Method for measuring and predicting gas-phase turbulence characteristics in gas-liquid two-phase jet flow

Abstract

The invention relates to the technical field of gas-liquid two-phase flow, and discloses a gas-phase turbulence characteristic measurement and prediction method in gas-liquid two-phase jet flow, which comprises the following steps: establishing a PDA (Personal Digital Assistant) system for air-assisted spray measurement, and measuring spray characteristic parameters for multiple times under the same working condition, the accuracy and the repeatability of the spraying characteristic parameters in the spraying field are verified; liquid drops with specific sizes in the spray field are selected as tracer liquid drops, the average value of the axial speed of the tracer liquid drops is calculated to be used for evaluating the local gas phase flow speed, and the terminal speed of the tracer liquid drops is calculated; calculating the Stokes number of the tracer liquid drop, comparing the Stokes number with a known flow following characteristic standard, and verifying the flow following characteristic of the tracer liquid drop in a gas phase; according to the gas-liquid two-phase flow field measuring device, the local gas-phase flow velocity is utilized, the characteristics of gas-phase turbulence motion are analyzed, a PDA system is utilized, gas-liquid two-phase flow field information is synchronously measured, and meanwhile the gas-liquid two-phase jet flow turbulence characteristics are measured.

Description

Technical Field

[0001] The invention relates to the technical field of gas-liquid two-phase flow, and more particularly to a method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet. 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 such conditions, the external gas phase flow will produce a strong shear effect on the droplets inside the spray, and the strong turbulent characteristics usually possessed by the high-speed gas jet will also induce turbulent breakup of the spray droplets. Therefore, the quantification and prediction of the external gas phase turbulence characteristics of the droplets in the gas-liquid two-phase jet is the key to a deep understanding and revealing of the characteristics of air-assisted spraying. Due to the recent progress in high-performance imaging and laser diagnostic technology, the commonly used methods for gas-liquid two-phase jet detection are particle image velocimetry (PIV) and phase Doppler anemometry (PDA), both of which have their own advantages and disadvantages. Therefore, in order to study the atomization mechanism, spray performance and spray droplet behavior in the spray area, it is necessary to select the appropriate detection technology according to the specific application requirements, flow characteristics and economy.

[0003] When the air-assisted injection system is working, the pre-mixed fuel and compressed air mixture will form a high-speed gas-liquid two-phase jet through the air nozzle, and gradually develop into the final visible spray. One of the notable features of the gas-liquid two-phase jet is the strong interaction between the gas phase and the liquid phase.

[0004] However, the prior art has the following problems:

[0005] 1. PIV cannot synchronously measure the liquid phase flow field information. PIV is a non-invasive fluid measurement technology that continuously captures the movement of particles dispersed in the fluid and uses image processing technology to analyze the movement of particles, thereby obtaining the velocity distribution and flow pattern of the fluid. However, in gas-liquid two-phase jets, PIV technology mainly measures the flow field of the gas phase. Because the presence of bubbles or droplets may cause uneven distribution or insufficient concentration of particles in the gas phase, which will affect the quality and tracking accuracy of the particle image, and the PIV system may find it difficult to accurately track the movement of the gas phase. In addition, PIV can only obtain the velocity information of the gas phase flow field, but cannot obtain the gas phase turbulence information; mainly because, in the case of high air flow velocity or high turbulence intensity, the PIV system requires high-resolution images and high-frame rate cameras to capture details. However, in gas-liquid two-phase flow, due to the presence of bubbles or droplets, the trajectory of the particles becomes complicated and the image becomes blurred, which affects the accuracy of turbulence measurement. Therefore, PIV technology mainly measures the velocity information of the gas phase flow field, and cannot achieve synchronous measurement of the liquid phase flow field information.

