Particle diffusional losses in confined flows such as pipes are important for identification of erosion or accretion build-up. Its applications are varied and prominent in industrial and biomedical applications where essentially, tubular geometries transport fluid-particle flows. Computational modelling of dilute suspensions of ultrafine and nano-scale particles dispersing in a fluid often use the Lagrangian particle tracking approach. The Euler Implicit scheme in FLUENT v16 was evaluated for tracking such particles through a standard pipe geometry. It was found that the particles' Brownian dispersion was dependent on the computational mesh, and the integration time step for a given particle size. For the pipe geometry well established analytical solutions for particle deposition efficiency are available to directly compare the simulated results. For irregular geometries (such as human respiratory anatomy) there are no analytical solutions and experimental deposition data is scarce and an alternative method is required to verify the simulation setup. In this case the computational models were set with zero velocity and the nanoparticles released from rest. The particle motion was then initiated purely by Brownian motion. The root mean square displacement was used to compare the time step size selector (either by the Length Scale Factor, L-s, or the Step Length Factor SLF approach in FLUENT; and a fixed time step using an in-house code 'PARTICLE'). This provides a modelling verification framework to determine the most appropriate time step selection that reproduces the particle Brownian dispersion behaviour correctly. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.