Abstract: Based on acoustic emission technique,the acoustic signals during pool boiling of water were measured and the relationship between boiling modes and acoustic signal characteristics was investigated by time-domain and frequency-domain analysis.The results showed that the values of acoustic energy,standard deviation(STD)and average absolute deviation(AAD)indicated extreme points when the boiling mode changed from natural convection to nucleate boiling at an excess temperature of 5℃.After transition boiling began at excess temperature of 30℃,the values of acoustic energy,STD and AAD changed irregularly.As excess temperature increased,the dominant frequency of acoustic signals showed two step decreases,at which excess temperatures were the turning points from natural convection to nucleate boiling and from nucleate boiling to transition boiling,respectively.Therefore,the step change of dominant frequency could be used as a criterion to identify the emergences of nucleate boiling and transition boiling.In comparison with the work published before,this work investigated transition boiling as well as natural convection and nucleate boiling.The criterion to determine boiling mode transition is proposed. Compared with conventional methods,this method was relatively fast,sensitive,accurate,non-intrusive and real-time online.

Key words: gas-liquid flow, heat transfer, nucleation, pool boiling of water, nucleate boiling, acoustic emission, dominant frequency

摘要： 采用声发射技术,结合频域和时域分析手段,研究了池内水沸腾过程中声信号变化与沸腾状态转变的对应关系。结果表明,当过热度为5℃时,沸腾由自然对流状态转变为核态沸腾状态,此时声信号能量、标准偏差和平均绝对偏差值均达到极值点;当过热度为30℃时,沸腾由核态沸腾状态转变为过渡沸腾状态,此时声信号能量、标准偏差和平均绝对偏差值开始出现剧烈的波动。与此同时,随过热度的增加,沸腾过程中声信号主频出现两次明显的阶跃性下降,分别对应于沸腾状态转变为核态沸腾和过渡沸腾。因此,声信号主频的阶跃性变化可作为沸腾状态转变的判据。与该领域的其他工作相比,不仅考察了自然对流状态和核态沸腾状态,还考察了核态沸腾向过渡沸腾的转变过程,同时提出了沸腾状态转变的判据。与传统方法相比,该法相对简单准确,具有灵敏度高、非侵入式、实时在线的优点。

关键词: 气液两相流, 传热, 成核, 池内水沸腾, 核态沸腾, 声发射, 主频