超临界压力有机工质管内对流传热及脉动流动机理研究

Due to the favorable thermo-economy, supercritical organic Rankine cycle (ORC) has been considered as one of the most promising solutions to convert medium/high temperature renewable energy or waste heat into electricity. Compared with other organic working fluids, hexamethyldisiloxane (MM) has relatively high thermostability and low critical parameters, making it more suitable for the medium/high temperature supercritical ORC system. However, as existing studies on supercritical flow and heat transfer mainly focus on specific working mediums and the general principle guiding the prediction of supercritical heat transfer coefficient is still missing, the optimize design of heat exchangers becomes one of the most challenging aspects related to the supercritical ORC modeling. By combining lab experiments, numerical modelling and theoretical analysis, the primary goal of this proposed work is to reveal the heat transfer and pulsatile flow mechanism of supercritical siloxane MM within a range of operating parameters of supercritical ORC. The main contents are as follows: (1) Based on the analysis of the supercritical flow pattern, heat transfer and pulsation characteristics, we will explore the effects of key parameters (including channel structure, flow direction, fluid parameters, and heat flux) on the supercritical heat transfer; (2) Through the quantification analysis and mathematical description of the abnormal heat transfer and pulastile flow at supercritical pressure, we will investigate the mechanism of two abnormal phenomenon and identify the key control factors; (3) By coupling thermodynamic and economic analysis models, we will study the influence of heat transfer characteristics on thermo-economy of supercritical ORC units. The outcomes of this work on the heat transfer and pulastile flow mechanism of supercritical siloxane MM can further supplement and enrich the supercritical flow and heat transfer theories, and provide a reference for the optimize design and operation of the medium/high temperature supercritical ORC system.

超临界ORC以其良好的热经济性成为面向中高温可再生及余热能源的潜在热电转化技术，六甲基二硅氧烷的综合理化性质与其运行条件相匹配，是循环适用有机工质。然而，目前针对部分典型流体的超临界流动与传热基础研究缺乏普适性，无法支撑中高温超临界ORC的优化设计及控制策略制定。为此，本项目拟采用实验、数值和理论分析相结合的研究方法，开展针对六甲基二硅氧烷的超临界压力管内对流传热及脉动流动机理研究。通过考察宽幅工况下工质的超临界压力传热、脉动特性及流动形态微观特征，阐明复杂边界条件耦合作用的影响机理；通过针对超临界压力传热异常和脉动边界的量化分析与数学描述，厘清超临界压力传热非常规现象的触发因子及其作用机制；通过建立中高温超临界ORC热力学和经济性耦合模型，揭示超临界压力工质传热特性对整机热经济性的影响规律。本项目的研究将深化对超临界流动与传热机理的认识，对发展中高温超临界ORC技术有着重要的学术意义。

超临界ORC以其良好的热经济性成为面向中高温可再生及余热能源的潜在热电转化技术，六甲基二硅氧烷的综合理化性质与其运行条件相匹配，是循环适用有机工质。然而，目前针对部分典型流体的超临界流动与传热基础研究缺乏普适性，无法支撑中高温超临界ORC的优化设计及控制策略制定。为此，本项目采用实验、数值和理论分析相结合的研究方法，开展针对六甲基二硅氧烷的超临界压力管内对流传热及脉动流动机理研究。通过考察宽幅工况下工质的超临界压力传热、脉动特性及流动形态微观特征，阐明复杂边界条件耦合作用的影响机理；通过针对超临界压力传热异常和脉动边界的量化分析与数学描述，厘清超临界压力传热非常规现象的触发因子及其作用机制；通过建立中高温超临界ORC热力学和经济性耦合模型，揭示超临界压力工质传热特性对整机热经济性的影响规律。所获得的关键成果包括：1）建立超临界压力流体流动与传热多功能实验台，首次获得热流密度、压力及流量等多因素耦合作用下的超临界压力硅油MM传热特性实验数据及其管内流体温度、密度、轴向速度、剪切应力、无量纲密度以及湍流强度等关键参数的分布规律；2）基于所获得的全局与局部参数，修正表征浮升力与热加速效应无量纲参数中的定性温度，提出浮升力、热加速效应、临界热流密度等无量纲参数对传热异常及流动不稳定的作用机制与阈值，结果得到他人实验数据的校验；3）建立基于热加速数的竖直微细管内超临界压力流体传热关联式，计算得到的努塞尔数与实验努塞尔数重合度较高，其较好地预测了管内局部传热强化、恶化、恢复等所有特征，关联式精度得到他人实验数据的校验；4）掌握超临界流体传热特性对ORC整机循环热经济性的影响规律；5）基于本项目共发表Q1区SCI收录论文3篇，EI收录1篇，国际会议1篇（获评优秀论文），参加会议并作报告3次，授权发明专利1项，培养研究生4名，项目负责人获得国防科学技术进步一等奖1项；6）项目所获得的超临界压力有机工质传热模型用于支撑某型号国内首套发动机预冷需求的燃油-空气换热器工程样机设计。本项目的研究将深化对超临界流动与传热机理的认识，对发展中高温超临界ORC技术及基于有机工质的空天飞行器综合热管理技术有着重要的学术意义。