Gas-liquid two-phase flow far-field droplet collision frequency and behavior prediction method

Abstract

The invention relates to the technical field of gas-liquid two-phase flow, and discloses a gas-liquid two-phase flow far-field liquid drop collision frequency and behavior prediction method, which comprises the following steps of: establishing a PDA (Personal Digital Assistant) system for air-assisted spray measurement, and measuring the diameter and the speed of a liquid drop in a spray field in real time; establishing a simplified model of liquid drop collision, and analyzing the influence of the ratio of the liquid drop property to the collision liquid drop size on the transition Weibull number to obtain a critical value after liquid drop collision; calculating the collision Weber number between different liquid drops by using the diameter of the liquid drops and the speed of the liquid drops measured in real time; comparing the calculated collision Weber number with a critical value, and carrying out statistical analysis on the number of liquid drops of different collision results to obtain frequencies of different collision results at different measurement volume positions; according to the method, the quantification of the front collision result probability of the liquid drops is realized by utilizing a PDA system for air-assisted spray measurement and a simplified model of liquid drop collision.

Description

Technical Field

[0001] The present invention relates to the technical field of gas-liquid two-phase flow, and more particularly to a method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow. Background Art

[0002] Gas-liquid two-phase jets exist in many natural, chemical and industrial processes, such as rain formation, wastewater treatment and spray atomization in combustion chambers. Among them, two-fluid atomization is an important category of gas-liquid two-phase jets. The spray produced by this atomization method essentially involves the randomness of the turbulence of the carrier phase (usually gas) and the polydispersity of the distribution of the dispersed phase (usually liquid), which is a relatively complex multiphase flow problem. As a two-fluid atomization method, air-assisted spraying is characterized by the high-speed gas jet inside the spray. Under this condition, the external gas phase flow will produce a strong shear effect on the droplets inside the spray, and the strong turbulence characteristics usually possessed by the high-speed gas jet will also induce turbulent breakup of the spray droplets. In addition, under the action of large-scale turbulent structures, the droplets inside the spray may also frequently collide and merge. At the same time, the presence of droplets will have a certain impact on the flow characteristics of the carrier phase itself, thus showing obvious two-way coupling characteristics. The area where the discrete droplets produced by air-assisted spraying undergo secondary breakup is basically close to the outlet plane of the nozzle. In the area far from the nozzle outlet, droplet collision and fusion are key factors affecting droplet dynamics and average droplet size. Therefore, the characterization of droplet dynamics inside the spray and the quantification of gas phase flow characteristics outside the droplets are the key to deeply understand and reveal the characteristics of air-assisted spray.

[0003] Saha et al. conducted experimental measurements on the far-field droplet collision dynamics of hollow cone swirl sprays and proposed a simplified method to calculate the possibility of fusion after droplet collision. This simplified method only considers droplets of the same size and in the form of head-on collisions; for head-on collisions of droplets, Jiang et al. found that the transition Weber number between different collision results is linearly correlated with the physical parameters of the droplets (the ratio of viscosity coefficient to surface tension coefficient (μ1 / σ1). However, Saha et al. used a single droplet to evaluate the possibility of droplet collision. In terms of expression, the relative velocity and size ratio of multi-scale and multi-dispersed collision droplets in actual sprays were ignored, resulting in an overly idealized and simplified calculation process, which would bring large errors in the actual calculation process.

[0004] Therefore, the current research provides a simple solution to the problem of droplet collision in the spray field. By using a single droplet to evaluate the possibility of droplet collision, the probability of droplet fusion after collision can be roughly estimated. However, this method does not adequately consider the multi-scale and multi-dispersed droplet characteristics in actual sprays, resulting in inaccurate estimates of the Weber number, and does not consider the relative velocity between droplets and the size of the droplet size ratio during the actual collision process. In addition, it is unclear how the basic theoretical research on droplet collision can be applied to the application research of air-assisted spray systems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for predicting the frequency and behavior of droplet collisions in the far field of gas-liquid two-phase flow, which utilizes a PDA system for air-assisted spray measurement and a simplified model of droplet collisions to achieve the quantification of the probability of droplet head-on collision results.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for predicting the frequency and behavior of droplet collisions in a far-field gas-liquid two-phase flow, comprising the following steps:

