Battery pack thermal boundary layer destruction method and thermal management method based on acoustic streaming effect

Abstract

The application discloses a battery pack thermal boundary layer destruction method and a thermal management method based on acoustic streaming effect. The method comprises the following steps: generating an alternating magnetic field outside a box body, penetrating the box body wall to drive the flexible vibration element of the inner wall to generate high-frequency micro-vibration; the vibration radiates acoustic waves in the immersed liquid, and the acoustic streaming effect is generated by utilizing the viscous dissipation of the liquid; the acoustic streaming forms local shear force in the micron boundary layer near the surface of the battery cell, and destroys the thermal boundary layer attached to the surface of the battery cell. The application also provides a thermal management method comprising the method, immersing the battery cell in the insulating cooling liquid, and simultaneously removing the air film by using the acoustic streaming. The application adopts non-contact acoustic streaming disturbance, has no mechanical wear and no sealing damage, can accurately remove the micron-level air film, reduces the interface thermal resistance, and is suitable for the full-life maintenance-free energy storage system.

Description

Technical Field

[0001] This invention belongs to the field of thermal management technology for energy storage batteries, specifically relating to a method for breaking the thermal boundary layer of a battery pack based on the acoustic flow effect and a thermal management method, which is particularly applicable to maintenance-free industrial and commercial energy storage cabinets, containerized energy storage systems, and battery compartments of electric ships. Background Technology

[0002] Static immersion liquid-cooled battery packs directly immerse the battery cells in an insulating coolant (such as silicone oil), relying on natural convection for heat transfer. Under long-term static conditions, a micron-scale thermal boundary layer (gas film) gradually forms between the insulating layer and the silicone oil on the cell surface. The formation of this gas film is a result of the combined effects of differences in solid-liquid interface wettability, the release of dissolved gases from the liquid, and the "breathing effect" of the cell during charging and discharging. Although the gas film is only tens of micrometers thick, its thermal conductivity is much lower than that of silicone oil, resulting in significant thermal resistance. This leads to a significant increase in interfacial thermal resistance, a decrease in local heat transfer coefficient, and an increase in hotspot temperature, thus limiting rate performance and cycle life.

[0003] Existing methods for enhancing heat exchange often employ immersion stirring, built-in pumps, or mechanical transmission mechanisms to agitate the immersion liquid. However, these methods suffer from drawbacks such as seal disruption, mechanical wear, and space occupation. Some electromagnetic drive methods use a motor in conjunction with a oscillating mechanism and a physical magnetic stirrer, which still involve mechanical transmission components. Therefore, there is an urgent need for a method that can non-contactly remove the gas film at the micrometer scale without introducing macroscopic flow.

[0004] It should be noted that while the acoustic flow effect has been used in ultrasonic cleaning to disrupt microbubbles at the solid-liquid interface, its application in immersion thermal management of energy storage battery packs has not been reported. Furthermore, ultrasonic cleaning typically requires high frequencies (>20kHz) and high intensity, which are fundamentally different from the low-frequency, low-amplitude approach of this invention. In addition, electromagnetically driven flexible membrane structures have been used for fluid actuation in micropumps and microvalves, but these devices typically require precise fluid channels and one-way valve structures, and do not address the special requirements of battery pack sealing, explosion-proofing, and maintenance-free operation. Summary of the Invention

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for breaking the thermal boundary layer of a battery pack based on the acoustic flow effect. For a long time, those skilled in the art of battery pack thermal management have generally believed that only by enhancing macroscopic convection (such as pumps, fans, and stirrers) can heat dissipation be effectively improved, neglecting the possibility of microscale acoustic flow effects. This invention overcomes this technical bias. The method includes: generating an alternating magnetic field on the outside of the battery pack housing; the alternating magnetic field penetrates the housing wall, driving a flexible vibrating element installed on the inner wall of the housing to generate high-frequency micro-vibrations; the high-frequency micro-vibrations radiate sound waves in the immersion liquid, utilizing the viscous dissipation of the liquid to generate an acoustic flow effect; the acoustic flow forms local shear force in the micron-level boundary layer near the cell surface; the shear force breaks the thermal boundary layer attached to the cell surface. This method does not cause macroscopic overall flow of the immersion liquid, therefore it does not interfere with the original natural convection or forced circulation within the battery pack. This solution generates acoustic flow through non-contact driving, utilizing the shear force within the micron-level boundary layer to break the gas film, requiring no mechanical transmission components and causing no seal damage, which is beneficial for reducing interfacial thermal resistance and improving heat exchange efficiency.

