Method for measuring and calculating ultrasonic cavitation micro-jet velocity

Abstract

The invention discloses a method for measuring and calculating the speed of an ultrasonic cavitation microjet, and belongs to the field of ultrasonic cavitation research. The method comprises the following steps: firstly, preparing an ultrasonic cavitation experiment test piece; then carrying out an ultrasonic cavitation experiment, characterizing the surface appearance of the experimental test piece after the experiment, and recording the size data of a cavitation pit caused by the ultrasonic cavitation microjet on the experimental test piece; combining a spherical indentation inversion calculation theory and a material constitutive equation to calculate an ultrasonic cavitation micro-jet velocity; and finally, according to the distribution rule and characteristics of the ultrasonic cavitation micro-jet velocity, the application scene of the ultrasonic operation condition is judged. The method breaks through the limitation that the existing method can only measure the speed of a single ultrasonic cavitation microjet, not only realizes the measurement of the speed of a plurality of ultrasonic cavitation microjets, but also avoids the direct measurement of a complex ultrasonic cavitation jet flow field, and greatly improves the efficiency and operability of the measurement of the speed of the ultrasonic cavitation microjets.

Description

Technical Field

[0001] The invention belongs to the research field of ultrasonic cavitation, and in particular relates to a method for measuring and calculating the velocity of ultrasonic cavitation micro jets. Background Art

[0002] Ultrasonic waves propagate in liquids in the form of periodic alternating pressure waves, which can form dense tiny cavitation bubbles in the liquid. These cavitation bubble nuclei grow in the negative pressure propagation zone and quickly collapse in the positive pressure zone. This process is called cavitation, accompanied by the generation of high-speed and high-pressure ultrasonic cavitation microjets. Since the speed of ultrasonic cavitation microjets can reach more than 1000m / s, they are widely used in fields such as material cleaning, surface modification and marine development. The speed of ultrasonic cavitation microjets is a key factor affecting their cleaning effect or material modification. For the application effect, too small an ultrasonic cavitation microjet speed will lead to problems such as low cleaning rate, insufficient cleaning depth and substandard material surface modification, but too large an ultrasonic cavitation microjet speed will often have a significant cavitation effect on the treated material. Therefore, accurately measuring the speed of ultrasonic cavitation microjets in different application scenarios is the key to ensuring its application effect.

[0003] During the ultrasonic cavitation process, usually multiple cavitation bubbles of different sizes and positions collapse at the same time to generate multiple ultrasonic cavitation microjets. Since the diameter and velocity of a single ultrasonic cavitation microjet are positively correlated with the collapse diameter of the cavitation bubble, there are obvious differences in the diameters and velocities of multiple ultrasonic cavitation microjets formed at the same time. At the same time, ultrasonic cavitation microjets are generally micrometer-scale, and conventional flow velocity measurement methods cannot achieve accurate measurement of microjets. At present, the measurement methods of ultrasonic cavitation microjets mainly rely on high-speed imaging technology, particle image velocimetry technology and numerical simulation methods, but these methods still have obvious defects. The high-speed imaging velocimetry method records the movement process of the gas-liquid interface during the collapse of the cavitation bubble, and determines the velocity of the ultrasonic cavitation microjet according to the displacement of the gas-liquid interface at different times. The velocity actually measured by this method is the contraction velocity of the cavitation bubble, not the velocity after the ultrasonic cavitation microjet is formed; the particle image velocimetry technology uses a high-speed camera to track the motion trajectory of the suspended particles in the microjet to measure the velocity of the ultrasonic cavitation microjet. Although this method can provide velocity information of the entire flow field, it is difficult to distinguish the motion trajectory of the suspended particles in the microjet for the dense cavitation microjet flow field formed in the liquid, resulting in large errors in the measurement results; the numerical simulation method has the advantages of convenience, speed and low cost compared to the experimental measurement method, but the current numerical simulation method usually only calculates the velocity of a single jet. The average velocity result of the ultrasonic cavitation microjet is calculated by combining theory and experience, which cannot fully describe the differences of the ultrasonic cavitation microjet. How to accurately describe dense and highly random ultrasonic cavitation microjets is still a major difficulty in numerical simulation, and it is also one of the key challenges for accurately calculating the velocity of ultrasonic cavitation microjets.

