A method and system for testing the fluctuation characteristics of a liquid film outside a horizontal pipe in dropwise flow

Abstract

A kind of drop flow horizontal tube outside liquid film fluctuation characteristic test method and system belong to drop flow fluctuation characteristic test technical field.The system includes horizontal heat pipe,drop flow liquid film,adjustable LED mounting plate,LED light source,CCD high-speed camera and computer;The outer edge of drop flow liquid film outside horizontal heat pipe is equipped with adjustable LED mounting plate,Two LED light sources are fixed on adjustable LED mounting plate,CCD high-speed camera is opposite to two LED light sources,CCD high-speed camera is connected with computer.Non-contact visual optical testing technology and scientific data processing method are used,which avoids the interference of direct contact measurement technology to thin fluctuation liquid film,realizes the accurate measurement of liquid film fluctuation characteristic along the three-dimensional direction of circumference and pipe length, solves the problem that experimental data of complex and random liquid film fluctuation characteristic is difficult to process,at the same time,It is beneficial to analyze the real mechanism and reason of drop flow liquid film heat transfer enhancement under actual fluctuation condition.

Description

TECHNICAL FIELD

[0001] The application relates to a kind of drop flow horizontal tube outside liquid film wave characteristic test method and system, belong to drop flow wave characteristic test technical field. BACKGROUND

[0002] Drop flow falling film evaporation has the advantages of small heat transfer temperature difference, high heat flux, high heat transfer coefficient, simple structure and low power consumption, and is widely used in petrochemical, refrigeration, nuclear engineering, aerospace, electronic chip heat dissipation and other fields. Under the periodic impact of liquid drops, the disturbance breaks the balance between gravity, viscous force, shear force and surface tension, and the instability of the liquid film surface forms a wave that significantly affects the velocity, temperature and pressure distribution in the liquid film outside the horizontal tube. The liquid film wave characteristic is closely related to the drop flow heat transfer process, so testing the liquid film wave characteristic outside the horizontal tube of drop flow is of great significance to the enhancement of drop flow heat transfer technology outside the horizontal tube.

[0003] Liquid film wave characteristic measurement techniques mainly include contact and non-contact methods. The liquid film outside the horizontal tube of drop flow is thin, and contact measurement techniques such as capacitance probe method and conductivity probe method will disturb the liquid film surface wave, so they are not suitable for measuring the liquid film outside the horizontal tube of drop flow. Non-contact measurement techniques such as fluorescence method, laser interference method and particle image velocimetry can directly measure the wave characteristics of single-frequency forced disturbance forming fixed-frequency sine wave or solitary wave on the liquid film surface. In the actual drop flow horizontal tube outside liquid film wave process, the liquid film is continuously subjected to multi-frequency wave, and there is nonlinear interaction between different frequency waves, causing the observed wave to be irregular, complex and random. Random wave data cannot be analyzed and processed. Developing a liquid film wave characteristic test method suitable for multi-frequency wave and a system for analyzing wave data has become a difficult problem to be solved.

[0004] Chinese patent CN116106023B proposes a three-dimensional test device and test method for liquid film wave frequency, which uses laser to measure the wave frequency of liquid film in various directions during the atomization process of centrifugal nozzle, pre-film nozzle and straight jet nozzle. This test method does not involve processing methods for multi-frequency wave random test data, and cannot be applied to analysis of drop flow liquid film random wave data.

[0005] Chinese patent CN105300504A proposes a method for measuring isolated wave frequency of flat wall falling liquid film, which uses a high-speed camera to take real-time pictures of liquid film flow on a vertical flat plate, and obtains isolated wave frequency by reading picture gray level. This test method is aimed at specific isolated waves in liquid film wave, and is not suitable for wave frequency measurement of multi-frequency natural wave.

[0006] The Chinese patent with publication number CN102175130B proposes a device and method for real-time measurement of interface wave fluctuation gas-liquid film thickness. The electric conductivity probe is used to measure the wave fluctuation gas-liquid film thickness in the pipe. The electric conductivity probe contacting the liquid film is not suitable for measuring the drop flow liquid film of thin liquid film.

[0007] The laser interference method is used to measure the average thickness of the liquid film in the postgraduate thesis "Laser Interference Measurement and Numerical Study of Dynamic Characteristics of Condensation Liquid Film Thickness in Tube" of Lanzhou University of Technology. The measurement of the wave fluctuation characteristics of the liquid film is not involved.

[0008] The laser-induced fluorescence particle method is used to measure the number of fluctuations occurring within a given time in "Experimental Study on Thin Liquid Film Thickness and Fluctuation of Annular Flow in Microchannels" in the 75th volume of the 11th issue of the Journal of Chemical Engineering in 2024. The focus is on the trend of the liquid film fluctuation frequency rather than the magnitude of the fluctuation frequency.

