Heat transfer characteristics of successive oil droplet impingement under minimum quantity lubrication

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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.