[0006] In one approach, the frequency of the high-frequency micro-vibration is from 100 Hz to 1 kHz, and the amplitude is from 50 μm to 200 μm. This approach defines an optimal range for frequency and amplitude, which is beneficial for achieving a balance between energy utilization efficiency, shear force magnitude, and safety.

[0007] In one approach, the alternating magnetic field is generated in an intermittent mode, with an interval period of 2-4 hours and a single operation duration of 2-5 minutes. This intermittent drive mode, matched to the several-hour timescale of film formation, helps to reduce the average power consumption to less than one-sixtieth of that of continuous drive, thus extending the system lifespan.

[0008] One approach also includes real-time monitoring of the cell's temperature distribution and dynamic adjustment of the frequency and / or amplitude of the alternating magnetic field based on the monitoring results, providing directional excitation only to hot spots. This approach achieves on-demand driving through temperature feedback, applying disturbances only to hot spots, which helps reduce average power consumption and improve temperature control accuracy.

[0009] One approach includes multiple flexible vibrating elements, each disposed on a different inner wall of the enclosure. The method further includes independently driving the alternating magnetic field corresponding to each flexible vibrating element, or adjusting the phase difference of the alternating magnetic fields to form a superposition of sound fields. Alternatively, multiple flexible vibrating elements are arranged in an array on the same inner wall of the enclosure, with each flexible vibrating element corresponding to a cell region. This approach employs multi-magnetic film coordination and phase control, which is beneficial for forming a uniform sound field or enhancing the acoustic flow intensity in a specific area, improving the air film clearance capability, and allowing for more precise hotspot management through array arrangement.

[0010] In one design, the flexible vibrating element is positioned below the liquid level on the inner wall of the tank, directly opposite the cell gap or the root of the cell terminal. This design ensures direct coupling of sound waves by placing the vibrating element below the liquid level, and direct alignment with the cell gap or terminal root minimizes the straight-line propagation path of the sound waves, thus improving energy utilization efficiency.

[0011] In one design, the flexible vibration element is a flexible magnetic diaphragm, which is integrally formed with the inner wall of the housing or fixedly attached by adhesive. This design uses a flexible magnetic diaphragm attached to the inner wall, occupying zero space and not interfering with the battery cell; both integral forming and adhesive methods can effectively transmit vibration energy.

[0012] In one approach, the alternating magnetic field is generated by an electromagnetic coil with a ferrite core. This approach uses an electromagnetic coil with a ferrite core, which helps to enhance the magnetic field strength, reduce magnetic leakage, and improve driving efficiency.

[0013] This invention also provides a thermal management method for an immersed liquid-cooled battery pack, comprising: immersing the battery cell in an insulating coolant; and generating acoustic flow in the boundary layer near the surface of the battery cell using any of the above methods, thereby utilizing the shear force of the acoustic flow to remove the gas film on the surface of the battery cell. This solution combines the acoustic flow gas film disruption method with immersion liquid cooling to achieve multi-scale coordinated thermal management, while leveraging the advantages of liquid cooling heat dissipation and precise acoustic flow gas film removal.

[0014] In one design, the insulating coolant is silicone oil, fluorinated liquid, or transformer oil, with a kinematic viscosity at 25°C ranging from 5 × 10⁻⁶ m² / s to 50 × 10⁻⁶ m² / s. This design limits the type and viscosity range of the coolant, which is beneficial for achieving an optimal balance between acoustic shear force and sound wave propagation distance.

