Effect of evaporating-condensing length ratio and heat flux on starting and operating characteristic of pulsating heat pipe

•Evaporating-condensing length ratio and heat flux are focused.•Start heat flux drops and then tends to be flat near a value with ratio increasing.•PHP transits to be unstable with heat flux increase, followed by direction variation.•The working heat flux region of PHP varies with the variation of length ratios.•PHP with length ratio of 0.8 possesses the optimal operating characteristic. Pulsating heat pipe is an efficient heat transfer technology applied in the fields of electronic cooling and energy utilization. There are many factors affecting the starting and operating characteristic, such as structure, working fluid, filling ratio and so on. In order to investigate the effect of heating length and heat flux, a series experiment was carried out on a closed loop pulsating heat pipe with six different length ratios of 0.3, 0.4, 0.6, 0.8, 1.0, 1.2. The results show that the length ratio and heat flux jointly affect the characteristic of PHP. The starting process of PHP experiences no pulsating, unstable pulsating and then stable pulsating at different length ratios with heat flux increasing. Starting heat flux drops firstly and then tends to be flat and stable near a certain value with increasing evaporating-condensing length ratio. The continuous increase of heating flux can lead to the transition of PHP from stable operating to unstable operating, and the transition occurs on the pulsating heat pipe with length ratio of 0.4 on the condition of heat flux increasing to 7.64 W/cm2. Working heat flux region of pulsating heat pipes varies with the variation of length ratios. The higher heat flux area is for the pulsating heat pipe with smaller length ratio, but the lower heat flux area is better for the PHP with larger length ratio. The optimal operating characteristic was obtained by pulsating heat pipe with length ratio of 0.8, and the thermal resistance reach to the lowest of 2.5 °C/W at heat flux of about 4.5 W/cm2.