Method for modulating gas-phase turbulence in gas-liquid two-phase flow

Abstract

The invention relates to the technical field of gas-liquid two-phase flow, and discloses a method for modulating gas-phase turbulence in gas-liquid two-phase flow, which comprises the following steps of: establishing a PDA (Personal Digital Assistant) system for air-assisted spray measurement, and respectively measuring speed information of a current-carrying phase and liquid drops in a spray field; analyzing the influence of the liquid drops on the flow field characteristics in combination with the speed information of the current-carrying phase and the liquid drops in the spray field; according to the mixing length theory of single-phase turbulence and in combination with the speed information of the current-carrying phase in the spray field, the segmented linear relation between the speed gradient and the pulsation speed of the current-carrying phase in different current-carrying gradient areas is analyzed; according to the invention, the PDA system for air-assisted spray measurement is utilized to realize synchronous measurement of gas-phase and liquid-phase flow field characteristics, and the influence of discrete liquid drops on a gas-liquid two-phase jet flow turbulence structure is analyzed.

Description

Technical Field

[0001] The invention relates to the technical field of gas-liquid two-phase flow, and more particularly to a method for modulating gas phase turbulence in 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. Considering that the actual fusion of colliding droplets requires a certain time scale, the droplet information obtained by the phase Doppler particle test has obvious instantaneous characteristics. Therefore, by analyzing the instantaneous characteristics of the droplets at the current spray measurement position, it can be considered that the droplets can maintain a relatively independent and complete discrete state in a short time. In this case, the air-assisted spray far from the nozzle outlet area will be classified as a droplet-carrying jet problem and has obvious turbulent diffusion flow characteristics, in which the gas phase can be regarded as a carrier phase (Carrier-phase), and the discrete droplets can be regarded as a discrete phase (Dispersed-phase). Therefore, elevating the air-assisted spray problem to a droplet-carrying jet problem and analyzing the basic characteristics of gas-phase turbulent diffusion inside the spray are the key to in-depth understanding and revealing the characteristics of air-assisted spray.

[0003] Since the turbulence of liquid droplets or solid particles in the prior art is a component of turbulent dispersed multiphase flows (TDMFs). When the gas phase is continuous, the actual flow can be divided into particle flow and droplet flow according to the type of discrete phase. The presence of particles / droplets usually has two opposite effects on the turbulent state of the carrier phase, namely enhancement or attenuation. Turbulence modulation is determined by multiple factors. Balachandar and Eaton summarized that turbulence modulation in suspensions is attributed to three main mechanisms: (a) enhanced dissipation due to the presence of particles; (b) kinetic energy transfer from particles to fluid; (c) formation of wakes and vortex shedding behind particles. Gai et al. reviewed the relevant research on turbulence modulation and emphasized that the main inducing factors of turbulence modulation include the destruction of velocity gradients and streamline curvature in the continuous phase, particle vortex shedding and attenuation of particle induced drag. At present, the factors that are widely believed to have an important influence on turbulence modulation include particle size, particle Reynolds number, particle Stokes number, mass loading and volume fraction. These factors have different effects on different actual flow conditions such as pipe flow, channel flow and jet. In some cases, their effects may even contradict each other, and current understanding of these phenomena remains limited.

[0004] Shinjo et al. used a direct numerical simulation (DNS) data set to study the effects of finite-sized droplets / ligaments on turbulence modulation and scalar mixing in dense fuel sprays. The results reveal the universal characteristics of isotropic turbulence in a region characterized by a relatively low droplet number density. However, the presence of droplets causes a slight change in the arrangement of strain eigenvectors with vorticity and mixing fraction gradients. In addition, the numerical simulation results of Dodd and Ferrante show that the decay rate of the turbulent kinetic energy of the two fluids increases with the increase of the droplet Weber number (based on the mean velocity of turbulence), the droplet-to-carrier density ratio, and the droplet-to-carrier viscosity ratio. Droplets increase the dissipation rate of turbulent kinetic energy by enhancing the local velocity gradient near the droplet interface. Rosa et al. studied the effect of bidirectional momentum coupling on droplet collision and coalescence and studied it using the Euler-Lagrangian joint numerical method. They found that the effects of bidirectional coupling on droplet aggregation and radial relative velocity were significant. Gai et al. conducted a numerical study on turbulence generated by industrial nozzles. Their results showed that the turbulent kinetic energy is mainly concentrated in the spray cone and decreases rapidly with the increase of the vertical distance from the nozzle. In addition, it was found that the spray droplet diameter has a significant effect on the turbulent kinetic energy. The water mist produced by these industrial nozzles will produce strong turbulence near the nozzle, and the turbulence intensity gradually decreases with the increase of the distance from the nozzle.