[0015] Compared to existing technologies, this invention offers the following advantages: It employs non-contact acoustic flow disturbance, eliminating the need for any perforations or mechanical transmission components in the enclosure, thus preserving the battery pack's sealing integrity and eliminating the risk of immersion fluid leakage; it completely eliminates mechanical transmission components such as motors, gears, oscillating mechanisms, and physical stirrers, fundamentally eliminating mechanical wear, jamming, and fatigue failure, achieving maintenance-free operation throughout the battery pack's lifespan; the acoustic flow effect acts only on the micron-level boundary layer of the cell surface, without causing macroscopic flow of the immersion fluid, avoiding the risk of mechanical impact on the cells, while precisely removing the gas film, reducing interface thermal resistance by 25%-40%, and lowering hotspot temperatures by 3-5°C; the intermittent operating mode reduces average power consumption to less than one-sixtieth of continuous operation, significantly extending system lifespan; temperature feedback control enables on-demand drive, further improving energy efficiency and temperature control accuracy. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a flowchart of a battery pack thermal boundary layer destruction method based on acoustic flow effect according to an embodiment of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application. Similar reference numerals and letters in the drawings indicate similar items. Once an item is defined in one drawing, it does not need to be defined again in subsequent drawings. The terms "upper," "lower," "left," "right," "vertical," "horizontal," etc., indicate the orientation or positional relationship based on the orientation shown in the drawings or conventional understanding, and are used only for description, not to indicate or imply that the device must have a specific orientation. In the absence of conflict, the embodiments and features of this application can be combined with each other.

[0019] The present invention will now be described in detail with reference to specific implementation methods. It should be noted that these implementation methods are only used to illustrate the present invention and should not be construed as limiting the scope of protection of the present invention.

[0020] I. Basic Steps of the Method The method for breaking the thermal boundary layer of an immersion battery pack based on the acoustic flow effect is performed according to the following steps.

[0021] Step S1: Generate an alternating magnetic field on the outside of the battery pack casing. This alternating magnetic field can be generated by an electromagnetic coil wound around a frame, with a ferrite core embedded inside to enhance the magnetic field strength. The coil is connected to a drive power supply via a cable, which outputs a sinusoidal or square wave current with adjustable frequency and amplitude. Alternatively, the alternating magnetic field can be generated by combining multiple permanent magnets with a drive coil, where the permanent magnets provide a bias magnetic field and the drive coil provides alternating excitation.

[0022] Step S2: The alternating magnetic field penetrates the box wall, driving a flexible vibrating element installed on the inner wall of the box to generate high-frequency micro-vibrations. The flexible vibrating element can be a flexible magnetic membrane with a thickness ranging from 0.1 mm to 0.5 mm, made of a flexible polymer matrix (such as fluorosilicone rubber) and magnetic filler (such as neodymium iron boron powder). To ensure an amplitude of 50-200 μm under an alternating magnetic field of 100 Hz-1 kHz (peak magnetic field strength approximately 50-200 mT), the volume fraction of the magnetic filler in the flexible magnetic membrane should be 15%-30%, with a remanence Br ≥ 0.6 T (such as ferrite) or Br ≥ 1.0 T (such as neodymium iron boron). If it is below this range, the amplitude is insufficient; if it is above this range, the brittleness of the magnetic membrane increases. The magnetic membrane is attached to the inner wall of the box, corresponding to the position of the outer coil. The magnetic field penetrates the plastic or non-magnetic metal box, acting on the magnetic filler inside the magnetic membrane, causing the magnetic membrane to undergo periodic bending deformation and displacement, thereby generating high-frequency micro-vibrations. The frequency and amplitude of the vibration are determined by the frequency of the driving signal and the amplitude of the current. According to electromagnetism, the magnetic driving force F_m on a flexible magnetic membrane in an alternating magnetic field is F_m ∝ B^2·A / μ_0, where B is the magnetic flux density and A is the area of ​​the magnetic membrane. In the frequency range of 100Hz-1kHz, the vibration amplitude of the magnetic membrane is proportional to the driving force and balances with the elastic restoring force k·x. Experimental calibration shows that when the coil current is 1A and the air gap in the magnetic core is 5mm, the surface area of ​​the magnetic membrane is B≈100mT, corresponding to an amplitude of approximately 100μm; when the current increases to 2A, B≈180mT, and the amplitude is approximately 180μm. Therefore, the amplitude can be linearly controlled by adjusting the coil current, with a control coefficient of approximately 1μm / mT.

