Study on microchannel cavitation phenomena based on experiment and simulation

Cavitation phenomenon is a common fluid mechanics phenomenon. Its special flow limiting structure generates abundant microchannel cavitation in the microchannel, and the existence of these cavitation flow field with complex flow field structure characteristics is formed in the microchannel. In order to better understand the cavitation phenomenon in the microchannel, this paper takes the microchannel as the research object, adopts the method of comparison between experimental research and numerical simulation, changes the cavitation number by adjusting parameters, and analyses the cavitation phenomenon of the microchannel under different working conditions. From the experimental point of view, a micro-channel with rectangular current limiting structure was designed. In order to observe and analyse the microchannel hydraulic cavitation phenomenon, the hydraulic cavitation micro-channel experiment platform was designed and built, which could analyse the cavitation flow pattern of the microchannel. The computational fluid dynamics simulation software Fluent was used to simulate and study the microchannel cavitation phenomenon characteristics by changing the boundary conditions. It is found that the cavitation phenomenon becomes more obvious as the number of cavitations decreases. Finally, the simulation results are compared with the experimental results, and the simulated cavitation flow pattern is in good agreement with the experimental results.


Introduction
Microchannel is a kind of flow channel structure with the size in the micron scale, which is widely used in the field of microfluidics, such as aerospace, biomedicine, and instrument testing [1][2].However, cavitation often occurs in microchannels under high-speed flow conditions, which is caused by the rapid evaporation of liquid to form bubbles because the local pressure of fluid is lower than the saturated vapor pressure of liquid [3].Recently, the broad applicability of cavitation within microfluidic systems across diverse domains has garnered increased attention.Its potential advantages have been harnessed across various areas, including augmenting heat transfer, advancing chemical engineering, enhancing water treatment, and dispersing nano-materials [4][5][6][7][8][9].Cavitation phenomenon will lead to many problems, such as increased flow resistance, decreased heat exchange efficiency, and even equipment damage, so it is important to conduct in-depth research on cavitation phenomenon in microchannels.
To understand and control cavitation phenomena in microchannels, researchers often employ a combination of experimental and numerical simulations.The experiment can provide the data of direct observation and measurement of cavitation phenomenon, while the numerical simulation can reveal the mechanism of cavitation and the characteristics of flow field.Through the comprehensive analysis of the experimental and simulation results, the influencing factors and development process of cavitation phenomenon in microchannels and its influence on fluid flow can be more comprehensively understood.
In the past few decades, many studies have been devoted to revealing the mechanism of cavitation in microchannels and exploring corresponding suppression and control methods.Among them, Barber J et al. [10] found through experimental research that fluid velocity, pressure and channel size are important parameters affecting cavitation phenomenon in microchannels.In addition, Kadam S T [11] used numerical simulation methods to analyze in detail the flow characteristics of bubble formation, growth and shedding in microchannels, providing important clues for further understanding of the mechanism of cavitation in microchannels.
In this paper, the cavitation phenomenon in microchannels is studied through the combination of experiment and numerical simulation.First adopts experimental means to study the micro-channel cavitation.Under the conditions of different fluid flow rates, the cavitation manifold is observed and recorded, and the simulation software Fluent is used to simulate the micro-channel cavitation, so as to explore the micro-channel cavitation phenomenon.By analyzing the mechanism and flow characteristics of cavitation phenomena, we aim to provide a scientific basis for further optimization of microchannel design and coping with cavitation problems.

Experimental setup
To facilitate the observation and analysis of micro-channel hydraulic cavitation phenomena, we devised and established an experimental platform dedicated to micro-channel hydraulic cavitation.This platform serves as a structured environment for the systematic exploration of micro-channel cavitation through experimental means.The configuration of the system is depicted in figure 1 and figure 2, showcasing its constituent components.System composition physical picture.The microchannel setup encompasses five parallel flow channels, each boasting dimensions of 26mm in length, 0.5mm in width, and 0.2mm in depth.Positioned 8.5mm downstream from the entrance of these microchannels is a rectangular constriction structure, which serves as a current limiter.This structure features dimensions including a throat length of 0.2mm, a width of 0.1mm, and a depth of 0.2mm.In order to facilitate the smooth flow of deionized water within the channels and enable accurate data measurement, the utilization of a protective encapsulation is deemed necessary.The microchannel assembly is composed of multiple integral elements, encompassing a microchannel chip, an upper cover, a resilient toughened glass layer, and a 0.1mm thick silicone skin.This amalgamation of components collaboratively constitutes the comprehensive microchannel structure.A visual depiction portraying the composition of the microchannel is effectively presented in figure 3.

Procedure
To ensure the integrity of the experimental setup, the initial step involved carefully adding deionized water to the tank, ensuring that the entire system was thoroughly filled to check for any potential leakage prior to the actual experiment.Subsequently, the inlet pump was activated, allowing the deionized water to flow into the system line.Powered by a pump, the deionized water passed through a series of components, including a filter and a glass rotor flowmeter, before entering the microchannel.
The flow rate was precisely controlled by adjusting the glass rotor flowmeter to achieve the desired experimental conditions.Once the system reached a stable state, data recording commenced, capturing relevant information.Simultaneously, meticulous adjustments were made to the position and focal length of the microscope to enable clear visualization of the flow patterns, which were documented using a high-speed camera.
Upon completion of the experiment, the pump and all other equipment were safely shut down.

