in the slit has an asymmetric distribution, and under the action of the Coanda effect, the flow will flow to one side at the slit outlet. When the high-temperature wall changes from the upper wall to the lower wall, the flow at the slit outlet will flow to the other side, thereby realizing the bidirectional deflection control of the slit flow by temperature, forming a temperature control switch based on the bistable slit flow.

[0059] 2Structural design:

[0060] In order to maximize the temperature regulation effect on the bistable slit flow, under the condition of ensuring continuous flow, this embodiment takes the 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 derivation:

[0069]

[0070] Where, μ(T) is the local fluid dynamic viscosity at x, μ(T1) and μ(T2) are the two wall dynamic viscosities 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. When the velocity boundary layer develops to the maximum displacement thickness, the boundary layer thickness no longer changes, and the flow field develops from the inlet section to the fully developed section. Therefore, the shortest 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 formula, the size of the slit heating length is obtained after calculation and rounding to the nearest integer while meeting the requirements of the large temperature difference control experiment. 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 flow attachment element outlet structure and structural parameters include: slit-slit width (2h), potential difference-potential difference height (s), splitter-split distance (H)-split angle (α), 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] Among them, A1 and A2 are the cross-sectional areas inside the slit and after the sudden expansion at the slit outlet, respectively, and U1 and U2 are the average flow velocities inside the slit and after the sudden expansion at the slit outlet, respectively. The potential difference s is related to the sudden expansion cross-sectional area A2. According to formula (2-9), the larger the potential difference, the greater the local pressure loss, and the farther the wall attachment point is from the outlet, which is not conducive to the wall attachment flow.

[0085] In order to strengthen 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 top of the splitter to the slit outlet. If the splitter distance is too short, the splitter will not be complete and the flow will be affected. If the splitter distance is too long, the flow will be oscillated when the flow is entrained near the two walls, and a stable flow with wall adhesion cannot be formed. In order to meet the requirement of stable wall adhesion and allow the fluid to enter the channel to the maximum 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 central streamline of the fluid, which forms an angle θ with the wall when attached to the wall. The size is affected by the slit outlet flow rate, 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 commonly used angle of the wall inclination angle α is 7° to 55°, and 12° is selected here. Assuming that the angle θ between the center streamline and the wall is 30°, the deflection radius R can be obtained as 4.36mm according to formula (2-10). Therefore, according to the geometric structure relationship, the calculation formula for the split wedge distance H is as follows:

[0091]

[0092] Where L is the width of the outlet channel, where 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. Considering the volume expansion 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 a bistable slit flow.

[0096] The two-dimensional numerical simulation method was used to test the flow deflection of the two walls of the device slit section under different temperature differences when the flow velocity at the slit inlet was 0.08m / s. Fig. 9 As shown in the figure, according to the test results, as the temperature difference between the two walls of the slit increases, the difference in the flow rate at the two outlets gradually increases, indicating that temperature can affect flow deflection. Among them, when the two walls are kept at the same temperature of 20°C, the ratio of the two outlet flow rates is 2.13; when one side of the wall rises to 60°C, the ratio of the flow rate flowing out along the high-temperature wall side to the flow rate on the low-temperature side can reach 50.95, which is about 24 times that of no temperature difference.

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

[0098] In addition to the flow deflection regulation function, the flow attachment switching of the slit outflow can also be achieved by temperature, thus forming a temperature-controlled switch based on bistable flow. Fig.10 As shown in the figure, numerical simulation has verified that when the high-temperature wall (60℃) is exchanged with the low-temperature wall (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] Through numerical simulation, it is verified that the bistable slit flow has temperature response characteristics. The temperature difference between the two walls of the slit can regulate the deflection and wall attachment of the slit outflow. Analogous to an electric switch, it can realize continuous flow regulation under temperature control and realize the opening and closing functions of the single-side flow path, so it can be used to design a temperature-controlled switch based on the bistable slit flow.

[0100] The above process provides the structure of the control element required to realize the switching of bistable slit flow by temperature, which is suitable for small-scale devices and enriches the original control methods, so that the bistable slit flow can be applied not only in the field of flow control, but also expand its application prospects in the field of thermal science. This temperature response mechanism and flow deflection characteristics make it possible to be used in the design and processing of thermal control devices, thermal sensor devices, thermal detection devices and thermal logic devices.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope 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 according to 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, and 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 according to the angle formed between 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 outlet channel.

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: 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.

4. The control element optimization method based on bistable slit flow switching according to claim 3, characterized in that: The velocity distribution is shown in the following formula: Where μ(T) is the local fluid dynamic viscosity at x, μ(T1) and μ(T2) are the two wall dynamic viscosities 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.

5. 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.

6. The control element optimization method based on bistable slit flow switching according to claim 4, 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.

7. 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.

8. 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 θ with the wall when attached to the wall. The wall attachment angle α, the slit width 2h and the deflection radius R satisfy the following formula:

9. The control element optimization method based on bistable slit flow switching according to claim 8, characterized in that: The angle range of the wall attachment inclination angle α is 7° to 55°, the splitter angle is 2α, and the value of the deflection radius R is obtained according to the selected wall attachment inclination angle.

10. 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 outlet 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.

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