[0023] Step S3: High-frequency micro-vibration radiates sound waves within the immersion liquid, utilizing the viscous dissipation of the liquid to generate an acoustic flow effect. The vibration of the magnetic membrane acts as a piston-like sound source, radiating sound waves into the immersion liquid. Due to the viscosity of the immersion liquid, sound waves dissipate energy during propagation, creating a velocity gradient along the propagation direction, thus generating a stable time-averaged flow, i.e., the acoustic flow effect. Taking dimethyl silicone oil (viscosity 10 cSt, 25℃) as an example, under the conditions of a driving frequency of 500 Hz and a magnetic membrane amplitude of 100 μm, the maximum acoustic flow velocity is calculated to be approximately 2.4 mm / s according to the acoustic flow theory formula u_max = (ρ·c·v^2) / (8·μ·f), the boundary layer thickness δ = sqrt(μ / (ρ·f)) ≈ 0.14 mm, and the shear force τ = μ·(u_max / δ) ≈ 0.17 Pa. After repeated testing, this shear force can stably peel off gas films with a thickness of 10-30 μm. The velocity distribution of the acoustic flow within the boundary layer on the cell surface is approximately a linear profile, increasing linearly from zero velocity at the cell surface (no slip boundary conditions) to the maximum acoustic flow velocity u_max at the outer edge of the boundary layer. Therefore, the shear force τ = μ·(u_max / δ) is a constant value and does not change with distance from the surface. This uniform shear force is beneficial for uniformly peeling off the entire gas film layer, rather than local tearing. The velocity field distribution of the acoustic flow is related to the frequency and amplitude of the sound field, as well as the viscosity coefficient of the liquid. Under typical operating conditions, the acoustic flow is mainly concentrated in the boundary layer near the magnetic film and the cell surface.

[0024] Step S4: Acoustic flow generates localized shear force within a micrometer-scale boundary layer near the cell surface. The boundary layer thickness is on the order of the sound wave wavelength, ranging from tens to hundreds of micrometers. The velocity gradient generated by the acoustic flow within this thin layer creates shear force, the magnitude of which can be estimated using hydrodynamic formulas. Taking silicone oil as an example, under conditions of 500 Hz frequency and 100 μm amplitude, the estimated acoustic flow shear force is approximately 0.17 Pa, sufficient to overcome the adhesion of the gas film (typically 0.05-0.1 Pa, according to literature reports). The interfacial adhesion between common cell insulating materials (PET, PI, ceramic coatings) and silicone oil is in the range of 0.03-0.2 Pa. The shear force generated by this method, 0.1-0.3 Pa (corresponding to frequencies of 500-1000 Hz and amplitudes of 100-200 μm), can cover most materials. For ceramic coatings with higher adhesion (approximately 0.2 Pa), the shear force can be increased to 0.3 Pa by increasing the amplitude to 200 μm or the frequency to 1 kHz.

[0025] Step S5: Shear force breaks down the thermal boundary layer (gas film) attached to the cell surface. After the gas film is peeled off or broken up, it floats to the liquid surface in the form of small bubbles or redissolves in the silicone oil. The cell surface resumes direct contact with the silicone oil, significantly reducing the interfacial thermal resistance and lowering the hot spot temperature. In this application, "thermal boundary layer" specifically refers to the micron-scale gas film layer formed on the cell surface due to gas evolution, rather than the temperature boundary layer. The thickness of this gas film is typically between 5-50 μm, depending on the settling time and the saturation of dissolved gas. The presence of the gas film increases the solid-liquid contact angle to over 90°, significantly reducing the heat transfer coefficient.

[0026] II. Optimized Parameters and Driving Mode The preferred frequency for high-frequency micro-vibration is 100Hz to 1kHz. Below 100Hz, the sound wave wavelength is too long (greater than 15mm in silicone oil), making it difficult to form an effective acoustic flow gradient within the micron-level boundary layer. Above 1kHz, the attenuation coefficient of the sound wave in silicone oil increases sharply (proportional to the square of the frequency), reducing energy utilization and potentially entering the sensitive frequency band of the BMS sampling circuit. The preferred amplitude is 50μm to 200μm. Below 50μm, the acoustic flow shear force is insufficient to damage the gas film; above 200μm, the vibration of the magnetic film may cause mechanical shock to the cell and accelerate the fatigue of the magnetic film material. In practical applications, the amplitude can be adjusted within this range according to the specific size of the battery pack and the type of cell.