[0008] Establish a PDA system for air-assisted spray measurement to measure the diameter and velocity of droplets in the spray field in real time;

[0009] A simplified model of droplet collision is established to analyze the effects of droplet properties and the size ratio of colliding droplets on the transition Weber number, and the critical value after droplet collision is obtained;

[0010] The Weber number of collisions between different droplets is calculated using the diameter and velocity of the droplets measured in real time;

[0011] The calculated collision Weber number is compared with the critical value, and the number of droplets with different collision results is statistically analyzed to obtain the frequency of different collision results at different measurement volume positions.

[0012] Furthermore, the PDA system for air-assisted spray measurement includes: a multi-line laser, an optical detector and a signal processor; the optical detector includes: a transmitter, a lens and a receiving probe;

[0013] The multi-line laser is used to generate a horizontally polarized light beam;

[0014] The horizontally polarized light beam is transmitted to the transmitter through an optical fiber and symmetrically intersects through a lens with a focal length of 310 mm. The intersecting light beams form an ellipsoidal measuring body with an axial length of 76×76×630 μm. When the droplets in the spray pass through the ellipsoidal measuring body, the laser beam is scattered by the droplets to form a scattered light signal. The receiving probe is placed at an angle of 70° to the emission axis of the multi-line laser.

[0015] The receiving probe is used to convert the scattered light signal into an electrical signal and transmit it to the storage machine;

[0016] The signal processor is used to filter, process, analyze and record the electrical signals in the storage unit.

[0017] Furthermore, the horizontally polarized light beam includes: two laser beams with a wavelength of λ=514.5 nm and two laser beams with a wavelength of λ=488 nm.

[0018] Further, the two laser beams with a wavelength of λ=514.5 nm are green laser beams, which are used to measure the axial velocity and diameter of the droplet;

[0019] The two laser beams with a wavelength of λ=488 nm are blue laser beams, which are used to measure the radial or tangential velocity of the droplet.

[0020] Furthermore, the influence of the analysis droplet properties and the collision droplet size ratio on the transition Weber number is expressed as:

[0021] We3=β×Oh+γ

[0022] Wherein, We3 is the transition Weber number of the third and fourth collision results of the droplet;

[0023] Where β and γ are given by the following formulas respectively

[0024]

[0025]

[0026] In the above formula, and is a dimensionless geometric parameter related to droplet deformation; among them, It is the ratio of the maximum radius to the minimum radius of the droplet, reflecting the flatness of the droplet; is the ratio of the height of the droplet to its maximum radius, indicating the aspect ratio of the droplet; is the ratio of the droplet bottom width to the maximum radius, which characterizes the expansion of the droplet bottom; is the ratio of the droplet top width to the maximum radius, indicating the shrinkage of the droplet top; It is the ratio of the radius of curvature of the side of the droplet to the maximum radius, indicating the degree of curvature of the side of the droplet; is the ratio of the radius of curvature of the front end of the droplet to the maximum radius, indicating the sharpness of the front end of the droplet; S0 and Sf represent the initial and final surface areas of the droplet, respectively; α is the defined viscous loss coefficient; Oh is the Ohnesorg number; Δ cot is the collision droplet size ratio.

[0027] Furthermore, the Weber number of collisions between different droplets is calculated and expressed as:

[0028]

[0029] In the above formula, B is the droplet collision influence parameter; We c is the collision Weber number; χ is the distance between the centers of the colliding droplets at the droplet relative velocity U r The projection in the direction of ; ρ1 is the droplet density; U r is the axial instantaneous velocity at the radial measurement position; σ is the surface tension coefficient; D i and D s Represent the diameters of large and small droplets, respectively.