[0005] And air-assisted spraying enhances the instability of the jet or spray by adding an additional pressurized gas flow (usually air) to promote the breakup of the liquid, thereby promoting the improvement of the atomization effect. In this process, the kinetic energy of the high-speed airflow and the strong shear stress present near the gas-liquid interface effectively shatter the liquid jet / sheet into ligaments, which are then further disintegrated and converted into dispersed droplets at a relatively low pressure compared to the single-fluid pressure atomization method. On the other hand, air-assisted spraying has complex multi-atomization mechanisms and gas-liquid two-phase flow characteristics. In the spray / flow field region away from the nozzle, the fully atomized spray is transformed into a jet filled with droplets with strong gas-liquid interaction. However, previous studies on air-assisted spraying have mainly focused on the basic characterization of spray characteristics, and rarely analyzed the effect of droplets on the jet turbulence in the framework of turbulent dispersed multiphase flow.

[0006] Prior art studies on turbulence modulation, especially in turbulent dispersed multiphase flows (TDMFs), have mainly focused on solid particles. Relatively speaking, the effects of droplets on gas turbulence are less studied due to the complexity of droplet behavior. Unlike solid particles, droplets in turbulent flows may undergo additional physical processes such as evaporation, deformation, coalescence, etc., which increase the complexity of the study. These complexities arise from the interaction between the droplets and the surrounding gas phase, including the transfer of mass and heat. The size of the droplets is closely related to the local flow characteristics, resulting in significant spatiotemporal variations in the distribution of droplets, resulting in polydisperse flows, which are characterized by significant concentration and flow modulation differences, which are expected to be significantly different from observations of gas-solid two-phase flows. In addition, when turbulent gas flows past stationary droplets, velocity gradients are formed around the droplets, resulting in uneven pressure distribution. External forces exerted on the droplets by continuous phases may cause the droplets to deform, and if these external forces exceed the internal forces and stresses that maintain the integrity of the droplets, the droplets may break. To accurately predict the deformation at the interface between the dispersed and continuous phases, the effects of surface tension as well as the viscosity and density differences between the two phases must be incorporated into the governing equations of motion.

[0007] In summary, the current research on turbulence modulation caused by droplets mainly relies on numerical simulation. In existing research, there are relatively few experimental verifications of turbulence modulation caused by spray droplets. Summary of the invention

[0008] The purpose of the present invention is to provide a method for modulating gas phase turbulence in gas-liquid two-phase flow, to realize the synchronous measurement of gas and liquid flow field characteristics by using a PDA system of air-assisted spray measurement, and to analyze the influence of discrete droplets on the turbulent structure of gas-liquid two-phase jet.

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

[0010] A method for modulating gas phase turbulence in a gas-liquid two-phase flow comprises the following steps:

[0011] A PDA system for air-assisted spray measurement was established, and the velocity information of the carrier phase and droplets in the spray field were measured respectively;

[0012] Combine the velocity information of the carrier phase and droplets in the spray field to analyze the influence of droplets on the flow field characteristics;

[0013] According to the mixing length theory of single-phase turbulence and combined with the velocity information of the carrier phase in the spray field, the piecewise linear relationship between the velocity gradient of the carrier phase and the pulsating velocity in different carrier gradient regions is analyzed.

[0014] 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;

[0015] The multi-line laser is used to generate a horizontally polarized light beam;

[0016] 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.

[0017] The receiving probe is used to convert the scattered light signal into an electrical signal and transmit it to the storage machine;

[0018] The signal processor is used to filter, process, analyze and record the electrical signals in the storage unit.

[0019] 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.

[0020] Furthermore, the recorded information includes: the diameter of the droplet, the speed of the droplet, and the time when the droplet passes through the ellipsoidal measuring body.

[0021] 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;

[0022] 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.

[0023] Further, the velocity information of the carrier phase and the droplets in the spray field is measured; wherein the velocity information of the droplets in the spray field is measured, specifically:

[0024] The axial measurement of the droplets in the spray field starts from the position x = 30 mm, and the average velocity distribution of multiple points in the transverse section of the spray at this position is integrated, expressed as:

[0025]

[0026] In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U g is the current phase velocity, r is the radial measurement position, U t，i is the axial velocity of the ith tracer droplet measured by the PDA system, is the arithmetic mean of the droplet axial velocity, R max x=30mm is the spray radius at the starting position.

