"Hydrothermal spallation drilling" is a possible alternative drilling technology that uses the properties of certain rock types to disintegrate into small fragments when heated up rapidly by a hot impinging fluid jet. Hot supercritical water jets are favored to provide the required heat for thermal rock fragmentation. However, the indispensable presence of a dense water-based drilling fluid during operation can cause considerable heat losses in the supercritical water jet before impingement on the rock surface. To predict these heat losses from the hot jet to the cold aqueous environment, a numerical model based on the commercial CFD tool ANSYS FLUENT (R) was established. Penetration lengths of the supercritical jet plume at near-critical pressures were determined numerically and validated with experimental values fora wide range of conditions. Experiments and simulations showed an acceptable agreement and the experimental trends were satisfactorily predicted by the model. (C) 2013 Elsevier B.V. All rights reserved.