Experimental Exploration And Theoretical Analysis On The Thermal Characteristics Of Flexible Heat Pipes

Heat pipe is a highly efficient passive heat dissipating device,which works by the latent heat of vaporization and condensation of the working fluid to transfer heat.The outstanding thermal characteristics of heat pipes attracts large attention from global profession.In the past decades,various heat pipes were designed and invented to function in manifold fields including waste heat recovery,solar energy exploitation as well as electronic cooling.Heat pipes technology used in electronic cooling absorbs the most attention.However,in some circumstances where heat dissipating space is limited,conventional heat pipes fails due to rigidity and inflexibility.For example,some cases demand heat pipes to be configured into“L”or“U”shape,and some other cases require flexibility of heat pipes in order to alleviate or eliminate the impacts brought by relative motion between evaporator and condenser.As such,flexible heat pipes emerge.So far,numerous works in regard to flexible heat pipes were reported,but the most challenging obstacle,the bending effects on flexible heat pipes,has not been comprehensively studied.In order to overcome such difficulty,this paper conducts a special bendable heat pipe to investigate the impacts of bending on its thermal performance.The length and outer radius of this heat pipe are 500 mm and 8 mm respectively with grooved wick structure.Customized testing platform for this heat pipe was proposed to systematically investigate the impacts of different bending angles and curvature radius on the thermal performance including evaporator temperature,maximum heat transfer capacity.The results show that,in horizontal placement,bending angles and curvature radius have negligible effects on the heat pipes'thermal performance.Specifically,at constant curvature radius and input power,when bending increases from 0~oto 180~o,the thermal resistance and evaporator temperature augments slightly,and the maximum heat transfer capacity diminish slightly.At constant bending angles,when the curvature radius declines from 2.4 cm to 2 cm and 1.6 cm,the thermal resistance and evaporator temperature increases slightly,and the maximum heat transfer capacity stay relatively unvaried.In conclusion,this type of bendable heat pipes is capable of being contorted to assemble into limited space to efficiently remove the local heat.In addition,in the field of microelectronics,light weight and highly packaging has become two main trends of development,which put higher requirements on the size of heat pipes Hence,ultra-thin heat pipes draw much interest from globe,and relative publications are reported largely.However,what's more challenging is flexibility has evolved into one of the development trends in microelectronics as could be proved by newly arising cutting-edge products like flexible touch screens,flexible batteries and deployable smartphones.This indicates the heat dissipating space will be not only restricted but also changeable.Consequently,this paper proposes the idea of flexible ultra-thin flattened heat pipes to surmount the aforementioned difficulties.The key point in regard to realizing flexibility of ultra-thin flattened heat pipes is how to characterize the effects of bending on the thermal performance of heat pipes,which directly determines the perspectives and methods for optimization of thermal performance in the process of actualizing flexibility.This paper thus selects a type of ultra-thin flattened heat pipes with 1.5 mm thickness to explore bending effects on its thermal performance.The samples were tested under the bending angle from 0 ~o to 180 ~o with three different curvature diameters:1.5 cm,2 cm,and 2.5 cm.The testing results demonstrate that at constant input power,larger bending angle or smaller curvature diameter cause the thermal resistance and evaporator temperature climb up.Besides,uniformity of axial temperature was upset by bending due to continuously expanded temperature differences in adiabatic section.Maximum heat transfer capacity declined for more than 90%when the bending angle changes from 0 o to 180 o.The most severe degradation of it sits within the bending angle of 0 ~o to 30 ~ofor more than 50%.But even at the largest bending deformation,the ultra-thin flattened heat pipes could still efficiently operate with low thermal resistance and low evaporator temperature within allowable input power.Finally,an empirical equation to predict the evaporator temperature at given input power,bending angle,and curvature diameter was proposed on the basis of experimental data.In the conclusion,this paper summarizes the effects of bending brought by inevitable bending deformation on the thermal performance of heat pipes in the process actualizing flexibility,and analyzes the reasons from physical principles so as to reveal the potentials as well as the trends of flexible heat pipes.