Implementation, demonstration and validation of a user-defined wall function for direct precipitation fouling in ANSYS fluent
Johnsen, Sverre G.; Pääkkönen, Tiina M.; Johansen, Stein T.; Keiski, Riitta L.; Wittgens, Bernd (2017-05-30)
JOHNSEN, S.G., PÄÄKKÖNEN, T.M., JOHANSEN, S.T., KEISKI, R.L. and WITTGENS, B., 2017. Implementation, demonstration and validation of a user-defined wall function for direct precipitation fouling in ansys fluent, J.E. OLSEN and S.T. JOHANSEN, eds. In: International Conference on Computational Fluid Dynamics in the Oil & Gas, Metallurgical and Process Industries 2017, SINTEF Proceedings, pp. 717-725.
Published in this repository with the kind permission of the publisher.
In a previous paper (Johnsen et al., 2015) and presentation (Johnsen et al., 2016), we developed and demonstrated a generic modelling framework for the modelling of direct precipitation fouling from multi-component ﬂuid mixtures that become super-saturated at the wall. The modelling concept involves the 1-dimensional transport of the ﬂuid species through the turbulent boundary layer close to the wall. The governing equations include the Reynolds-averaged (RANS) advection-diffusion equations for each ﬂuid species, and the axial momentum and energy equations for the ﬂuid mixture. The driving force for the diffusive transport is the local gradient in the species’ chemical potential. Adsorption mechanisms are not modelled per se, but the time-scale of adsorption is reﬂected in the choice of Dirichlet boundary conditions for the depositing species, at the ﬂuid-solid interface.
In this paper, the modelling framework is implemented as a user-deﬁned function (UDF) for the CFD software ANSYS Fluent, to act as a wall boundary condition for mass-transfer to the wall. The subgrid, 1-dimensional formulation of the model reduces the computational cost associated with resolving the ﬁne length-scales at which the boundary-layer mass transfer is determined, and allows for efﬁcient modelling of industry-scale heat exchangers suffering from fouling.
The current paper describes the modelling framework, and demonstrates and validates its applicability in a simpliﬁed 2D heat exchanger geometry (experimental and detailed CFD modelling data by Pääkkönen et al. (2012, 2016)). By tuning the diffusivity, only, good agreement with the experimental data and the detailed CFD model was obtained, in terms of area-averaged deposition rates.
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