A Computational Fluid Dynamic (CFD) modelling able to describe a discharge process of a Molten Salts (MS) Thermal Energy Storage system with integrated Steam Generator (TES-SG) is presented in this paper. This simulation model has been validated with experimental results obtained at the PCS Facility located at the ENEA Casaccia Research Centre in Rome. The numerical results of this model (MS velocities, temperatures and boundary heat flux distributions within the SG) have been used to derive a dimension-less form for describing fluid flow, by Reynolds number, Re, and heat transfer between MS and helical-coil tube bundle SG, by Nusselt number, Nu. The obtained correlation is: Nu = 0.3146Re(0.54)Pr(0.36), where Re and Nu are based on the outer diameter of the helical coils. It is valid for 400 to 1200 Reynolds numbers, 4 to 11 Prandtl numbers, an average winding angle of 2 degrees and an outer diameter of the helical coils of 0.0127 m. The correlation here proposed can be very useful for sizing a vertical helical-coil tube bundle SG immersed in a Molten Salts tank of a commercial CSP plant for a specific nominal power. (C) 2015 Elsevier Ltd. All rights reserved.