[0004] Based on this, this method fully considers the shortcomings of the existing methods, obtains the cavitation morphology characteristics caused by ultrasound on the material surface, combines the material constitutive equation, and inversely calculates the ultrasonic cavitation microjet velocity. This method breaks through the limitation of the existing method that can only measure a single ultrasonic cavitation microjet velocity. It not only realizes the measurement of multiple ultrasonic cavitation microjet velocities, but also avoids the direct measurement of complex cavitation jet flow fields, greatly improving the efficiency and operability of ultrasonic cavitation microjet velocity measurement. Summary of the invention

[0005] The present invention aims to provide a method for measuring and calculating the velocity of ultrasonic cavitation microjet, so as to achieve accurate measurement of the velocity of ultrasonic cavitation microjet. Through this method, it is possible to ensure that ultrasonic cavitation achieves the expected effect in various application scenarios, and improve its application effect and reliability.

[0006] A method for measuring and calculating the velocity of ultrasonic cavitation microjet, the method comprising the following steps:

[0007] Step 1: Conduct an ultrasonic cavitation experiment, characterize the morphology of the experimental specimen using an electron microscope and a scanning electron microscope, determine the size and distribution of the cavitation pits caused by the ultrasonic cavitation microjet on the experimental specimen, and count the diameter and depth of the cavitation pits. The experimental specimens need to be prepared before the ultrasonic cavitation experiment is carried out. Referring to JB / T6037 "Laboratory Full Immersion Corrosion Test of Metal Coverings", common metals are selected as experimental materials, and they are processed into 30mm×15mm×3mm experimental specimens. First, petroleum ether is used to wash off the surface protective oil film, and then 800-2000 mesh sandpaper is used to polish it smooth, and then acetone solution and anhydrous ethanol are used to clean it in turn, and it is blown dry with cold air. Before the experiment, an electron microscope is used to observe the morphology of the experimental specimen. The ultrasonic cavitation experiment is to put the prepared experimental specimen into an ultrasonic treatment tank, keep the distance between the experimental specimen and the ultrasonic transducer unchanged at 3mm, set the ultrasonic operating power, frequency and processing time, and after the experiment is completed, the experimental specimen is cleaned with anhydrous ethanol, blown dry with cold air, and then the cavitation pit size is counted.

[0008] Step 2: Calculate the ultrasonic cavitation microjet velocity based on the statistical cavitation pit diameter and depth data, combined with the micron indentation inversion calculation theory and the material constitutive equation of the experimental specimen. The specific calculation process is as follows:

[0009] S41, for a single cavitation pit, first calculate the equivalent force area A of the cavitation pit formed by the experimental specimen p , as shown in formula 1:

[0010]

[0011] Where, d p is the diameter of the cavitation pit, m; h p is the depth of cavitation pit, m;

[0012] S42, combined with the material constitutive equation of the experimental specimen, describes the elastic and plastic deformation process of the experimental specimen caused by the impact of ultrasonic cavitation microjet, and calculates the equivalent impact stress of the experimental specimen when the cavitation pit is formed, as shown in formula (2):

[0013]

[0014] In the formula, σ 0 is the yield strength of the test specimen, Pa; ε p is the strain of a single cavitation pit in the experimental specimen; A is the strain hardening coefficient; n is the strain hardening exponent; B is the strain rate hardening coefficient; is the dimensionless strain rate, usually defined as formula (3):

[0015]

[0016] In the formula, is the current strain rate; is the reference strain rate;

[0017] S43, according to the micron indentation inversion analysis theory, the impact load L on the experimental specimen to form a cavitation pit p As shown in formula (4):

[0018] L p =σ p A p (4)

[0019] In the formula, σ p is the equivalent impact stress of the experimental specimen when the cavitation pit is formed, Pa;

[0020] S44, considering the impact cross section of the jet to be circular, the impact load L caused by the ultrasonic cavitation microjet is calculated by combining the geometric dimensions of the ultrasonic cavitation microjet and the impact stress. jet , as shown in formula (5):

[0021]

[0022] Where R is the radius of the ultrasonic cavitation microjet, m; S is the circumference of the ultrasonic cavitation microjet impact area, m; σ jet is the ultrasonic cavitation microjet water impact stress, Pa; d jet is the diameter of ultrasonic cavitation microjet, d jet =d p ;

