Method for controlling millimeter-level bubble separation speed by using laser

Abstract

The invention relates to a method for controlling the separation speed of millimeter-level bubbles by using laser, and belongs to the technical field of fluid dynamics and photo-thermal regulation and control. The method comprises the following steps: defining an average speed within 250 microseconds after bubbles are completely separated from a solid-liquid interface as a separation speed; laser with the wave band of 808 nm is adopted to irradiate a solid-liquid interface, and dynamic balance of buoyancy borne by bubbles, surface adhesion, photo-thermal flow driving force and heat exchange flow resistance is changed by cooperatively regulating and controlling the laser power density (42.2-171.5 W / cm < 2 >), the surface roughness (the number of abrasive paper meshes is 60-10000) and the thickness of an absolute ethyl alcohol solution (5-40 mm). Experiments show that the separation speed is in a trend of increasing and then decreasing (the peak value is at 151W / cm < 2 >) along with the laser power density, is increased along with the reduction of the roughness, and is increased and then decreased (the peak value is at 20mm) along with the thickness of the solution; and under the optimal condition, the average separation speed reaches 49.03 mm / s. The invention discloses a mechanism for regulating and controlling bubble separation by laser, and provides a new technical support for an efficient heat exchange system and microfluid equipment.

Description

1. Name

[0001] A method for controlling millimeter-scale bubble departure velocity by laser 2. TECHNICAL FIELD

[0002] The present application relates to the field of fluid mechanics and photo-thermal regulation, and particularly to a method for controlling millimeter-scale bubble departure velocity by laser. 3. BACKGROUND

[0003] Bubble departure behavior has an important influence on boiling heat transfer efficiency. The contribution of bubble departure process to the heat flux from the heat transfer interface to the liquid can reach 61%, and in experiments, the temperature difference near the bubble bottom neck can be observed to exceed 20K. In addition, the bubble departure phenomenon shows important application value in the fields of hydrolysis hydrogen production, microfluidics, photocatalysis and coal mine flotation. Previous studies have shown that photo-regulation technology has gradually become a research hotspot in the field of bubble departure regulation due to its advantages such as point-to-point precise control, remote control ability and anti-electromagnetic interference. However, previous studies have focused on promoting the bubble departure process, and the results have mainly focused on the departure diameter, and the study of bubble departure velocity and millimeter-scale bubbles commonly used in actual industrial applications is relatively scarce. Especially in the fields of fluid mechanics, micro-robot driving and micro-scale heat conduction, there is an urgent need for millimeter-scale bubble departure regulation. 4. SUMMARY

[0004] To solve the above problems and shortcomings of the prior art, the present application provides a method for controlling millimeter-scale bubble departure velocity by laser.

[0005] To achieve the above-mentioned purpose, the present application provides the following solutions: a method for controlling millimeter-scale bubble departure velocity by laser. The method comprises:

[0006] The average velocity of the bubble within 250us after the bubble completely departs from the solid-liquid interface is defined as the bubble departure velocity. That is, the departure velocity is calculated by dividing the distance moved by the bubble within 250us after the bubble completely departs from the solid-liquid interface by 250us.

[0007] Mechanism of laser regulation of bubble departure velocity. To reveal the internal mechanism, the present application constructs a bubble force analysis model based on fluid dynamics theory. That is, when the laser regulates the bubble departure, the bubble is mainly affected by four forces, i.e. buoyancy, surface adhesion force, photo-thermal flow driving force and heat exchange flow resistance. Among them, the buoyancy and the photo-thermal flow driving force promote the bubble departure, the surface adhesion force and the heat exchange flow resistance hinder the bubble departure, and when the bubble is in a critical state, the forces are balanced. With the increase of laser irradiation time and solid-liquid interface temperature, the photo-thermal flow driving force gradually increases, which actively breaks the force balance state, the bubble leaves the solid-liquid interface, the surface adhesion force and the heat exchange flow resistance disappear instantly, the bubble floats and accelerates, and the bubble departure process is completed.

