Investigation of Entropy Generation Through the Operation of an Unlooped Pulsating Heat Pipe

In the present study, an unlooped pulating heat pipe has been considered with two liquid slugs and three neighboirng vapor plugs.The governing equations such as momentum, energy and mass equations are solved explicitly except the energy equation of liquid slugs.The aim of the present study is to calculate the entropy generation through the performance of a pulsating heat pipe. Additionally, the effects of different pipe diameters and evaporator temperatures have been investigated. Besides, Bejan number has been derived for each case study to investigate the share of heat transfer in entropy generation.  According to the results, by increasing the pipe diamtere the sensible and latent heat transferred into the pulsating heat pipe enhance and the liquid slugs oscillate in high amplitudes. On the other hand, the entropy generation value increases as the pipe diameter increases. The evaluated Bejan numbers show that the viscous effects share in entropy generation decreases as the pipe diameter increase. Previous studies reports that there is a threshold of pulsating heat pipe diameter to operate. However, the results of the present paper demonstrate that the using pulsating heat pipes are not reasonable in small diameters. Moreover, the results show that the heat removing performance of pulsating heat pipe improves as the temperature difference of the evaporator and condenser increases. The calculated results demonstrate increment in total entropy generation in high evaporator temperatures. Moreover, the Bejan number will increase by any increment in the evaporator temperature and this phenomenon reveals the insignificant role of viscous effects in high evaporator temperatures. To validate the calculations, the results have been compared to the previous ones. This comparison shows very good agreement between them.