[0006] 2. PDA cannot synchronously measure gas phase flow field information. PDA relies on light scattering of particles to measure velocity and diameter; in gas-liquid two-phase jets, liquid phase particles are usually larger in size and can effectively scatter light, while bubbles or gas bubbles in the gas phase may be smaller or irregular in size, making their light scattering characteristics very different from those of liquid phase particles; the gas phase is too small or the intensity of reflected light is too weak, making it difficult to accurately measure gas phase flow field information. In addition, in gas-liquid two-phase jets, the light scattering signals of gas and liquid phase particles may be aliased, making it difficult to separate the scattering signals of the two particles for independent measurement. PDA systems are usually designed to measure particle properties of a single phase (such as a liquid phase). For the case of a mixture of gas and liquid phases, signal analysis and separation become complicated. Although PDA technology performs well in the measurement of liquid phase particles, it is challenging to synchronously measure the flow field information of the gas phase in a gas-liquid two-phase jet, and other specialized technologies or algorithms are usually required in combination with PDA systems to solve these problems.

[0007] Therefore, in existing gas-liquid two-phase jets and sprays, droplets are dispersed as discrete phases in a continuous gas phase. The velocity characteristics of gas phase flow are crucial for further understanding of droplet-gas interactions and droplet secondary breakup analysis. However, under experimental conditions, it is still difficult to accurately measure the droplet velocity and diameter while simultaneously measuring the motion velocity around the droplet. Summary of the invention

[0008] The purpose of the present invention is to provide a method for measuring and predicting the gas phase turbulence characteristics in a gas-liquid two-phase jet, which utilizes a PDA system for air-assisted spray measurement to achieve simultaneous measurement of the gas-liquid two-phase flow field information while also measuring the turbulence characteristics of the gas-liquid two-phase jet.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet comprises the following steps:

[0011] Establish a PDA system for air-assisted spray measurement and measure the spray characteristic parameters multiple times under the same working conditions to verify the accuracy and repeatability of the spray characteristic parameters in the spray field;

[0012] Select droplets of a specific size in the spray field as tracer droplets, calculate the average value of the axial velocity of the tracer droplets to evaluate the local gas phase flow velocity, and calculate the terminal velocity of the tracer droplets;

[0013] Calculate the Stokes number of the tracer droplet and compare it with a known flow-following characteristic standard to verify the flow-following characteristics of the tracer droplet in the gas phase;

[0014] The local gas phase flow velocity is used to analyze the characteristics of gas phase turbulent motion.

[0015] 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;

[0016] The multi-line laser is used to generate a horizontally polarized light beam, including: two laser beams with a wavelength of λ=514.5nm and two laser beams with a wavelength of λ=488nm;

[0017] 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.

[0018] The receiving probe is used to convert the scattered light signal into an electrical signal and transmit it to the storage machine;

[0019] The signal processor is used to filter, process, analyze and record the electrical signals in the storage unit.

[0020] 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;

[0021] 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.

[0022] Furthermore, the spray characteristic parameters are measured multiple times under the same working conditions to verify the accuracy and repeatability of the spray characteristic parameters in the spray field, specifically:

[0023] The droplet velocity on the spray axis is measured multiple times, the average value is calculated, and the uncertainty U is quantified based on the standard error SE , used to verify the accuracy and repeatability of spray characteristic parameters in the spray field;

[0024] The average droplet velocity is The expression is:

[0025]

[0026] In the above formula, The average velocity of the droplets to the left of the center of the spray axis, is the average velocity of droplets to the right of the center of the spray axis;

[0027] in,

[0028]

[0029] In the above formula, f D is the Doppler frequency, λ is the wavelength of the green laser beam, and θ is the crossing angle of the two laser beams emitted by the transmitter;

[0030] Where the uncertainty U is quantified according to the standard error SE , the expression used to verify the accuracy and repeatability of the spray characteristic parameters in the spray field is:

[0031]

[0032] Where N is the number of experiments to measure the droplet diameter and velocity, X i is the measurement result of each experiment of measuring the droplet diameter and velocity, X represents the arithmetic mean of the measurement results of multiple measurements of the droplet diameter and velocity, and U SE It is a dimensionless result obtained by calculating the ratio of the actual error value to the average value of multiple measurements.

