Thermal Performance of a Closed-Loop Pulsating Heat Pipe With Multiple Heat Sources

A closed-loop pulsating heat pipe with multiple heat sources (CLPHP w/MHS) was invented to be used as a heat transfer medium between a number of heat sources to a single heat sink. However, an issue on the suitable heat source arrangement that causes the heat pipe to have the highest thermal performance was suspicious. The CLPHP w/MHS was made of a copper capillary tube with 32 turns. There were three heat sources with nonidentical input heat flux installed along a longitudinal axis in the evaporator section. Experimental investigations were conducted by permuting the heat sources into six unduplicated arrangements. For the vertical CLPHPs, the highest thermal performance is achieved when heat sources are arranged in consecutive order ascending from the lowest heat flux at the inlet of the evaporator section, since working fluid is promoted to circulate in complete one direction and then the heat can transfer more continuously. Finally, for the horizontal CLPHPs, the highest thermal performance is achieved when the heat sources are arranged in opposite order to the case of vertical CLPHPs, that is, descending from the highest heat flux, since working fluid pulsates with no intermission stop and this causes the heat transfer to be not interrupted.     

Investigation of the Startup Condition of a Closed-Loop Oscillating Heat Pipe

This article develops a concept for a suitable startup condition for a closed-loop oscillating heat pipe (CLOHP). This concept was developed by using visual data and the thermodynamics theory for predicting the amount of vapor evaporation and condensation in a CLOHP. The visual data indicated that the key to a suitable startup is the amount of net vapor expansion in the evaporator and the amount of net collapsed vapor in the condenser. Initial dryout, an event that occurs after a startup failure, results when the net vapor expansion is higher than the amount of net vapor collapsed. This situation obstructs the replacement process. This is a mechanism in which the volume of mixture from the condenser section flows to the evaporator section to replace the volume of mixture that leaves the evaporator section. When the replacement process is impeded, all of the liquid in the evaporator section evaporates and the evaporator section is not refilled by the mixture from the condenser section. The evaporator section is then filled with vapor and initial dryout occurs. In addition, this article presents a mathematical model that predicts the operating temperature for a suitable startup condition. This prediction can be used to avoid a startup failure of a CLOHP. When comparing the model with that of the experimental data, a 16% error range was attained.     

Mathematical Modeling of Closed-End Pulsating Heat Pipes Operating with a Bottom Heat Mode

The mathematical model of a closed-end pulsating heat pipe (CEPHP) with a bottom heat mode at different inclination angles was constructed. The closed-end pulsating heat pipe was modeled with specified assumptions that were observed visually (i.e., the scaling factor for geometrical size and the frequency of bubble generation inside the liquid slugs). The solution for all of the basic governing equations of liquid film, liquid slugs, and vapor plugs, in which the effects of surface tension, viscous friction of the working fluid, and perfect gas were included, has been numerically obtained by solving a series of ordinary differential equations by means of the explicit method. However, the solution for the momentum equation of liquid slugs was numerically obtained by solving a series of partial differential equations by using the implicit method. Results from the model clearly simulated the dynamics of the internal working fluid in the CEPHP. Moreover, the results were compared with existing experimental data, and good agreement was found with an error range of ± 13%. It was also noted that the maximum heat transfer rate of the CEPHP with bottom heat mode occurred at the highest evaporator temperature (150°C for this study) and inclination angles of 70–80 degrees from horizontal axis. The boiling frequencies in this range of inclination angles were observed by visual experiment and seen to be at their highest values. This has been justified by the higher amount of liquid in the evaporator section as well as the change in flow pattern to a stratified flow (inclination tube).     

Correlation to Predict the Maximum Heat Flux of a Vertical Closed-Loop Pulsating Heat Pipe

The objective of this study is to experimentally investigate the effect of various parameters on the maximum heat flux of a vertical closed-loop pulsating heat pipe (CLPHP) and the inside phenomena that cause maximum heat flux to occur. A correlation to predict the maximum heat flux using the obtained results was also established. Quantitative and qualitative experiments were conducted and analyzed. A copper CLPHP and a transparent high-temperature glass capillary tube CLPHP were used in the quantitative and qualitative experiments. From the study, it was found that when the internal diameter and number of meandering turns increased, the maximum heat flux increased. However, when the evaporator section length increased, the maximum heat flux decreased. The maximum heat flux of a CLPHP occurs due to the dry-out of liquid film at the evaporator section. This occurs after a two-phase working fluid circulation changes flow pattern from countercurrent slug flow to co-current annular flow, because the vapor velocity increases beyond a critical value. A correlation to predict the maximum heat flux obtained from this study was developed.