[0030] Further, the calculated collision Weber number is compared with the critical value, and the number of droplets of different collision results is statistically analyzed; wherein the calculated collision Weber number is compared with the critical value, including: comparing the diameter ratio of the colliding droplets with the critical value;

[0031] Among them, different collision results include: fusion, separation or fragmentation between different droplets, specifically:

[0032] The diameter ratio of the colliding droplets is compared with the critical value. When the diameter ratio of the colliding droplets exceeds the critical value, it is considered to be fused;

[0033] When the diameter ratio of the colliding droplets does not exceed the critical value, the droplets are considered to be separated.

[0034] Furthermore, statistical analysis is performed on the number of droplets of different collision results, including: calculating the probability of collision and fusion between different droplets, expressed as:

[0035]

[0036] In the above formula, N1 is the number of droplets whose collision Weber number is less than We1; N 2-3 is the number of droplets between We2 and We3; N s is the total number of sampled droplets; where We2 is the transitional Weber number between the collision droplet size ratio and the second collision result and the third collision result; We3 is the transitional Weber number between the collision droplet size ratio and the third collision result and the fourth collision result.

[0037] Furthermore, the number of droplets of the different collision results is statistically analyzed to obtain the frequency of different collision results at different measurement volume positions; wherein the frequency of different collision results at different measurement volume positions is expressed as:

[0038]

[0039] Where N1 is the number of droplet collision results, N s is the number of droplets resulting from all collisions.

[0040] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0041] (1) The present invention constructs a PDA system dedicated to air-assisted spray measurement, which not only realizes the synchronous measurement of gas and liquid flow field characteristics, but also provides important theoretical support for in-depth understanding of the spray atomization mechanism, and also helps to further improve the atomization efficiency of dual-fluid spray;

[0042] (2) The present invention combines the droplet energy balance principle and the scale analysis technology to achieve a quantitative analysis of the probability of a droplet head-on collision result;

[0043] (3) The present invention also quantifies the probability of droplet collision away from the nozzle outlet based on the droplet data collected by the PDA system and the relative velocity of the droplets. In addition, the present invention also deeply considers the influence of the droplet size ratio on the collision result, and constructs a theoretical model of the droplet size ratio, and applies it to the analysis of the probability of droplet collision results in the air-assisted spray system, realizing the accurate quantification of the probability of droplet head-on collision. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0045] The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to the present invention is further described below in conjunction with the accompanying drawings;

[0046] Figure 1 It is a schematic diagram of the overall process of the method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow provided by the present invention;

[0047] Figure 2 It is a simplified model diagram of droplet collision in the method for predicting the frequency and behavior of droplet collision in the far field of gas-liquid two-phase flow provided by the present invention;

[0048] Figure 3 It is the spray droplet collision result in the gas-liquid two-phase flow far-field droplet collision frequency and behavior prediction method provided by the present invention; wherein, (a)-(d) are the collision results of n-octane at different positions, and (e)-(h) are the collision results of n-dodecane at different positions;

[0049] Figure 4 It is a probability distribution diagram of the spray droplet collision result in the gas-liquid two-phase flow far-field droplet collision frequency and behavior prediction method provided by the present invention; wherein (a) is a probability distribution diagram of the collision result of n-octane, and (b) is a probability distribution diagram of the collision result of n-dodecane;

[0050] Figure 5 It is a schematic diagram of the results of hydrocarbon fuel droplet collisions under standard atmospheric pressure in the gas-liquid two-phase flow far-field droplet collision frequency and behavior prediction method provided by the present invention. DETAILED DESCRIPTION

[0051] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0052] In order to better understand the purpose, structure and function of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings.

[0053] like Figure 1 As shown, the present invention proposes a method for predicting the frequency and behavior of droplet collisions in a far-field of a gas-liquid two-phase flow, comprising the following steps:

[0054] Establish a PDA system for air-assisted spray measurement to measure the diameter and velocity of droplets in the spray field in real time;

[0055] A simplified model of droplet collision is established to analyze the effects of droplet properties and the size ratio of colliding droplets on the transition Weber number, and the critical value after droplet collision is obtained;

[0056] The Weber number of collisions between different droplets is calculated using the diameter and velocity of the droplets measured in real time;

[0057] The calculated collision Weber number is compared with the critical value, and the number of droplets with different collision results is statistically analyzed to obtain the frequency of different collision results at different measurement volume positions.