[0008] Further, the buoyancy expression is:

[0009]

[0010] Where d b is the bubble detachment diameter, for simplifying calculation, the bubble detachment can be defined as a standard sphere, p l and p g are the solution density and the bubble density in the bubble respectively, and g is the gravity acceleration.

[0011] Further, the surface adhesion force expression is:

[0012] F s = p c d c 2 s b in L where d D is the bubble contact diameter on the solid-liquid interface, s is the gas-liquid surface tension, and theta is the contact angle of the bubble on the solid-liquid interface.

[0013] Where d c is the bubble contact diameter on the solid-liquid interface, s is the gas-liquid surface tension, and theta is the contact angle of the bubble on the solid-liquid interface.

[0014] Further, the photothermal flow driving force and the heat exchange flow resistance are both generated by the liquid flow close to the bubble, and the essence is the drag force of the surrounding liquid on the bubble, so they can be given by the drag force formula, and specifically:

[0015]

[0016] Where R b , p L , and C D represent the characteristic height (bubble radius) of the bubble, the mass density of the surrounding liquid, and the resistance coefficient determined by the Reynolds number Re respectively, u is the liquid velocity, and v is the bubble velocity.

[0017] The millimeter-level bubble detachment velocity is jointly controlled by changing the surface roughness, the solution thickness, and the laser power density.

[0018] Further, the laser source adopts 808nm waveband laser, which has strong stability and adjustable power, and provides reliable energy support for controlling the bubble detachment. In the application, the laser power density is 42.2W / cm 2 , 65.4W / cm 2 , 87W / cm 2 , 108W / cm 2 , 129.2W / cm 2 , 151W / cm 2 , and 171.5W / cm 2 .

[0019] Further, by changing the laser power density, the millimeter-level bubble detachment speed can be regulated. 2 Further, by changing the laser power density, the millimeter-level bubble detachment speed can be regulated.

[0020] Further, the surface roughness is characterized by different grit sandpaper, and the grit number of the sandpaper is used to characterize the surface roughness, that is, the number of holes per square inch, and the smaller the grit number, the rougher the surface.

[0021] Further, by changing the laser power density, the millimeter-level bubble detachment speed can be regulated.

[0022] Further, the solution is anhydrous ethanol solution, and the solution thickness is 5mm, 20mm, 30mm, 40mm.

[0023] Further, by changing the solution thickness, the millimeter-level bubble detachment speed can be regulated.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The present application focuses on the regulation of bubble detachment process by laser, and by systematically exploring the kinetic characteristics of bubble detachment speed and diameter, the influence law of surface roughness, solution thickness and laser power density on bubble detachment behavior is revealed.

[0026] 2. Based on the control variable method, the optimal conditions for promoting bubble detachment are determined: under the conditions of 20mm solution thickness, 60 grit sandpaper surface and 151W / cm 2 laser power density, the average bubble detachment speed can reach 49.03mm / s, the maximum detachment speed can reach 51.31mm / s, and the average detachment diameter is 1.31mm. 5. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to make the technical solutions in the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description only represent some of the embodiments of the present application, and all other embodiments obtained by a person of ordinary skill in the art without creative effort on the basis of the drawings also belong to the protection scope of the present application.

[0028] Figure 1 A schematic diagram of an experimental device for a method for regulating millimeter-scale bubble detachment speed by using a laser and a microscale schematic diagram of a bubble detachment process are provided in the present application. Among them, 1, 808 laser beam; 2, square cuvette; 3, anhydrous ethanol solution; 4, high-speed camera; 5, bubble generated by laser radiation; 6, sandpaper; 7, microscale schematic diagram of the bubble detachment process.

[0029] Figure 2 A timing chart when a high-speed camera captures a bubble detachment process is provided in the present application.

[0030] Figure 3 A schematic diagram of forces acting on a bubble during detachment is provided in the present application.

