Piston cooling effect is one of the key factors affecting the reliability of internal combustion engine. In order to investigate the influence of oscillatory flow structure of engine oil in the cooling oil gallery of piston on the heat transfer enhancement, and to reveal the oscillatory heat transfer mechanism of gas-liquid two-phase flow, the oscillatory flow structure of gas-liquid two-phase flow in the simplified rectangular cavity was obtained by high-speed photography technology. The effects of oscillatory structure of two - phase flow on average heat transfer coefficient of wall surface at different oscillation frequencies and water filling ratios were analyzed, and the effects of different oscillation frequencies and water filling ratios on maximum bubble diameter and mixing ratio were also studied. Results show that oscillation frequency is the primary factor influencing the oscillatory heat transfer effect of gas-liquid two-phase flow, the higher the oscillation frequency, the higher the average heat transfer coefficient of wall surface. The average heat transfer coefficient of wall surface will decrease if the water filling ratio is extra large or small, which is the highest at water filling ratio of 40%-60%. (C) 2019 Elsevier Ltd. All rights reserved.