Heat Transfer Characteristics Of Oscillating Heat Pipes With Hybrid Working Fluid Of Liquid Metal

Oscillating heat pipe(OHP),with the characteristics of a capillary-free structure,various tube forms,and high heat transfer performance,is an effective heat transfer device that solves the current problem of high heat flux under the micro/nano scale.Thermophysical properties of conventional working fluids are limiting the miniaturization and heat transfer performance of the OHPs.Through the coupling of high thermal conductivity liquid metal and surfactant aqueous solution with low surface tension,this thesis intends to control the size distribution of liquid metal micro-nano droplets by the surfactant aqueous solution and input power.6-turn flat plate oscillating heat pipes were investigated visually and experimentally with hybrid working fluids composed of liquid metal and sodium sulfate(SDS)or sodium dodecylbenzene sulfonate(SDBS).The purpose is to explain the mechanism of self-dispersion and oscillation motion of liquid metal in an OHP and the influence of hybrid working fluids on the heat transfer performance.The main research content and conclusions of this thesis are as follows:(1)Set up a small-scale experiment platform,collect visual and heat transfer data under different working fluid ratios of liquid metal and surfactant and analyze the self-dispersion and distribution of liquid metal droplets through the visual data system.Combined with the heat transfer data and the intensity of the droplets' movement,the method of multiple oscillations of liquid metal micro-nano droplets to enhance the heat transfer of the oscillating heat pipe is further explained.(2)Based on the visualization results,the dispersion mechanism of liquid metal droplets in the oscillating heat pipe is studied.The experimental results show that the liquid metal is dispersed into spherical droplets under the action of the shear force generated by the steam expansion.Coalescence of small liquid metal droplets are reduced due to the repulsion force acting on the interfaces of liquid metal droplets by the covered surfactant molecules,resulting in oscillating flows with many small liquid metal droplets in the OHPs.In SDBS solution,micron-sized droplets can be dispersed.In the case of surfactant SDS,liquid metal can be dispersed into nanometer droplets with a particle size of 410-520 nm.(3)Under the mixed working fluid composed of liquid metal galinstan and SDBS,the optimum surfactant concentration of liquid metal droplets' size distribution is 0.4 wt.%.Under this surfactant concentration,the average radius of the liquid metal droplets is in the range of 0.2 to 0.6 mm at the heat input of 260 W to 380 W,within the best oscillation phenomenon.When the filling amount of liquid metal is 5% and the mass fraction of SDBS solution is 0.4 wt%,the oscillation heat pipe has the best heat transfer performance,and the thermal resistance is reduced by up to 14.4% compared with the pure water working fluid.(4)OHP with galinstan and SDS as the working fluids is investigated visually and experimentally under different liquid metal filling ratios and heat input.It is hard for the droplets to coalescence because of exited surfactant aqueous solution.When the filling ratios of liquid metal is 20-25%,the high viscosity and mass of liquid metal spherical droplets hinder the oscillating motion of the hybrid working fluids easily and thus increase the thermal resistance of oscillating heat pipes.When the filling ratios was 5-10%,the hybrid working fluids coupled the high thermal conductivity of the liquid metal and the heat transfer performance was improved observably and the thermal resistance was reduced by 22.0% at most when the liquid metal filling ratio was 5%.When the filling amount of liquid metal is5%,it has best heat transfer performance under both hybrid working fluids of SDBS and SDS surfactant aqueous solution.