Advanced development of high pressure turbines requires accurate predictions of film cooling flow. However, the length scales inherent to film cooling flows produce a large disparity compared to those of the mainstream flow field. To address this computational modelling challenge, an immersed mesh block (IMB) methodology has been initiated (Lad and He, 2011) which uses the much refined mesh around cooling holes to be mapped into the base mesh which tends to be much coarser for blade aerodynamic designs. Both the base mesh flow field and that of the IMB are solved simultaneously. By employing a simultaneous two-way coupling, the flow physics in and around cooling holes is able to interact with the mainstream, hence the length scales of both types of flow, as well as their interactions, are appropriately captured and resolved. The present work is aimed to develop a new numerical scheme for enforcing conservation at the interfacing boundary between the immersed cooling block and the base mesh, as well as, carry out a systematic validation and application of the IMB method for some well-established film-cooling experimental configurations (cylindrical and fan-shaped holes) at different blowing ratios. During the validation process, the mesh counts/resolution requirements for consistent cooling predictions for design analyses are established. The method is then applied to a transonic HPT stage. Its steady and unsteady flows are investigated. The results consistently demonstrate the effectiveness and applicability of the conservative IMB method, and indicate, for the first time, some interesting and relevant unsteady film-cooling behaviour.