[0058] The PDA system for air-assisted spray measurement includes: a multi-line laser, an optical detector and a signal processor; the optical detector includes: a transmitter, a lens and a receiving probe;

[0059] The multi-line laser is used to generate a horizontal polarized light beam, including: two laser beams with a wavelength of λ=514.5nm and two laser beams with a wavelength of λ==488nm;

[0060] The laser beam is transmitted to the transmitter through an optical fiber and symmetrically intersects through a lens with a focal length of 310 mm. The intersecting beams form an ellipsoidal measuring body with an axial length of 76×76×630 μm. When the droplets in the spray pass through the ellipsoidal measuring body, the laser beam is scattered by the droplets to form a scattered light signal. The receiving probe is placed at an angle of 70° to the emission axis of the multi-line laser.

[0061] The receiving probe is used to convert the scattered light signal into an electrical signal and transmit it to the storage machine;

[0062] The signal processor is used to filter, process, analyze and record the electrical signals in the storage unit.

[0063] The two laser beams with a wavelength of λ=514.5 nm are green laser beams, which are used to measure the axial velocity and diameter of the droplet;

[0064] The two laser beams with a wavelength of λ=488 nm are blue laser beams, which are used to measure the radial or tangential velocity of the droplet.

[0065] The influence of the analysis droplet properties and the collision droplet size ratio on the transition Weber number is expressed as:

[0066] We3=β×Oh+γ

[0067] Among them, β and γ are given by the following formulas:

[0068]

[0069] in, and is a dimensionless geometric parameter related to droplet deformation; among them, It is the ratio of the maximum radius to the minimum radius of the droplet, reflecting the flatness of the droplet; is the ratio of the height of the droplet to its maximum radius, indicating the aspect ratio of the droplet; is the ratio of the droplet bottom width to the maximum radius, which characterizes the expansion of the droplet bottom; is the ratio of the droplet top width to the maximum radius, indicating the shrinkage of the droplet top; It is the ratio of the radius of curvature of the side of the droplet to the maximum radius, indicating the degree of curvature of the side of the droplet; is the ratio of the curvature radius of the front end of the droplet to the maximum radius, indicating the sharpness of the front end of the droplet; S0 and S f represent the initial and final surface areas of the droplet, respectively; α is the defined viscous loss coefficient; Oh is the Ohnesorg number; Δ cot is the collision droplet size ratio; a is the maximum radius of the droplet.

[0070] The Weber number of collisions between different droplets is calculated as:

[0071]

[0072] In the above formula, B is the droplet collision influence parameter; We c is the collision Weber number; χ is the distance between the centers of the colliding droplets at the droplet relative velocity U r The projection in the direction of ; ρ1 is the droplet density; U r is the axial instantaneous velocity at the radial measurement position; σ is the surface tension coefficient; D i and D s Represent the diameters of large and small droplets, respectively.

[0073] In this embodiment, droplets with a diameter less than 5 μm are small droplets, and droplets with a diameter not less than 5 μm are large droplets.

[0074] like Figure 5 As shown in Figure 2, for the head-on collision of a droplet, experimental studies have found that the transition Weber number between different collision results is related to the physical parameters of the droplet (the ratio of the viscosity coefficient to the surface tension coefficient μ l / σ l ) is linearly correlated. The transition Weber number We1 between collision result I and collision result II shows a linear correlation with μ l / σ l The transition Weber number We2 between collision result II and collision result III is found to decrease with μ l / σ l The increase shows an obvious linear increasing trend. In addition, it is found that the transition Weber number We3 between collision result III and collision result IV is not only affected by the properties of the droplet, but also related to the Ohnesorg number of the colliding droplet. According to the measurement results of the critical collision Weber number and the Ohnesorg number obtained from the experimental data, it can be concluded that the relationship between the two is We3 = 30 × Oh + 15. The Ohnesorg number is defined as Oh = 16μ l / (ρ l σR s ) 1 / 2 , where μ l is the droplet dynamic viscosity, R s =D s / 2 represents the radius of the small droplet in the colliding droplet.

