Start-up And Operating Performance Of The Pulsating Heat Pipe And Its Visualization Research

A visual experimental investigation was conducted to study the start-up and operating characteristics of pulsating heat pipe (PHP). The behaviors of the vapor plug and the liquid slug were investigated. Combined with the data of the thermal laboratory bench and the existing results, the start-up and operating performance is analyzed in this paper.The sudden change of the flow pattern of the PHP during the start-up stage was observed. Also the behaviors of the working substance such as combination of the vapor plugs or liquid slugs, working substance of liquid state flowing back to the U-turns of the heating section etc. which are helpful for the redistribution of the different states of working substance and the start of the PHP were observed. Three different parameters, the highest start temperature, the minimum start power and the start time, are used to analyze the start-up performance. The PHP is easier to start with relatively lower filling ratio with lower start temperature and start power. The way the PHP starts changes from temperature mutant start-up to temperature smooth start-up with the heating power increasing, which may results in the start temperature falling. However, with the heating power increasing continuously, the start temperature would be raised. Therefore, the performance of the PHP is better when the heating power is moderate or relatively higher while the start temperature may be too high when the heating power continuously increases.Different flow patterns such as bubbly flow, plug-like flow, mixed flow and annular flow were seen in the stable operating stage. The flow direction of working substance may change when the heating power changes or when the heating power is high. There is more gaseous state working substance which flows more quickly and has a relatively higher temperature in the pipes in which the working substance goes up.The PHP with higher filling ratios can adapt to a wider range of heating power, though operation may be unstable within certain range of heating power. Relative to the heating section, the way of heating or temperature of the cooling water temperature may cause the greater temperature pulsation amplitude in the cooling section. Compared to acetone, distilled water with larger latent heat of vaporization shows a higher performance when the heating power is higher. The performance of the PHP under high heating power is analyzed in this paper on the basis of the visualization phenomena, experimental results of the thermal laboratory bench and a theoretical model. An irrational aspect of the model is put forward and the way to improve it is proposed.