[0027] Further, the velocity information of the carrier phase and the droplets in the spray field is measured; wherein the velocity information of the carrier phase in the spray field is measured, specifically:

[0028] The dimensionless pulsation velocity of the current-carrying phase is defined as the ratio of the root mean square of the pulsation velocity of the current-carrying phase to the cross-sectional average velocity at the starting position. The expression is:

[0029] u′ g =u g，rms / U g，0 (4)

[0030]

[0031] In the above formula, u′ g is the pulsating speed of the current-carrying phase, u g，rms is the root mean square of the pulsating velocity of the current-carrying phase, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U t，i is the axial velocity of the i-th tracer droplet measured by the PDA system; is the arithmetic mean of the droplet axial velocity;

[0032] The average velocity gradient of the current-carrying phase is expressed by the dimensionless parameter ζ, which is calculated as follows:

[0033]

[0034] In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, is the average velocity of the current-carrying phase, R max x = 30 mm is the spray radius at the starting position, and r is the radius of the droplet.

[0035] Furthermore, the analysis of the influence of droplets on flow field characteristics includes:

[0036] The influence of droplets on gas flow velocity, analysis of the influence of droplets on the turbulence level of gas flow, the influence of droplets on the axial velocity of the carrier phase, the influence of droplets on the average axial velocity of the carrier phase under different fuel conditions, the influence of droplet diameter on the average velocity of the carrier phase and the influence of droplet diameter on the pulsating velocity of the carrier phase in the same measurement plane.

[0037] Furthermore, the piecewise linear relationship between the velocity gradient and the pulsating velocity of the current-carrying phase in the different current-carrying gradient regions is analyzed, specifically:

[0038] Analyze the correlation between the pulsating velocity and velocity gradient of the carrier phase in the weak gradient region of single-phase and two-phase jets;

[0039] The correlation between the pulsating velocity and velocity gradient of the current-carrying phase at different axial positions in the strong gradient region of single-phase jet and two-phase jet is analyzed.

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

[0041] (1) By building a PDA system for air-assisted spray measurement, the synchronous measurement of gas and liquid flow field characteristics is achieved, which provides a theoretical basis for a deeper understanding of the atomization mechanism, thereby further improving the atomization effect of dual-fluid spray.

[0042] (2) Small spray droplets (D < 5 μm) with good followability were used to simulate the movement of a single-phase jet. The carrier phase velocity was characterized and normalized. The effects of droplets on the gas phase turbulence modulation under different experimental conditions were compared and analyzed. The two-phase jet formed by air-assisted spray was divided into three regions according to the overall distribution of the average velocity. It was confirmed that the presence of droplets would increase the turbulence level of the gas phase flow to a certain extent.

[0043] (3) According to the turbulent velocity gradient in the flow field, it is divided into a weak gradient region close to the spray axis and a strong gradient region far away from the spray axis, and a linear test is performed on the pulsating velocity and velocity gradient of the two-phase flow field. It is found that there is a certain degree of piecewise linear correlation between the two, which is different from the assumption in the classical mixing length theory that the pulsating velocity of single-phase turbulence is proportional to the velocity gradient. 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 gas phase turbulence modulation method in the gas-liquid two-phase flow of 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 gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0047] Figure 2 It is a distribution diagram of average velocity of the carrier phase in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0048] Figure 3 It is a distribution diagram of the pulsating velocity of the carrier phase of the single-phase jet and the two-phase jet in the gas-phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0049] Figure 4 1 is a graph showing the change in axial velocity gradient of the current-carrying phase in different measurement planes in the gas-phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention, wherein (a) is the change in axial velocity gradient of a single-phase jet, and (b) is the change in axial velocity gradient of a two-phase jet;

[0050] Figure 5 It is a distribution diagram of average axial velocity of the carrier phase under different injection pulse widths in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0051] Figure 6 It is a distribution diagram of average axial velocity of the carrier phase under different test fuel types in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0052] Figure 7 It is a distribution diagram of average axial velocity of the carrier phase under different jet pulse widths in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0053] Figure 8 It is a shear zone width diagram of different fuel injection pulse widths in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention;

[0054] Fig. 9 It is a pulsation velocity distribution diagram of the carrier phase in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention, wherein (a) is different injection pulse widths, (b) is different test fuel types, and (c) is different injection pulse widths;

[0055] Fig.10 It is a distribution diagram of the relationship between the pulsating velocity of the carrier phase and the velocity gradient in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention, wherein (a) is a weak gradient area, and (b) is a strong gradient area;

[0056] Fig.11 It is the correlation between the pulsating velocity of the carrier phase and the velocity gradient at different axial positions in the weak gradient region in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention, wherein (a)-(b) are single-phase jets, and (e)-(h) are two-phase jets;

[0057] Fig.12 It is the correlation between the pulsating velocity of the carrier phase and the velocity gradient at different axial positions in the strong gradient region in the gas phase turbulence modulation method in the gas-liquid two-phase flow provided by the present invention, wherein (a)-(d) are single-phase jets, and (e)-(h) are two-phase jets. DETAILED DESCRIPTION

[0058] 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.

