| The development of efficient heat transfer mechanisms are urgently required to transfer heat from the chip to the heat sink on account of the miniaturization and performance improvement of electronic equipment.Pulsating or oscillating heat pipe(PHP or OHP)is a very promising solution that can deliver a large amount of heat between heat source and cold source with little temperature difference.However,no external energy is needed.In the past few decades,a large number of theoretical and experimental researches have been carried out to understand the operating characteristics of PHP.And improving the heat transfer performance of pulsating heat pipes has become the research goal of many scholars.In order to solve the shortcomings of experimental research,a three-dimensional pulsating heat pipe with wall is proposed in present work.The numerical simulation is used to investigate the start-up characteristics of the pulsating heat pipe under different initial pressures.Moreover,thermal resistance of PHP with different concentrations of surfactant(CTAC)is compared under the condition of varying input power ranged 10 W to 40 W.The present work focuses on the interaction of viscosity and surface tension in respect of flow resistance.Then,the indirect effects of surface tension on wall wettability are summarized.The formation of thin liquid film which effects the drying limit under different surface tensions are analyzed.Meanwhile,the heat transfer performance and oscillation motion of PHP with super-hydrophilic,hydrophilic,hydrophobic and super-hydrophobic surfaces are studied.The differences of bubble nucleus and motion characteristics are systematically explained,and the influence of dynamic contact angle hysteresis on capillary resistance is also analyzed.Finally,the dry-out heat load of different wettability surfaces are compared in this work.Within the scope of present work,the results show that the PHP with the initial pressure of 0.01 MPa is difficult to start under low heating power(10 W),while it’s absolutely easier under low initial pressure(0.07 MPa,0.05 MPa).The surfactant concentration has a significant impact on the heat transfer performance of the pulsating heat pipe.It mainly affects the flow process by changing liquid surface tension and viscosity.Under low heating power,the driving force in the tube is not strong enough,so the addition of surfactant can’t enhance the heat transfer,even may hinder the fluid movement and deteriorate the heat transfer due to viscosity.For example,the thermal resistance of the pulsating heat pipe with 2000 ppm CTAC has not reduced under all conditions in present work.Selecting suitable concentration can reduce the thermal resistance by about 4.78%.At the same time,the surfactant can affect the wettability by reducing the surface tension,and improve the anti-drying ability of the pulsating heat pipe.Within the scope of present work,compared with PHP with hydrophilic(including superhydrophilic)surface,it has been found that(including superhydrophobic)PHP with hydrophobic surface shows lower thermal resistance at heat load of 10 W.But at high heat load(20W-40W),the PHP with hydrophilic surface has better heat transfer properties,whose thermal resistance reduces up to 10.8%.At the same time,it’s also discovered the differences in advancing and receding angles(dynamic contact angle hysteresis)on different wettability,which leads to various capillary resistance.Moreover,the switch of flow direction is observed in the PHP with hydrophobic surface,while the stable directional circulation is always maintained in the PHP with hydrophilic surface.The vapor-liquid interface of the hydrophilic surface is concave and the hydrophobic surface is convex.More liquid plugs in PHP with hydrophobic surface result in larger capillary resistance than that in PHP with hydrophilic surface.Finally,due to low flow resistance and the effect of liquid film,PHP with hydrophilic surface can absolutely lift the dry-out heat load. |
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