Abstract:

The temperature signals in pulsating heat pipe (PHP) exhibit more complex transient fluctuation characteristics, which can be better analyzed by using continuous wavelet transform (CWT) method. Based on the visualization experiment (glass PHP), the wavelet analysis method was used to investigate the PHP temperature oscillation signal and flow pattern identification. The results revealed that the dominant frequency of the temperature signals is affected by the sampling frequency, wall material and heat flux. A sampling frequency of 1 Hz may lead to temperature signal distortion at high heat flux ($q$=2.65—3.18 W/cm²), and it is recommended to use a sampling frequency of 10 Hz or higher. The thermal inertia of the glass material introduces signal distortion in wall temperature measurements, particularly at high heat flux. With increasing heat flux ($q$=0.35—3.18 W/cm²), the fluctuation amplitude of the fluid temperature decreases, the frequency increases, and the dominant frequency of the temperature signal increases (0.02—3.88 Hz). Correspondingly, the internal flow of the PHP has gradually transformed from a slug flow to the annular flow, and the large one-direction circulation flow appears. The dominant frequency derived from the fluid temperature signal inside the tube can be generalized to copper tubes or other metal PHP, which helps to identify the internal flow pattern and the change in the flow regime, and to better understand their heat transfer characteristics.

Key words: pulsating heat pipe, wavelet analysis, signal dominant frequency, flow, two-phase flow, heat transfer

摘要：

脉动热管中的温度波动信号具有较复杂的瞬态波动特征，采用连续小波变换方法可以较好地分析此类信号特征。采用全玻璃脉动热管，在可视化实验基础上，应用小波分析方法，重点研究PHP温度振荡信号及流型识别。结果表明，采集频率、管壁材料以及热通量等会影响温度信号的主频值。当热通量较高（$q$=2.65~3.18 W/cm²）时，1 Hz的采集频率容易导致信号失真，应采用10 Hz及以上的采集频率。玻璃管的热惰性会导致温度信号失真，尤其在高热通量（$q$=2.65~3.18 W/cm²）时不可忽略。总体来看，随着热通量的增加，蒸发温度波动幅度减小、频率增加。当热通量从0.35 W/cm²增加到3.18 W/cm²，流体温度信号的主频值从0.02 Hz增加到3.88 Hz。相应地，管内流型由弹状流逐渐向环状流转变，并出现单向大循环流。由管内流体温度信号求得的主频值可推广至铜管或其他金属材料的脉动热管，有助于识别其内部流型及流态变化，更好地理解其传热特性。

关键词: 脉动热管, 小波分析, 信号主频, 流动, 两相流, 传热