[0059] 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.

[0060] The present invention proposes a method for modulating gas phase turbulence in gas-liquid two-phase flow, which uses phase Doppler particle analysis technology to measure and deeply analyze the turbulence characteristics of gas-assisted spray, focusing on the influence of discrete droplets on the turbulent structure of gas-liquid two-phase jet. Figure 1 As shown:

[0061] 1. First, the PDA system for air-assisted spray measurement is used. The system mainly consists 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) is used to generate a horizontally polarized beam (output power is 0.8W), which is divided into four beams, including two laser beams with wavelengths λ = 514.5nm and two laser beams with wavelengths λ = 488nm; among them, the laser beam with wavelength λ = 514.5nm is a green laser beam, which is used to measure the axial velocity and diameter of the droplet; the laser beam with wavelength λ = 488nm is a blue laser beam, which is used to measure the radial or tangential velocity of the droplet. The laser beam is transmitted to the transmitter through an optical fiber and symmetric phase angle through a lens with a focal length of 310mm. The intersecting beams will form an ellipsoidal measurement 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 scattered signal. The spatial position of the measuring body is adjusted by a displacement system with a movement accuracy of up to 0.1 mm.

[0062] 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, which can achieve a 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 which are distributed opposite to the transmitting probe and the receiving probe.

[0063] 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%.

[0064] 2. Characterize the carrier phase velocity under different test conditions and parameters in the spray field:

[0065] As shown in Table 1, the specific calculation process is as follows:

[0066] Since the PDA has a large measurement error for the droplet concentration area near the nozzle outlet, the axial measurement position in the spray field starts from x = 30 mm, and the integral of the average velocity of multiple points in the spray transverse section at this position is:

[0067]

[0068] In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U g is the current phase velocity, r is the radial measurement position, U t，i is the axial velocity of the ith tracer droplet measured by the PDA system, is the arithmetic mean of the droplet axial velocity, R max x = 30 mm is the spray radius at the starting position;

[0069] The dimensionless current-carrying phase pulsation velocity is defined as the ratio of the root mean square of the current-carrying phase pulsation velocity to the cross-sectional average velocity at the starting position, and the expression is:

[0070] u′ g =ug，rms / U g，0 (4)

[0071]

[0072] In the above formula, u′ g is the pulsating speed of the current-carrying phase, u g，rms is the root mean square of the pulsating velocity of the current-carrying phase, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U t，i is the axial velocity of the i-th tracer droplet measured by the PDA system; is the arithmetic mean of the droplet axial velocity.

[0073] The average velocity gradient of the current-carrying phase is expressed by the dimensionless parameter ζ, which is calculated as:

[0074]

[0075] In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, is the average velocity of the current-carrying phase, R max x = 30 mm is the spray radius at the starting position, and r is the radius of the droplet.

[0076] Table 1 Experimental conditions and parameters

[0077]

[0078] 3. Analyze the influence of droplets on flow field characteristics

[0079] Attached Figure 2 The average velocity distribution of the air-assisted spray in the working condition No. 1 along different axial positions and radial positions in the half cross section. The cross-sectional average velocity at the initial axial position is used for normalization; the axial sampling position is expressed as x / ΔL, where ΔL = 30 mm is the axial measurement position interval of the experimental setting, and the radial position is uniformly expressed as the actual axial position and the inner diameter d of the air nozzle outlet. i The normalized results are shown in Figure 2.

[0080] By setting T f = = 1.0ms to achieve the minimum injection amount, thus approximating it to a single-phase gas result; when T f >1.0mms, the droplet diameter is less than 5μm, which is similar to the other results of two phases; when T f >1.0ms, droplet diameter greater than 5μm is approximately a two-phase droplet result. Droplet velocity data information U measured by PDA t，i , substituting into formula (2)-(3) to obtain the velocity information of the current-carrying phase, such as Figure 2 shown.

[0081] like Figure 2 As shown in the figure, from the overall distribution of the average velocity, the spray field can be divided into three areas: A, B, and C. Among them, the dividing line between the AB area refers to the line connecting the corresponding radial position points when the spray axial velocity reaches 95% of the maximum value in this area; and the dividing line of the BC area represents the line connecting the radial positions when the spray axial velocity is reduced to 0 (as shown in the attached figure). Figure 2 ), we can get: the average axial velocity in region A (mainstream region) is relatively large, while the axial velocity in region C is basically 0; the axial velocity in region B is the transition between regions AC, and there can be an obvious velocity gradient. Therefore, region B is defined as the shear zone, and its region width is represented by δ. By comparing the single-phase jet and the two-phase jet, it can be found that in the mainstream region and the partial shear zone close to the mainstream region, the other average axial velocities of the two-phase jet and the average axial velocity of the droplets are basically consistent, and are significantly higher than the average axial velocity of the single-phase gas jet, indicating that the presence of droplets will help to increase the gas phase flow velocity in the spray field.