[0033] Further, the average value of the axial velocity of the tracer droplet is used to evaluate the local gas phase flow velocity and calculate the terminal velocity of the tracer droplet; wherein the expression for calculating the average value of the axial velocity of the tracer droplet to evaluate the local gas phase flow velocity is:

[0034]

[0035] In the above formula, U g is the local gas phase flow velocity, U t is the axial velocity of the tracer droplet, is the arithmetic mean of the axial velocities of n tracer droplets with a diameter less than 5 μm;

[0036] The expression for calculating the terminal velocity of the tracer droplet is:

[0037]

[0038] In the above formula, C d is the drag coefficient, U term is the terminal velocity of the tracer droplet, D t is the diameter of the tracer droplet, ρ l is the density of the droplet, ρ g is the gas phase density.

[0039] Furthermore, the expression for calculating the Stokes number of the tracer droplet is:

[0040]

[0041] In the above formula, τ0 is the droplet response time, τ t is the turbulent flow time scale;

[0042] Among them, the expression of droplet response time τ0 is:

[0043]

[0044] In the above formula, ρ l and μ g Represent the density of the droplet and the dynamic viscosity of the air, D t is the diameter of the tracer droplet.

[0045] Furthermore, the characteristics of gas phase turbulent motion are analyzed by using the local gas phase flow velocity, specifically:

[0046] The gas phase turbulence intensity in the spray field is obtained by calculating the ratio of the root mean square of the gas phase flow pulsation velocity to the local gas phase flow velocity.

[0047] The Reynolds number of the local gas phase flow is calculated using the maximum turbulence scale and the local gas phase flow velocity.

[0048] Furthermore, the gas phase turbulence intensity τ in the spray field is obtained by calculating the ratio of the root mean square of the pulsating velocity of the gas phase flow to the local gas phase flow velocity. tur The expression is:

[0049]

[0050] In the above formula, τ tur is the gas phase turbulence intensity, μ g，rms is the root mean square value of the pulsating velocity of the gas phase flow, U gis the local gas phase flow velocity;

[0051] Among them, the pulsating velocity root mean square value μ g，rms The expression is:

[0052]

[0053] In the above formula, is the arithmetic mean of the axial velocities of n tracer droplets with a diameter less than 5 μm, U t，i is the axial velocity of the ith tracer droplet.

[0054] Furthermore, the Reynolds number Re of the local gas phase flow is calculated by using the maximum turbulence scale and the local gas phase flow velocity. g The expression is:

[0055]

[0056] In the above formula, Re g is the Reynolds number of the local gas phase flow; μ g is the dynamic viscosity of air; U g is the local gas phase flow velocity; L 1 / 2 It is the half width of the spray.

[0057] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0058] (1) The present invention not only realizes the synchronous measurement of gas and liquid flow field characteristics by building a PDA system for air-assisted spray measurement, providing a theoretical basis for in-depth understanding of the atomization mechanism, but also further improves the atomization effect of dual-fluid spray.

[0059] (2) The present invention uses small-sized droplets captured by the PDA test system as tracer droplets to characterize the air flow velocity and the Stokes number of the droplets in the spray field, and verifies the accuracy of the tracer droplets in characterizing the local gas phase flow velocity. The results show that the tracer accuracy error of this method in characterizing the local gas flow velocity is less than 1%, realizing the mathematical description of the gas phase turbulent motion characteristics.

[0060] (3) Based on the measurement method proposed in the present invention, not only the simple measurement and prediction of the gas phase turbulence characteristics are achieved, but also by selecting small-sized droplets as tracer droplets and using the Stokes number of the droplets to verify the tracer accuracy, the synchronous measurement of the gas-liquid two-phase flow field information is achieved; the entire measurement process in the present invention also has the advantages of being simple, easy to understand and time-saving. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] 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.

[0062] The following is a further description of the method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to the present invention in conjunction with the accompanying drawings;

[0063] Figure 1 It is a schematic diagram of the overall process of the method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet provided by the present invention;

[0064] Figure 2 It is a principle diagram of PDA measurement in the method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet provided by the present invention;

[0065] Figure 3 It is a schematic diagram of the principle of using PDA to measure droplets in the method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet provided by the present invention, wherein (a) is a single droplet light scattering mode, and (b) is the variation of scattered light intensity with scattering angle for different scattering modes;

[0066] Figure 4 It is a schematic diagram of determining the spray half-width in the method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet provided by the present invention.

