In hydrogen safety analysis, structure response due to the pressure and thermal loads from the combustion is of great concern. It is of high significance to understand not only the combustion process itself, but also the heat losses from the combustion products to the solid structures which may have strong impacts on the pressure and temperature decays. In many previous numerical simulations, heat losses from turbulent hydrogen flames to the confinement structures were usually considered to be negligible or less important. However, it has been revealed by many experimental studies that modeling of heat losses from the combustion products is important for accurate predictions. Our objectives are to study the importance of various heat transfer mechanisms and their relative contributions to the total energy losses. Numerical investigations on the mechanisms of heat losses caused by propagating turbulent flames were performed using a semi-implicit pressure-based all-speed CFD code GASFLOW-MPI. Heat losses from turbulent sonic flames to the structures of the ENACCEF facility at IRSN were studied. It appears that the effect of heat losses on the flame propagation properties is not significant. However, the impacts of heat losses on the pressure peak and pressure decay after hydrogen combustions should not be neglected. It indicates from our simulation results that the convective heat transfer and thermal radiative heat transfer are the main contributors of the total energy losses to the Structures of ENACCEF. In our cases, the effect of steam condensation heat transfer is relatively small but not negligible. The relative contributions of various heat transfer mechanisms could be different in other experimental facilities with various geometrical configurations, various internal structures, and various optical and thermal characteristics of the burnable gas mixtures. In general, it is suggested to include the heat transfer mechanisms in order to improve the reliability and accuracy of numerical analyses of hydrogen safety issues. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.