[0082] According to the above formula (4), we can know that:

[0083] u g =u g，rms / U g，0

[0084] Among them, u′ g is the pulsating speed of the current-carrying phase, u g，rms is the root mean square of the pulsating velocity of the current-carrying phase, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U t，i is the axial velocity of the i-th tracer droplet measured by the PDA system; is the arithmetic mean of the droplet axial velocity.

[0085] Therefore, the droplet velocity U obtained by PDA sampling is t，i The pulsating velocity distribution of the carrier phase of the single-phase Case 0 and Case 1 two-phase jets is calculated by using equations (2)-(5), as shown in Figure 2. Figure 2As shown. It can be found that in almost all positions along the spray axis and radial direction, the pulsating velocity of the carrier phase under the two-phase jet condition is greater than that of the single-phase jet. It can be concluded that the presence of droplets will help to increase the turbulence level of the gas phase flow to a certain extent. In addition, by comparing the distribution of the pulsating velocity along the radial direction at the same axial position, it can be found that, whether it is a single-phase jet or a two-phase jet, the pulsating velocity of the carrier phase approximately maintains a constant level within a certain range close to the spray axis, that is, a weak pulsating velocity gradient area. After exceeding a certain radial position, the pulsating velocity decreases significantly until it reaches 0, thus presenting a strong pulsating velocity gradient area. The dividing line between the above two obvious pulsating velocity gradient difference areas is as follows: Figure 2 As shown by the double-dotted line in the figure, the dividing line is the radial position corresponding to the pulsation speed dropping to 0.9 times the maximum value.

[0086] And according to formula (6), we can know:

[0087]

[0088] In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, is the average velocity of the current-carrying phase, R max x = 30 mm is the spray radius at the starting position, and r is the radius of the droplet.

[0089] in, according to Figure 1 Between adjacent sampling points The ratio of the size to the distance is calculated. Thus, the axial velocity gradient of the current-carrying phase of the single-phase jet (Case 0) and the two-phase jet (Case 1) in different measurement planes is calculated, and the results are shown in Figure 4 As shown. It can be found that under both jet conditions, the axial velocity gradient can show a relatively obvious negative peak. Moreover, at the same axial position, the peak value of the axial velocity gradient of the carrier phase under the two-phase jet condition is larger than that of the single-phase jet, and the fluctuation range of the value is also larger than that of the single-phase jet. This shows that due to the presence of droplets, the axial velocity of the carrier phase tends to have a more obvious velocity gradient. In addition, it is found by comparison that the maximum negative peak value of the velocity gradient in different measurement planes under the two jet conditions appears at r / d i =1, and decreases with the increase of x / ΔL, indicating that the trend of axial velocity change is gradually weakening.

[0090] According to the U calculated from Case 0-Case 3 in Table 1 g，0 , and calculated by equations (2)-(3) The average axial velocity of the current-carrying phase under different injection pulse widths is plotted, such as Figure 5As shown. It can be found that the increase of injection pulse width will lead to the increase of the average axial velocity of the carrier phase. In summary, it is known that the increase of injection pulse width will directly lead to the increase of the average diameter of the spray droplets, and the larger droplet diameter has a more significant effect on the increase of the average velocity of the carrier phase.

[0091] According to the U calculated for Case 0, Case 1, Case 4 and Case 5 in Table 1 g，0 , calculated by formula (2)-(3) The average axial velocity distribution of the carrier phase under different test fuel conditions is plotted, such as Figure 6 Compared with the injection pulse width, the influence of fuel properties on the average axial velocity of the carrier phase is not very significant. According to the calculations of Case 0, Case 6-Case 8 in Table 1, The average axial velocity distribution of the carrier phase under different jet pulse width conditions is plotted, such as Figure 7 As shown. The results show that the effect of the jet pulse width on the average axial velocity of the carrier phase is less obvious. In addition, it can be found that when x / ΔL<6, the average axial velocity of the carrier phase in the mainstream area under different jet pulse widths is quite different. When x / ΔL is greater than 6, the average axial velocity of the carrier phase under different jet pulse widths is almost the same.