[0027] Unlike ultrasonic cleaning (typically with frequencies >20kHz and amplitudes >1mm), this method uses mid-to-low frequencies of 100Hz-1kHz and micro-amplitude of 50-200μm. The resulting acoustic flow is limited to the micron-level boundary layer on the surface of the battery cell, without causing macroscopic cavitation or severe disturbances in the liquid, thus avoiding mechanical damage to the battery cell and the immersion fluid. The maximum velocity of the acoustic flow generated by this method is approximately 2.4mm / s, and the boundary layer thickness is 0.14mm. Based on this, the local Reynolds number Re = ρ·u·δ / μ ≈ 960 × 0.0024 × 0.00014 / 0.01 ≈ 0.032, which is much lower than the laminar flow critical value of 2000, belonging to typical creeping flow (Stokes flow), without generating macroscopic turbulence. In contrast, the flow velocity generated by existing mechanical stirring is typically >100mm / s, and the Reynolds number is >1000, belonging to macroscopic flow. Therefore, this method causes disturbances at the microscale without interfering with the overall temperature stratification of the immersion fluid.

[0028] Alternating magnetic fields can be generated in an intermittent mode. Using a 100Ah lithium iron phosphate prismatic cell, the cell surface was monitored by optical coherence tomography (OCT) at 25℃ and 0.5C float charge. Results showed that the average gas film thickness increased from 5μm to 8μm (no significant increase) after 2 hours of rest, and to 25μm after 4 hours (significant increase in thermal resistance). After initiating vibration at 500Hz and 100μm amplitude, the gas film thickness decreased to 12μm within 2 minutes (scavenging rate approximately 50%), and to below 1μm after 5 minutes (scavenging rate >95%). Therefore, the gas film requires continuous rest for 2 to 4 hours to significantly form and lead to a decrease in heat transfer performance, while disrupting the formed gas film only requires 2 to 5 minutes of vibration excitation. A timer can be set to automatically start the drive power supply every 2 to 4 hours, operating continuously for 2 to 5 minutes before stopping. The average power consumption of this intermittent mode is only one-sixtieth of that of the continuous operation mode, making it highly suitable for power-sensitive long-life energy storage systems. Intermittent mode can be used in combination with temperature feedback mode: it normally operates in intermittent mode, and when the temperature sensor detects an abnormally high temperature, an additional disturbance is temporarily added.

[0029] As a preferred option, the intermittent cycle can be dynamically adjusted according to the average temperature of the battery cell: when the average temperature is below 30℃, the cycle is 4 hours; when it is between 30-40℃, the cycle is 3 hours; and when it is above 40℃, the cycle is 2 hours. The duration of a single operation can also be adjusted according to the rate of temperature change: if the temperature difference rises rapidly (>0.5℃ / min), the duration of a single operation can be extended to 5 minutes.

[0030] III. Temperature Feedback Control To further improve energy efficiency and temperature control accuracy, the temperature distribution of the battery cell can be monitored in real time, and the frequency and / or amplitude of the alternating magnetic field can be dynamically adjusted based on the monitoring results, with directional excitation only applied to hot spots. The temperature sensor can be an NTC thermistor or a PT100 platinum resistance thermometer, attached to the center of the large surface area of ​​the battery cell or placed at the inlet and outlet of the battery module. The temperature sampling frequency should be no less than 1Hz, with a sampling accuracy of ±0.5℃. The control logic can employ a threshold comparison method or a PID algorithm. An example of PID control is as follows: Set the target temperature T_set = average cell temperature + 1℃, and the deviation e(t) = T_i(t) - T_set. The driving frequency f = 500 + 50 * e(t) (Hz), limited to the range of 100-1000Hz; the driving current I = 1 + 0.2 * e(t) (A), limited to the amplitude of 50-200μm. When |e| < 0.5℃ for 30 seconds, the drive is stopped. Alternatively, a simpler threshold comparison method can be used: calculate the average temperature T_avg of all cells and the temperature difference ΔT = T_i - T_avg for each cell. When ΔT exceeds the first threshold (e.g., 2°C), the coil corresponding to that cell region is activated; when ΔT falls below the second threshold (e.g., 0.5°C), the drive is stopped. The drive frequency can be set to a fixed value (e.g., 500Hz) or dynamically adjusted according to the temperature difference—the larger the temperature difference, the higher the drive frequency or the larger the amplitude.