[0075] The calculated collision Weber number is compared with the critical value, and the number of droplets of different collision results is statistically analyzed; wherein the calculated collision Weber number is compared with the critical value, including: comparing the diameter ratio of the colliding droplets with the critical value;

[0076] Among them, different collision results include: fusion, separation or fragmentation between different droplets, specifically:

[0077] The diameter ratio of the colliding droplets is compared with the critical value. When the diameter ratio of the colliding droplets exceeds the critical value, it is considered to be fused;

[0078] When the diameter ratio of the colliding droplets does not exceed the critical value, the droplets are considered to be separated.

[0079] The number of droplets of different collision results is statistically analyzed, including: calculating the probability of collision and fusion between different droplets, expressed as:

[0080]

[0081] In the above formula, N1 is the number of droplets whose collision Weber number is less than We1; N 2-3 is the number of droplets between We2 and We3; N s is the total number of sampled droplets.

[0082] According to the collision results of the droplets, the number of droplets with different collision results is statistically analyzed to obtain the frequencies of different collision results at different measurement volume positions; wherein the frequencies of different collision results at different measurement volume positions are expressed as:

[0083]

[0084] Among them, N1 is the number of certain collision results of droplets, N s is the number of droplets resulting from all collisions.

[0085] The present invention further provides the following embodiments:

[0086] The PDA system based on air-assisted spray measurement first measures the droplet information at different positions in the spray field and establishes a simplified model of droplet collision and non-collision. Finally, in order to quantify different collision results, the droplet collision results are analyzed and the probability of different collision results at different measurement volume positions is described. The specific steps include:

[0087] 1. Use PDA to measure the information of droplets in the spray field:

[0088] The PDA system for air-assisted spray measurement is mainly composed of PDA, fuel injection system, constant volume bomb, control and acquisition system. In the experiment, a multi-line laser (Coherent Innova 70C argon ion laser) generates a horizontally polarized beam (output power of 0.8W), which is divided into four beams, including two laser beams with a wavelength of λ = 514.5nm and two laser beams with a wavelength of λ = 488nm. Among them, the green laser beam with a wavelength of 514.5nm is used to measure the axial velocity and diameter of the droplets, while the blue laser beam with a wavelength of 488nm is used to measure the radial or tangential velocity of the droplets. The laser beam is transmitted to the transmitter through an optical fiber and intersects symmetrically through a lens with a focal length of 310mm. The intersecting beams will form an ellipsoidal measuring body with an axial length of 76×76×630μm. The receiver probe is placed at an angle of 70° to the transmitting axis to effectively capture the first-order scattering signal. The spatial position of the measuring body is adjusted by a displacement system, and its movement accuracy can reach 0.1mm.

[0089] During the measurement, the spatial position of the injection system remains fixed. The transmitter and receiver are respectively installed on the two cantilevers of the 3D displacement system, and the computer-controlled displacement system is used to achieve precise positioning of the measurement body, with an achievable displacement resolution of 0.1mm. For PDA measurements under different back pressure environments, it is necessary to carry out them in a dedicated constant volume bomb. The PDA test bomb has three windows, each of which is equipped with high-strength quartz glass. Two of the windows are distributed directly opposite the transmitter probe and the receiver probe.

[0090] When the spray droplets pass through the measuring body, the receiving probe converts the scattered light signal into an electrical signal and transmits it to the storage machine. The data processing software used in parallel with the PDA measurement interface provides powerful data filtering and analysis capabilities. In the experiment, the droplet information is processed, displayed and recorded. The recorded information includes the droplet diameter, velocity and the time when it passes through the measuring body. Preliminary measurement and analysis of the droplet size show that the droplet diameter is mainly distributed within 50μm, and the percentage of droplets larger than this diameter is negligible. In addition, the droplet velocity measurement range is set to -50 to 180m / s. Under each working condition, no less than 20,000 droplets are collected to minimize sample errors to meet statistical calculation requirements. The tolerance for non-spherical droplets of the sampled droplets is set to 5%.