[0092] In summary, under the current experimental conditions, the fuel properties and the injection pulse width have little effect on the average axial velocity of the carrier phase of the air-assisted spray. Therefore, two groups of different injection pulse widths (T f =30ms and T f =70ms) and calculate the shear zone width of the spray field as a function of x / ΔL, such as Figure 8 shown.

[0093] The shear zone width of the spray field is expressed as:

[0094] According to the attached Figure 2 According to the average axial velocity, the spray field is divided into mainstream area A, shear area B, and stagnation area C. It can be seen that the average axial velocity in area A is relatively large, while the axial velocity in area C is basically 0. The axial velocity in area B is the transition between areas AC. Therefore, the width of area B is the width of the adjacent Figure 2 The vertical distance of the location of area B in the middle.

[0095] The results show that the variation trend of the shear zone width of the two-phase jet with x / ΔL is more gentle than that of the single-phase jet, and the injection pulse width is T f = 7.0ms compared to the shearing zone width at T f=3.0ms is larger, but the δ change trends of the two are almost the same. The above results show that compared with the single-phase jet, in the gas-liquid two-phase jet formed by air-assisted spray, the presence of droplets will cause axial stretching of the shear zone of the carrier phase, while inhibiting its diffusion in the radial direction. As a result, the shear zone of the two-phase jet becomes moderately slender compared to the single-phase jet. In the present invention, the spray adopts a vertical downward spraying method, and a large number of droplets are dispersed in the carrier phase of the generated two-phase jet. Compared with the single-phase jet, the droplets and the droplet clusters formed due to the inhomogeneity will be subject to the gravity effect that cannot be ignored, and thus cause axial stretching of the shear zone of the carrier phase. Under the same conditions, this droplet gravity effect will be more significant at a larger injection pulse width.

[0096] According to the U calculated from Case 0 to Case 8 in Table 1 g，0 , u calculated by formula (2)-(5) g，rms , draw the pulsating velocity distribution of the current-carrying phase under all current experimental conditions, such as Fig. 9 As shown. Similar to the result of the average axial velocity, the increase of the injection pulse width will also lead to an increase in the pulsating velocity of the carrier phase in the same measurement plane. This result shows that the larger-scale spray droplets have a certain enhancement effect on the turbulence of the carrier phase of the air-assisted spray. In addition, there are also certain differences in the pulsating velocity of the carrier phase under different test fuels, which is different from its effect on the average velocity of the carrier phase. Fig. 9 (c) shows the distribution of carrier phase pulsation velocity under different jet pulse widths. It can be seen that under the same conditions, the increase of jet pulse width will reduce the pulsation velocity of the carrier phase. This is because under the same injection pulse width, a larger jet pulse width will lead to a decrease in the actual injection oil-gas ratio, that is, a decrease in the carrier ratio. Under the same jet pulse width, an increase in the injection pulse width will also lead to an increase in the carrier ratio. Therefore, the effects of the jet pulse width and the injection pulse width on the pulsation velocity of the carrier phase can be explained by the actual spray carrier ratio.

[0097] At the same time, it can be seen that under different experimental conditions, the pulsating velocity distribution of the carrier phase exists and is attached. Figure 3 The same weak pulsating velocity region and strong pulsating velocity region. The pulsating velocity of the carrier phase in the jet region remains relatively stable within a certain range close to the spray axis position, and shows a weak velocity gradient, which is caused by the annular nozzle outlet structure adopted by the air nozzle. Compared with the jet structure with a clear cone structure and the steeper velocity distribution in the jet cross section produced by the traditional flat nozzle, this annular nozzle is more inclined to form a relatively stable velocity distribution at the axis of the jet. In the area where the spray is close to the external stagnant ambient gas, the pulsating velocity will have a larger gradient.

[0098] 4. Analyze the piecewise linear relationship between the velocity gradient of the current-carrying phase and the pulsating velocity

[0099] According to the mixing length theory of single-phase turbulence, the pulsating velocity is proportional to the velocity gradient. In addition, the study shows that the linearity of the two will be affected by the current ratio. When the current ratio is less than 0.5, the pulsating velocity of the carrier phase and the average axial velocity will maintain a relatively ideal linear relationship; when the current ratio is greater than 0.5, the linearity of the two will deteriorate significantly.

[0100] Due to the special intermittent injection method, the minimum adjustable injection time (T f =1.0ms, the nozzle cannot open normally after this time), the equivalent current ratio under the current experimental conditions is greater than 0.5. Therefore, it can be assumed that the gas phase fluctuation velocity and the average axial velocity in the current gas-assisted spray will not meet the ideal overall linearity. In addition, the flow field characteristics results show that with the increase of r / d i With the increase of r / d, the average velocity gradient of the carrier phase first decreases and then increases, showing an obvious non-monotonic trend. i As the value of increases, the fluctuation speed of the carrier phase first maintains a relatively constant value, and then gradually decreases. Therefore, it can be inferred that the two have poor linearity as a whole.

