Visualization study on the flow characteristics of a nitrogen pulsating heat pipe

An experimental setup for visualization study of a nitrogen pulsating heat pipe (PHP) is developed. The PHP is a flat-plate PHP fabricated using MEMS technology with silicon base plate and BF33 glass cover plate. The hydraulic diameter of the channel is 667 μm and the overall dimension of the PHP is 70 × 25 × 2 mm3. The filling ratios (FRs) in this study are measured with visualization images. Six filling ratios are adopted ranging from 15% ∼ 94% and the PHP only fails to start-up with 15% FR. Flow images of nitrogen PHPs are obtained for the first time. Four types of flow patterns are observed. The variations of flow characteristics and fluid motion with heat loads and FRs are analyzed. The fluid motion diagram and flow pattern proportion are extracted from the visualization results as well. Large amplitude oscillating motion is rarely observed in the nitrogen PHP. And annular flow is found to be dominant in low and medium FR working conditions, which indicates the one-dimensional slug flow model may have a narrow application range for the flat-plate nitrogen PHP.