[0023] S45, the impact load L caused by the ultrasonic cavitation microjet jet Equal to the impact load L on the experimental specimen to form a cavitation pit p , the dynamic water impact pressure of ultrasonic cavitation microjet can be calculated by formula (6):

[0024]

[0025] According to the water hammer pressure calculation formula, the impact pressure of ultrasonic cavitation microjet on the experimental specimen is related to the ultrasonic cavitation microjet velocity v jet At the same time, the surrounding fluid medium of the micron-scale ultrasonic cavitation microjet is static when it is formed. Compared with the micron-scale ultrasonic cavitation microjet, the fluid in the static area can be regarded as an infinite area. Due to the viscosity and surface tension of the liquid, the blocking effect of the static flow domain on the microjet cannot be ignored. Therefore, the attenuation coefficient is introduced to characterize the velocity loss in the ultrasonic cavitation microjet process, as shown in formula (7):

[0026]

[0027] In the formula, ρ L and ρ S are the density of cleaning medium and test piece, kg / m 3 ;c L and c S are the sound propagation speed in the cleaning medium and the test specimen, m / s; μ eff and λ eff are the viscosity attenuation coefficient and surface tension correction factor respectively.

[0028] The viscosity attenuation coefficient can be calculated by formula (8):

[0029]

[0030] In the formula, C 1 is a dimensionless coefficient; μ is the viscosity of the cleaning medium, Pa·s; D is the diameter of the ultrasonic cavitation microjet, D=1 / 10R max ; σ is the static surface tension of the cleaning medium, N / m; R max It is the maximum radius of cavitation bubble growth, that is, the cavitation bubble collapse radius, m.

[0031] The surface tension correction factor λ eff It can be calculated by formula (9):

[0032]

[0033] In the formula, C 2 is a dimensionless coefficient.

[0034] Combining equations (7) to (9), the ultrasonic cavitation microjet velocity v is calculated jet .

[0035] Step three, statistically analyze the ultrasonic cavitation microjet velocity results, and analyze the distribution law and characteristics of the ultrasonic cavitation microjet velocity in combination with the geometric morphology characteristics of different cavitation pits, and then determine the applicable scenarios of ultrasonic operating conditions.

[0036] The present invention adopts the above technical solution to achieve the following beneficial effects:

[0037] (1) The present invention proposes a method for measuring and calculating the velocity of ultrasonic cavitation microjets. The velocity of ultrasonic cavitation microjets is calculated by measuring the size characteristics of cavitation pits caused by ultrasonic cavitation microjets on the material surface, combining the micron indentation inversion analysis theory and the material constitutive equation.

[0038] (2) The method for measuring and calculating the ultrasonic cavitation microjet velocity proposed in the present invention has the advantages of strong operability, high precision, and high efficiency. Combined with the micron indentation inversion analysis theory and the material constitutive equation, it not only realizes the measurement of multiple ultrasonic cavitation microjet velocities during the ultrasonic cavitation process, but also avoids the direct measurement of complex cavitation flow fields, greatly improving the efficiency and operability of ultrasonic cavitation microjet velocity measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Purpose of the drawings: In order to more clearly illustrate the embodiments and technical solutions of the present invention, the drawings required for use in the embodiments will be simply marked and introduced below.

[0040] Figure 1 It is a flow chart of the ultrasonic cavitation microjet velocity measurement method.

[0041] Figure 2 This is the surface morphology of the experimental specimen before the ultrasonic cavitation microjet experiment in the embodiment of the present invention.

[0042] Figure 3 It is the microscopic morphology of cavitation pits formed on the surface of the experimental specimen after the ultrasonic cavitation microjet experiment in the embodiment of the present invention.

[0043] Figure 4 It is a characterization of the morphology and structure of the cavitation pits formed on the surface of the experimental specimen in the embodiment of the present invention.

[0044] Figure 5 This is the velocity distribution result of the ultrasonic cavitation microjet in the embodiment of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described below in conjunction with the accompanying drawings, but the present invention is not limited to the following embodiments.

