Research For The Heat Transfer Performance Of Gravity Heat Pipes With Hydrophilic And Hydrophobic Properties

As an efficient heat exchange element,gravity heat pipe has been widely applied in the field of heat transfer due to its simple structure,easy processing,low cost,low thermal resistance,excellent performance of heat transfer and high reliability.Meanwhile,the gravity heat pipe is related to boiling and condensation heat transfer,and the heating surface with hydrophilic and hydrophobic properties has an important effect on boiling and condensation.Therefore,this paper proposes a new idea of heat transfer enhancement by gravity heat pipe,which combines the wetting characteristics with the gravity heat pipe.The inner surface of the gravity heat pipe is treated with a chemical etching and coating method to make the surface hydrophilic or hydrophobic.A combination of experimental research and numerical simulation was used to analyze and study the effects of hydrophilicity,hydrophobicity,and combination of hydrophilicity and hydrophobicity on the start-up and steady-state heat transfer characteristics of the gravity heat pipe.Therefore,the research of this topic has important theoretical and practical significance.Each of twelve gravity heat pipes filled with a certain amount of deionized water,which has the characteristics of 9.52 mm outer diameter,8.32 mm inner diameter,a 100mm-long evaporator,a 50mm-long adiabatic and a 100mm-long condenser,was selected as the main research object of experimental research and numerical simulation in this paper.Two methods of chemical etching and coating were used to vary the wettability that was quantified as the contact angle inside evaporator and condenser sections.We designed and fabricated gravity heat pipe heat pipes with different contact angles for experiments,and an experimental platform for wetting the gravity heat pipe system was also set up to study and analyze the start-up characteristics and the steady-state heat transfer characteristics of gravity heat pipes.Finally,according to the experimental data,the evaporator and condenser heat transfer coefficients,total heat transfer coefficient,thermal resistance and effective thermal conductivity of gravity heat pipes were selected as the key parameters for evaluating the heat transfer performance of the heat pipe.First of all,we experimentally investigated the start-up process of gravity heat pipes with various wettabilities under different heating conditions,and analyzed the effect of surface wettability on the start-up characteristics of the heat pipe and the wall superheat degree of the evaporator.From the analysis of the average temperatures of the evaporator and condenser sections and the startup time spent on the evaporator sections,we found that introducing hydrophilic properties inside the evaporator section not only significantly shortened the start-up time but also decreased the start-up temperature.The start-up time spent on the evaporator section of heat pipe with smooth surface was longer than that of heat pipe with hydrophilic properties only,but shorter than that of heat pipe with hydrophobic properties at different input powers.At the same input power,the start-up time increased with rising contact angle on the evaporator sections.As the contact angles on the evaporator sections of HP1-HP6 increased,the wall superheat degree ? gradually rose.With increasing input power,the wall superheat degree ? on the hydrophobic evaporator section was elevated more obviously than that on the hydrophilic one.For heat pipes with combined hydrophilic and hydrophobic properties,the start-up time spent on the evaporator section with hydrophobic properties was longer than that of the hydrophilic evaporator section.Meanwhile,the start-up time of heat pipe with hydrophilic properties was shorter than that of heat pipe with smooth surface.The modification played a significant role in promoting the start-up process.Secondly,the heat transfer characteristics of gravity heat pipes(HPs)modified with inner surfaces of various wettabilities were experimentally studied,and we found that,for HP1-HP3 and HP8-HP10,introducing hydrophilic properties inside the evaporator sections increased the heat transfer coefficients of both evaporator and condenser at the same power.The evaporator and condenser heat transfer coefficients of HP8 and HP9 with combined hydrophilic and hydrophobic properties were higher than those of HP7 with smooth surface.The thermal resistances of HP1-HP3 with hydrophilic properties were lower than those of HP4-HP6 with hydrophobic properties at different powers.At the same power,as the contact angles on the evaporator sections of HP1 to HP6 increased gradually,the overall thermal resistances were elevated.The thermal resistance of HP7 with smooth surface was lower than those of HP4-HP6 with hydrophobic properties but higher than those of HP8 and HP9 at the same power.The thermal resistances of HP8 and HP9 may be lower than that of HP1 if the input power was further increased.In addition,as the contact angles of HP1-HP6 increased,the effective thermal conductivities gradually declined at all heat input powers.The effective thermal conductivities of HP1-HP3 with hydrophilic properties exceeded those of HP4-HP6 with hydrophobic properties and HP7 with smooth surface at different input powers.At the same input power,the effective thermal conductivities of HP8-HP10 were higher than those of HP4-HP6 modified with the hydrophobic properties,HP7 with smooth surface,HP11 and HP12 but lower than that of HP1.Finally,based on the theoretical analysis and experimental research of the internal evaporation and condensation process of the gravity heat pipe,a heat transfer numerical model of gravity heat pipes modified with the wettability is established.The influence of the hydrophilic,hydrophobic properties and combination with the wettability on heat transfer performance of heat pipes at stable state were studied and analyzed by combining the experimental results with the simulation results.By comparing the experimental results with the simulation results,it is found that the experimental results are consistent with the simulation results,and the simulation results are also verified.The results of research show that the total hydrophilic treatment method is superior to the full hydrophobic treatment method in improving the heat transfer capacity of the heat pipe,whereas the full hydrophobic treatment will inevitably weaken the heat transfer capacity of the heat pipe,indicating that the hydrophilic property has a positive effect on the enhancement of heat transfer capability of the heat pipe.It is also found that the full hydrophilic treatment and the hydrophilic treatment of the evaporator section and the hydrophobic treatment of the condenser section help to increase the condenser heat transfer coefficient and enhance the heat exchange capacity,and the combination of hydrophilic on evaporator section and hydrophobic properties on condenser section is better than the combination of hydrophobic on evaporator section and hydrophilic properties on condenser section in improving the heat transfer performance of the heat pipe.Meanwhile,heat pipe with combination hydrophilic on the evaporator and hydrophobic properties on the condenser has a higher heat transfer capacity than that of smooth heat pipe,indicating the combination of hydrophilic on the evaporator and hydrophobic properties on the condenser has the effect of enhancing the heat transfer of the heat pipe and reducing the thermal resistance.