Mechanisms of particle fouling on structured heat exchanger surfaces

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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.