[0046] The ultrasonic cavitation microjet velocity measurement method can be used to measure the ultrasonic cavitation microjet velocity, such as Figure 1 As shown, in combination with a specific embodiment, the following implementation steps are included:

[0047] Step 1: Preparation of experimental specimens. Refer to JB / T6037 "Laboratory Full Immersion Corrosion Test of Metal Coverings". Common aluminum alloy plates are selected as experimental specimens and processed into 30mm×15mm×3mm specimens. First, use petroleum ether to wash away the surface protective oil film, then use 800#~2000# sandpaper to polish it smooth, then use acetone solution and anhydrous ethanol to clean it in turn, and blow dry with cold air. Before the experiment, use an electron microscope to observe the morphology of the experimental specimens. Figure 2 This is the microscopic morphology of the test piece before the experiment began.

[0048] Step 2: ultrasonic treatment. Place the prepared test piece into an ultrasonic treatment tank. Use pure water as the cleaning medium. The ultrasonic operating power is 2000W, the ultrasonic frequency is 20kHz, and the treatment time is 60s. During the experiment, keep the distance between the test piece and the ultrasonic transducer at 3mm. After the treatment, clean the test piece with anhydrous ethanol and blow dry with cold air.

[0049] Step 3: Use scanning electron microscopy to characterize the morphology of the experimental specimen after the experiment, determine the size and distribution of the cavitation pits caused by the ultrasonic cavitation microjet on the experimental specimen, and count the diameter and depth of the cavitation pits formed on the surface of the experimental specimen. Figure 3 It is the microscopic morphology of the surface of the experimental specimen after the experiment. Figure 4 It is the result of characterizing the geometric morphology of cavitation pits.

[0050] Step 4: According to the diameter and depth data of the cavitation pits formed on the surface of the experimental specimen, combined with the micron indentation inversion calculation theory and the material constitutive equation, the ultrasonic cavitation microjet velocity is calculated, and the results are shown in Table 1. In this embodiment, the cleaning medium density ρ L =1000kg / m 3 , the experimental specimen density ρ S =2700kg / m 3 , the sound propagation speed c in the cleaning medium L =1482m / s, the sound propagation velocity c in the experimental specimen S =3040m / s, the yield strength of the aluminum alloy test piece σ 0 =350mPa, the strain hardening coefficient A =330mPa, the strain hardening exponent n =0.42, the strain rate strengthening coefficient B =0.02, the reference strain rate

[0051] Table 1 Cavitation size on the surface of the experimental specimen and ultrasonic cavitation microjet velocity

[0052]

[0053]

[0054] Step 5: Count the results of ultrasonic cavitation microjet velocity and analyze the distribution law and characteristics of ultrasonic cavitation microjet velocity. Figure 5 It is the distribution law of ultrasonic cavitation microjet velocity.

[0055] according to Figure 5 It can be judged that the ultrasonic operating parameters in this embodiment can produce a strong corrosive effect on the aluminum alloy material. Therefore, the ultrasonic operating conditions in this embodiment are not suitable for cleaning the aluminum alloy test piece material, but can be used for surface modification of the aluminum alloy test piece material.

[0056] The present invention provides a method for indirectly measuring the velocity of ultrasonic cavitation microjets by combining the micron indentation inversion analysis theory and the material constitutive equation. The method avoids direct measurement of micron-scale ultrasonic cavitation microjets and simultaneously realizes the measurement of multiple ultrasonic cavitation microjets, greatly improving the operability and efficiency of ultrasonic cavitation microjets velocity measurement.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. All other embodiments obtained by ordinary technicians in this field without making creative work should fall within the scope of protection of the embodiment scheme of this specification.

Claims

1. A method for measuring and calculating the velocity of ultrasonic cavitation microjet, characterized in that: The following steps are involved: Step 1: Conduct an ultrasonic cavitation experiment, use an electron microscope and a scanning electron microscope to characterize the morphology of the experimental specimen after ultrasonic cavitation treatment, determine the size and distribution of the cavitation pits caused by the ultrasonic cavitation microjet on the experimental specimen, and collect data on the diameter and depth of the cavitation pits; Step 2, calculating the ultrasonic cavitation microjet velocity according to the statistical cavitation pit diameter and depth data, combined with the micron indentation inversion calculation theory and the material constitutive equation of the experimental specimen; Step three, statistically analyze the ultrasonic cavitation microjet velocity results, and analyze the distribution law and characteristics of the ultrasonic cavitation microjet velocity in combination with the geometric morphology characteristics of different cavitation pits, and then determine the applicable scenarios of the ultrasonic operating conditions.

