Study On Mixture-to-oil Migration And Resuspension Characteristics Of Nanoparticles During Boiling Process Of Nanorefrigerant

Nanorefrigerant has better heat transfer performance than conventional refrigerants.It is expected to be used in refrigeration systems to greatly improve the energy efficiency,which has gained worldwide attention.To apply nanorefrigerant to the actual refrigeration system,it is necessary to ensure the stability of nanorefrigerant during the cycle.The stability of nanorefrigerant depends on the migration and loss performance of nanoparticles during system operation.Mixture-to-oil migration is the main route for nanoparticles to participate in the cycle.And the nanoparticles that deposited on heating surface are expected to work again through resuspension behavior.Therefore,it is necessary to investigate the mixtureto-oil migration and resuspension characteristics of nanoparticles.In order to investigate the mixture-to-oil migration and resuspension characteristics of nanoparticles during boiling process,researches were started by experiments and theoretical analysis.The main works and conclusions in this thesis are reviewed as follows:(1)In order to improve the migration-to-oil performance,the surface modification methods were explored.The nanoparticles were modified by 5 different surface modifiers.Then the particle size distribution and mixtureto-oil migration ratio were measured.The effect of grafting methods and alkyl chain length on mixture-to-oil migration characteristic was investigated.(2)The research on migration-to-oil characteristic shows that the organic surface modification can effectively inhibit the aggregation of nanoparticles during boiling process,which increases of mixture-to-oil migration ratio.Grafting by covalent bond works better than electrovalent bond on the improvement of mixture-to-oil migration ratio.The longer the grafted alkyl chain is,the higher the mixture-to-oil migration ratio is.The correlation for nanorefrigerant-oil mixture modified by silane coupling agent is applicable to the modified nanoparticles with different alkyl chain length and initial concentration.The relative error is within ±5%.(3)In order to investigate the resuspension characteristic of deposited nanoparticles during boiling process,the resuspending experimental rig was designed and built.The resuspension behavior and the microstructure of deposited layers were visually observed.The mechanism of resuspension behavior was analyzed according to the resuspension phenomenon.And a mathematic model was set to quantitatively describe the effect of deposited layer density and heat flux on resuspension ratio.(4)The research on resuspension characteristic shows that the porous structure is a main factor for deposited nanoparticles to resuspend during boiling process.The flow of refrigerant between pores and the disturb of bubble growth and motion on the liquid flow provide the driving force for resuspension behavior.The resuspension ratio increases with the increase of heat flux.Under low heat flux conditions,resuspension ratio decreases with the increase of deposited layer density.Under medium heat flux conditions,resuspension ratio firstly increases and then decreases with the increase of deposited layer density.Under high heat flux conditions,resuspension ratio increases with the increase of deposited layer density.The mathematic model built describes the effect of resuspension ratio on deposited layer density and heat flux well.The prediction for resuspension ratio agrees with 88% of the experimental data to within-12% ~+8%.