外胀式螺旋波纹管螺旋-脱涡耦合流动下强化换热机理研究

At present, heat transfer enhancement technology for high efficient heat transfer components is mainly materialized from studying the surface characteristics of the components, the flow characteristic and heat transfer mechanism in the components are therefore of great scientific importance. Considering the varying physical properties of the high temperature and pressure gas and the radiation characteristics of the high temperature wall as well as the alternation, polydirection, asymmetry and tridimensional diversion of the helical flow and eddy flow, this project studies the flow characteristics and heat transfer mechanisms in the outward convex helical corrugated tube with the help of experiment and simulation. A PIV (Particle Image Velocimetry) system is used to capture the accurate helical-trapped coupled vortex structures, and then the macroscopic characteristics of the flow inside the tube will be obtained. The LES (Large Eddy Simulation) simulation results are compared with the experimental data which is used to verify the accuracy of the simulation model. The LES model is used to study change rules of the flow shape, inner flow and heat transfer parameters. Monte Carlo combined with finite volume method are used to study the effect of radiation energy on the surface heat transfer. Response surface methodology is used to obtain the relationship between objective functions and effect factors, and field synergy and entransy dissipation are used for the optimal control and design of the flow and heat transfer. Results obtained from this study would contribute to the understanding of the relationship among the flow, heat transfer and related parameters of the helical-eddy flow, and also contribute to the design and optimization of high efficient heat exchangers.

高效换热元件强化换热技术目前普遍基于表面性能参数进行研究，因此，揭示换热元件内部流动特征及传热输运机理具有重要的科学意义。本课题针对外胀式螺旋波纹管中螺旋流和脱涡流的交替性、多向性、非对称性和转向立体性等特征，考虑高温带压气体物性变化及高温壁面的辐射特性，结合实验和理论研究其内部流动和强化传热机理。采用粒子图像测速仪（PIV）获得流体的宏观流动特征，精确捕捉螺旋-脱涡流动的涡旋结构。建立LES数值模型并与实验结果对比，验证模型的正确性，基于该模型考察流体流型、内部流动和传热参数的变化规律；采用蒙特卡洛法结合有限体积法，研究辐射能对表面传热的影响；采用响应曲面法确定目标函数和影响因子的函数关系，根据场协同和火积耗散最小原则实现流阻和传热的最优控制及优化。研究结果对深入理解螺旋-脱涡耦合流动状态下性能参数与流动、传热参数的关联具有重要的理论价值，在换热器的设计与优化具有广阔的应用前景。

课题围绕一种新型外胀式螺旋波纹管强化换热元件，对其在脱涡-螺旋流耦合下的强化换热机理和特性展开数值模拟和实验研究。该强化换热元件是通过液压胀型工艺加工而成，适合勇于高温、高压、含灰分流体的工作环境。通过实验对外胀式螺旋波纹管换热元件的传热、阻力特性进行了测试，并对数值模拟方法进行了验证。采用数值方法，对螺旋波纹管内的复杂流动机制和局部换热、阻力特性进行了研究，并分析了流动对换热和阻力的影响。从热力学第二定律的角度，采用换热熵产模型，对管内强化换热机理进行分析。最后，基于响应曲面法和多目标优化理论，建立了以换热性能、阻力性能和能量效益为三目标的评价体系，取代以往的单一目标评价方法。结果表明：（1）螺旋波纹管、贯轴式波纹管和光管的平均换热性能、阻力性能的实验结果和数值模拟的比较验证了数值模拟方法的准确性，二者误差不超过10%；（2）通过实验和模拟研究，发现螺旋波纹管的换热性能略小于贯轴式波纹管，阻力性能有较大程度的下降，最终螺旋波纹管的综合换热性能整体优于贯轴式波纹管，这是由于螺旋流的产生对换热和阻力均有削弱作用，但对阻力的削减远大于换热性能的削减；（3）局部换热性能在波结迎风面处，受湍流冲击壁面作用达到最强，局部阻力性能在光滑壁面的边界层内和二次流涡核处最大；（4）螺旋波纹管综合换热性能呈现区间最优原则，即在小雷诺数下大波结高度和小波结间距更佳，在大雷诺数下小波结高度和大波结间距更佳，其中波高=2mm、波距=20mm和波高=2mm、波距=30mm的管型，在雷诺数为3800-43800下整体最优；（5）基于多目标遗传算法得到螺旋波纹管以努塞尔特数、摩擦因子和综合传热系数为三目标的帕累托最优解集，并从平衡角度出发得到最优区间。