Study On The Performances Of Nanofluid Pulsating Heat Pipes

Pulsating heat pipe(PHP)is capable of high heat flux transfer.Nanofluid has higher thermal conductivity than the base liquid.Nanofluid PHPs become the focus of heat transfer technology.Supported by National Natural Science Foundation of China^*,a study on the inner flow and heat transfer performance of nanofluid PHP was carried out in this thesis.Firstly,nanofluids were produced by adding different kinds of nanoparticles to distilled water respectively with different volume fraction.It was found that when the nanofluid PHP was not working,nanoparticles would settle down to the bottom of liquid plugs,or would cling on the inner wall,and vapor bubbles of different size formed in the liquid plugs.For TiO₂/H₂O nanofluids,suspension could be improved by ultrasonic and pH value adjustment.Secondly,based on the transient hot-wire theory,a liquid thermal conductivity meter was established and adjusted to measure the conductivities of nanofluids.It was found that the thermal conductivities of TiO₂/H₂O nanofluids at 0.1%,0.5%and 1.0%volume fraction were 1.8%,5.9%and 7.0%higher than that of distilled water.Thirdly,glass PHPs were fabricated for a visualization experimental investigation on the inner flow and heat transfer.When heated,nanoparticles would be adsorbed on the inner wall of heating section.The oscillating motion in PHP could excite and enhance the suspending stability.The suspension of nanofluids would decrease as temperature increased. When the heat load was high enough,working fluid moved fast,and disturbed bubbles could generate in the liquid plugs.Compared to the distilled water PHP,nanofluid PHP could work in lower heat loads,and had shorter starmp time,lower thermal resistance,and temperature oscillation of smaller amplitude and higher frequency.At the working temperature of 110℃,the thermal resistance of PHP charged with distilled water was 0.23K/W,and the thermal resistance would decrease to 0.11K/W and 0.13K/W for PHPs filled with 1.0%volume TiO₂/H₂O and CuO/H₂O respectively.The thermal resistance was cut down 50%compared with their base liquid.Fourthly,one copper PHP was fabricated to further study the performance.When the inclination angle was 90°for the bottom heating,the PHP had the best performance. Compared with distilled water,the performance of nanofluid PHP was less sensitive to the inclination angle;nanofluid PHP had lower thermal resistance if the heat load was not very high;the temperature oscillation amplitude of nanofluid PHP was smaller and more stable. At a head load of 150W and a working temperature of 109℃,the thermal resistance were 0.035K/W and 0.030K/W for distilled water and 0.5%volume TiO₂/H₂O nanofluid respectively.Finally,a model based on the artificial neural network(ANN)was accomplished.By the results of this model,the best ratio of nanoparticles was between 0.1%and 0.3%volume fraction,and more nanoparticles were not good to the performance of nanofluid PHP.The analysis results agree well with the experiment data.The innovations in this thesis include:(1)TiO₂/H₂O nanofluids of various concentrations were made,and their suspensions in PHP were studied.(2)Glass PHPs were fabricated and visualized to investigate the flow patterns of nanofluid PHP,and the results were compared with that of distilled water.(3)One copper nanofluid PHP was experimented,the results showed that it had better performances than the base fluid.(4)A model based on the artificial neural network(ANN)was established,the concentration of nanofluids for better performance PHP was deducted.The results could be referenced for the PHP optimization.