Study On The Mechanism Of Thermal-Induced Composition Separation Of Zeotropic Working Fluids In Nano T-Junctions

Composition adjustable thermodynamic cycles based on zeotropic working fluids is a research hotspot and development direction in the field of high efficiency conversion of low and medium grade energy.In these cycles,the composition separation is a key thermodynamic process,and the T-junction is a key component for separation.However,at the present stage,the clarification on mechanisms of the composition separation in T-junctions is still missing.Alternatively,composition separation on the nanoscale via T-junctions would guide a new direction,and the mechanism can be revealed in terms of the intermolecular interaction and molecular distribution.This study covers the following aspects: Firstly,thermophysical properties,including vapor-liquid equilibrium and transport properties,is the foundation of the composition separation.Therefore,force fields and methods of molecular simulation were evcaluated.Secondly,study on separation in a single-walled nanotube was proposed.The feasibility of separation of zeotropic working fluids in nanotubes was demonstrated and the factors affecting the separation performance were discussed.Thirdly,taking the double-walled nanotube as an example,study on separation in a multi-walled nanotube was proposed.During the study on separation in multi-walled nanotubes,due to the difference of vibration characteristics between double-walled nanotube and single-walled nanotube,the forces exerted on molecules were different as well.Therefore,the vibrational thermal conduction of materials was conducted.As for thermophysical properties of working fluids,the prediction accuracy of different types of zeotropic working fluids with different force fields was compared,and the selection criteria of force fields for different types of working fluids were proposed.It provides a guidance for the selection of force fields and methods for the molecular simulation of separation of zeptropic working fluids in nanotubes.Furthremore,the transport and composition separation of zeotropic working fluids in a single-walled T-shaped carbon nanotube was conducted.When liquid HFC32/HC600 a mixture with the mass fraction of 0.5/0.5 flows into the T-shaped carbon nanotube,effects of the external temperature difference was analyzed,and the transport and composition separation characteristics in impacting or branching singlewalled nano T-junctions was compared.The results indicated that when an external temperature difference applied to the nano T-junction,a satisfactory separation performance can be obtained,due to the asymmetric free energy change of each composition,and different interactions of the T-shaped carbon nanotube on the two types of molecules.Moreover,the impacting T-shaped carbon nanotube performs better than the branching T-shaped carbon nanotube,and the mass fraction of HFC32 in the outlet of the impacting one can reach 89.88%.The vibration of nanotubes affects the transport and composition separation of zeotropic working fluids,and the vibrations are different between the single-walled and the double-walled nanotubes.The study on the thermal conduction of multi-walled nanotubes was conducted,in order to clarify the mechanism of composition separation in multi-walled nanotubes.The factor affecting the thermal condution in this study is“twist angles”.Firstly,the in-plane thermal conductivity of multi-layer graphene decreases first and then increases as the twist angle increases from 0° to 30°.There is a maximum value when the twist angle is 0°,and a local maximum value when the twist angle is 30°.Meanwhile,taking Si and SiC as the object of study,it is found that the resonance of phonons with the same frequency can enhance the thermal transport between two materials.Furthermore,studies on thermal conduction of multi-walled nanotubes can be carried out when curling multi-layered two-dimensional materials into multi-walled nanotubes.In this study,effects of interlayer twist angle on the thermal conductivity of 4 types of nanotubes,including double-walled carbon nanotube,double-walled boron nitride nanotube,boron nitride nanotube coaxially wrapped by carbon nanotube and carbon nanotube coaxially wrapped by boron nitride nanotube,was investigated.The results indicated that among the four types of nanotubes,the double-walled carbon nanotube has the highest thermal conductivity.With a constant chiral angle of the outer tube,the thermal conductivity increases as the chiral angle of the inner tube increases.However,there is enhancement effect due to the resonance of phonons with the same frequency.Finally,considering the transport and composition separation in a single-walled nanotube and vibration thermal conduction of milti-walled nanotubes,the transport and composition separation of zeotropic working fluids in a multi-walled nanotubes was conducted.The results indicated that the addition of the outer tube to the single-walled T-shaped carbon nanotube enhances the interaction force of the nanotube to molecules,and the flow resistance correspondingly increases,resulting in smaller number of molecule in both outlets.Furthermore,the mechanisms of the transport and the composition separation were analyzed from phonon induced force and the energy of mixture molecules in T-shaped carbon nanotube point of view.