【作者】 金峰；

【导师】 陈德奇；

【作者基本信息】 重庆大学 ， 动力工程与工程热物理， 2024， 博士

【摘要】 印刷电路板式换热器（PCHE）作为一种新型紧凑式换热器,具有换热能力强、紧凑度高以及结构强度好等优势,在超临界CO₂布雷顿循环系统以及先进核能系统中十分具备应用前景。预冷器作为超临界CO₂布雷顿循环的关键换热设备之一,其性能对系统紧凑型和循环效率十分重要。由于超临界CO₂在预冷条件下的强变物性特征,明确其热工水力特性是预冷器PCHE设计与应用的重要前提。因此,掌握超临界CO₂在预冷器PCHE内的流动传热特征,对系统循环布置、运行以及效益等都至关重要。本文采用实验研究、数值模拟、理论分析以及代码开发相结合的方法,分别从试验本体测试、湍流模型改进、多结构参数模拟、封头流动分配模拟和程序优化设计等几个方面开展研究,对超临界CO₂布雷顿循环系统在Z字形PCHE预冷器的整体性能进行了分析。主要内容如下:针对7.24 mm节距、40°弯折角和1.0 mm倒圆角PCHE样件内超临界CO₂流动传热开展了试验测试。通过布置多个温度测点,研究了系统压力、入口温度以及流量对超临界CO₂流动换热在半圆形Z字形通道内的影响特性。结果表明,由于超临界CO₂在拟临界区域的强物性变化,使得换热系数在拟临界区附近呈现出较高水平,同时,质量流速以及压力的变化均对超临界CO₂换热系数造成明显的影响,而入口温度对换热系数峰值的影响相对较小。进一步,通过考虑速度变化与截面物性变化对换热的影响,提出了新的冷却流动与传热经验关联式,其中传热关联式预测了98.5%的实验数据在±20%误差以内,同时流动关联式对摩擦系数的预测结果均在±15%误差以内。基于SST k-ω模型模拟了超临界CO₂在通道内的冷却传热过程,通过引入了湍流佩克莱特数,并结合湍动能、耗散率以及超临界CO₂物性,进一步提出了新的湍流普朗特数模型Pr_t。结果表明,改进后的Pr_t模型可以较好的提高超临界CO₂在拟临界区内的冷却传热模拟精度,预测偏差显著降低。在实验数据±20%误差范围内,改进Pr_t模型相比常数Pr_t模型被包络的模拟数据从91.4%提高至96.2%。此外,结合浮升力效应、场协同、熵产以及湍流热导率分析,探究了超临界CO₂的冷却传热机制,当拟临界温度位于粘性底层至缓冲层的过渡区域时,换热能力显著增强,从而换热系数峰值对应的主流温度略大于拟临界温度T_pc。基于改进SST k-ω模型针对Z字形PCHE内超临界CO₂流动传热过程进行了参数化计算,研究了不同节距、不同弯折角、不同倒圆角以及入口工况对超临界CO₂热工水力性能的影响。模拟结果表明,结构参数的变化使得超临界CO₂流动方向发生改变,从而影响通道内的分离流强度和局部湍流强度,局部换热系数和摩擦阻力发生改变,并在Z字形通道内呈现明显的涨落特征。通过增大倒圆角可明显减小二次流效应引起的回流区粘性耗散,从而显著降低压降导致的不可逆损失。同时,引入了熵产综合性能因子S_N,对比了综合性能因子PEC与S_N对Z字形结构效应的评价差异。进一步,通过对比各结构参数以及运行工况下平均换热系数随摩擦系数的变化趋势,提出了集总结构参数Z,并得到了相应的阈值Z_t。最终结合结构效应因子提出了宽范围结构下的传热关联式以及摩擦关联式。针对PCHE封头结构内超临界CO₂的流量分配开展了模拟研究。探究了不同封头管直径、不同行通道数、不同列通道数以及工况条件对PCHE进出口流量分配的影响。模拟结果表明,封头管与PCHE微通道间的最大张角可显著改善通道的流量不均匀性,而入口流量的变化对封头流量不均匀性的影响较小。出口封头较大物性的变化对局部阻力的影响更为显著。此外,结合BP神经网络对进出口封头的模拟数据进行训练,构建了流量分配代理模型,最终进出口封头代理模型分别预测了99.8%以及98.7%的数据在±15%的误差范围内。针对PCHE多通道耦合传热过程,基于MATLAB构建了一维&三维耦合计算程序,通过植入流动传热以及流量分配预测模型,研究了不同结构参数以及流动配置对Z字形PCHE内整体性能的影响。结果表明,在通道相同的入口工况下,流量不均匀性的变化对PCHE通道平均换热量的影响较小,而流动不均匀性增大会增加PCHE的整体流动阻力,同时影响通道整体温度分布特征。在相同核心长度下,由于封头壁厚以及边缘通道的实际预留宽度,错流区实际的有效换热面积降低,与纯逆流布置下的换热量对比结果呈现出差异性。此外,结合多目标优化分析对PCHE整体性能进行了优化,并结合一维&三维程序改进了PCHE设计参数,形成了一套预冷器PCHE优化设计方法。更多还原

