Method and system for testing fluctuation characteristics of liquid film outside drip flow horizontal tube

Abstract

The invention discloses a method and a system for testing fluctuation characteristics of a liquid film outside a drip flow horizontal tube, and belongs to the technical field of drip flow fluctuation characteristic testing. The system comprises a horizontal heat transfer tube, a drop-shaped flow liquid film, an adjustable LED mounting plate, an LED light source, a CCD high-speed camera and a computer, the adjustable LED mounting plates are mounted at two ends of the outer edge of the drop-shaped flow liquid film outside the horizontal heat transfer tube, the two LED light sources are fixed on the adjustable LED mounting plates, and the CCD high-speed camera directly faces the two LED light sources and is connected with the computer. A non-contact visual optical test technology and a scientific data processing method are adopted, interference of a direct contact measurement technology on a thin fluctuating liquid film is avoided, accurate measurement of the fluctuation characteristics of the liquid film outside the pipe in the circumference and the three-dimensional direction of the pipe length is achieved, the problem that complex and random experimental data of the fluctuation characteristics of the liquid film are difficult to process in the prior art is solved, and the experimental accuracy is improved. And meanwhile, the real mechanism and reason of enhanced heat transfer of the drop-shaped flow liquid film under the actual fluctuation condition can be analyzed.

Description

Technical Field

[0001] This invention relates to a method and system for testing the fluctuation characteristics of the external liquid film of a droplet-like flow in a horizontal tube, belonging to the technical field of droplet-like flow fluctuation characteristic testing. Background Technology

[0002] Tear-flow falling film evaporation has advantages such as small heat transfer temperature difference, high heat flux density, high heat transfer coefficient, simple structure, and low power consumption, and is widely used in petrochemical, refrigeration, nuclear engineering, aerospace, and electronic chip heat dissipation fields. In the case of droplet-flow horizontal tube external liquid film flow, the periodic impact of droplets disrupts the balance between gravity, viscous force, shear force, and surface tension. The instability of the liquid film surface causes fluctuations that significantly affect the velocity, temperature, and pressure distribution within the liquid film outside the horizontal tube. The liquid film fluctuation characteristics are closely related to the droplet-flow heat transfer process; therefore, testing the droplet-flow external liquid film fluctuation characteristics is of great significance for enhancing droplet-flow heat transfer technology outside horizontal tubes.

[0003] Liquid film fluctuation characteristics measurement techniques mainly include contact and non-contact methods. The liquid film outside a droplet-flow horizontal tube is thin, and contact methods such as capacitance probes and conductivity probes disturb the surface fluctuations, making them unsuitable for measuring the liquid film outside a droplet-flow horizontal tube. Non-contact methods such as fluorescence methods, laser interferometry, and particle image velocimeters can directly measure the fluctuation characteristics of a fixed-frequency sinusoidal wave or isolated wave formed on the liquid film surface by a single-frequency forced disturbance. In actual droplet-flow horizontal tube liquid film fluctuation processes, the liquid film is continuously subjected to multi-frequency fluctuations, and nonlinear interactions exist between fluctuations of different frequencies, causing the experimentally observed fluctuations to exhibit irregularity, complexity, and randomness, making it impossible to analyze and process the random fluctuation data. Developing a system suitable for testing the liquid film fluctuation characteristics of multi-frequency fluctuations and analyzing the fluctuation data has become an urgent problem to be solved.

[0004] Chinese patent CN116106023B discloses a three-dimensional testing device and method for liquid film fluctuation frequency. It utilizes laser to measure the fluctuation frequency of the liquid film along various directions during atomization using centrifugal nozzles, pre-filming nozzles, and direct-fire nozzles. However, this testing method does not address the processing of random fluctuation test data and cannot be applied to the analysis of random fluctuation data of droplet-flow liquid films.

[0005] Chinese patent CN105300504A proposes a method for measuring the frequency of isolated waves in a liquid film falling on a flat wall. It uses a high-speed camera to capture real-time images of the liquid film flow on a vertical flat plate and obtains the frequency of the isolated wave by reading the grayscale level of the image. This test method is designed for specific isolated waves in liquid film fluctuations and is not suitable for measuring the wave frequency of multi-frequency natural fluctuations.

[0006] Chinese patent CN102175130B discloses a device and method for real-time measurement of the thickness of a gas-containing liquid film with interfacial fluctuations. It uses a conductivity probe to measure the thickness of the gas-containing liquid film with fluctuations inside the tube. The conductivity probe is in contact with the liquid film and is not suitable for measuring the thickness of a thin liquid film, such as a droplet-like liquid film.