[0031] IV. Multi-magnetic film synergy and phase control When multiple flexible vibrating elements are installed on the inner walls of the enclosure, the alternating magnetic field corresponding to each vibrating element can be driven independently. For example, when the cell temperature in the left side of the enclosure is detected to be too high, only the coil corresponding to the left wall is driven; when the cell temperature in the central area is detected to be too high, the coils on both sides can be driven simultaneously, causing the sound flows from two directions to superimpose in the central area, generating a stronger shear force. By adjusting the phase difference of each alternating magnetic field, the distribution of the sound field can be further controlled. The phase difference φ can be adjusted within the range of 0° to 360°. When φ=0°, the sound pressure amplitude increases to twice that of a single source after the two sound waves are superimposed in the central area, and the local shear force increases by about 30%-50%; when φ=180°, a standing wave is formed, the sound flow at the node is almost zero, but the sound flow at the antinode is enhanced. In actual control, the controller can achieve a 1° precision adjustment by adjusting the phase register of the drive signal. Multiple magnetic membranes can also work alternately in a time sequence. For example, the left magnetic membrane can be driven to work for 30 seconds, and then the right magnetic membrane can be driven to work for 30 seconds. This cycle is repeated to avoid the sound waves interfering with each other and causing an uncertain sound field distribution, while also causing the immersion liquid to generate a larger overall disturbance.

[0032] As an alternative, multiple small magnetic films can be arranged in an array on the same inner wall of the enclosure. Each magnetic film corresponds to a cell area and is driven independently, enabling more precise hotspot management. Independent driving of multiple magnetic films can be achieved using a multi-channel electromagnetic coil array. Each coil is controlled by an independent H-bridge driver chip (such as the DRV8871), and the current of each channel is controlled separately through the PWM output port of a microprocessor (such as the STM32F103). When multiple coils need to be driven simultaneously, time-division multiplexing or phase synchronization control can be used. To avoid electromagnetic crosstalk between coils, the coil spacing should be greater than 5 times the coil diameter, or a magnetic shielding partition can be used.

[0033] V. Magnetic Film Arrangement and Structure To maximize sound wave transmission efficiency, the flexible vibrating element should be positioned below the liquid surface on the inner wall of the enclosure. Above the liquid surface, sound waves must pass through an air layer; due to the significant difference in acoustic impedance between air and liquid (approximately 3500 times), most of the energy is reflected. Below the liquid surface, sound waves couple directly into the immersion liquid. Simultaneously, the vibrating element should be directly aligned with the cell gap or the root of the cell's electrode. The cell gap represents the shortest path for sound wave propagation, while the electrode root is the area where air film most easily accumulates.

[0034] The flexible vibration element is preferably a flexible magnetic membrane, which can be connected to the inner wall of the housing in two ways. One method, integral molding, is suitable for plastic housings. During injection molding, the magnetic membrane is embedded into the inner wall, eliminating the connection interface and achieving the highest vibration transmission efficiency. In integral molding, a 0.2mm thick pre-cured flexible magnetic membrane is placed in the injection mold. The injection material is PC / ABS alloy, the melt temperature is 260℃, the mold temperature is 80℃, the injection pressure is 80MPa, and the holding time is 5 seconds. After molding, the magnetic membrane is firmly bonded to the inner wall of the housing, with an interface shear strength ≥2MPa and a vibration transmission efficiency (magnetic membrane amplitude / magnetic field induced amplitude at the coil) ≥0.9. The other method, adhesive bonding, is suitable for metal housings. An oil-resistant and high-temperature-resistant adhesive is used to bond the magnetic membrane to the inner wall, with the adhesive layer thickness controlled within the range of 0.05mm to 0.15mm. The matrix material of the magnetic film can be fluorosilicone rubber, nitrile rubber, silicone rubber, etc., and the magnetic filler can be neodymium iron boron powder (grade N35 or higher), samarium cobalt powder (Sm2Co17), or ferrite powder. When the magnetic film thickness is 0.2mm, the amplitude can reach about 100μm under the drive of 500Hz frequency and 1A current.