[0067] In the figure: 1. Transmitter; 2. Measuring body; 3. Nozzle; 4. Receiving probe; 5. Detector; 6. Spray. DETAILED DESCRIPTION

[0068] 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.

[0069] 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.

[0070] The measurement process and principle of PDA are as follows Figure 2 As shown, two laser beams emitted by the transmitter intersect at a point in space at a crossing angle θ and form an ellipsoidal measurement volume 2 (measurement / probe volume). is the angle between the laser emission direction and the axis of the receiving probe, ψ is the angle between the detectors 5 in the receiving probe; the coherent laser in the measuring body 2 will form interference fringes, and the fringe spacing can be determined by the laser wavelength λ and the crossing angle θ.

[0071] PDA measurement technology is based on the principle of scattered light interference. When light hits a spherical droplet, the surface of the droplet reflects part of the light, while the other part of the light enters the droplet through refraction and undergoes forward and backward refraction multiple times in the droplet. Figure 3 As shown in (a). The intensity and proportion of scattered light can be calculated by the Fresnel coefficients. This intensity is unevenly distributed in different directions. The main intensity of scattered light is affected by the incident angle, polarization state, and the relative refractive index between the measured particle and the surrounding medium. According to Snell's law, the size of the scattering angle can be determined, and the phase is determined by the optical path difference of the light. As the scattering angle changes, the intensity of scattered light in different scattering modes will also change. This change law is Figure 3 As shown in (b), the intensity of the scattered light is mainly concentrated in the first three order scattering modes.

[0072] In actual measurement operations, in order to effectively reduce the impact of multi-order reflected and refracted light and ensure that the received light intensity is as high as possible, the first-order refracted light signal is usually selected as the received signal. When the angle between the incident light and the receiver axis is 70°, the intensity of the reflected light is reduced to the minimum, and the best signal-to-noise ratio can be achieved at this time.

[0073] In summary, the present invention aims at the problem that the existing PIV and PDA technologies cannot realize the synchronous measurement of gas phase and liquid phase in gas-liquid two-phase jet, and proposes a method for measuring and predicting gas phase turbulence characteristics in gas-liquid two-phase jet. The PDA system based on air-assisted spray measurement first measures the droplet information at different positions in the spray field, uses small droplets to trace the gas phase motion characteristics, and then characterizes the gas phase turbulent motion characteristics. Finally, the Stokes number (Stk) of the droplets is collected for feasibility verification, as follows:

[0074] 1. Build a PDA system for air-assisted spray measurement

[0075] 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 droplet, while the blue laser beam with a wavelength of 488nm is used to measure the radial or tangential velocity of the droplet. The laser beam is transmitted to the transmitter 1 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 2 with an axial length of 76×76×630μm. The receiving probe 4 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.

[0076] During the measurement, the spatial position of the injection system remains fixed. The transmitter 1 and the receiver are respectively installed on the two cantilevers of the 3D displacement system, and a computer-controlled displacement system is used to achieve precise positioning of the measuring body, with an achievable displacement resolution of 0.1 mm. For PDA measurements under different back pressure environments, it is necessary to carry out 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 transmitting probe and the receiving probe 4.

[0077] 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 unit. 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, speed and the time when it passes through the measuring body, such as Figure 2 As shown. Preliminary measurement and analysis of the droplet size showed 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 the sample error to meet the statistical calculation requirements. The non-spherical droplet tolerance of the sampled droplets is set to 5%.

