Turbine blades and vanes of modern gas turbines are exposed to high thermal loads. Therefore, cooling systems are required to reduce the gas turbine component temperatures under most engine operating conditions. This has driven manufactures to develop sophisticated cooling methods which minimize the use of coolant to maximize engine efficiency by enabling further increases in operating temperatures. The cooling designers use correlations developed using simplified geometries and apply them to the complex channel geometries of the actual cooling passages. There is a large amount of literature on approximate models of internal cooling passages by using square or rectangular channels, with or without various turbulators. While this information provides valuable information of the flow and heat transfer characteristics that can be expected, there is limited literature on the flow and heat transfer characteristics inside real turbine internal cooling systems. This thesis investigates the cooling performance of engine realistic internal cooling passages for turbine.This thesis provides a better understanding of cooling technology under an engine realistic features. An industrial gas turbine 1st stage blade multi-pass system was experimentally tested using a large scale (2.5 times) transparent acrylic model at engine representative Reynolds number and heat transfer and pressure drop results compared to Computational Fluid Dynamics predictions. The internal passages of a real turbine engine are usually highly complex and is common to use small features, such as turbulators, pedestals and impingement holes to enhance heat transfer. In addition, numerous film holes are usually included which draw fluid along the length of the cooling passage. These features combine to create a complex 3D cooling passage flow quite different to simple duct flow. The effect of streamwise fluid temperature variation on the local heat transfer coefficient measurements in transient heat transfer tests in long channels is addressed. For a long flow passage, a significant amount of heat is absorbed from the flow into the test surface as the flow progresses downstream. This results in a decreasing flow temperature in the streamwise direction at any instant in time, while the gas temperature at all streamwise positions increase with time. The effect of the selection of reference temperature for the determination of heat transfer coefficient is compared.The significant enhancement in heat transfer occurs immediately at the 180° turn into the passage, mostly due to the turn’s entrance effect as well as the enhanced turbulated flow conditions after the turn. This secondary flow promotes flow mixing, resulting in a heat transfer enhancement. In higher Reynolds number flow, the fully developed profile in the second passage (after turn) was reached at a longer distance compared to that with lower Reynolds number flow. The results capture the developing size and strength of the vortical structures in secondary flow at downstream of the turn. The local flow field was shown to be strongly coupled to the enhancement of heat transfer coefficient. Some cases show heat transfer coefficient on the smooth surface at the entrance after the turn slightly higher values than ribbed case. It is noted that the rib enhanced heat transfer after the turn region is less than the heat transfer coefficient enhancement by the turn. The effect of rib location after the turn on heat transfer enhancement was discussed.A rapid design approach for U-bend is proposed to design a minimum pressure loss in the turbine internal cooling system. In the multi-pass system, a major pressure drop is noted in the turn region for both smooth and ribbed channels. Some pressure recovery is observed just downstream of the turn. The total pressure loss for the flow in a bend region is a critical design parameter, as it augments the pressure required at the inlet of the cooling passage, resulting in a lower thermal efficiency. 2D and 3D U-bend configurations are designed by the rapid preliminary design approach. The results indicate that topology optimization can come up with preliminary design concepts for minimizing pressure loss of internal cooling passages. Computational simulations are carried out on rapid preliminary designed 2D and 3D U-bend configurations to validate the proposed design methodology.