纳米颗粒在超临界CO2中的构型及热物性受控机制

Nanoparticles (NPs) can play an important role in the field of thermophysical properties regulation and heat transfer enhancement of supercritical fluids (SCFs). However, both NPs and SCFs have complex properties and are susceptible to different factors, which make the research of the mixture’s dispersion stability and the regulation mechanism of the thermal properties more challenging. When NPs are suspended in SCFs，lots of factors，such as their structure, morphology, spacial distribution and interfacial property etc, can crucially affect their dynamic behavior and thermal properties enhancement. In this project, the development of accurate force fields will serve as the beginning, to clearly explore the dispersion stability mechanism，the study about the interfacial property and effective heat conduction model for the single NP, along with the potential energy function and microscopic interaction for the two-body NPs will be conducted gradually and thoroughly in the SC-CO2, which always show a drastic variation for the thermal properties. Furthermore, a reduced and effective coarse-grained force field for the NP is accurately fitted, through carefully and comprehensively analyzing the different interactions. Base on it, the dynamic behavior (aggregation, breakage etc.) of NPs and the heat conduction model for the nanoclusters are studied at the mesoscale. The multi-scale simulations are further performed to acquire the bulk thermal and heat transfer properties of the supercritical nanofluid. Finally, the mechanism using NPs to regulate the thermal properties of the supercritical nanofluid are well understood and an accurate and reasonable calculation model for the thermal properties of the supercritical nanofluid are also developed, which can provide a high efficient and quick way to acquire the fundamental data for further studying of the rheology and heat transfer enhancement properties.

纳米颗粒在超临界流体热物性调控及传热强化方面可发挥重要作用。二者自身特性复杂和影响因素众多，使得其混合流体的分散稳定与物性受控机理研究极具挑战。纳米粒子在超临界流体中特有的构形、空间分布、界面特性及微观作用力等是其动力学行为与热性能强化的决定性因素。本项目拟以高分辨率力场构建为基础，由微观尺度的单粒子界面特性与有效热传导模型研究，到双粒子间势能函数与微观作用力描述，逐步深入揭示不同特性纳米粒子在强变物性特征的超临界CO2流体中的分散稳定机理。进而通过对粒子微观受力的综合评判，简化并精确拟合有效粗粒化力场，以此展开介观尺度的粒子动力学行为（聚集、破裂等）及纳米团簇传热模型的研究，以及微-介观跨尺度的超临界纳米流体体相热物性及传热特性的模拟与实验验证，最终实现纳米粒子调控其热物性机理的深入认知，提出与超临界纳米流体热性能相关的有效理论计算关联模型，为其流变与传热强化特性的研究提供基础数据。

超临界流体技术不仅在超临界萃取、材料制造、化学反应等方面有着广泛应用，在先进的能源转换与利用领域也具有重要应用。其广泛的应用前景本质上得益于超临界流体的热物理性质。以CO2为例，其在超临界态不仅具有类似于液相的密度、气体的粘度和扩散系数，而且溶解能力强、表面张力为零，潜在具有较好的传热传质特性。然而，超临界CO2流体显著的变物性特性使其比常规流体具有更复杂的流体与传热特性。针对超临界CO2传热化问题，本项目创新性地提出采用纳米颗粒来调控超临界CO2流体热物性，并对其潜在的调控机理和机制展开探索性的研究。首先，基于精确分子力场模型的构建或选取，从微观上揭示了超临界CO2流体中不同活性剂分子在单粒子表面的自组装行为、空间结构及界面特性；研究结果表明：将氟化的表面活性剂掺混于超临界CO2流体中时，可明显改善贵金属纳米粒子在超临界CO2流体中的分散稳定性。其次采用MD方法探究了两粒子间相互作用及CO2分子空间分布特性，并充分考虑温度、密度等热力学参数及粒子自身属性（尺寸、形状、类型等）的影响，获取粒子间势能大小随距离变化的函数关系；最终基于对精细力场的细致分析及粗粒化处理，采用介观的PBE和跨尺度模拟方法探究多体粒子在超临界CO2 流体中的分散、聚集及沉降行为，及其与体相流体热物性的相互作用机制。本项目研究不仅有助于深入认识超临界CO2流体在能源动力应用领域的传热特性及技术瓶颈，同时基于纳米颗粒的热物性调控机制研究也将有助于克服或缓解其作为传热介质时所存在的传热恶化问题，同时为换热设备的优化设计及能源系统热力效率的提升提供理论支撑和技术指导，项目成果有望在航天动力推进技术、超临界CO2流体冷却、纳米材料制备等领域进行推广应用。