A highly efficient aircraft gas turbine engine requires proper utilisation of coolant supplied from the various stages of the compressor to the high-temperature working turbine blade. In the present work, a compound rib-dimple configuration is explored for heat transfer enhancement in the internal channel of gas turbine blades. A compound rib-dimple configuration consisting of multiple mini 45° V ribs and spherical dimples is fixed on the walls of high aspect ratio (AR) coolant channels to promote turbulent mixing and heat transfer augmentation. The comparable size of ribs and dimples is considered to increase the flow interaction between dimples and ribs. The numerical investigation is carried out to study the induced flow structure and to study the heat transfer parameters. Simulations have been performed for different Reynolds numbers in the range of 20,0000 to 70,000. The k-ε turbulence model with a scalable-wall function is utilised to model the turbulent flow. The V-rib present in front of the spherical-dimple creates strong down-wash flow, which disturbs the recirculation region inside the dimple. A pair of longitudinal vortices is generated above the dimple, promoting turbulent mixing between mainstream and near-wall flow. This three-dimensional flow effect enhances the intensity and uniformity of heat transfer. Further, Nusselt number and thermal-hydraulic performance (THP) are calculated for four high AR channels (4-6.5) with rib heights of 1- 1.5 mm. The maximum Nusselt number is obtained for channel with AR = 6.5 and 1.5 mm rib height, while the same channel with 1 mm rib height shows the highest THP. The THP of channel with AR 6.5, 1 mm rib height, improves by 2.36- 3.54 %, 9.49-10.15 % and 13.37-14.49 % with respect to channels with AR 6, 5 and 4, respectively.