During oil–gas minimum lubrication, lubricating oil droplets are easily formed into hollow oil droplets containing bubbles when disturbed by a high-speed airflow. Microbubbles have an important influence on the heat transfer characteristics and movement of multiple oil droplets successively impinging on an oil film. In this work, the behavior of multiple oil droplets successively impacting an oil film is numerically simulated on the basis of the coupled level set-volume fraction method, and the influences of different bubble distributions on the heat transfer characteristics of double oil droplets successively impinging on the oil film are investigated. The formation mechanism of some unique heat transfer phenomena in the impingement process is discussed, and the influences of different bubble distribution forms on the geometric size of the thermal wake and cooling effect of the impingement area are analyzed. Results showed that a “cicada wing-like” thermal wake appears during the falling process of high-temperature oil droplets. The combined effects of heat transfer, flow field, and air flow separation behavior are the main reasons behind this wake. During the falling and spreading process of solid and hollow oil droplets, the velocity gradient difference at the tail of the oil droplet affects the geometric size of the wake. In the later stages of the impingement process, a vortex is formed in the impingement pit under the combined action of the space flow field and the pressure field. This vortex strongly improves the heat flux density in the impingement area. Different bubble distribution forms have different effects on the cooling and heat dissipation effect during impingement, and hollow oil droplets are unfavorable for cooling and heat dissipation.
The electric field-driven droplet formation technique can effectively improve the formation throughput and control droplet size, which is important for the application of micro-scale droplets in biopharmaceuticals and chemical analysis. In this paper, the droplet formation characteristics in T-junction microchannel under the action of electric field are investigated by coupling a three-dimensional lattice Boltzmann method (3D LBM) with leaky dielectric model, focusing on the effects of electric capillary number, flow ratio and viscosity ratio on the droplet size. It is shown that as the electrical capillary number increases, the non-uniformly distributed electric force stretches the dispersed phase to form a Taylor cone and increases shear force at the interface of the two liquids to overcome the surface tension force. This facilitates the transition from squeezing to dropping and reduces the droplet size. At high flow ratios, increasing the electric capillary number leads to a pinning effect between the dispersed phase and the wall, which intensifies the compression of continuous phase on the neck of dispersed phase, resulting in a significant decrease in droplet size. As the viscosity ratio increases, the vortex resistance caused by electrical force decreases, and thus the electric field effect will dominate the droplet formation process.
:In the oil-air lubrication system, the effective lubrication film is the key to ensure the lubrication effect, and its formation quality is closely related to the spreading flow process of the hollow oil droplets impacting the wall in the conveying airflow. To
Combining different nonsmooth surface structures on the friction pair can form a synergy effect of pressure and increase both the pressure balance and clearance flow characteristics. In view of the above findings, a composite-grooved structure, which combines with a helical groove and an annular groove, is designed in this paper and applied in a piston-copper pair. Compared with the existing related research, this paper has more scientific methods and multifaceted systematic. The numerical simulation method is applied to analyze the flow characteristics of six composite-grooved pistons from the aspects of pressure distribution, tilting torque and clearance leakage, the pressure balance and clearance flow. Results show that different groove patterns and different groove designs have different effects on the effectiveness of piston-copper pairs. Through comprehensive analysis, the specific influence of different groove patterns and groove designs is given, and the design instructions for improving the pressure balance and clearance flow characteristics of the piston-copper pair are obtained. Therefore, the result and methodology presented in this paper are expected to be used to design new-type piston.
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