Correlation to predict thermal characteristics of pulsating heat pipes with long evaporator section

In terms of employing closed-loop pulsating heat pipes as heat transfer devices in solar water heaters, the evaporator should be extremely long according to the length of the solar collector. However, there is a noticeable shortage of Semi-Empirical Correlations for predicting the thermal performance of extra-long closed-loop pulsating heat pipes utilized for heat absorption within solar collectors. The primary objective of this study was to formulate a Semi-Empirical Correlation specifically designed for predicting the thermal efficiency in extra-long closed-loop pulsating heat pipe configurations. This study extensively examined the thermal efficiency of a comprehensive design of closed-loop pulsating heat pipe samples when implemented in an evacuated-tube solar water heater system strategically designed for individual households. The specified maximum length for the evaporator section of the closed-loop pulsating heat pipes is set at 1.50 m, in contrast to the conventional design, which typically features an evaporator section of approximately 0.15 m. Furthermore, the investigation encompasses a number of sets, varying as one, two, and four. Therefore, a Semi-Empirical Correlation derived from the experimental data concerning seven pertinent non-dimensional parameters is presented, demonstrating a standard deviation percentage of 4.4 %. Among these parameters, the number of sets emerged as the most influential, inducing a maximum percentage change in the heat rate of water of 20.47 %. Moreover, compared with other studies utilizing pulsating heat pipes in solar water heaters, the correlation exhibited a 1.52 % error. These findings suggest that the developed Semi-Empirical Correlation is a valuable tool for predicting the thermal performance of an extra-long closed-loop pulsating heat pipe. It is particularly well suited for estimating the thermal behavior of closed-loop pulsating heat pipes with extended configurations, showcasing its enhanced accuracy and relevance in capturing the distinction of heat transfer in extra-long closed-loop pulsating heat pipe systems.