Reactor performance of confined jet mixers for continuous hydrothermal flow synthesis of nanomaterials is investigated for the purpose of scale-up from laboratory scale to pilot-plant scale. Computational fluid dynamics (CFD) models were applied to simulate hydrothermal fluid flow, mixing and heat transfer behaviours in the reactors at different volumetric scale-up ratios (up to 26 times). The distributions of flow and heat transfer variables were obtained using ANSYS Fluent with the tracer concentration profiles being simulated via solving the species equations. The predicted temperature distributions under various volumetric scale-up ratios were compared with the available experimental data, and good agreements reached. The mixing between supercritical water jet and precursor stream with different scale-up ratios was examined in detail to identify the effect of scale-up ratios on hydrodynamic and thermodynamic features. The findings indicate that slightly weaker mixing was observed at the pilot plant scales, but the momentum dominated turbulent flow in the reactors and the same order of magnitude of mixing levels at both laboratory and pilot plant size scales could lead to similar quality nanoparticles to be manufactured under the investigated volumetric scale-up ratios and operating conditions, which is supported by experimental observation from literature. (C) 2015 Elsevier B.V. All rights reserved.