Mechanisms of particle fouling on structured heat exchanger surfaces

结垢，即在传热和/或传质表面上形成沉积物，是能源和化学工程过程的设计和操作的基本问题。 由此造成的经济和生态破坏是巨大的。颗粒结垢，即悬浮颗粒在载液中的沉积，如砂或泥（见CNOWA 2006），与结晶结垢一起成为结垢问题的主要来源。 结构化热交换器表面（例如肋、翅片或凹坑）通常用于增加热传递速率，同时将额外的摩擦损失保持在最小值。 与其他涡流发生器相比，交错凹坑在能量效率方面显示出最佳可能的解决方案，因为热释放的增加近似等于摩擦阻力的增加。尽管在操作过程中了解了污垢的重要性和由此产生的问题，但在设计过程中，最大限度地提高传热和最小化水力损失是最重要的。在先前的研究项目“颗粒污垢对结构化传热表面的影响”中，颗粒污垢的影响可以通过空间和时间的高分辨率欧拉-拉格朗日大涡模拟和详细的实验研究来系统地分析，在建议的后续研究项目中，它的目的是通过内部的影响来扩展所开发的数值方法，颗粒碰撞和壁粗糙度。基于模拟的沉积颗粒局部分布和短时间段的积灰层厚度，提出了一种新的多尺度算法，用于将积灰层厚度外推到长时间段，具有实际意义。此外，连续相和分散相之间的动量交换将被研究，以估计不同的颗粒质量负荷的局部湍流的调制。将进行平行测量，以解决和确定时间和空间的三维污垢表面覆盖。基于热阻、质量热阻和面积热阻的污垢热阻必须组合成一个整体污垢状态的一致图形，作为一个基本的结果，将深入了解考虑颗粒污垢的凹坑表面的物理性能。此外，还将确定和鉴定数量/操作编号，以便表征凹坑表面的热和流体动力学行为。所获得的见解的目的是填补现有的知识缺乏，并将提供额外的数量的依赖于颗粒污垢的热工水力性能的确定。

Fouling, i.e. the formation of deposits on heat- and/or mass transferring surfaces is a fundamental problem for the design and operation of energy and chemical engineering processes. The resulting economic and ecological damage is enormous. Particulate fouling, the deposition of the suspended particles in the carrier fluid, such as sand or mud (see VDI WA 2006), is together with crystalline fouling the main source of fouling problems. The structured heat exchangers surfaces e.g. ribs, fins or dimples are commonly utilized to increase heat transfer rates while keeping the additional friction losses at a minimum. Staggered dimples show the best possible solution in terms of energy efficiency compared to other vortex generators, since the increase of the heat release is approximately equal to the increase of the frictional resistance. Despite the knowledge about the significance of fouling and the resulting problems during the operation, the maximization of heat transfer and minimization of hydraulic losses are in the foreground in the design process. In the previous research project “Effects of particulate fouling on structured heat transfer surfaces”, the influence of particulate fouling could be analyzed systematically for dimpled surfaces using spatial and temporal highly resolved Euler-Lagrangian Large-Eddy simulations and detailed experimental investigations.Within the proposed follow-up research project it is intended to extend the developed numerical methods by the influence of inter-particle collisions and wall roughness. Based on the simulated local distribution of deposited particles and the integral fouling layer thickness for short fouling intervals, a new multi-scale algorithm for the extrapolation of the fouling layer thickness towards long intervals which are of practical importance. Moreover the momentum exchange between the continuous and dispersed phase will be investigated in order to estimate the modulation of local turbulence for different particle mass loadings. Parallel measurements will be carried out to resolve and determine the temporal and spatial three-dimensional fouling surface coverage. The thermal, mass and area based fouling resistance have to be assembled to a consistent figure of the total fouling state.As a fundamental result, a deep insight into the physical performance of dimpled surfaces considering particulate fouling will be obtained. Furthermore, quantities/operating numbers will be identified and qualified which allows the characterization of the thermal and fluid dynamic behavior of dimpled surfaces. The acquired insights are aimed to fill up the existing lack of knowledge and will provide additional quantities for the determination of the thermo-hydraulic performance in dependence of particulate fouling.