Investigations Of The Operational Performance Of Closed Loop Pulsating Heat Pipes

In recent years, thermal management of microelectronics is becoming a major feasibility bottleneck and has shown the limitations and shortcomings of conventional solutions. Market expectations are posing a simultaneous challenge of increased power levels coupled with high heat fluxes. Active and passive systems incorporating mini-/micro channel flows are gaining ground to meet the challenges. Both single phase and two-phase flows are under consideration with the latter proving to be better alternatives. Inline with these developments are the Pulsating Heat Pipes (PHPs), which are very attractive entrants in the family of closed passive two-phase heat transfer systems. These devices have already shown very high promise for terrestrial applications. They also have a potential for thermal control applications for space.Although grouped as a subclass of the overall family of heat pipes, PHPs have their own structural features and operational principle. These apparently simple looking cooling devices have offered considerable challenges in phenomenological and theoretical understanding. Research activity in this area has steadily increased after their introduction. Yet, complete design rules and optimization procedures are still not available, more quantitative data are waited to be obtained for real time application. Highlighting major progress and milestones achieved in the development of this promising technology of PHPs in the last decade, the thesis presents a comprehensive investigation on the closed loop pulsating heat pipes (CLPHPs), including experiments and mathematic modeling. Here are general outlines:Firstly, two flat plate CLPHPs have been designed, fabricated and tested for flow visualization coupled with standard thermometry. Both were made of aluminum with overall size 180×120×3mm~3, while one specimen had a total of 40 parallel rectangular channels with cross section 2×2mm~2, and another had a total of 66 parallel rectangular channels with cross section 1×1mm~2. Ethanol was used as the working fluid. As the results, flow pattern and its transitions were observed, parametric influences on thermal performance were investigated, such as filling ratio, heat input flux and operational orientations. In addition to, thermal performances of the two flat plate CLPHPs were compared with respective to the influence of geometric parameters, such as the channel cross section and the number of channels.Secondly, two tube CLPHPs have been fabricated and tested. Both specimens consisted of a total of 40 serpentine copper tubes with the inner diameter of 1mm and 2mm respectively. Ethanol, water and R123 were employed as the working fluids. As the results, effects of inner diameter, heat input flux, thermal properties of the working fluid and its filling ratio, operational orientations on thermal performance were investigated. Moreover, structural and operational differences existed in flat plate and tube type CLPHPs were investigated. It is found that the sharp-angled corners of rectangular cross section render the operational characteristics of the flat plate CLPHPs quite different from circular cross section devices for the lower filling ratio.Lastly, based on experimental results on a single closed loop PHPs, a two-phase flow modeling using homogenous and separated two-fluid models is attempted for the single loop system. As in the case of two-phase thermosyphons, only the gravity head is assumed to be providing the driving potential. It is demonstrated that vital information can indeed be extracted from such a modeling approach and this brings us one step closer to the real-time operation. For the further improvement of the model, the 'void fraction constraint' needs to be implemented; also, 'the dynamic pressure term' needs to be considered.