| Nanorefrigerant has been regareded as a promosing technique for increasing the energy efficiency of refrigeration systems due to its better heat transfer efficiency compared with the conventional refrigeration working fluids.Currently,the stability of nanorefrigerant and the wetting characteristics of the nanoparticle deposition layer formed by nanorefrigerant are two of the most concerning problems about nanorefrigerant.Specifically,the key problems of the nanorefrigerant stability are the mechanisms of the nanoparticle kinetic behaviors,including aggregation,inter-phase migration,and resuspension.In this thesis,the above problems are addressed experimentally and theoretically.1)The stability of nanorefrigerant/oil mixture was quantitatively evaluated under alternating-phase-change condition.It was shown that the stability of a nanorefrigerant/oil mixture under alternating-phase-change condition was worse than that under non-phase-change condition,but it can be improved by decreasing heating temperature or cooling temperature,increasing oil concentration,or decreasing particle concentration.2)To study the nanoparticle behavior of aggregation,the size of nanoparticle aggregate was dynamically measured using the dynamic light scattering（DLS）technique,and a new aggregation model was developed to predict the size of aggregate in nanorefrigerant.It was found that the existing model that was based on classical DLVO theory cannot precisely reflect the effect of oil on aggregation.The new model proposes that the difference of the particle affinity of refrigerant molecules and oil molecules leads to the competitive adsorption on the particle surface,which differs the composition of adsorption layer with that of bulk liquid,and affects the interactions between particles.Based on these considerations,the new model incorporates the adsorption layer sub-model to calculate the composition and thickness of the adsorption layer,and to calculate the particle interactions with modified DLVO equations.The results showed that the new model well reflects the effects of oil on nanoparticle aggregation,and the prediction for particle size agrees with 92%of the experimental data to within±10%.3)To study the nanoparticle behavior of inter-phase migration,an experimental apparatus for nanoparticle migration during boiling process was set up,and the proportions of the migrated nanoparticles through different routes were measured.It was found that during boiling process,nanoparticles can not only migrate to vapor phase（a.k.a.“mixture-to-vapor migration”）but also migrate to oil phase（a.k.a.“mixture-to-oil migration”）.For common operating conditions,the measured mixture-to-oil migration ratios were always greater than the mixture-to-vapor migration ratios,indicating the mixture-to-oil migration is the main route for nanoparticle migration during boiling phase change.However,the mixture-to-oil migration ratio were still low especially at low oil concentrations,which cannot meet the application requirements for nanorefrigerant.It was believed that the nanoparticle aggregation and sedimentation within the oil excess layer and the nanoparticle deposition within the evaporation micro-liquid-layer are the main causes for the low mixture-to-oil migration ratio.At this point,the surface modification technique was employed to increase the mixture-to-oil migration ratio.The TiO2 nanoparticles was grafted with alkyl chain,which was shown to inhibit the aggregation within the oil excess layer during boiling and increase the mixture-to-oil-migration ratio significantly for the nanorefrigerants containing mineral oil.4)To study the nanoparticle behavior of resuspension,a visualized pool boiling rig was built,and the resuspension process of deposited nanoparticles during boiling was observed for the first time.Also,the nanoparticle resuspension ratio was measured for different boiling duration,heat flux,and particle deposition density.The results showed that the heat flux was the main influential factor on resuspension,i.e.,the greater the heat flux,the higher the resuspension ratio.The influence of particle deposition density on resuspension is related to heat flux:for small heat flux,the resuspension ratio decreases with the increase of deposition density;for medium heat flux,the resuspension ratio firstly increases and then decreases with the increase of deposition density;for large heat flux,the resuspension ratio increases with the increase of heat flux.It was believed that the loose,porous structure of the nanoparticle deposition layer plays a key role in the resuspension phenomenon of deposited nanoparticles,and that the liquid impingement formed during bubble generation and the liquid disturbance formed by bubble departure are the main mechanisms for nanoparticle resuspension.A mathematical model was established,and the model was shown to well describe the influences of heat flux and particle deposition density on particle resuspension ratio.5)The wetting behaviors of pure refrigerant and refrigerant/oil mixture on commonly-rough surfaces（CRS）,nanoparticle deposition surface（NDS）,hierarchical micro/nanostructured surfaces（HMS）,and fluorinated hierarchical micro/nanostructured surfaces（FHMS）were quantitatively investigated by visualization experiments and image processing techniques.The effects of oil concentration,surface roughness,roughness structure,surface chemical properties on the wetting characteristic parameters（i.e.,contact line velocity,contact angle,capillary rise height）were quantitatively analyzed,and the wetting mechanisms of refrigerant/oil mixture on various surfaces were discussed.A novel phenomenon called“liquid film climbing the wall”was observed for the first time,which was considered to be responsible for the higher wettability of refrigerant/oil mixture compared with pure refrigerant.On the other hand,the wetting behavior of pure refrigerant or refrigerant/oil mixture on the CRS,NDS,and HMS were similar:“liquid film climbing the wall”phenomenon occurred,and the wettability of refrigerant/oil mixture on these surfaces were positively correlated to the surface roughness.However,the“liquid film climbing the wall”phenomenon didn’t occur on the FHMS,which was attributed to its refrigerant-philic and oleophobic nature. |