[0007] The graduate thesis of Lanzhou University of Technology, entitled "Laser Interferometric Measurement and Numerical Study on Dynamic Characteristics of Condensate Film Thickness in Pipes", uses the laser interferometric method to measure the average thickness of the liquid film, but does not involve the measurement of the liquid film fluctuation characteristics.

[0008] The paper "Experimental Study on Thickness and Fluctuation of Thin Liquid Film in Circular Flow in Microchannel" published in Volume 75, Issue 11 of the Journal of Chemical Industry and Engineering (2024) used the laser-induced fluorescence particle method to measure the number of fluctuations within a given time period. The focus was on the trend of the liquid film fluctuation frequency rather than the magnitude of the fluctuation frequency.

[0009] The graduate thesis of Dalian University of Technology, entitled "Experimental Study on Flow and Heat Transfer Characteristics of Fluctuating Condensate Film", measures the thickness of the liquid film on the substrate of condensate inside the tube based on optical total internal reflection technology and measures the wave frequency, wave velocity and wavelength of the fluctuating liquid film based on LED / phototransistor optical testing technology. The experimentally measured film thickness is the average value of the substrate condensate film thickness rather than the instantaneous value of the film thickness. The method of measuring the fluctuating characteristics inside the tube cannot be used to measure the fluctuating liquid film outside the opaque tube. The measured liquid film wave frequency is simplified to the average value of the wave frequency. There is a lack of data processing method to obtain multi-frequency fluctuating characteristics based on the instantaneous liquid film thickness.

[0010] The problems with the test method for the external liquid film fluctuation characteristics of droplet flow in a horizontal tube are as follows: (1) Since the liquid film outside the horizontal tube of droplet flow is thin, non-contact measurement must be used to avoid disturbing the surface fluctuation of the liquid film. Contact measurement techniques such as capacitance probe method and conductivity probe method are not suitable for measuring the fluctuation characteristics of the liquid film outside the horizontal tube of droplet flow.

[0011] (2) Due to the irregularity, complexity and randomness of the multi-frequency fluctuations of the liquid film outside the horizontal tube of the droplet flow observed in the experiment, it is impossible to analyze and process the random fluctuation data of the liquid film outside the horizontal tube of the actual droplet flow. Therefore, the existing liquid film fluctuation characteristic test methods are mostly for measuring the fluctuation characteristics of the liquid film with periodic single-frequency forced disturbance, or for simplifying the actual multi-frequency fluctuation characteristic measurement data. There is a lack of liquid film fluctuation characteristic test methods and data processing methods suitable for multi-frequency fluctuations. Summary of the Invention

[0012] To overcome the shortcomings of existing technologies, this invention proposes a method and system for testing the wave characteristics of the liquid film outside a droplet-flow horizontal tube. It employs non-contact visual optical testing technology to measure the wave data such as film thickness, wavelength, wave velocity, and wave frequency of the liquid film outside the actual droplet-flow horizontal tube. A method for decomposing and processing multi-frequency wave experimental data is established to decompose random and complex wave data into a combination of single-frequency wave data of different frequencies.

[0013] To achieve the above objectives, the present invention adopts the following technical solution: a method for testing the fluctuation characteristics of the external liquid film of a droplet-like flow horizontal tube, comprising the following steps: S1. Adjust the adjustable LED mounting plate along the circumference and length of the horizontal heat transfer tube, with the two LED light sources facing the position of the droplet-shaped liquid film to be measured. S2. The diameter of the reflected aperture formed by the reflection of light from the LED light source on the surface of the droplet-shaped liquid film is recorded by a CCD high-speed camera. The reflected aperture diameter image recorded by the S3 and CCD high-speed camera is processed by a computer to amplify the tiny changes in liquid film thickness into a reflected aperture diameter signal. The relationship between the reflected aperture diameter and the reflection path length is used to obtain the time-domain fluctuating liquid film thickness outside the droplet-like flow horizontal tube and its corresponding frequency number within the test time. S4. Use Fourier transform to convert the time-domain fluctuating liquid film thickness into the frequency-domain fluctuating liquid film thickness; S5. Establish two LED light sources facing the cross-correlation function of the time-domain fluctuating liquid film thickness at different times. Calculate the wave velocity and wavelength of the fluctuating liquid film and its corresponding frequency number within the test time based on the delay time between the peak values ​​of the cross-correlation function. S6. Based on the frequency of different wave velocities, the complex and random droplet-like horizontal pipe external liquid film fluctuations are decomposed into multiple single-frequency fluctuation combinations, and the weight values ​​corresponding to each fluctuation and the fluctuation base are calculated according to the frequency of different wave velocities.

