含不凝性气体的蒸汽气泡凝结过程研究

利用高速摄像仪对高过冷度下含不凝性气体的蒸汽气泡冷凝及破裂过程进行可视化研究,以分析不凝性气体对气泡微细化沸腾(MEB)过程的影响。实验结果表明:初始不凝性气体体积份额x0小于2.5%时,气泡突然破碎成大量微小气泡;x0在2.5%~7.5%之间时,较大气泡只会分裂成数个小气泡;x0大于7.5%时,气泡界面非常稳定,不会发生破碎和分裂现象。此外,当蒸汽气泡中含有较多不凝性气体时,气泡凝结过程减弱,液体对气泡的惯性冲击减小,气泡不易破裂。由此可表明,在气泡微细化沸腾发生时,不凝性气体的存在会阻碍加热面上气泡的破碎,从而降低传热能力。

Study on Condensation Process of Vapor Bubble Containing Noncondensable Gas

A visualized investigation was performed to study the condensation and collapse processes of vapor bubbles at high liquid subcooling with a high-speed video camera (Fastcam SA5). Experiment results show that a vapor bubble will collapse to many microbubbles rapidly when the initial volumetric fraction of noncondensable gas in the bubble, x₀, is less than 2.5%. Instead, a large bubble will split into just several tiny bubbles when x₀ is between 2.5% and 7.5%. Further, x₀ increases exceeding 7.5%, the bubble surface will be very stable, and no collapse or fragmentation occurs. The noncondensable gas in a vapor bubble reduces the condensation at the bubble surface, which weakens the inertial shock of the liquid on the surface and makes the bubble more stable. This may elucidate that the presence of noncondensable gas will inhibit the bubble collapse in microbubble emission boiling and deteriorate the heat transfer performance consequently.