Evaporating Temperature Uniformity of the Pulsating Heat Pipe with Surfactant Solutions at Different Concentrations

The evaporating section of the pulsating heat pipe (PHP) is in direct contact with the electronics when it is used for heat dissipation, and thus the evaporating temperature uniformity has an important effect on the safe and reliable operation of electronic equipment. On the basis of these conditions, an experimental study on the evaporating temperature uniformity of the PHP with surfactant solutions at different concentrations was conducted at the heat fluxes of (1911–19 427) W/m2. Sodium stearate was utilized for the solute; the surfactant solutions were prepared with the concentrations of 0.001 wt%, 0.002 wt%, and 0.004 wt%, respectively, and the filling ratios of the PHP were 0.31, 0.44 and 0.57, respectively. The experimental results revealed that under all tested working conditions, the highest temperature always appeared in the intermediate zone of the evaporating section. As the heat flux increased, the temperature differences among different zones rose initially and then reduced due to the change of the flow motion and the flow pattern. The evaporating temperature uniformity of the sodium stearate solutions-PHP was better than that of the deionized water-PHP, which suggested that the evaporating temperature uniformity might be improved through decreasing the surface tension. Furthermore, combined with the effect of surface tension and viscosity, for different filling ratios, the required concentration was different when the best evaporating temperature uniformity was achieved. To be specific, when the filling ratio were 0.31 and 0.44, the best evaporating temperature uniformity was achieved at the concentration of 0.004 wt%, while at the filling ratio of 0.57, the best evaporating temperature uniformity was attained at the concentration of 0.002 wt%.