[0078] 2. Extract and process spray droplet information

[0079] In order to ensure sufficient droplet sampling data, multiple injections need to be performed under the same working conditions to ensure that the cumulative number of captured droplets is not less than 20,000. The PDA measurement body position mainly includes different axial positions x and different radial positions r along the spray axis. Referring to the time evolution of the spray morphology, the PDA capture process of droplets can also be divided into three stages. After the nozzle is opened, the spray passes a certain time T. s Therefore, the injection time can be shorter than T s The period of time is called the first stage, when the nozzle is open but the PDA does not capture valid droplet information. s After that, the droplet sampling process enters the second stage. In this stage, a large number of droplets begin to continuously pass through the PDA measurement body, and this process lasts for about 2.5ms, that is, the actual opening time of nozzle 3, the jet holding pulse width T0 + the injection holding pulse width T a . After the air nozzle is closed, the sampling process enters the third stage. At this time, the spray 6 process is forcibly terminated, and the droplets lose their energy source and begin to diffuse freely. It should be noted that in the third stage, there are still relatively many droplets suspended around the measuring body and may be detected by the laser. However, the speed of these suspended droplets is relatively small. Therefore, in order to facilitate data statistics and calculations, the second stage is used as the effective stage for droplet sampling, that is, only the droplets detected by the laser from the time when spray 6 begins to contact the measuring body to the time when nozzle 3 is closed are considered. In order to ensure the accuracy and repeatability of the experimental data, each set of working conditions needs to be repeated 3 times. The average values ​​of the relevant spray characteristic parameters of multiple experiments are calculated, and the uncertainty of the experiment is quantified according to the standard error (SE). The calculation method is as follows:

[0080]

[0081] Where N is the number of experiments, X i is the measurement result of each experiment, It represents the arithmetic mean of multiple experimental measurement results. In addition, the uncertainty U SE It is a dimensionless result obtained by calculating the ratio of the actual error value to the average value of multiple measurements.

[0082] 3. Selection of tracer droplets and velocity characterization

[0083] PDA is used to simultaneously measure the widely distributed droplet size and velocity, and relatively small droplets are used as tracer particles to further determine the gas phase flow in a simple and convenient way. Sufficiently small droplets (<5μm) are used to characterize the local gas phase flow velocity. Therefore, the local gas phase flow velocity can be expressed as:

[0084]

[0085] Among them, U t represents the axial velocity of the tracer droplet, is the arithmetic mean of the axial velocities of n tracer droplets with a diameter less than 5 μm. t The terminal velocity of the tracer droplet is calculated as:

[0086]

[0087] Among them, C d is the drag coefficient, U term is the terminal velocity of the tracer droplet, D t is the diameter of the tracer droplet, ρ l is the density of the droplet, ρ g is the gas phase density.

[0088] 4. Verify the tracer accuracy

[0089] In air-assisted spraying, droplets of different sizes are dispersed in the gas phase jet. Therefore, the gas phase is regarded as the continuous phase or carrier phase, and the droplet / liquid phase is regarded as the dispersed phase. The movement of the droplets is usually determined by the degree to which the droplets accompany the external gas phase flow, which can be characterized by the dimensionless Stokes number (Stk). Droplets with low Stk tend to follow the external flow; while droplets with large Stk are easily dominated by their own inertia and tend to move along their original trajectory. When small-sized droplets are used to characterize local gas flows, it is generally believed that the Stokes number of these small droplets should be as small as possible to provide as high a tracking accuracy as possible.

[0090] Stk is usually defined as the droplet response time τ0 and the turbulent flow time scale τ t The ratio is as shown in formula (4):

[0091]

[0092] For a spherical droplet, the droplet response time τ0 can be used as the time constant of the droplet in the Stokes flow, as shown in equation (5):

[0093]

[0094] Among them, ρ l and μ g Represent the density of the droplet and the dynamic viscosity of the air, D t is the diameter of the tracer droplet.

[0095] Turbulent flow time scale τ tIt can be obtained by calculating the ratio of the maximum size of the maximum turbulent eddy to the root mean square of the gas phase flow pulsation velocity on the axis. Since the dispersion characteristics of local particles are mainly dominated by the large-scale turbulent structure in the gas phase flow, such as Figure 4 Therefore, the half width of the spray (expressed as L 1 / 2 L represents the maximum scale of turbulent structure. 1 / 2 It can be quantified by measuring the radial airflow velocity in the spray field. 1 / 2 It can be defined as the distance between the positions where the airflow velocity on both sides of the spray drops to half the axis velocity. In the spray visualization study, large-scale vortex structures with symmetrical distribution can be seen on both sides of the spray head under specific working conditions. The droplet velocity is measured by the PDA test system. The droplet velocity is mainly determined by the laser wavelength λ and the intersection angle θ of the two laser beams emitted by the transmitter. Therefore, the present invention uses the spray half-width to characterize the maximum turbulence scale. Taking x=50mm as an example, the attached Figure 4 is the calculation method of the large eddy length scale in the spray, L 1 / 2 The specific determination steps are as follows:

[0096] (1) Measure the average droplet velocity at multiple points in a mirror image direction of the spray

[0097] (2) Find the average velocity of the droplets at the left and right positions on the spray axis to be the same and to be reduced to half position;

[0098] (3) The distance between the left and right positions is defined as the spray half-width L 1 / 2 .