[0014] Furthermore, the thickness of the fluctuating liquid film outside the droplet-like horizontal tube is:

[0015] In the formula, The thickness of the fluctuating liquid film; The diameter of the reflecting aperture; This refers to the distance between the LED light source and the outer wall of the horizontal tube. Let be the angle of incidence of light entering the liquid film from air. The angle of reflection of light off the horizontal tube wall. Let be the refractive index of the liquid film. is the refractive index of air.

[0016] Further, in step S4, the time-domain fluctuating liquid film thickness is converted to the frequency-domain fluctuating liquid film thickness using Fourier transform. The conversion formula is as follows:

[0017] In the formula The thickness of the liquid film is a frequency-domain fluctuation. The thickness of the liquid film fluctuates in the time domain; The frequency is the wave frequency.

[0018] Furthermore, in step S5, the cross-correlation function of the time-domain fluctuating liquid film thickness is:

[0019] Wave speed is:

[0020] Wavelength is:

[0021] In the formula, The cross-correlation function of time-domain fluctuating liquid film thickness; The liquid film thickness is the time-domain fluctuation of the first LED light source; The liquid film thickness is a time-domain fluctuation of the second LED light source; For testing time; This refers to the time delay between the peaks of the cross-correlation function; The distance between the first LED light source and the second LED light source; Wave speed; λ is the wavelength.

[0022] A test system for testing the fluctuation characteristics of the external liquid film of a droplet-like flow horizontal tube, which implements the above-mentioned test method; The system includes a horizontal heat transfer tube, a droplet-shaped liquid film, an adjustable LED mounting plate, a first LED light source, a second LED light source, a CCD high-speed camera, and a computer. The adjustable LED mounting plate is installed at both ends of the droplet-shaped liquid film on the outer surface of the horizontal heat transfer tube. The two LED light sources are fixed on the adjustable LED mounting plate. The CCD high-speed camera is positioned opposite the two LED light sources and is connected to the computer.

[0023] Furthermore, the adjustable LED mounting plate is positioned along the circumference and length of the horizontal heat transfer tube.

[0024] The beneficial effects of this invention are as follows: It employs non-contact visual optical measurement technology, based on the difference in the reflection path length of LED light from the fluctuating surface of the droplet-like liquid film, magnifying the minute liquid film thickness into a reflected aperture diameter signal recorded by a CCD high-speed camera. This allows for precise measurement of the temporal and spatial distribution of the fluctuating liquid film thickness outside the horizontal tube, avoiding interference from direct contact measurement techniques on the thin fluctuating liquid film. Based on the experimentally measured time-domain fluctuating liquid film thickness, the transient values ​​of the wave frequency, wave velocity, and wavelength of the fluctuating liquid film, along with the corresponding frequency counts within the test time, are calculated using Fourier transform and cross-correlation function methods. This achieves precise measurement of the fluctuating characteristics of the liquid film outside the tube along the three-dimensional directions of the circumference and tube length. Furthermore, the Gaussian function method couples the liquid film fluctuating characteristic data with the corresponding frequency counts within the test time. Based on the peak shape of the wave velocity frequency, the fluctuation is sequentially decomposed into combinations of single-frequency disturbances with different weight values. This solves the problem of difficulty in processing experimental data on complex and random liquid film fluctuating characteristics, facilitating the analysis of the true mechanism and reasons for enhanced heat transfer in droplet-like liquid films under actual fluctuating conditions, and providing a theoretical basis for enhanced heat transfer technology in horizontal tubes using droplet-like liquid films. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a test system for the fluctuation characteristics of an external liquid film in a horizontal tube with droplet flow.

[0026] Figure 2 This is a waveform frequency distribution diagram of wave velocity measured at a certain location in a undulating liquid film.

[0027] Among them, (a) is a distribution diagram of the frequency of different wave velocities measured at a certain location of the undulating liquid film; (b) is a diagram of the base wave velocity with the highest frequency and the lowest wave velocity; (c) is a diagram of medium amplitude wave velocity with intermediate frequency and wave velocity; and (d) is a diagram of large amplitude wave velocity with the lowest frequency and the highest wave velocity.

