基于相变换热的瞬变脉动内燃机废气余热能的高效热电转换

Due to factors such as the engine working process and changing load, the exhaust thermodynamic parameters have transient pulsation, which brings difficulties for efficient thermoelectric conversion of exhaust waste heat. It is one of the key technical problem need to solve for engine exhaust waste heat thermoelectric recycle. Considering on the mutual coupling factors among engine exhaust strong unsteady characteristic, large temperature gradient along exhaust flow direction, strong temperature dependent properties of thermoelectric materials, and optimal module size corresponding to peak power characteristic, this project is mainly to develop the efficient thermoelectric conversion method and theory when considering on the actual engine operation condition. This project proposes a new asymmetric-rib cascade thermoelectric structure using phase-change heat transfer and explores its thermoelectric conversion method. The new thermoelectric structure proposed in this project has its advantage: (1) the thermoelectric module and exhaust channel are separated, which is helpful to implement structure arrangement and enhance heat transfer; (2) the phase transition working medium is taken as the intermediate heat transfer media, which can realize a relatively stable generator heat source, and reduce the effect of exhaust strong transient pulse to thermoelectric generator; (3) taking sectional cascade thermoelectric structure combined with a variety of phase change working mediums and thermoelectric materials, which is helpful to realize an efficient thermoelectric conversion. The project is mainly to solve a series of scientific problems such as realizing the relatively steady characters corresponding to transient pulse heat, exploring the phase change heat transfer mechanism under non-steady condition, developing efficient cascade structure. This study can provide efficient design mechanism guidance for all kinds of engine waste heat recycling technology, and it has important scientific significance in promoting the progress of the thermoelectric conversion technology.

由于内燃机工作过程和负荷变化等因素影响，排气管内气流热力参数具有瞬变脉动性，严重影响了利用内燃机排气进行温差发电的转换效率。本项目基于内燃机排气自身强非稳态性、尾气流动方向沿程大温度梯度特性、热电材料物性强温度依存性等因素间具有相互耦合作用，拟开展内燃机实际运行工况下的余热高效热电转换理论和实验的研究。为此，提出了利用非对称肋式相变换热梯级温差发电结构的思路，其特点在于：（1）热电模块与尾气通道相分离，宜于结构布置和实现强化换热；（2）采用相变工质作为中间传热介质，缓解排气瞬态脉动性的影响；（3）采用多种相变工质与多种热电材料相结合的分段式梯级温差发电结构，实现高效热电转换。项目重点围绕瞬态脉动热源的相对稳态化、非稳态相变传热机理、多梯级结构热电转换机理等一系列科学问题展开研究。本研究对于推动热电转换技术在内燃机余热回收领域利用具有指导意义，也具有重要的科学价值。

利用温差发电器进行内燃机排气余热发电可有效提高其燃油利用率，同时降低排气对环境的不良影响。然而，受内燃机排气自身热物性的限制，排气传热性能较差；且受内燃机工作过程中负荷变化等因素的影响，排气热力学参数具有瞬变脉动特性，严重影响了排气余热温差发电器的发电效率。为了从根本上解决该问题，提高温差发电效率，本项目首先通过理论分析对常规的排气温差发电器的性能优化方式和评估方法进行了研究；在此基础上，提出解决方案，并先后通过数值研究和实验研究方法进行了方案有效性和可行性分析及验证。主要研究成果如下：(1) 对传统排气温差发电系统采用的排气侧强化换热性能优化方法提出“净功比”的量化评价指标，并通过理论推导得到其计算公式；(2) 提出了基于实际汽车驾驶工况的排气参数加权分析方法，并根据该方法进行了排气温差发电器性能分析和模块面积优化；(3) 提出了在传统温差发电器中加入相变装置的结构优化方案，并对优化后的相变式温差发电器进行了数值研究，与传统温差发电器相比，相变式温差发电器在使用更少的模块的情况下，大大提升了输出功率和发电效率，同时，提高了模块工作温度的均匀性；(4) 开展了相变腔传热特性实验研究，得到受限空间内沸腾和冷凝换热受热流密度、充液率、面积比等因素的影响规律；(5) 开展了相变式温差发电器实验，实验结果表明，实验研究范围内，随着模块数量减少，模块工作温度及冷热端温差升高，输出功率和发电效率均升高，验证了本项目所提优化方案的有效性和可行性。本研究对于推动热电转换技术在内燃机余热回收领域的应有具有指导意义，对受限空间内沸腾和冷凝耦合相变传热特性的研究具有重要的科学价值。