[0031] Figure 4 Data analysis when laser power density is changed to regulate bubble detachment in the present application.

[0032] Figure 5 Data analysis when surface roughness is changed to regulate bubble detachment in the present application.

[0033] Figure 6 Data analysis when solution thickness is changed to regulate bubble detachment in the present application. 6. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments only represent some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort also belong to the protection scope of the present application.

[0035] EMBODIMENT

[0036] The present application provides a method for regulating millimeter-scale bubble detachment speed by using a laser. The inventor of the present application has conducted experiments, which can be divided into four steps: solution preparation and device construction, placement and debugging of the experimental device, bubble generation and collection, and data processing and analysis.

[0037] Solution preparation and device setup: First, an appropriate amount of absolute ethanol is accurately measured and placed in a square cuvette, forming a liquid layer of the appropriate thickness (solution thickness of 5 mm, 20 mm, 30 mm, 40 mm) required for the experiment. Then, sandpaper (sandpaper grits of 60, 200, 400, 600, 800, 1000, 3000, 5000, 7000, 10000) is placed in the square cuvette and ensured to be flat at the bottom of the ethanol solution.

[0038] Device setup and debugging: The square cuvette containing ethanol solution and sandpaper is placed firmly on the experimental table. Turn on the lighting equipment to ensure that the experimental area has sufficient light illumination to facilitate subsequent high-speed video capture of various phenomena during the experiment. At the same time, adjust the position and angle of the camera to ensure it can be aligned with the sandpaper surface and the area where the bubble may move. Finally, position and fix the laser light source so that its beam can accurately irradiate the specific position of the sandpaper surface, providing the required excitation energy for the bubble generation, growth, and detachment process (laser power density of 42.2 W / cm 2 , 65.4 W / cm 2 , 87 W / cm 2 , 108 W / cm 2 , 129.2 W / cm 2 , 151 W / cm 2 , 171.5 W / cm 2 ).

[0039] Bubble generation and collection: After the experimental conditions are stable and the equipment is debugged, turn on the laser light source. The sandpaper surface triggers the nucleation and growth of bubbles under the action of the laser. When the bubble growth is complete, it begins to detach from the sandpaper surface and exhibits a vertical upward movement in the ethanol solution. At this time, the high-speed camera captures the bubble detachment and upward movement process at a frame rate of 4000 frames / s, ensuring that the bubble's shape, position, and movement state at different times can be recorded completely.

[0040] Data processing and analysis: The high-speed video of the bubble is analyzed by the image processing software accompanying the high-speed camera to obtain the size and detachment speed of the bubble. The detachment speed is calculated by dividing the distance moved by the bubble after completely detaching from the solid-liquid interface by 250 μs.

[0041] Finally, the experimental device is shown in Figure 1 , including 808 laser beam 1, square cuvette 2, absolute ethanol solution 3, high-speed camera 4, laser radiation generated bubble 5, sandpaper 6, and bubble detachment process microscale schematic 7.

[0042] The application uses a high-speed camera to implement real-time dynamic capture on the millimeter bubble detachment process, and the acquisition speed can reach 4000 frames / s. Figure 2 It can be seen that the process of complete bubble detachment from the solid-liquid interface only needs one frame interval, and the corresponding actual time is 250us, which is extremely short. To further explore the dynamic performance in the bubble detachment process, the average speed within 250us after the bubble completely detaches from the solid-liquid interface is defined as the bubble detachment speed.

[0043] As shown in Figure 3 , when the laser controls the bubble detachment, it is mainly affected by four forces, i.e. buoyancy, surface adhesion, photothermal flow driving force and heat exchange flow resistance.

[0044] As shown in Figure 4 , by changing the laser power density, the millimeter bubble detachment speed can be controlled. When the laser power density increases from 42.2W / cm 2 to 171.5W / cm 2 , the average bubble detachment speed shows a trend of first increasing and then decreasing, and the peak value appears at 151W / cm 2 .