2. The method for measuring and calculating the velocity of ultrasonic cavitation microjet according to claim 1, characterized in that: According to the statistical data of cavitation pit diameter and depth, combined with the spherical indentation inversion calculation theory and the material constitutive equation of the experimental specimen, the specific steps are as follows: (1) First, based on the statistical cavitation diameter and depth data, for a spherical cavitation pit, the equivalent force area A of the cavitation pit formed by the experimental specimen is calculated. p , as shown in formula 1: Where, d p is the diameter of the cavitation pit, m; h p is the depth of cavitation pit, m; (2) Combined with the material constitutive equation of the experimental specimen, the elastic and plastic deformation process of the experimental specimen caused by the impact of ultrasonic cavitation microjet is described, and the equivalent impact stress of the experimental specimen formed by cavitation pit is calculated, as shown in formula (2): Where σ0 is the yield strength of the test specimen, Pa; ε p is the strain of a single cavitation pit in the experimental specimen; A is the strain hardening coefficient; n is the strain hardening exponent; B is the strain rate hardening coefficient; is the dimensionless strain rate, usually defined as formula (3): In the formula, is the current strain rate; is the reference strain rate; (3) According to the equivalent force area A of the cavitation pit p And the equivalent impact stress σ of the experimental specimen to form cavitation pits p , combined with the micron indentation inversion analysis theory, the impact load L on the experimental specimen to form the cavitation pit is calculated p The impact load L on the experimental specimen to form the cavitation pit p As shown in formula (4): L p =s p A p (4) In the formula, σ p is the equivalent impact stress of the experimental specimen when the cavitation pit is formed, Pa; (4) Considering that the jet impact cross section is circular, combined with the ultrasonic cavitation microjet diameter d jet and impact stress σ jet Calculation of the impact load L caused by ultrasonic cavitation microjet jet , the diameter of the ultrasonic cavitation microjet is equal to the diameter of the cavitation pit, and the calculation expression is shown in formula (5): Where R is the radius of the ultrasonic cavitation microjet, m; S is the circumference of the ultrasonic cavitation microjet impact area, m; σ jet is the ultrasonic cavitation microjet water impact stress, Pa; d jet is the diameter of ultrasonic cavitation microjet, m; (5) The impact load L caused by the ultrasonic cavitation microjet jet Equal to the impact load L on the experimental specimen to form a cavitation pit p , the dynamic water impact pressure of ultrasonic cavitation microjet can be calculated by formula (6): The impact pressure of ultrasonic cavitation microjet on the experimental specimen and the velocity v of ultrasonic cavitation microjet jet At the same time, the surrounding fluid medium of the micron-scale ultrasonic cavitation microjet is static when it is formed. Compared with the micron-scale ultrasonic cavitation microjet, the fluid in the static area can be regarded as an infinite area. Due to the viscosity and surface tension of the liquid, the blocking effect of the static flow domain on the microjet cannot be ignored. Therefore, the attenuation coefficient is introduced to characterize the velocity loss in the ultrasonic cavitation microjet process, as shown in formula (7): In the formula, ρ L and ρ S are the density of cleaning medium and test piece, kg / m 3 ;c L and c S are the sound propagation speed in the cleaning medium and the test specimen, m / s; μ eff and λ eff are the viscosity attenuation coefficient and surface tension correction factor respectively. The viscous attenuation coefficient μ eff It can be calculated by formula (8): Where C1 is a dimensionless coefficient; μ is the viscosity of the cleaning medium, Pa·s; D is the diameter of the ultrasonic cavitation microjet, m; σ is the static surface tension of the cleaning medium, N / m; R max It is the maximum radius of cavitation bubble growth, that is, the cavitation bubble collapse radius, m. The surface tension correction factor λ eff It can be calculated by formula (9): Where C2 is a dimensionless coefficient. Combining equations (7) to (9), the ultrasonic cavitation microjet velocity v is calculated jet .

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