Heat Transfer Enhancement Characteristics Investigation Of Gravity Heat Pipe With Internal Helical Microfin

Recently gravity heat pipe (GHP) has been widely used in many industrial heat transfer applications. With energy conservation and emission reduction drawing more attention, how to enhance the performance of GHP has been a research issue. According to the research conducted and literatures reviewed by the author, this research draws a conclusion to classify the heat transfer enhancement technologies as using highly efficient working fluid, inner surface treatment, inserting the inner pipe in the thermosyphon, and other methods. Especially, inner surface treatment is manufactured more simply and applied more widely in comparison with other methods.This research focuses on gravity heat pipe with internal helical microfin (GHPIHM). The heat transfer characteristics of GHPIHM are investigated in various conditions of different parameters, different sectional arrangements of the internal helical microfin, and different working fluids, which are also compared with those of smooth gravity heat pipe (GHP). The heat transfer enhancement mechanism of GHPIHM is analyzed as well. The main work and conclusions of this research are as follows:1. The manufacturing process of heat pipes is designed and meantime a series of GHPs and GHPIHMs are fabricated. This research designs and builds up two experimental apparatus in accordance with the structure of heat pipes. The experimental schemes, data processing and uncertainty are presented and analyzed in the end.2. The experimental results show that the internal helical microfin can improve the start-up and steady-state performance. The influences of the inclination angle on boiling heat transfer and condensation heat transfer are different. The more the cooling water flow rate is, the worse heat transfer characteristics are. With the filling ratio increasing, the evaporation heat transfer coefficient decreases firstly and then increases, while the condensation heat transfer coefficient reduces gradually. The increase of the aspect ratio leads to the improvement of heat transfer coefficient and the decrease of temperature in evaporator section.3. The influences of sectional arrangements of the internal helical microfin on operating temperature, temperature distribution, and heat transfer coefficient are studied. The experiments show that the internal helical microfin is able to reduce the temperature in adiabatic section. And GHPHIMs which are arranged with internal helical microfins in adiabatic and condensation sections possess the best heat transfer characteristics. The above results are due to the fact that the internal helical microfin can enlarge heat transfer areas, make the liquid film thinner and generate the local secondary flow.4. The water, self-rewetting fluid, acetone, and butanol are used as working fluids. The experimental results present the self-rewetting fluid can enhance the heat transfer of GHP in horizontal level owing to the inverse Marangoni effect of self-rewetting fluid. Because working fluids have diflerent transfer capacities and specific heat capacities in vertical level, the heat transfer characteristics ranking of GHPs and GHPIIIMs is as follows:water>acetone>self-rcwetting fluid>butanol.