[0101] Therefore, the present invention is to determine the pulsating velocity u′ in the weak gradient and strong gradient regions of the current-carrying phase pulsating velocity. g The linearity test is performed separately with the velocity gradient ζ.

[0102] Among them, u′ g According to formula (2), we know:

[0103] u′ g =u g，rms / U g，0

[0104] Among them, u′ g is the pulsating speed of the current-carrying phase, u g，rms is the root mean square of the pulsating velocity of the current-carrying phase, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U t，i is the axial velocity of the i-th tracer droplet measured by the PDA system; is the arithmetic mean of the droplet axial velocity, that is

[0105] The average velocity gradient of the current-carrying phase is expressed by the dimensionless parameter ζ, which is calculated as follows:

[0106]

[0107] in, According to the attached Figure 2 Between adjacent sampling points It is calculated by the ratio of the size to the distance.

[0108] Therefore, plotting the pulsating velocity u′ g The relationship with the velocity gradient ζ is shown in Figures (10)-(12).

[0109] in, Fig.10 (a) shows the relationship between the pulsating velocity and velocity gradient of the carrier phase in the weak gradient region of the single-phase jet and the two-phase jet. It can be seen that u′ under the two jet conditions g and ζ show a relatively obvious linear relationship. Among them, the linearity of the single-phase jet is better than that of the two-phase flow, and the slopes of the linear fitting curves of the two are quite different. The relationship between the pulsating velocity and velocity gradient of the carrier phase of the single-phase jet and the two-phase jet in the strong gradient area is shown in Figure 2. Fig.10 Similarly, it can be found that no matter it is a single-phase jet or a two-phase jet, u′ g and ζ show a relatively obvious linear correlation. At the same time, it can be noticed that compared with the weak gradient, the u′ of the single-phase jet and the two-phase jet in the strong gradient region is g The linear difference between ζ and ζ is smaller.

[0110] Attached Fig.11 is the correlation between the pulsating velocity of the carrier phase and the velocity gradient at different axial positions in the weak gradient region. The selected internal axial positions of the jet include x / ΔL=2, x / ΔL=4, x / ΔL=6 and x / ΔL=8, and the selected working conditions include single-phase jet and two-phase jet (T f =3.0ms).

[0111] In general, u′ g and ζ show a relatively obvious linear correlation. At a position relatively close to the nozzle outlet (abef), u′ g The slope of the linear fitting function of and ζ is very close to 0. This phenomenon also shows that the pulsating velocity of the carrier phase hardly changes with the change of the axial velocity gradient. At the measurement position relatively far away from the nozzle (cdgh), the u′ of the single-phase jet and the two-phase jet is g The slope of the linear fitting line of and ζ is not equal to 0. In the single-phase jet, u′ at x / ΔL=6 and x / ΔL=8 g The slopes of the linear fitting lines are close to those of the ζ linear fitting lines, while the slopes of the two are quite different in the two-phase jet.

[0112] The linear correlation results of the pulsating velocity of the current-carrying phase and the velocity gradient at different axial positions in the strong pulsating velocity gradient region are as follows: Fig.12 As shown. It can be found that in the strong gradient area, u′ gand ζ also show a relatively obvious linear correlation, and the slopes of the straight lines obtained by linear fitting are all negative. This indicates that the pulsating velocity of the carrier phase will decrease with the increase of the axial velocity gradient. The above results show that for the gas-liquid two-phase jet formed by air-assisted spraying, the pulsating velocity of the carrier phase in the spray field and the velocity gradient do not satisfy the overall linear correlation within the jet cross section in a certain radial direction. However, in the weak gradient area and the strong gradient area obtained by dividing according to the gradient difference of the pulsating velocity, u′ g Both ζ and ζ show a relatively obvious piecewise linear correlation.

[0113] In summary, the present invention has the following detailed technical effects:

[0114] (1) Since the synchronous measurement of the flow field characteristics of two fluids is affected by many factors, the present invention realizes the synchronous measurement of the gas and liquid flow field characteristics by building a PDA system for air-assisted spray measurement, which provides a theoretical basis for a deep understanding of the atomization mechanism, thereby further improving the atomization effect of two-fluid spray.

[0115] (2) Since droplets in turbulence may undergo some additional physical processes compared to solid particles, the study of turbulence modulation of TDMFs carrying droplets is complicated. To this end, the present invention uses small spray droplets (D<5μm) with good followability to simulate the movement of a single-phase jet, characterizes and normalizes the carrier phase velocity, compares and analyzes the effects of droplets on gas-phase turbulence modulation under different experimental conditions, and divides the two-phase jet formed by air-assisted spray into three regions according to the overall distribution of the average velocity. The results show that the presence of droplets will increase the turbulence level of the gas-phase flow to a certain extent.

