The experimental program performed in the Upper Plenum Test Facility (UPTF) was designed to investigate the flow behavior in the primary system of a PWR during a LOCA, aiming to extend the existing experimental database and assist in the development and assessment of advanced computational tools. In particular, the main objective of the UPTF-TRAM during the C1 series of tests was to investigate the multidimensional effects under full-scale conditions of the ECC injection due to a small-break LOCA. In this context, Computational Fluid Dynamics (CFD) simulations can provide high-fidelity, detailed descriptions of the fluid flow and, therefore, can be used to supplement the evaluation of the phenomena of interest. In the present article, single-phase flow simulations of the UPTF-TRAM Test C1 Run 21a2, conducted with the computational tool OpenFOAM, are presented. The original UPTF design geometry was simplified and adapted for a CFD simulation, preserving the geometrical details of the cold legs, downcomer, and core inlet regions. Conjugate heat transfer simulations were performed to include the effects of transient heat transfer across the core barrel, and different boundary conditions were evaluated. Numerical results show that the ECC injection produces a cold front that impinges on the core barrel and descends through the downcomer with a swirling displacement. Temperature evolution in selected positions in the downcomer shows a satisfactory agreement with the experimental results, especially in the measurement locations close to the affected cold leg.