[0028] In the diagram: 1. Horizontal heat transfer tube, 2. Droplet-shaped liquid film, 3. Adjustable LED mounting plate, 4. First LED light source, 4a. Second LED light source, 5. CCD high-speed camera, 6. Computer. Detailed Implementation

[0029] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] Figure 1A test system for testing the fluctuation characteristics of the external liquid film of a droplet-like flow horizontal tube is shown. The system includes: a horizontal heat transfer tube 1, a droplet-like flow liquid film 2, an adjustable LED mounting plate 3, a first LED light source 4, a second LED light source 4a, a CCD high-speed camera 5, and a computer 6. The adjustable LED mounting plate 3 is installed at both ends of the droplet-like flow liquid film 2 on the outer surface of the horizontal heat transfer tube 1. The first LED light source 4 and the second LED light source 4a are fixed on the adjustable LED mounting plate 3. The position of the adjustable LED mounting plate 3 is adjusted along the circumference and length of the horizontal heat transfer tube 1. The instantaneous three-dimensional distribution of the liquid film thickness of the droplet-like flow liquid film 2 is measured by adjusting the position of the LED light source. The CCD high-speed camera 5 is directly facing the LED light source and is connected to the computer 6.

[0031] The testing method for this testing system adopts the following steps: S1. Adjust the adjustable LED mounting plate 3 along the circumference and length of the horizontal heat transfer tube 1, with the two LED light sources facing the position of the droplet-shaped liquid film 2 to be measured. S2. The fluctuations on the surface of the droplet-shaped liquid film 2 outside the horizontal heat transfer tube 1 cause the liquid film thickness to exhibit irregular changes. The diameter of the reflected aperture formed by the light from the LED light source reflected on the surface of the droplet-shaped liquid film 2 is recorded by the CCD high-speed camera 5. The reflected aperture diameter image recorded by the S3 and CCD high-speed camera 5 is processed by the computer 6 to amplify the minute changes in liquid film thickness into a reflected aperture diameter signal. The relationship between the reflected aperture diameter and the reflection path length is used to obtain the time-domain fluctuating liquid film thickness outside the droplet-like flow horizontal tube and its corresponding frequency number within the test time.

[0032] Thickness of the fluctuating liquid film outside the horizontal tube for droplet flow:

[0033] In the formula, For the thickness of the fluctuating liquid film, ; The diameter of the reflecting aperture. ; The distance between the LED light source and the outer wall of the horizontal tube. ; Let be the angle of incidence of light entering the liquid film from air. The angle of reflection of light off the horizontal tube wall. Let be the refractive index of the liquid film. is the refractive index of air.

[0034] S4. The time-domain fluctuating liquid film thickness is converted to the frequency-domain fluctuating liquid film thickness using Fourier transform to obtain the wave frequency of different fluctuating liquid film thicknesses. The conversion formula is as follows:

[0035] In the formula For frequency domain fluctuations, the liquid film thickness is... ; For time-domain fluctuations in liquid film thickness, f is the wave frequency, in Hz.

[0036] S5. Establish the cross-correlation function between the thickness of the fluctuating liquid film at different times when two LED light sources are facing each other. Calculate the wave velocity and wavelength of the fluctuating liquid film and its corresponding frequency number within the test time based on the delay time between the peak values ​​of the cross-correlation function.

[0037] Cross-correlation function of time-domain fluctuating liquid film thickness:

[0038] Wave speed is:

[0039] Wavelength is:

[0040] In the formula The cross-correlation function of time-domain fluctuating liquid film thickness; The thickness of the liquid film in the 4-time domain fluctuation of the first LED light source. ; For the second LED light source, the liquid film thickness fluctuates in the 4a domain. T represents the test time, in seconds. The time delay between the peaks of the cross-correlation function, in seconds; The distance between the first LED light source 4 and the second LED light source 4a. v is the wave speed. ; λ is the wavelength, m.

[0041] S6. Based on the frequency of different wave velocities, the complex and random fluctuations of the liquid film outside the horizontal tube in the droplet flow are decomposed into multiple combinations of single-frequency fluctuations. Figure 2 Figure (a) shows the distribution of different wave velocities at a certain location in the undulating liquid film. The irregular distribution in the figure can be decomposed into three waves based on the peak shape of the wave velocity frequencies: Figure 2 (b) represents the base wave velocity, which has the highest frequency and lowest velocity. Figure 2 (c) represents medium amplitude wave speeds with moderate frequency and speed. Figure 2 In the middle (d), the wave velocity with the lowest frequency and the highest wave velocity is the large amplitude wave velocity. The weight values ​​corresponding to each wave and wave base are obtained according to the frequency of occurrence of different wave velocities.