[0116] (3) The present invention divides the flow field into a weak gradient region close to the spray axis and a strong gradient region far away from the spray axis according to the turbulent velocity gradient in the flow field; and performs a linear test on the pulsating velocity and velocity gradient of the two-phase flow field, and finds that there is a certain degree of piecewise linear correlation between the two, but this is different from the assumption in the classical mixing length theory that the pulsating velocity of single-phase turbulence is proportional to the velocity gradient.

[0117] 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 modulating gas phase turbulence in a gas-liquid two-phase flow, characterized in that: The following steps are involved: A PDA system for air-assisted spray measurement was established, and the velocity information of the carrier phase and droplets in the spray field were measured respectively; Combine the velocity information of the carrier phase and droplets in the spray field to analyze the influence of droplets on the flow field characteristics; According to the mixing length theory of single-phase turbulence and combined with the velocity information of the carrier phase in the spray field, the piecewise linear relationship between the velocity gradient of the carrier phase and the pulsating velocity in different carrier gradient regions is analyzed.

2. The method for modulating gas phase turbulence in gas-liquid two-phase flow according to claim 1, 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 modulating gas phase turbulence in gas-liquid two-phase flow according to claim 2, 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 modulating gas phase turbulence in gas-liquid two-phase flow according to claim 3, characterized in that: The recorded information includes: the diameter of the droplet, the velocity of the droplet, and the time when the droplet passes through the ellipsoidal measuring body.

5. The method for modulating gas phase turbulence in gas-liquid two-phase flow according to claim 4, 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.

6. The method for modulating gas phase turbulence in gas-liquid two-phase flow according to claim 5, characterized in that: The velocity information of the carrier phase and the droplets in the spray field is measured; wherein the velocity information of the droplets in the spray field is measured, specifically: The axial measurement of the droplets in the spray field starts from the position x = 30 mm, and the average velocity distribution of multiple points in the transverse section of the spray at this position is integrated, expressed as: In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U g is the current phase velocity, r is the radial measurement position, U t，i is the axial velocity of the ith tracer droplet measured by the PDA system, is the arithmetic mean of the droplet axial velocity, R max x=30mm is the spray radius at the starting position.

7. The method for modulating gas phase turbulence in gas-liquid two-phase flow according to claim 6, characterized in that: The velocity information of the carrier phase and the droplets in the spray field is measured; wherein the velocity information of the carrier phase in the spray field is measured, specifically: The dimensionless pulsation velocity of the current-carrying phase is defined as the ratio of the root mean square of the pulsation velocity of the current-carrying phase to the cross-sectional average velocity at the starting position. The expression is: in g =in g，rms / IN g，0 (4) In the above formula, u′ g is the pulsating speed of the current-carrying phase, u g，rms is the root mean square of the pulsating velocity of the current-carrying phase, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, U t，i is the axial velocity of the i-th tracer droplet measured by the PDA system; is the arithmetic mean of the droplet axial velocity; The average velocity gradient of the current-carrying phase is expressed by the dimensionless parameter ζ, which is calculated as follows: In the above formula, U g，0 is the cross-sectional average velocity of the current-carrying phase at the starting position x = 30 mm, is the average velocity of the current-carrying phase, R max x = 30 mm is the spray radius at the starting position, and r is the radius of the droplet.

8. The method for modulating gas phase turbulence in gas-liquid two-phase flow according to claim 1, characterized in that: The influence of the droplets on the flow field characteristics is analyzed, and the specific analysis contents include: The influence of droplets on gas flow velocity, analysis of the influence of droplets on the turbulence level of gas flow, the influence of droplets on the axial velocity of the carrier phase, the influence of droplets on the average axial velocity of the carrier phase under different fuel conditions, the influence of droplet diameter on the average velocity of the carrier phase and the influence of droplet diameter on the pulsating velocity of the carrier phase in the same measurement plane.

9. The method for modulating gas phase turbulence in gas-liquid two-phase flow according to claim 1, characterized in that: The piecewise linear relationship between the velocity gradient and the pulsating velocity of the current-carrying phase in the different current-carrying gradient regions is analyzed, specifically: Analyze the correlation between the pulsating velocity and velocity gradient of the carrier phase in the weak gradient region of single-phase and two-phase jets; The correlation between the pulsating velocity and velocity gradient of the current-carrying phase at different axial positions in the strong gradient region of single-phase jet and two-phase jet is analyzed.

Images