厌氧消化中多相非牛顿流体流动、传热与生化反应动力学的耦合机理研究

Anaerobic digestion is a biochemical technology that converts biomass to methane under oxygen free conditions. An extensive investigation of anaerobic methane fermentation requires identifying the relationship between the physical environment and the biological processes. To increase the methane production, a computational fluid dynamics (CFD) technique along with theoretical analysis and experimental measurement is used to characterize the bacterial fermentation mechanisms. This is further complicated as it is intertwined with the three-phase flow of gas-solid-liquid and heat transfer in anaerobic digesters, in which the liquid phase exhibits a non-Newtonian fluid behavior. To develop an appropriate model for the multiphase flow, twelve turbulence models are evaluated by comparing the frictional pressure drops of the three-phase flow in a horizontal pipe obtained from CFD simulation results with those from experimental data. The flow patterns in a small reactor with a mechanical impeller and a gas diffuser are captured by particle image velocimetry and high-speed photographic techniques. The measured flow profiles and several flow images are used to check the simulations quantitatively and qualitatively. To improve mixing performance, the vertical draft tube that is employed is numerically and experimentally studied. Further, the biochemical reaction that consists of twelve biological parameters and eight ionic compounds is predicted based on the converged flow and temperature fields. The biogas yields in a complete-mix digester with and without draft tube mixing are measured to verify the simulation reliability. By integrating multiphase flow and heat transfer with bioreaction in anaerobic digesters, the CFD methodology may be applicable to the study of other bioreactors.

厌氧消化是一种在无氧环境下将生物质转化为甲烷的生化技术。本项目拟通过理论分析、试验以及计算流体力学（CFD）方法来揭示厌氧反应器内的微生物发酵与含有非牛顿流体特性的气-固-液三相流动和传热的耦合机理，解析物理环境与生物反应过程的内在联系，从而提高厌氧消化产甲烷率。首先，联合 CFD 模拟和试验测试，对比分析水平圆管内三相流动的沿程压力损失，评估12种湍流模型对三相流动的适应性，从而提出一种适用于多相流的湍流模型。在此基础上，进一步研究该模型在小型搅拌式反应器应用的可行性，并结合试验结果对该模型进行改进。其次，在置入气液下送强化器的情况下，研究其对混合性能的影响。最后，分析含有12种生化参数和8个离子化合物全混式厌氧反应器内的生物气产量，并研究气液下送强化器置入前后对于生物燃气产量的影响。项目旨在系统掌握厌氧反应器内多相流动和传热与生化反应的耦合机理，为其他生物反应器的研究提供参考。

厌氧发酵中气体-固体-非牛顿流体三相共存体系与生化反应动力学耦合研究具有重大的应用背景，国内外学者现有研究成果大部分仅停留在对单相牛顿流体物理过程的探索上，对厌氧发酵耦合机理的研究不够全面。针对以上问题，本项目以搅拌式厌氧发酵作为产甲烷的其中一种混合方式，系统性地展开了涉及多相非牛顿流体流动、传热、微生物及化学反应等多学科交叉融合下的机理研究，深化对厌氧发酵反应器内产气机理的认识，实现较低能源投入和较高能源产出的良性循环。根据本项目研究结果，可以得出以下结论：1）计算流体力学方法是预测搅拌式消化器中流动模式和混合时间的一种经济方法；2）搅拌桨叶转速增加导致混合强度增加，但高于最佳阈值以上，会对甲烷的产量造成负面影响，存在最佳甲烷产量的混合强度阈值；3）无法仅通过提高搅拌桨叶的转速来提高日甲烷产量；4）粪肥的流变特性随着混合时间变化；5）混合有助于减少厌氧消化器的发沫，并且应避消化器负载过大；6）利用计算流体动力学模型来表征厌氧消化器中由机械叶轮驱动的固体悬浮液。研究了非牛顿流体行为对多相流动特性的影响。结合切线方法和均匀性指数方法可以预测最小叶轮速度，避免固体堆积。.另外，本项目研究证实，每个厌氧消化装置都存在一个混合强度阈值，超过这个阈值，增加混合强度是能量浪费，不会增加甲烷产量，而是可能减少甲烷产量。以kw/h为单位的净能量生产的结果证实理想的最佳甲烷生产混合强度在50rpm到100 rpm范围内。而使用净能量概念计算甲烷的生产是确定每个厌氧消化的混合模式、混合强度、混合时间和混合间隔的最佳标准。

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