Oscillating heat pipes : study of their oscillating motion using neutron imaging and application
Metadata[+] Show full item record
[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Neutron imaging produces unique images of objects making it a useful nondestructive technique with various applications in science and industry. Oscillating heat pipes (OHP) are being studied as a higher performance way of cooling high power electronics. The oscillating motion of the liquid in an OHP has a significant role in the performance. However, the fluid dynamics in an OHP are very chaotic and they are not fully understood. In these studies, the dynamics of both the liquid and vapor phases were studied using neutron imaging in two OHPs, one using water as the working fluid and one using acetone. The resulting data were compared with external temperature data to investigate the workings of the OHPs and also compared to a numerical model. The results show that higher heat input causes more active oscillating motion of the liquid slugs, resulting in better performance of the OHP. It was inferred that higher heat input causes more thin film evaporation, resulting in more active oscillating motion. In the OHPs in this study, the number of liquid slugs is essentially same as the number of turns whether the liquids oscillate or not. The oscillating motion was divided into three types: vibration, pulsation, and pseudo-circulation. Pseudo-circulation motion is the dominant one for achieving higher performance of the OHP. For the acetone-OHP, the oscillating motion of the liquid slugs was periodic and systematic over some periods of time. The temperature of several thermocouples had the same frequency as the oscillating motion of the liquids in the acetone-OHP. For those thermocouples, the temperature fluctuations and the oscillating motion of the liquid were synchronized with each other. The frequency, phase shift and amplitude of the oscillating motion of the acetone OHP were modeled by assuming that the driving force is the pressure difference between adjacent vapor bubbles and the number of liquid slugs is same as the number of turns. The results successfully modeled the oscillating motion of the liquid slugs, which was observed in the experimental data. The phase shift and amplitudes showed a good agreement with the experimental data, however, the frequency did not show as good of agreement. The modeling showed that the minimum number of turns for active oscillating motion was 6. It also showed that a change in the condenser length has a greater effect on the oscillating motion of the liquid than a change in the evaporator length, and the liquid actively oscillates when the filling ratio is between 40% and 50%. The modeling of this study shows the promising possibility of predicting the motions of the liquid in an OHP, which will aid in designing OHPs for particular applications. A solar hot water system using an OHP was also fabricated and tested. Oscillating motion of the fluid in the OHP was successfully observed via temperature measurements despite its unusual dimensions. It was inferred that there may be two modes of oscillating motion, which are systematic. The maximum overall efficiency obtained was 60%. The experiments showed that an OHP might be utilized to achieve a higher efficiency solar hot water system in the future.
Access is limited to the campuses of the University of Missouri.