This study investigates experimentally the heat transfer characters of a volumetrically-heated melt pool in LWR lower plenum under different cooling boundary conditions both in 2D and 3D vessel geometries and with simulant melts with and without potential of crust formation. A survey of existing heat transfer correlations based on individual experimental definitions is firstly given. The inconsistency in parameter definitions in Nu-Ra correlations is addressed. The general difference of upward heat transfer behavior in term of Nu depending on the existence of crust is discussed and explained. In several serials of LIVE3D and LIVE2D experiments, in which different combination of external cooling and top cooling were performed, and both simulants with crust formation and without crust formation were used, the influence of cooling boundary conditions, the vessel geometry and the simulants on the overall upwards and downwards heat transfer as well as on the melt temperature and heat flux distribution have been analyzed. This paper provides some explanations about the discrepant among the exiting heat transfer correlations and recommends most suitable descriptions of melt pool heat transfer under different accident management scenarios.