Experimental investigation on heat transfer and flow patterns of pulsating heat pipe assisted by ultrasonic cavitation

•The impacts of ultrasound on the bubble and flow of PHP were visually studied.•Influence of ultrasound on the position and velocity of working fluid was revealed.•The comprehensive effect of the nanofluid and the ultrasonic on PHP was presented.•PHP with ultrasonic exhibited short start-up time and low start-up temperature.