Experimental Study On Application And In-tube Flow Boiling Heat Transfer Of R600a And Nanorefrigerationoil Mixtures

The nano-refrigeration-oil with good stabilitycan be prepared by adding nanoparticles into based refrigeration oil with certain techniques.It is an innovative means to reduce the energy consumption of refrigeration equipment by using nano-refrigeration-oil.The performance of refrigeration oil can be improved by adding nanoparticles,especially the friction and heat transfer performance.And it is expected to be a potential way to optimize performance of the refrigeration compressor and the heat exchangersimultaneously.But this kind of research is still at the starting stage at present.Therefore,it is necessary to carrary out the basic research on using nanorefrigeration-oil in refrigeration system.The action mechanism of the nano-refrigeration-oil in refrigeration system is explored progressively a seriers of experimental investigations in four aspects.In this paper,we study on the preparation methods of nano-refrigeration-oil with good stability,the thermal properties and flow boiling heat transfer characteristics of nano-refrigeration-oil,and the practical applications of nanorefrigeration-oil in the refrigerator.In the research of prepareing nano-refrigeration-oil,we prioritized the suitable nanoparticles and surfactents by means of measuring the sedimentation.Then the wet modification technology and 1,3-dipolar cycloaddition reactionwere introduced to modify the surface of nanoparticles.After that,Ultraviolet-visible spectrophotometry and Dynamic light scattering were employed to analyse the dispersion stability of nanoparticles in the refrigeration oil.And mass spectrum,nuclear magnetic resonance and Fourier-transform infrared spectrometerwere used to explore the mechanism of stability experimentally.Finally,we prepared graphite nano-refrigeration-oil with silane coupling agent KH570 as surfactant,fullerence nano-refrigeration-oil containing perfluorinated octyl chain,and metal oxidenano-refrigeration-oil with polyvinylpyrrolidone as surfactant.All of these nano-refrigeration-oils can keep stable dispersion for a long period of time.We also pointed out that sterically hindered effect is the main mechanism why the surface modified nanoparticles can disperse in the refrigeration oil stablely.Regarding to the thermal properties of nano-refrigeration-oil,oscillation tubedensimeter,rotational viscometer,thermal constant analyzer were employed to measure the density,viscosity and thermal conductivity of nano-refrigeration-oils with different concentrations,respectivily.The influence of particle morphology on the density and viscosity of graphite nano-refrigeration-oil was investigated experimentally.And the influence of different kinds of nanoparticles on the thermal conductivity was studied as well.Correlations of the density,the viscosity and the thermal conductivity were developed as a function of nanoparticles concentration,particle size and shape.The results showed that the density,viscosity and thermal condictivity of nano-refrigeration-oil increased with respect to increasing nanoparticles mass fraction.The densities of graphite nano-refrigeration-oil adding plate-shaped graphite were higher than those with spherical graphite at the same mass fraction,while the viscosity showed the opposite trend.The thermal conductivity of nano-refrigeration-oil shows a downward trend when the particle size increases at the same volume fraction.While at same particle size it improves with the thermal conductivity of particle material.The theoretical results of the correlations agreed well with experimental results.In the research of flow boiling heat transfer and pressure drop charactersistics of nano-refrigeration-oil/regrigerant mixture,an experimental setup was designed and built in this paper.The flow boiling heat transfers as well as the friction pressure drops of mixtures inside a horizontal smooth tube were tested.A calculation of correlation based on the concentration of nanoparticles and thermal properties of mixture was proposed for quantitatively predicting the flow boiling heat transfer characteristic of mixture.The results showed that the flow boiling heat transfer coefficient and frictional pressure drop of nano-refrigeration-oil/regrigerant mixture are larger than those of pure refrigerant and refrigerant/oil mixture,and the enhancements decreased with the increase of mass flux.When the concentration of nanoparticles increased,the enhance factor also improved.But when the vapor quality rose,the enhance factor show the trend of increased atfirst and the decreased.Under the experimental conditions,the boiling heat transfer coefficient increased 19% at 0.2wt% of nanoparticles,while the frictional pressure drop increased 36.6%.The mechanism of nanoparticles on the boiling heat transfer and friction pressure drop can be explained in three sections.There are the thermal properties of mixture,the boiling heat transfer characteristic and the change of heating surface.The theoretical results of the correlations agreed well with experimental results.According to the research of applicating nano-refrigeration-oil in the refrigerator,we select two household refrigerators(Haier BCD-206TAS).A refrigerant test system wasbuilt to test and analyze the performance of a domestic refrigerator using nanolubricants with mass fractions of 0-0.5wt%.The results showed that graphite nanolubricants work normally and safely in the refrigerator.There were certain reductions on pull down time,discharge pressure,discharge temperature,suction pressure,and compressor temperature.There were also certain improvements on storage temperature and freeze capacity.In addition,power consumption of the refrigerator decreased 4.55% by using graphite nanolbricant with a mass fraction of 0.1%.Besides,the refrigerator performced better by using fullerenenano-refrigeration-oil,thepower consumption of the refrigeratordecreased 6.1% a mass fraction of 0.1%.Adding certain amount of nanoparticles can reduce the friction coefficient of base refrigeration oil,enhance the boiling heat transfer of the mixture as well as improve the refrigeration cycle.These may decreasethe frictional loss and the frictional heatof compressor,which lead to a reductionon puwer consumption.These results could confirm that the graphite nanolubricant can improve the performance of domestic refrigerator employing R600 a as working fluid.