Investigation On Phase Distribution Of Supercritical Fluid In Nanoscale

Supercritical fluids(SF)widely exists in nature and is used as a working medium for various power cycles and wastewater treatment,etc.The traditional thermal theory treats SF as a single-phase fluid without bubbles and interfacial phenomenon,and it is difficult to accurately describe the energy transfer and conversion of SF.In the 1960s to 1970s,the concept of supercritical pseudo-boiling was put forward based on the phenomenon that both supercritical and subcritical boiling heat transfer may have wall temperature peaks occur.But until now,the connotation and extension of supercritical pseudo-boiling have not been given internationally.This paper studies the SF phase distribution,pseudo-boiling transition temperature and heat transfer characteristics in unconfined and confined spaces.Three regimes of liquid-like,two-phase-like and gas-like are divided at the nanoscale,and the characteristics and laws of the two-phase-like region are explored.The main contents and innovations of this paper are described as follows:The density spatiotemporal distribution and nonlinear analysis of time series curves.At present,molecular-scale studies reveal that SF possesses heterostructure properties.In view of the existing research on SF density fluctuations mainly focused on the qualitative analysis of specific working conditions,the molecular dynamics simulation method is used to quantitatively analyze the density fluctuations,covering the ranges of densities 0.5ρc-1.4ρc(where pc is the critical density)at the critical temperature Tc.The deviations display M-shape distribution with increases of average densities,and the maximum deviation occurs at ρave=0.8ρc.In the range of the potential function φ(r),the potential energy and interaction force between two atoms depends on the interatomic distance and affects the arrangement of particles,which is called the "potential induction" mechanism.Statistical physics points out that fluctuation is the basic property of all matter,and fluctuations produce microscopic inhomogeneity,especially in the critical region called the "critical fluctuation" mechanism.Under the combined action of the two mechanisms,the strongest heterogeneity appears not at the critical point,but below the critical point atρave=0.8ρc.In addition,according to nonlinear dynamic analysis,it is found that different simulation conditions present chaos or random characteristics.The research results significantly deepen the fundamental understanding of SF and provide a guide theoretically for SF’s practical application.Pseudo-boiling transition temperature and nanoscale bubblelike in supercritical argon.The phase distribution characteristics of SF in unconfined space are investigated by molecular dynamics simulation.Pressure and temperature are well controlled and with periodic boundary conditions applied in all the box surfaces.Subcritical boiling takes place at a saturation temperature corresponding to subcritical pressure.This paper finds that supercritical pseudo-boiling occurs in a temperature range.An onset pseudo-boiling temperature and a termination pseudo-boiling temperature are defined as Ts and Te,respectively.We determine Ts and Te using three different approaches of neighboring molecules method,the radial distribution function method and the two-body excess entropy method,and find consistent outcomes.The two transition temperatures divide the whole phase diagram into three regimes of liquid-like,two-phase-like,and gas-like.In the two-phase-like regime,nano bubblelike are observed to have two distinct characteristics:(1)The particle distribution in the bubblelike is sparse,showing gas-like properties.(2)Bubblelike has an obvious curved interface.Nonlinear dynamics demonstrate chaotic behavior in the two-phase-like regime,similar to the two-phase regime in the subcritical domain.This study reveals that SF has significant multiphase flow characteristics,laying a foundation for establishing supercritical multiphase flow theory.The spatiotemporal density,phase distribution and hydrogen bond structure for supercritical water.Experiments and quantum calculations well understand the properties of a single water molecule,but a fluid containing a large number of water molecules,and its structure and dynamics are not understood yet.There is remains largely unknown territory in density fluctuation,phase distribution,and hydrogen bond structure of supercritical water.The density fluctuation and phase distribution of fluid containing hydrogen bonds are investigated based on the rigid SPC/E water model.Quantitative analysis of system density fluctuations is performed by square root error and maximum structure factor.The results show that the increase of average densities and temperatures suppresses the local density oscillation,different from the M-type evolution law of supercritical argon.The three-regime-model of supercritical water is established on the neighboring molecules method and the radial distribution function method.The relationship between hydrogen bonds and density,temperature and pressure is revealed.During the evolution process,one and more hydrogen bonds will be broken,and the broken hydrogen bonds will form new hydrogen bonds with new molecules or exist as isolated molecules.Hydrogen bonds are continuously broken up and re-organize in supercritical water dominate the local density fluctuation,which is also the main reason for the difference between the density fluctuation of supercritical water and argon.Our work deepens the fundamental understanding of supercritical water and demonstrates the generalized multiphase character of SF.The phase distribution and heat transfer characteristics of SF in confined space.With the rapid development of nano-device,the physical phenomena near the nanoscale fluid-solid interface have become very important.Shale gas reduces CO2 emissions while satisfying economic development,and SF fracturing technology effectively enhances shale gas recovery,carbon capture,and storage in nanostructures.Therefore,the SF phase distribution and heat transfer characteristics in confined space are investigated in this paper.The effects of wall wettability,wall temperature,and fluid’s initial state on phase distribution are considered.Under the action of wall wettability,the fluids in different initial states will self-organize to form new phase distribution.A liquid-like layer will form on the strong wettability wall surface,similar to the annular flow under subcritical pressure.At the same time,a gas-like layer is formed on the weakly wetting wall surface,similar to the Leidenfrost phenomenon under subcritical pressure.Affected by the temperature of the wall,a thicker(thinner)liquid-like(gas-like)layer is formed on the cold wall than hot wall.The teat transfer characteristics of SF have the following rules:(1)The temperature jump and thermal resistance length at the fluid-solid interface increase with the weakening of the wall wettability,contrary to the conclusions at subcritical pressure.(2)The heat flux decreases with the decrease of the wall wettability and the initial system density,and the difference between different initial states decreases at weak wall wettability.In addition,different simulation processes follow the principle of minimum energy.The evolution process of fluid phase distribution satisfies the minimum energy principle under different simulation conditions.This work aims to reveal the SF phase distribution and heat transfer characteristics in confined space and provide theoretical support for the design and application of SF equipment.