纳米流体在金属泡沫内强化换热及沉积规律的基础研究

With the improvement of thermal performance and size miniaturization for thermal equipments, conventional heat transfer enhancement techniques no longer satisfy related requirements. Metal foam with excellent thermal performance is an ideal material for making efficient compact heat exchange equipments. It is shown that the main thermal resistance of convective heat transfer in metal foams is in the fluid side, from which the fluid thermal conductivity can be enhanced by adding nanoparticles into base-fluid (nanofluid). As a new-type heat transfer enhancement technology, flow and heat transfer of nanofluid in metal foams is accompanied with deposition of nanoparticles. In this project, systematic theoretical, numerical and experimental study for this new-type heat transfer enhancement form will be performed from two aspects, "heat transfer mechanisms of nanofluid flowing through metal foams" and "deposition characteristics of nanofluid in metal foams and related mechanisms". Flow and heat transfer characteristics in macro-scale and micro-scale will be investigated for nanofluid in foams and convective heat transfer correlations will be developed. Investigations will be performed on deposition characteristics of nanoparticles on the surface of foam-ligaments and related compatibility. Numerical model of heat and mass transfer coupling process for nanofluid in foams will be developed. The influencing rule of nanoparticle-deposition on flow and heat transfer will be explored. The present research can not only present the physics on heat and mass transfer of nanofluid in metal foams but also provide experimental and theoretical fundamentals for optimizing the design of efficient compact thermal equipments.

随换热设备换热性能提高及其体积小型化趋势，传统强化换热技术已难以满足要求。具备优良传热性能的金属泡沫是制造高效紧凑式换热设备的理想材料。研究表明，金属泡沫内对流换热热阻主要在流体中，可采用在基液中添加纳米颗粒(纳米流体)的做法来强化流体导热。作为一类新型强化传热技术，纳米流体在金属泡沫内流动换热会伴随纳米颗粒沉积。本项目从“纳米流体在金属泡沫内强化换热机理”、“纳米颗粒在金属泡沫内沉积规律及机理”两方面对这类新型强化换热形式进行系统的理论、数值和实验研究。拟开展纳米流体在泡沫内宏观和微观流动换热特性研究，总结纳米流体在泡沫内对流换热关联式，开展纳米颗粒在骨架表面的沉积特性及相关配伍性研究，建立纳米流体在泡沫内热质传递耦合数值模型，探索纳米颗粒沉积对流动换热的影响规律。本项目研究不仅揭示纳米流体在金属泡沫内热质传递的物理机制，而且为高效紧凑式换热设备优化设计提供实验和理论基础。

金属泡沫有较高固体导热系数和比表面积，而一般流体导热系数较低，金属泡沫对流传热的主要热阻在流体一侧，这可通过流体中加入高导热纳米颗粒来降低流体侧热阻。金属泡沫内纳米流体传热是很有前景的双重强化传热形式，在紧凑换热器、高热流电子散热器方面有潜在应用。本项目对多孔介质内纳米流体流动、热质传输进行了系统研究。基于两方程模型研究了填充固体石蜡的金属泡沫复合材料在非稳态过程中的热响应特性，详细讨论了金属泡沫的孔隙率、孔密度、热扩散率, 以及石蜡中含纳米颗粒添加物对复合材料热响应特性的影响程度。实际应用中, 应综合考虑相关影响因素以使复合材料的导热性能达到最优。对于金属泡沫对流传热，通过对比局部非热平衡/热平衡模型分析了各参数对局部非热平衡特性的影响，发现孔隙率和导热系数比是影响的两大主要影响参数，金属泡内局部非热平衡效应随孔隙率降低、孔密度减小、导热系数比增大、雷诺数减小而逐渐增强。研究了纳米流体在填充金属泡沫的通道内的流动与传热特性，结果表明纳米流体和金属泡沫都有非常好的传热效果，雷诺数、达西数和纳米颗粒体积分数都会影响换热性能，孔隙率增大却导致换热性能变差。在获得了较好传热性能的同时，压降也有较大的增加，因此要综合考虑实际应用中的流动传热。对纳米流体在高温金属骨架表面的常压池沸腾换热现象进行了实验研究，分析了泡沫厚度、孔密度对沸腾换热的影响，发现纳米颗粒虽然增强了流体导热能力但增大了流体粘度，同时颗粒沉积还增加了表面热阻并改变了汽化核心分布。此外，纳米颗粒体积分数太高则导致纳米流体的非牛顿特性，如果要通过增加纳米颗粒体积分数来增强换热，应将研究重点防在纳米流体的非牛顿特性上。对于阻力较大的多孔泡沫，未来研究重点应放在多孔介质孔隙结构梯度设计方面以减小流动阻力。总之，纳米流体和金属泡沫有很明显的强化传热的作用，在未来传热设备中能有较好的应用。