[0091] For the convenience of theoretical description and mathematical calculation, this analysis method satisfies the following basic assumptions:

[0092] (a) The independent droplets captured by PDA are all considered to be standard spheres, and the collision or even fusion between the droplets at the moment of capture is ignored;

[0093] (b) Ignoring the influence of the PDA measurement volume size and the actual droplet path in the measurement volume, it is assumed that droplets of different sizes and speeds pass through the PDA measurement volume in sequence along the same path within their respective measurement times;

[0094] (c) Assuming that droplet collision only occurs between two droplets captured by the PDA in chronological order, a head-on collision will occur when the velocity of the droplet captured at the last moment (represented by U1) is lower than the velocity of the droplet chasing behind (represented by U2). On the other hand, when the velocity of the droplet captured at the last moment is greater than the velocity of the droplet chasing behind, a collision will not occur (here, the secondary collision between the colliding droplets or the possible impact of droplets from other directions on the droplets on the path are uniformly ignored).

[0095] 2. Establish a simplified model of droplet collision

[0096] Based on the above assumptions, a simplified model of droplet collision and non-collision is established, such as Figure 2 In the current simplified collision model analysis method, the head-on collision between droplets of unequal sizes is also considered. The dynamic process of the head-on collision between droplets of unequal sizes is experimentally studied, and the collision droplet size ratio Δ cot =D l / D s and the Weber number of the droplet We c，s A new droplet collision result diagram is constructed in the parameter space of . The results show that the collision droplet size ratio has almost no effect on the transition Weber number We2 between collision result II and collision result III. However, with Δ cot As the droplet size ratio Δ cot The influence of is small, but it is greatly affected by the droplet properties. For We3, the droplet size ratio Δ cot The impact on droplet fusion. Therefore, for each droplet collision evaluation in the current air-assisted spray, the impact of the front and rear droplet size ratio on We3 needs to be considered. Here, the values ​​of We1 and We2 of the tested fuel n-octane are set to 3.84 and 5.76, respectively, while the values ​​of We1 and We2 of the tested fuel n-dodecane are set to 2.40 and 8.64, respectively.

[0097] A theoretical model based on droplet energy balance and scale analysis is proposed to explain the dependence of We3 on the collision droplet size ratio, which is expressed as:

[0098] We3=β×Oh+γ (3)

[0099] Where β and γ are given by the following formulas respectively

[0100]

[0101] Here and is a dimensionless geometric parameter related to droplet deformation; S0 and S f represent the initial and final surface areas of the droplet, respectively; α is a defined viscous loss coefficient.

[0102] The relationship between the geometric parameters and the size ratio was measured experimentally, and these parameters were fitted with a second-order polynomial. Note that in the first stage of the collision process, the dimensionless parameter characterizing the ratio of the large droplet protrusion diameter to the small droplet diameter Will follow Δ col Exceeding a certain range (1<Δ col <4.1) and becomes negative. Therefore, it can be considered that when the diameter ratio of the colliding droplets exceeds the critical value of 4.1, due to the relatively large difference in droplet size, the small droplets are easily wrapped and absorbed by the large droplets during the collision process. Therefore, the present invention uniformly regards the collision results under such conditions as fusion.

[0103] 3. Analysis of droplet collision results

[0104] The droplet diameter D measured by PDA s and speed U r , substitute into formula (1) to calculate We c The We1 and We2 values ​​of the tested fuel n-octane are 3.84 and 5.76 respectively, while the We1 and We2 values ​​of the tested fuel n-dodecane are 2.40 and 8.64 respectively. Figure 5 , compared with We c and the size of We1 and We2, we can get: We c When We1<We, the droplets undergo (I) permanent fusion after slight deformation; c When <We2, the droplet bounces off; We c When >We2, the droplets undergo (III) permanent fusion after large deformation or (IV) separation. Based on formula (3), when the diameter ratio of the colliding droplets exceeds the critical value of 4.1, the collision result under this condition is uniformly regarded as fusion. Therefore, when We c >We2, and when the droplet diameter ratio obtained from the droplet diameter information obtained by PDA sampling exceeds the critical value of 4.1, the droplets undergo permanent fusion after large-scale deformation (III). The rest of the cases are considered to be droplet separation (IV).

