Visualized experimental study on the hydro-thermal performance of a dual-condenser pulsating heat pipe with a forced oscillatory flow system

• The performance of a dual-condenser pulsating heat pipe (DCPHP) under forced oscillatory flow was investigated. • The oscillatory flow of 1 and 1.5 Hz stimulated better thermal performance of the DCPHP compared to 0.5 and 2 Hz. • Horizontal operation DCPHP had lower evaporator temperature difference but higher thermal resistance than vertical. • The influence mechanisms of oscillatory flow frequency and gravity on the DCPHP were revealed. This paper presented an experimental investigation of a visualized dual-condenser pulsating heat pipe with a forced oscillatory flow system. The hydro-thermal performance of the pulsating heat pipe under different oscillatory flow frequencies (0.5 to 2 Hz), varying heating powers (2 to 6 W), and distinct orientations (vertical and horizontal) was discussed. The wall temperature distribution, heat transfer capacity, vapor–liquid flow behaviors, and bubble morphology characteristics were studied. Experimental results demonstrated that increased heating power led to higher wall temperature, greater temperature difference in the evaporator section, and lower thermal resistance of the pulsating heat pipe. As the oscillatory flow frequency increased, the evaporator temperature difference and thermal resistance first decreased and then increased. Compared with the vertical orientation, the horizontally oriented pulsating heat pipe showed a higher wall temperature, more symmetrical temperature distribution, 10.1 % to 49.5 % smaller evaporator temperature difference, and 12.9 % to 30.5 % greater thermal resistance. Some flow behaviors were observed through the visualization method, including bubble volume variation, bubble coalescence/breakup, dryout, and nucleate boiling. Moreover, the bubble morphology exhibited distinct differences when the pulsating heat pipe was oriented horizontally and vertically. The mechanisms of oscillatory flow frequency and gravity acting on the pulsating heat pipe were revealed. Moderate frequency improved performance through sensible convective heat transfer and liquid rewetting, while excessive frequencies suppressed bubble growth and latent heat transfer. The vertical orientation cases had better performance due to gravity-assisted rewetting, while horizontal operation cases suffered performance deterioration from non-uniform circumferential liquid film distribution and increased dryout risk.