[0035] After continuous vibration for 10^8 cycles at 500Hz and 100μm amplitude (approximately 55 hours of continuous operation), the magnetic membrane showed no visible cracks, and the amplitude attenuation was less than 5%. Calculated based on intermittent mode (3 minutes of operation every 3 hours), this is equivalent to a 60-year service life, meeting the full lifespan requirements of the energy storage system.

[0036] VI. Device for Generating Alternating Magnetic Fields The alternating magnetic field is preferably generated by an electromagnetic coil with a ferrite core. The ferrite core uses manganese-zinc or nickel-zinc ferrite, which has high permeability (initial permeability approximately 2000-5000), confining the magnetic flux within the magnetic circuit and reducing leakage flux. The coil has 100 to 500 turns and a drive current of 0.5A to 3A. As an alternative, a combination of a permanent magnet and a drive coil can be used. The permanent magnet provides the bias magnetic field, and the drive coil provides the alternating excitation, allowing for greater magnetic force without increasing the number of coil turns.

[0037] VII. Thermal Management Methods for Immersed Liquid-Cooled Battery Packs A complete thermal management method for submerged liquid-cooled battery packs comprises two layers. The first layer involves immersing the battery cells in an insulating coolant, utilizing the liquid's high specific heat capacity and thermal conductivity for basic heat dissipation. The coolant can be silicone oil (such as dimethyl silicone oil, viscosity 10 cSt), fluorinated liquid (such as 3M Novec 7100), or transformer oil (such as No. 25 transformer oil), and its kinematic viscosity at 25°C should preferably be between 5 × 10⁻⁶ m² / s and 50 × 10⁻⁶ m² / s. For viscosities below 5 × 10⁻⁶ m² / s (such as low-viscosity fluorinated liquids), the acoustic shear force decreases proportionally to viscosity, and below 0.05 Pa, it cannot effectively disrupt the gas film. For viscosities above 50 × 10⁻⁶ m² / s (such as high-viscosity silicone oils), the acoustic attenuation coefficient α∝μ·f² increases by a factor of 3 at 1000 Hz, and the effective range is shortened from 100 mm to below 30 mm, failing to cover distant cells. Therefore, choosing this viscosity range can simultaneously ensure shear force and propagation distance. The second layer: Using any of the aforementioned methods, acoustic flow is generated in the boundary layer near the cell surface, utilizing acoustic shear force to remove the gas film on the cell surface. The two methods work synergistically: immersion liquid cooling handles overall heat dissipation, while the acoustic flow effect removes localized gas films. They do not interfere with each other because the acoustic flow only acts on the micron-level boundary layer, without altering the macroscopic flow characteristics of the liquid, thus achieving multi-scale thermal management.

[0038] VIII. Performance Verification and Adaptability to Extreme Operating Conditions A static submerged battery pack model was constructed under laboratory conditions. The casing was made of transparent plastic, filled with silicone oil, and housed multiple battery cells. An electromagnetic coil was installed on the outside of the casing, and a flexible magnetic film was attached to the corresponding position on the inner wall of the casing. A sinusoidal drive signal with a frequency of 500Hz and a current of 1A was applied to the coil, causing the magnetic film to vibrate at a micro-vibration of approximately 100μm. High-speed imaging of the battery cell surface showed that the attached gas film was peeled off within a few minutes. Thermocouples were used to measure the surface temperature of the battery cells. Under constant current discharge conditions of 1C at room temperature (25℃), the hot spot temperature dropped by 3-5℃ within 3 minutes, and the interfacial thermal resistance decreased by 25%-40% (measured using a thermal resistance tester, according to ASTM D5470 standard, by contacting the battery cell with a heat flow sensor, applying a pressure of 50N, and measuring the thermal resistance under steady-state conditions. Comparing before and after the start-up acoustic flow, the thermal resistance decreased from an initial value of approximately 120cm²·K / W to 70-90cm²·K / W, a decrease of 25%-40%. The test was repeated 3 times, and the standard deviation was less than 5%). The effectiveness of this method can be verified by comparing temperature data with and without the acoustic flow effect.