[0009] The postgraduate thesis "Experimental Study on Flow and Heat Transfer Characteristics of Fluctuation Condensation Liquid Film" of Dalian University of Technology measures the thickness of the condensation liquid film at the bottom of the tube based on the optical total reflection technology and measures the wave frequency, wave speed and wave length of the fluctuation liquid film based on the LED / phototransistor optical testing technology. The film thickness measured in the experiment is the average value of the thickness of the condensation liquid film at the bottom rather than the instantaneous value of the film thickness. The method of measuring the fluctuation characteristics in the tube cannot be used for the fluctuation measurement of the liquid film outside the opaque tube. The measured liquid film wave frequency is simplified as the average value of the wave frequency, and there is a lack of data processing method for obtaining the multi-frequency fluctuation characteristics based on the instantaneous liquid film thickness.

[0010] The problems of the test method for the fluctuation characteristics of the liquid film outside the horizontal tube of drop flow are as follows:

[0011] (1) Since the liquid film outside the horizontal tube of drop flow is thin, a non-contact method must be used for measurement to avoid disturbing the surface fluctuation of the liquid film. The contact measurement technology such as the capacitance probe method and the electric conductivity probe method is not suitable for the measurement of the fluctuation characteristics of the liquid film outside the horizontal tube of drop flow.

[0012] (2) Due to the irregularity, complexity and randomness of the multi-frequency fluctuation of the liquid film outside the horizontal tube of drop flow observed in the experiment, the random fluctuation data of the actual liquid film outside the horizontal tube of drop flow cannot be analyzed and processed. Therefore, the existing liquid film fluctuation characteristic test methods are mostly for measuring the fluctuation characteristics of the periodically single-frequency forced disturbance liquid film, or for simplifying the measurement data of the actual multi-frequency fluctuation characteristics. There is a lack of liquid film fluctuation characteristic test method and data processing method suitable for multi-frequency fluctuation. SUMMARY

[0013] 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.

[0014] 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:

[0015] 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.

[0016] 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.

[0017] 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.

[0018] S4. Use Fourier transform to convert the time-domain fluctuating liquid film thickness into the frequency-domain fluctuating liquid film thickness;

[0019] 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.

[0020] 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.

[0021] Furthermore, the thickness of the fluctuating liquid film outside the droplet-like horizontal tube is:

[0022]

[0023]

[0024] 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.

[0025] 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:

[0026]

[0027] 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; It is the wave frequency.

[0028] Furthermore, in step S5, the cross-correlation function of the time-domain fluctuating liquid film thickness is:

[0029]

[0030] Wave speed is:

[0031]

[0032] Wavelength is:

[0033]

[0034] 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.

[0035] 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;

[0036] 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.

[0037] Furthermore, the adjustable LED mounting plate is positioned along the circumference and length of the horizontal heat transfer tube.

[0038] The beneficial effects of this invention are as follows: It employs non-contact visual optical measurement technology, leveraging the difference in the reflection path length of LED light from the fluctuating surface of the droplet-like liquid film. This magnifies the minute liquid film thickness into a reflected aperture diameter signal recorded by a CCD high-speed camera, accurately measuring 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

[0039] 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.

[0040] Figure 2 This is a waveform frequency distribution diagram of wave velocity measured at a certain location in a undulating liquid film.

[0041] 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.

[0042] 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

[0043] 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.

[0044] 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.

[0045] The testing method for this testing system adopts the following steps:

[0046] 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.

[0047] 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.

[0048] 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.

[0049] Thickness of the fluctuating liquid film outside the horizontal tube for droplet flow:

[0050]

[0051] 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.

[0052] 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:

[0053]

[0054] In the formula, For frequency domain fluctuations, the liquid film thickness is... ; For time-domain fluctuations in liquid film thickness, ; The frequency is expressed in Hz.

[0055] 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.

[0056] Cross-correlation function of time-domain fluctuating liquid film thickness:

[0057]

[0058] Wave speed is:

[0059]

[0060] Wavelength is:

[0061]

[0062] 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. ; 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. ; For wave speed, ; For wavelength, .

[0063] 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.

[0064] 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.

[0065] 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. 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. The refractive index of air; 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. The thickness of the liquid film fluctuates in the time domain; For wave frequency; 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.

2. A test system for the fluctuation characteristics of an external liquid film in a horizontal tube with droplet-like flow, characterized in that: This system implements the test method described in claim 1; 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.

3. The test system for the fluctuation characteristics of the external liquid film of a droplet-like flow horizontal tube according to claim 2, 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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