Data measurement
The total liquid flow in the micro-channel was measured by a glass rotor flowmeter.The glass rotor flowmeter used in this experiment is LZB-4, with a maximum measuring range of 16L/h, a maximum allowable working pressure of 1MPa and an accuracy of 0.64L/h.According to the measured liquid flow rate, the average liquid flow rate through the current limiting structure can be calculated by the following formula: The average velocity of the section of current-limiting structure can be calculated by the following formula: Here, cross A is the throat cross-sectional area of the current-limiting structure, and vol U is the volume flow rate.
The cavitation number in microchannel flow is defined as: Where out P is the fluid pressure at the outlet of the microchannel, and v is the average flow velocity at the throat of the current-limiting structure.

Numerical method
In this paper, we employed a two-dimensional microchannel with a current limiting structure to simulate the cavitation manifold.To enhance computational efficiency, we utilized a half-structure physical model, reducing the computational workload.The VOF model was employed to simulate the multiphase flow, while the Transition SST model was chosen as the turbulence model.For the cavitation model, we opted for the S-S model.
To investigate the effect of various flow rates on cavitation, we varied the cavitation number by adjusting the flow rate, while keeping the temperature at 20 degrees and the outlet pressure at standard atmospheric pressure.Specifically, we selected simulation conditions corresponding to cavitation number of 0.0793.To ensure the reliability of our results, we conducted a grid independence verification and determined that a grid size of 65715 was sufficient.

Experimental results
Based on the experimental measurements of liquid flow data in the microchannel, where the temperature and outlet pressure are kept constant, we can determine the liquid flow rate and cavitation number at the current limiting structure using formula (1) and formula (2), as presented in table 1.The data clearly demonstrate that as the liquid flow rate increases, the corresponding flow rate accelerates, while the cavitation number progressively decreases.4 presents the experimental visualization results depicting various cavitation numbers.The visualization clearly illustrates that as the cavitation number decreases, the presence of cavitation inside the microchannel becomes increasingly pronounced.

Simulated result
We compared our simulation results with experimental data, as depicted in figure 5.This comparison allowed us to evaluate the agreement between the simulated cavitation manifold and the experimental findings, providing valuable insights into the accuracy and validity of our study.

Conclusions
In short, when the cross section of the current-limiting structure is constant, it can be concluded from formula (1) that the liquid velocity is faster as the flow rate increases.When the temperature and outlet pressure in the system remain constant, formula (2) shows that as the flow rate of the main pipe gradually increases, the liquid velocity in the current limiting structure also increases.Therefore, this leads to a reduction in local pressure, which triggers a reduction in the number of cavitations, and ultimately leads to a more pronounced cavitation phenomenon.
Reflecting on these findings, it becomes evident that the interplay between flow rate, liquid velocity, and cavitation has significant implications.The initial insight into how changes in flow rate impact cavitation provides a foundation for deeper exploration.Considerations such as the structural integrity of the conduit, the nature of the liquid itself, and the potential consequences for downstream processes also come into play.
Furthermore, the link between flow rate and cavitation points toward potential strategies for control and optimization.Engineers and designers could use this understanding to mitigate the adverse effects of cavitation by carefully adjusting the flow rate within acceptable limits.Conversely, deliberately increasing the flow rate might be leveraged in certain scenarios to induce controlled cavitation, which could have applications in fields such as material processing or mixing.
In essence, the initial analysis underscores the intricate relationship between flow dynamics and cavitation.Subsequent reflections emphasize the broader ramifications and practical applications of this phenomenon, paving the way for innovative approaches to fluid system management and process enhancement.

Figure 1 .
Figure 1.Schematic diagram of experimental platform and system composition.

Figure 2 .
Figure 2. System composition physical picture.The microchannel setup encompasses five parallel flow channels, each boasting dimensions of 26mm in length, 0.5mm in width, and 0.2mm in depth.Positioned 8.5mm downstream from the entrance of these microchannels is a rectangular constriction structure, which serves as a current limiter.This structure features dimensions including a throat length of 0.2mm, a width of 0.1mm, and a depth of 0.2mm.In order to facilitate the smooth flow of deionized water within the channels and enable accurate data measurement, the utilization of a protective encapsulation is deemed necessary.The microchannel assembly is composed of multiple integral elements, encompassing a microchannel chip, an upper cover, a resilient toughened glass layer, and a 0.1mm thick silicone skin.This amalgamation of components collaboratively constitutes the comprehensive microchannel structure.A visual depiction portraying the composition of the microchannel is effectively presented in figure3.

Figure 5 .
Figure 5.Comparison between experimental and simulation results.The obtained results clearly demonstrate a strong correlation between the experimental cavitation manifold diagram, the simulated gas phase volume fraction diagram and the simulated velocity diagram, providing solid evidence to support the accuracy and reliability of the conducted experiment.

Table 1 .
Velocity and cavitation number at different flow rates.