A coupled DEM-CFD analysis of asphaltene particles agglomeration in turbulent pipe flow

We performed numerical investigations of the agglomeration behaviour of asphaltene particles in a turbulent pipe flow by using a commercial discrete element modelling (DEM) software, EDEM, coupled with a computational fluid dynamics (CFD) software package, FLUENT, in four-way coupling. We describe the particle–particle and particle–wall interactions to the Hertz–Mindlin approach extended with the Johnson–Kendall–Roberts contact force model. The very small size of asphaltene particles, however, requires minor time steps resulting in excessively long computing times. To reduce the latter, we developed a novel scaling method in which (a) the value of the shear modulus is reduced while the non-dimensional Cohesion number is kept constant by also reducing the value of the interfacial energy, and (b) the values of the friction parameters are adapted. We calibrated our scaling method by means of two tests, viz. the stop distance of a single particle sliding and rolling over a flat surface, and a common angle of repose test, both for particles 100 and 60 µm in size. As a result, we were able to increase both the Rayleigh time step and the DEM time step by a factor of 25. The eventual DEM/CFD simulations for 5 µm particles in a turbulent pipe flow still took almost 4 months on a 48 cores workstation. They resulted in visualisations and in quantitative data on, among other things, number and size (gyration radius) of agglomerates, their fractal dimension, and a coordination number, all as a function of time.