| Along with the extensive use of devices with high performance and high integration,the heat accumulation due to the high power density has become a bottleneck restricting the development of electronic and laser systems.Spray cooling as an effective high heat flux cooling technology,has a broad application prospect in solving heat issues in limited space.In order to solve the problems in the research on heat transfer of nanofluid spray cooling,based on the research on heat transfer enhancement of spray cooling by surfactant as dispersant,the mechanism and effect of heat transfer enhancement of surfactant are systematically studied theoretically and experimentally in this thesis.Some surfactants are often added to nanofluids as dispersants to prevent the accumulation and precipitation of nanoparticles and ensure the stability of nanofluids when nanofluids are used as coolant.In view of the uncertainty of heat transfer enhancement in the existing researches on nanofluid spray cooling,the experimental study was conducted to study the heat transfer enhancement effects of three commonly used dispersants Tween20,PVP and SDS on nanofluid spray cooling.It was found that the heat transfer of aqueous nanoparticles without the use of surfactants was weaker than that of pure water.The addition of three kinds of surfactants significantly enhanced the heat transfer ability of the working medium compared to the nanoparticles alone,and it was affected by the concentration of surfactants.The optimal concentration of dispersant Tween20 was 45 ppm,and compared with the nanoparticle aqueous solution and water,the average heat transfer coefficient increased by 29.6%and 14.9%respectively.The optimal concentration of dispersant PVP was 300 ppm,and compared with the nanoparticle aqueous solution and water,the average heat transfer coefficient increased by 22.5%and 8.6%respectively.The optimal concentration of dispersant SDS was 800 ppm,and compared with the nanoparticle aqueous solution and water,the average heat transfer coefficient increased by 68.7%and 49.5%respectively.Among the three kinds of surfactants,SDS has the best heat transfer enhancement effect on nanofluid spray cooling,mainly because Al2O3 nanoparticles are positively charged in the water medium and have strong adsorption effect on the anionic groups ionized by SDS in aqueous solution.In order to study the strengthening mechanism of surfactants on spray cooling heat transfer,this thesis established analysis model of the high alcohols surfactant of spray cooling heat transfer based on previous measured data of spray characteristics of high alcohols surfactant and mathematical models of the heat transfer processe of spray cooling.And the accuracy of the spray cooling heat transfer model is verified and confirmed that its average error is less than 10%.By using the above model to analyze the characteristics of the liquid film formed in the spray process,it was found that after adding high alcohol surfactant,the thickness of the liquid film increased obviously and the flow of the liquid film was getting faster.Thicker and faster liquid film was conducive to better flow and heat removal.The study of the effect of spray characteristics and fluid properties on heat transfer results showed that the improvement of heat transfer performance of high alcohol surfactants is mainly related to the increase of droplet kinetic energy and the decrease of surface tension.Comprehensive consider the effect of droplet kinetic energy and surface tension,Weber number is an important parameter of working fluid,the study found that the addition of high alcohol surfactant increased Weber number of the working fluid,and the distribution of Weber number was similar with the distribution of heat transfer results.Therefore increase of Weber number is one of the strengthening mechanism of high alcohol surfactants on spray cooling heat transfer,and Weber number can be used to estimate the effect of the working fluid on the spray cooling heat transfer. |
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