[0105] Therefore, the n-octane and n-dodecane spray droplets at different sampling axial positions were placed on the U r and D sThe collision results in the parameter space of were statistically calculated, and the distribution of the collision results of n-octane and n-dodecane spray droplets calculated at different spray axis positions was obtained, as shown in Figure 3 By comparing the distribution of droplet collision results at different axial positions, it can be found that: U r and D s The numerical range of decreases and increases with the increase of x / d0.

[0106] In order to quantify the Figure 3 Different collision results will be attached Figure 3 The number of droplets in the four collision situations was statistically analyzed and the frequencies of different collision results at different measurement volume positions were calculated, as shown in formula (6):

[0107]

[0108] Among them, N l is the number of certain collision results of droplets, N s is the number of droplets resulting from all collisions.

[0109] From this, the frequencies of each type of droplet collision results calculated for different spray axis positions are plotted, as shown in the attached figure. Figure 4 As shown. According to the statistical results, it can be found that the possibility of fusion of droplets with large deformation is the highest, followed by the fusion of droplets with small deformation. The probability of separation after droplet collision and fusion is relatively low. For n-octane spray, at different x / d0 positions, the probability of separation after bounce and fusion is less than 10%, and the change with x / d0 is not obvious. The possibility of droplet collision fusion caused by the larger droplet size ratio is stable at around 20%. However, it can be seen that when x / d0 is less than 12, the large deformation fusion and small deformation fusion of droplets at different axis positions show a relatively stable data trend, and then the probability of fusion decreases and increases respectively. For n-dodecane spray, when x / d0 is less than 8, the probability of all collision results shows a relatively obvious monotonic change trend. It can also be noted that when x / d0 is greater than 12, the probability of large deformation fusion and small deformation fusion of droplets decreases and increases respectively.

[0110] The phenomenological explanation of the above calculation results is mainly based on the consideration of the microscopic droplet velocity and size distribution range of the spray centerline. When the amount of sampled droplet data is relatively large, the results of statistical calculations generally show the overall characteristics of the data without being affected by a single droplet. Therefore, it can be considered that the We c In the calculation of , the relative velocity U of adjacent droplets obtained by sampling r The influence on the overall calculation result can be ignored, and the diameter of the small droplet D s Will mainly determine We cThe actual value of . According to the analysis of the droplet diameter distribution, as x / d0 increases, the droplet diameter tends to be distributed in a smaller diameter range. Therefore, the We c As x / d0 increases, it tends to decrease. Since the transition Weber numbers We1 (between small deformation fusion and bounce) and We2 (between bounce and large deformation fusion) are fixed, under a fixed x / d0, the probability of large deformation fusion in the droplet collision results decreases, while the probability of small deformation fusion increases, such as Figure 3 As shown. In addition, since the transition Weber number range of n-dodecane droplets is larger than that of n-octane, at the same spray centerline position, the proportion of n-dodecane droplets that bounce off is significantly higher than that of n-octane. From the results of generalized droplet collision fusion (i.e., considering the sum of all possible droplet fusion results), it can be found that the probability of n-octane and n-dodecane spray droplet collision fusion results is high.

[0111] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for predicting the frequency and behavior of droplet collisions in the far field of gas-liquid two-phase flow, characterized in that: The following steps are involved: Establish a PDA system for air-assisted spray measurement to measure the diameter and velocity of droplets in the spray field in real time; A simplified model of droplet collision is established to analyze the effects of droplet properties and the size ratio of colliding droplets on the transition Weber number, and the critical value after droplet collision is obtained; The Weber number of collisions between different droplets is calculated using the diameter and velocity of the droplets measured in real time; The calculated collision Weber number is compared with the critical value, and the number of droplets with different collision results is statistically analyzed to obtain the probability of different collision results at different measurement volume positions.

2. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 1 is characterized in that: The PDA system for air-assisted spray measurement includes: a multi-line laser, an optical detector and a signal processor; the optical detector includes: a transmitter, a lens and a receiving probe; The multi-line laser is used to generate a horizontally polarized light beam; The horizontally polarized light beam is transmitted to the transmitter through an optical fiber and symmetrically intersects through a lens with a focal length of 310 mm. The intersecting light beams form an ellipsoidal measuring body with an axial length of 76×76×630 μm. When the droplets in the spray pass through the ellipsoidal measuring body, the laser beam is scattered by the droplets to form a scattered light signal. The receiving probe is placed at an angle of 70° to the emission axis of the multi-line laser. The receiving probe is used to convert the scattered light signal into an electrical signal and transmit it to the storage machine; The signal processor is used to filter, process, analyze and record the electrical signals in the storage unit.

3. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 2 is characterized in that: The horizontally polarized light beams include: two laser beams with a wavelength of λ=514.5nm and two laser beams with a wavelength of λ=488nm.

4. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 3 is characterized in that: The two laser beams with a wavelength of λ=514.5 nm are green laser beams, which are used to measure the axial velocity and diameter of the droplet; The two laser beams with a wavelength of λ=488 nm are blue laser beams, which are used to measure the radial or tangential velocity of the droplet.

5. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 1, characterized in that: The influence of the analysis droplet properties and the collision droplet size ratio on the transition Weber number is expressed as: We3=β×Oh+γ Wherein, We3 is the transition Weber number of the third and fourth collision results of the droplet; Where β and γ are given by the following formulas respectively In the above formula, and is a dimensionless geometric parameter related to droplet deformation; among them, It is the ratio of the maximum radius to the minimum radius of the droplet, reflecting the flatness of the droplet; is the ratio of the height of the droplet to its maximum radius, indicating the aspect ratio of the droplet; is the ratio of the droplet bottom width to the maximum radius, which characterizes the expansion of the droplet bottom; is the ratio of the droplet top width to the maximum radius, indicating the shrinkage of the droplet top; It is the ratio of the radius of curvature of the side of the droplet to the maximum radius, indicating the degree of curvature of the side of the droplet; is the ratio of the curvature radius of the front end of the droplet to the maximum radius, indicating the sharpness of the front end of the droplet; S0 and S f represent the initial and final surface areas of the droplet, respectively; α is the defined viscous loss coefficient; Oh is the Ohnesorg number; Δ cot is the collision droplet size ratio.

6. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 5, characterized in that: The Weber number of collisions between different droplets is calculated as: In the above formula, B is the droplet collision influence parameter; We c is the collision Weber number; χ is the distance between the centers of the colliding droplets at the droplet relative velocity U r The projection in the direction of ; ρ1 is the droplet density; U r is the axial instantaneous velocity at the radial measurement position; σ is the surface tension coefficient; D i and D s Represent the diameters of large and small droplets, respectively.

7. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 6, characterized in that: The calculated collision Weber number is compared with the critical value, and the number of droplets of different collision results is statistically analyzed; wherein the calculated collision Weber number is compared with the critical value, including: comparing the diameter ratio of the colliding droplets with the critical value; Among them, different collision results include: fusion, separation or fragmentation between different droplets, specifically: The diameter ratio of the colliding droplets is compared with the critical value. When the diameter ratio of the colliding droplets exceeds the critical value, it is considered to be fused; When the diameter ratio of the colliding droplets does not exceed the critical value, the droplets are considered to be separated.

8. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 7, characterized in that: The number of droplets of different collision results is statistically analyzed, including: calculating the probability of collision and fusion between different droplets, expressed as: In the above formula, N1 is the number of droplets whose collision Weber number is less than We1; N 2-3 is the number of droplets between We2 and We3; N s is the total number of sampled droplets; where We2 is the transitional Weber number between the collision droplet size ratio and the second collision result and the third collision result; We3 is the transitional Weber number between the collision droplet size ratio and the third collision result and the fourth collision result.

9. The method for predicting the far-field droplet collision frequency and behavior of gas-liquid two-phase flow according to claim 8, characterized in that: The number of droplets of the different collision results is statistically analyzed to obtain the frequency of different collision results at different measurement volume positions; wherein the frequency of different collision results at different measurement volume positions is expressed as: Where N1 is the number of droplet collision results, N s is the number of droplets resulting from all collisions.

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