[0045] As shown in Figure 5 , by changing the surface roughness, the millimeter bubble detachment speed can be controlled. When the sandpaper grit increases from 60 to 10000, the corresponding roughness decreases, and the average bubble detachment speed shows a downward trend, and reaches the minimum at 7000.

[0046] As shown in Figure 6 , by changing the solution thickness, the millimeter bubble detachment speed can be controlled. When the solution thickness increases from 5mm to 40mm, the average bubble detachment speed shows a trend of first increasing and then decreasing, and reaches the peak at 20mm.

[0047] The above describes in detail a method for controlling the millimeter bubble detachment speed by using laser according to the embodiments of the application. In this paper, specific examples are applied to describe the phenomena and implementation modes of the application in detail. The above embodiment is only used to help understand the purpose and advantages of the application; meanwhile, for those skilled in the art, the scheme of the application can be modified or replaced, and the actual operation and use fields will also change accordingly, which are all covered in the scope of the present application.

Claims

1. A method for controlling the detachment speed of millimeter-level bubbles using laser, characterized in that: The following steps are involved: (a) The average velocity of the bubble within 250 μs after it completely leaves the solid-liquid interface is defined as the bubble detachment velocity, which is calculated as follows: Where Δx is the distance the bubble moves within 250 μs after detachment; (b) An 808nm laser is used to irradiate the solid-liquid interface, changing the dynamic balance of the buoyancy, surface adhesion, photothermal flow driving force, and heat exchange flow resistance of the bubble. When the laser irradiation causes the photothermal flow driving force to increase to break the force balance, the bubble detaches from the solid-liquid interface, the surface adhesion and heat exchange flow resistance disappear, and the bubble accelerates to float up and complete the detachment process. (c) By regulating the laser power density, surface roughness, and solution thickness, the millimeter-scale bubble detachment speed can be controlled, so that millimeter-scale bubbles can leave the solid-liquid interface at different detachment speeds, and the maximum detachment speed under the optimal conditions can be further determined.

2. The method according to claim 1, characterized in that The laser power density is 42.2W / cm 2 Up to 171.5W / cm 2 Adjustable within the range.

3. The method according to claim 1, characterized in that The surface roughness is characterized by the mesh number of sandpaper, which ranges from 60 mesh to 10,000 mesh, with smaller mesh numbers indicating greater roughness.

4. The method according to claim 3, characterized in that The mesh numbers of the sandpaper include 60 mesh, 200 mesh, 400 mesh, 600 mesh, 800 mesh, 1000 mesh, 3000 mesh, 5000 mesh, 7000 mesh and 10000 mesh.

5. The method according to claim 1, wherein The solution is an absolute ethanol solution, and the thickness ranges from 5 mm to 40 mm.

6. The method according to claim 5, characterized in that The solution thickness includes 5mm, 20mm, 30mm, and 40mm.

7. The method according to claim 1, characterized in that The force when the bubble detaches satisfies the following model: buoyancy: Surface adhesion: F s =πd c σsinθ Driving force of light and heat flow and resistance of heat exchange flow: where d b is the bubble detachment diameter, ρ l and ρ g are the density of the solution and the density of the bubble inside the bubble, respectively, and g is the acceleration of gravity d c is the contact path length of the bubble at the solid-liquid interface, σ is the gas-liquid surface tension, θ is the contact angle of the bubble at the solid-liquid interface, R b , ρ L 、C D They represent the characteristic height of the bubble (bubble radius), the mass density of the surrounding liquid, and the drag coefficient determined by the Reynolds number Re, u is the liquid velocity, and v is the bubble velocity.

8. The method according to claim 1, characterized in that By changing the laser power density, the millimeter-scale bubble detachment speed can be controlled.

9. The method according to claim 1, characterized in that By replacing sandpaper with different mesh sizes, the bubble detachment speed at the millimeter level can be adjusted.

10. The method according to claim 1, characterized in that By changing the solution thickness, the millimeter-scale bubble detachment speed can be controlled.

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