[0042] The shear pressure drop calculated based on different wave velocities after decomposition was compared with the experimental values. The pressure drop at the wave base was 14.2 Pa, with a weight of 58% based on the frequency of wave occurrence. The pressure drop and weight for medium-amplitude waves were 25.2 Pa and 32%, respectively, while the pressure drop and weight for large-amplitude waves were 41.9 Pa and 10%, respectively. The weighted average pressure drop was 20.5 Pa, while the experimentally measured pressure drop was 24.1 Pa. The pressure drop calculated using the average was 13.5 Pa. The comparative analysis shows that the calculated pressure drop after decomposition significantly improved the agreement with the measured value compared to the data before decomposition, demonstrating the good rationality and accuracy of this data processing technique.

[0043] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for testing the fluctuation characteristics of the liquid film outside a horizontal tube in a droplet-like flow, characterized in that, Includes the following steps: S1. Adjust the adjustable LED mounting plate along the circumference and length of the horizontal heat transfer tube, with the two LED light sources facing the position of the droplet-shaped liquid film to be measured. S2. The diameter of the reflected aperture formed by the reflection of light from the LED light source on the surface of the droplet-shaped liquid film is recorded by a CCD high-speed camera; The reflected aperture diameter image recorded by the S3 and CCD high-speed camera is processed by a computer to amplify the tiny changes in liquid film thickness into a reflected aperture diameter signal. The relationship between the reflected aperture diameter and the reflection path length is used to obtain the time-domain fluctuation liquid film thickness outside the droplet-like flow horizontal tube and its corresponding frequency number within the test time. S4. Use Fourier transform to convert the time-domain fluctuating liquid film thickness into the frequency-domain fluctuating liquid film thickness; S5. Establish the cross-correlation function of the time-domain fluctuating liquid film thickness at different times when the two LED light sources are facing each other. Calculate the wave velocity and wavelength of the fluctuating liquid film and its corresponding frequency number within the test time based on the delay time between the peak values ​​of the cross-correlation function. S6. Based on the frequency of different wave velocities, the complex and random droplet-like horizontal pipe external liquid film fluctuations are decomposed into multiple single-frequency fluctuation combinations, and the weight values ​​corresponding to each fluctuation and the fluctuation base are calculated according to the frequency of different wave velocities.

2. The method for testing the fluctuation characteristics of the external liquid film of a droplet-like flow in a horizontal tube according to claim 1, characterized in that, The thickness of the fluctuating liquid film outside the droplet-like horizontal tube: ； ； In the formula, The thickness of the fluctuating liquid film; The diameter of the reflecting aperture; This refers to the distance between the LED light source and the outer wall of the horizontal tube. Let be the angle of incidence of light entering the liquid film from air. The angle of reflection of light off the horizontal tube wall. Let be the refractive index of the liquid film. is the refractive index of air.

3. The method for testing the fluctuation characteristics of the external liquid film in a horizontal tube with a droplet-like flow according to claim 2, characterized in that, In step S4, the time-domain fluctuating liquid film thickness is converted to the frequency-domain fluctuating liquid film thickness using Fourier transform. The conversion formula is as follows: ； In the formula, The thickness of the liquid film is a frequency-domain fluctuation. denoted as the time-domain fluctuating liquid film thickness; f is the wave frequency.

4. The method for testing the fluctuation characteristics of the external liquid film of a droplet-like flow in a horizontal tube according to claim 3, characterized in that, In step S5, the cross-correlation function for the time-domain fluctuating liquid film thickness is: ； Wave speed is: ； Wavelength is: ； In the formula, The cross-correlation function of time-domain fluctuating liquid film thickness; The liquid film thickness is the time-domain fluctuation of the first LED light source; The liquid film thickness is a time-domain fluctuation of the second LED light source; For testing time; This refers to the time delay between the peaks of the cross-correlation function; The distance between the first LED light source and the second LED light source; Wave speed; λ is the wavelength.

5. A test system for the fluctuation characteristics of the external liquid film of a droplet-like flow horizontal tube, characterized in that: This system implements the test method described in any one of claims 1-4; The system includes a horizontal heat transfer tube, a droplet-shaped liquid film, an adjustable LED mounting plate, a first LED light source, a second LED light source, a CCD high-speed camera, and a computer. The adjustable LED mounting plate is installed at both ends of the droplet-shaped liquid film on the outer surface of the horizontal heat transfer tube. The two LED light sources are fixed on the adjustable LED mounting plate. The CCD high-speed camera is positioned opposite the two LED light sources and is connected to the computer.

6. The test system for the fluctuation characteristics of the external liquid film of a droplet-like flow horizontal tube according to claim 5, characterized in that: The adjustable LED mounting plate is positioned along the circumference and length of the horizontal heat transfer tube.

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