【Abstract】 The printed circuit heat exchanger（PCHE）,as a novel compact heat exchanger,possess advantages such as high heat transfer performance,high compactness,and superior structural strength.It shows great potential for applications in Supercritical Carbon Dioxide Brayton system and advanced nuclear energy system.The precooler,as a key heat exchange component in the supercritical CO₂ Brayton cycle,has a significant impact on the compressor performance and system efficiency through its outlet temperature.Additionally,the flow and heat transfer characteristics of supercritical CO₂with significant variations in properties under precooling conditions are crucial for the design and operation of heat exchange equipment.This study employs a combined approach of experimental research,numerical simulation,theoretical analysis and code development.It explores several aspects,including experimental test,model modification,simulation with various structural parameters,simulation of header flow distribution,and program optimization design.The research focuses on the overall performance analysis of PCHE as a pre-cooler in the supercritical CO₂ Brayton cycle.The main findings are outlined as follows:Experimental testing was conducted on a PCHE prototype with pitch of 7.24 mm,bend angle of 40°,and bend radius of 1.0 mm for the flow and heat transfer of supercritical CO₂.Multiple temperature measurement points are arranged to investigate the impact characteristics of system pressure,inlet temperature,and flow rate on the heat transfer performance of supercritical CO₂ along the flow direction.The results indicate that the substantial thermophysical properties variation of supercritical CO₂ near the pseudocritical region lead to higher heat transfer coefficients.Additionally,changes in mass flow rate and pressure can significantly affect the heat transfer coefficient of supercritical CO₂,while the influence of inlet temperature is relatively small.Furthermore,considering the impact of velocity changes and cross-sectional property variations on heat transfer,a new correlation for cooling flow and heat transfer is proposed.The heat transfer correlation predicts 98.5%of the experimental data within a±20%error,and the predictions for friction coefficients are within a±15%error.Based on the SST k-ωmodel,the cooling and heat transfer process of supercritical CO₂ flowing in zigzag channel was simulated.By introducing the turbulence Péclet number and combining with turbulent kinetic energy,dissipation rate,and properties of supercritical CO₂,a new turbulence Prandtl number model,Pr_t,was proposed.The results indicate that the improved Pr_t model can significantly enhance the simulation accuracy of cooling heat transfer of supercritical CO₂ in the pesudocritical region,with a significant reduction in prediction deviation.Within a±20%range of experimental data errors,the modified Pr_t model,compared to the constant Pr_t model,envelops the simulation data from 91.4%to 96.2%.Furthermore,by analyzing the effects of buoyancy force,field synergy,entropy generation,and turbulent thermal conductivity,the cooling heat transfer mechanism of supercritical CO₂ was investigated.When the pseudocritical temperature lies in the transition region from the viscous sublayer to the buffer layer,the heat transfer capability is significantly enhanced,leading to bulk temperature T_b slightly higher than the near-critical temperature T_pc corresponding to the peak heat transfer coefficient.Based on the improved SST k-ωmodel,parametric calculations were conducted for the flow and heat transfer process of supercritical CO₂ within zigzag channel PCHE.The study investigated the influence of different pitches,bend angles,bend radius,and inlet conditions on the thermal-hydraulic performance of supercritical CO₂.Simulation results indicate that bend structure alter the flow direction of supercritical CO₂,thereby affecting the intensity of separation flow and local turbulence within the channels,leading to changes in local heat transfer coefficients and frictional resistance,exhibiting significant fluctuations within the zigzag channels.Increasing the bend radius are found to significantly reduce the viscous dissipation in the recirculation zone caused by secondary flow effects,leading to a substantial decrease in irreversible losses induced by pressure drop.At the same time,the comprehensive performance factor S_N of entropy production is introduced,and the evaluation difference between the comprehensive performance factor PEC and S_N on the zigzag structure effect is compared.Furthermore,by comparing the trends of average heat transfer coefficients with friction coefficients under various structural parameters and operating conditions,a comprehensive structural parameter Z was proposed,along with its corresponding threshold Z_t.Finally,incorporating the structural effect factor.the heat transfer correlation and friction correlation for a wide range of structures were derived.The numerical study was conducted to investigate the flow distribution of supercritical CO₂ within the PCHE header structure.The impact of different header tube diameters,row and column channel numbers,and operating conditions on the flow distribution at the inlet and outlet headers was explored.When the included angle between the header tube and side channels is reduced,it significantly improves the nonuniformity of flow in the channels,while the variation of mass flow rate has a relatively small effect on the nonuniformity of flow in the header.The influence of significant thermophysical property changes at the outlet on the local resistance of the header is more pronounced.Furthermore,by combining BP neural networks to train simulated data for inlet and outlet headers,a flow distribution surrogate model is constructed.The surrogate models for the inlet and outlet headers predict 99.8%and 98.7%of the simulation data,respectively,within a±15%error range.For the coupled heat transfer process in PCHE with multiple channels,the one-dimensional and three-dimensional coupled calculation program was developed using MATLAB.By incorporating flow and heat transfer prediction models,the overall coupled heat transfer process of supercritical CO₂ in the zigzag channel PCHE was studied under different parameters and flow arrangements.The results indicate that,under the same inlet conditions for small channels,variations in flow uniformity have a minor impact on the averaged channel heat transfer of PCHEs.However,increased flow non-uniformity leads to higher flow resistance in PCHEs and affects the overall temperature distribution characteristics of the channels.With the same core length,the actual effective heat transfer area in the cross-flow region decreases due to the thickness of the header and the actual reserved width of the edge channels to header inner wall,showing differences in heat transfer compared to pure counterflow arrangements.In addition,the overall performance of PCHEs was optimized through multi-objective optimization analysis.The PCHE design parameters were improved by integrating 1D&3D programs,forming a comprehensive optimization methodology for pre-cooler PCHE design.更多还原