[0039] Control group: Same battery pack, same discharge conditions, no acoustic current activation. After 4 hours of rest, the cell hotspot temperature rose to 52℃, and the interface thermal resistance remained at 120cm²·K / W. Experimental group: Acoustic current activation was performed. After 5 minutes, the hotspot temperature dropped to 47℃, and the thermal resistance dropped to 80cm²·K / W. The comparison shows that acoustic current reduced the thermal resistance by 33% and the temperature dropped by 5℃. Repeated three times yielded consistent results.

[0040] At -20℃, the viscosity of silicone oil increases to over 100 cSt, leading to increased acoustic attenuation. To compensate, the driving frequency can be reduced to 100-300 Hz, and the amplitude increased to 150-200 μm. Experiments show that this method can still reduce the hotspot temperature by 2-3℃ under -20℃ and 1C discharge conditions. Under high-frequency charge / discharge conditions (4 full charge / discharge cycles per day), gas film formation accelerates, allowing the interval period to be shortened to 1-2 hours while maintaining effective heat dissipation. During cell charge / discharge, electrode volume changes cause internal pressure fluctuations, promoting dissolved gas precipitation. Especially at high discharge rates, the breathing effect intensifies, accelerating gas film formation. The interval period of this method can be adaptively adjusted according to the charge / discharge rate: 4 hours for below 0.5C, 2 hours for 1C, and 1 hour for 2C. Simultaneously, acoustic flow is continuously activated for the first 10 minutes after discharge begins to suppress initial gas film formation.

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for breaking the thermal boundary layer of a battery pack based on the acoustic flow effect, characterized in that, Includes the following steps: Step S1 generates an alternating magnetic field on the outside of the battery pack casing; The alternating magnetic field described in step S2 penetrates the box wall and drives the flexible vibration element installed on the inner wall of the box, which can be driven by the magnetic field, to generate high-frequency micro-vibration. The high-frequency micro-vibration described in step S3 radiates sound waves in the immersion liquid, generating an acoustic flow effect by utilizing the viscosity dissipation of the liquid. The acoustic flow described in step S4 creates localized shear forces within the micron-level boundary layer near the cell surface; Step S5 describes the shear force that destroys the thermal boundary layer attached to the surface of the battery cell.

2. The method according to claim 1, characterized in that, The frequency of the high-frequency micro-vibration is from 100 Hz to 1 kHz, and the amplitude is from 50 μm to 200 μm.

3. The method according to claim 1, characterized in that, The alternating magnetic field is generated in an intermittent mode, with an intermittent period of 2-4 hours and a single working time of 2-5 minutes.

4. The method according to claim 1, characterized in that, Also includes: The temperature distribution of the battery cell is monitored in real time, and the frequency and / or amplitude of the alternating magnetic field are dynamically adjusted according to the monitoring results, with directional excitation only applied to hot spots.

5. The method according to claim 1, characterized in that, The method includes multiple flexible vibration elements, which are respectively disposed on different inner walls of the housing; the method further includes: independently driving the alternating magnetic field corresponding to each flexible vibration element, or adjusting the phase difference of each alternating magnetic field to form a superposition of sound fields; or, multiple flexible vibration elements are disposed on the same inner wall of the housing to form an array, with each flexible vibration element corresponding to a cell area.

6. The method according to claim 1, characterized in that, The flexible vibration element is disposed below the liquid level on the inner wall of the box and is directly opposite the gap between the battery cells or the root of the battery cell's terminal post.

7. The method according to claim 1, characterized in that, The flexible vibration element is a flexible magnetic membrane, which is integrally formed with the inner wall of the box or fixedly attached by adhesive.

8. The method according to claim 1, characterized in that, The alternating magnetic field is generated by an electromagnetic coil with a ferrite core.

9. A thermal management method for an immersion liquid-cooled battery pack, characterized in that, include: The battery cells are immersed in insulating coolant; Using the method described in any one of claims 1 to 8, an acoustic flow is generated in the boundary layer near the surface of the battery cell, and the gas film on the surface of the battery cell is removed by the shear force of the acoustic flow.

10. The method according to claim 9, characterized in that, The insulating coolant is silicone oil, fluorinated liquid, or transformer oil, and its kinematic viscosity at 25°C is 5×10^{-6}m^2 / s to 50×10^{-6}m^2 / s.

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