Research On Heat Transfer Performance Of Helical-Rib Gravity Heat Pipes With Small Diameters

Gravity heat pipe is regarded as a special superconductive heat transfer component. With its high thermal conductivity, excellent uniformity of temperature distribution, changeable heat flux, and flexible adjustable resistance on both sides of evaporation section and condensation section, gravity heat pipe has gradually attracted people's attention and been widely used in many fields of industry applications. As the further developments of energy efficient recovery and enhanced heat transfer technology, the ordinary gravity heat pipe is not able to meet the requirements of heating and cooling under certain conditions. An enhanced gravity heat pipe with higher heat transfer performance is urgently needed. In recent years, both domestic and overseas researchers have already started to investigate the heat transfer enhancement techniques of gravity heat pipe theoretically and experimentally. Many experiments have proved that the heat transfer enhancement inside the gravity heat pipe is able to improve the boiling and condensation heat transfer performances radically. Therefore, a relatively new type of gravity heat pipe with helical-rib structure inside was presented and investigated experimentally and theoretically in this paper. Firstly, a group of closed and open helical-rib gravity heat pipes were studied by experiments. The heat transfer performances of helical-rib gravity heat pipes with the corresponding smooth gravity heat pipes under the same working conditions were also compared. The results showed that helical-rib structure could shorten the start-up time of closed and open gravity heat pipe. But there were greater temperature fluctuation in closed helical-rib gravity heat pipe than smooth heat pipe. The helical-rib structure could enhance both boiling and condensation heat transfer in gravity heat pipe and reduce the thermal resistence of gravity heat pipe. Then the mechanism of helical geometric parameters effecting on both boiling and condensation heat transfer were also be analyzed based on the experiments. It was also pointed out that the diameter of a heat pipe had more effects on heat transfer characteristics than helical geometric parameters. Therefore, two enhancement coefficients including diameter factor were introduced to modify the theoretical formulations so as to fit the experimental data closely. The results of fitting curves showed that the modified formulations were capable to approximate the heat transfer performance in a specific range and the maximum relative error was less than 20 percent. Subsequently, heat transfer performances of open helical-rib gravity heat pipes with the corresponding closed helical-rib gravity heat pipes were compared under the same working conditions in this paper. It was found that the heat transfer performance of open helical-rib gravity heat pipes were lower than the closed helical-rib gravity heat pipes' obviously when the heat load was low. With the increasing of heat load, the heat transfer performance of open helical-rib gravity heat pipes became better and better. When the working condition reaching the first transitional point of the open gravity heat pipe, their heat transfer performance even exceeded the closed gravity heat pipes gradually and their thermal resistence were smaller than the closed gravity heat pipes. The boiling heat transfer coeffient of open gravity heat pipes was close to closed gravity heat pipes while the condensation heat transfer coeffient was higher than closed gravity heat pipes. Therefore, it was suitable for applying open helical-rib gravity heat pipe to a high heat load system.