[0099] Using n-octane and n-dodecane as test liquids, the Stk of all droplets collected at the axial sampling position was calculated. The results showed that the Stk of the tracer droplets in the air-assisted spray of n-octane and n-dodecane did not exceed 0.1. Therefore, it can be considered that the tracer accuracy error of using droplets with a diameter of 0-5μm to characterize the local gas flow velocity is less than 1%.

[0100] 5. Characterization of gas phase turbulent motion characteristics

[0101] Assuming that these small-scale droplets can completely follow the airflow to determine the local airflow velocity in the instantaneous spray field. Therefore, the root mean square value of the pulsating velocity of the gas phase flow is:

[0102]

[0103] in, is the arithmetic mean of the axial velocities of n tracer droplets with a diameter less than 5 μm, U t，i is the axial velocity of the ith tracer droplet.

[0104] The velocity of the tracer droplet can effectively characterize the local gas phase flow velocity. Therefore, the gas phase turbulence intensity in the spray (expressed by τ tur The value of pulsation velocity (expressed as rms) can be obtained by calculating the ratio of the corresponding gas phase flow pulsation velocity to the average gas flow velocity, as follows:

[0105]

[0106] Use spray half width L 1 / 2 To characterize the maximum turbulence scale and the flow velocity U obtained by the tracer droplet characterization g , calculate the Reynolds number Re of the local gas phase flow g ,for:

[0107]

[0108] From the above formula, we can see that under fixed fuel conditions, Re g Mainly affected by U g With L 1 / 2 The impact of the product.

[0109] In summary, the present invention has the following detailed technical effects:

[0110] (1) In order to achieve the synchronous measurement of the gas and liquid flow field characteristics in air-assisted spraying, the present invention constructs a dedicated PDA measurement system. By considering a variety of possible influencing factors, it provides important theoretical support for in-depth analysis of the atomization mechanism and helps to further improve the atomization efficiency of dual-fluid spraying.

[0111] (2) In a gas-liquid two-phase jet, droplets are distributed as discrete phases in a continuous gas phase. The velocity characteristics of the gas phase flow are crucial for a deep understanding of the interaction between droplets and gas and the secondary breakup process of droplets. Small-sized droplets captured by the PDA test system were used as tracer droplets to characterize the air flow velocity and the Stokes number (Stk) of the droplets in the spray field, and the accuracy of the tracer particle method for characterizing the gas phase flow velocity was verified. The results show that the tracer accuracy error of this method in characterizing the local gas flow velocity is less than 1%. The Reynolds number of the local gas phase flow calculated based on the flow velocity of the tracer droplets and the half-width of the spray field realizes a mathematical description of the gas phase turbulent motion characteristics.

[0112] 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 measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet, characterized in that: The following steps are involved: Establish a PDA system for air-assisted spray measurement and measure the spray characteristic parameters multiple times under the same working conditions to verify the accuracy and repeatability of the spray characteristic parameters in the spray field; Select droplets of a specific size in the spray field as tracer droplets, calculate the average value of the axial velocity of the tracer droplets to evaluate the local gas phase flow velocity, and calculate the terminal velocity of the tracer droplets; Calculate the Stokes number of the tracer droplet and compare it with a known flow-following characteristic standard to verify the flow-following characteristics of the tracer droplet in the gas phase; The local gas phase flow velocity is used to analyze the characteristics of gas phase turbulent motion.

2. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 1 is characterized in that: The PDA system for air-assisted spray measurement comprises: a multi-line laser, an optical detector and a signal processor; the optical detector comprises: a transmitter (1), a lens and a receiving probe (4); The multi-line laser is used to generate a horizontally polarized light beam, including: two laser beams with a wavelength of λ=514.5nm and two laser beams with a wavelength of λ=488nm; The laser beam is transmitted to the transmitter via an optical fiber and symmetrically intersects via a lens with a focal length of 310 mm. The intersecting light beams form an ellipsoidal measuring body (2) with an axial length of 76×76×630 μm. When droplets in the spray pass through the ellipsoidal measuring body (2), the laser beam is scattered by the droplets to form a scattered light signal. The receiving probe (4) 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 measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 2, 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.

4. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 3 is characterized in that: The spray characteristic parameters were measured multiple times under the same working conditions to verify the accuracy and repeatability of the spray characteristic parameters in the spray field, specifically: The droplet velocity on the spray axis is measured multiple times, the average value is calculated, and the uncertainty U is quantified based on the standard error SE , used to verify the accuracy and repeatability of spray characteristic parameters in the spray field; The average droplet velocity is The expression is: In the above formula, The average velocity of the droplets to the left of the center of the spray axis, is the average velocity of droplets to the right of the center of the spray axis; in, In the above formula, f D is the Doppler frequency, λ is the wavelength of the green laser beam, and θ is the crossing angle of the two laser beams emitted by the transmitter; Where the uncertainty U is quantified according to the standard error SE , the expression used to verify the accuracy and repeatability of the spray characteristic parameters in the spray field is: Where N is the number of experiments to measure the droplet diameter and velocity, X i is the measurement result of each experiment for measuring the droplet diameter and velocity, It represents the arithmetic mean of the measurement results of multiple measurements of droplet diameter and velocity, U SE It is a dimensionless result obtained by calculating the ratio of the actual error value to the average value of multiple measurements.

5. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 1, characterized in that: The average value of the axial velocity of the tracer droplet is used to evaluate the local gas phase flow velocity and calculate the terminal velocity of the tracer droplet; wherein the expression for calculating the average value of the axial velocity of the tracer droplet to evaluate the local gas phase flow velocity is: In the above formula, U g is the local gas flow velocity, U t is the axial velocity of the tracer droplet, is the arithmetic mean of the axial velocities of n tracer droplets with a diameter less than 5 μm; The expression for calculating the terminal velocity of the tracer droplet is: In the above formula, C d is the drag coefficient, U term is the terminal velocity of the tracer droplet, D t is the diameter of the tracer droplet, ρ l is the density of the droplet, ρ g is the gas phase density.

6. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 1, characterized in that: The expression for calculating the Stokes number of the tracer droplet is: In the above formula, τ0 is the droplet response time, τ t is the turbulent flow time scale; Among them, the expression of droplet response time τ0 is: In the above formula, ρ l and μ g Represent the density of the droplet and the dynamic viscosity of the air, D t is the diameter of the tracer droplet.

7. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 1, characterized in that: The characteristics of gas phase turbulent motion are analyzed by using the local gas phase flow velocity, specifically: The gas phase turbulence intensity in the spray field is obtained by calculating the ratio of the root mean square of the gas phase flow pulsation velocity to the local gas phase flow velocity. The Reynolds number of the local gas phase flow is calculated using the maximum turbulence scale and the local gas phase flow velocity.

8. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 7, characterized in that: The gas phase turbulence intensity τ in the spray field is obtained by calculating the ratio of the root mean square of the pulsating velocity of the gas phase flow to the local gas phase flow velocity. tur The expression is: In the above formula, τ tur is the gas phase turbulence intensity, μ g，rms is the root mean square value of the pulsating velocity of the gas phase flow, U g is the local gas phase flow velocity; Among them, the pulsating velocity root mean square value μ g，rms The expression is: In the above formula, is the arithmetic mean of the axial velocities of n tracer droplets with a diameter less than 5 μm, U t，i is the axial velocity of the ith tracer droplet.

9. The method for measuring and predicting gas phase turbulence characteristics in a gas-liquid two-phase jet according to claim 7, characterized in that: The Reynolds number Re of the local gas phase flow is calculated by using the maximum turbulence scale and the local gas phase flow velocity. g The expression is: In the above formula, Re g is the Reynolds number of the local gas phase flow; μ g is the dynamic viscosity of air; U g is the local gas phase flow velocity; L 1 / 2 It is the half width of the spray.

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