Improving the thermal efficiency of gas turbine engines requires high inlet temperature, which often imposes high thermal stress and heat load on the engines. This requires more efficient cooling systems to enhance the operation and overall performance of the gas turbine engines. Among the existing heat transfer approaches for blade cooling, turbulated rib structures promote heat convection by dismantling the boundary layer to give room for enhancing turbulence and secondary flow. The thermal performance of rib-cooled channels is mainly influenced by rib configurations and geometric parameters like rib pitch, angle and channel aspect ratio.
Dimple structures that function as vortex generators in the cooling passages have been adopted to increase turbulent mixing in the near-wall flow region. Numerous experimental studies have demonstrated the effectiveness of dimple structures in improving heat transfer performance. Nevertheless, despite the significant amounts of studies on the characteristics and principles of heat transfer enhancement on rib turbulators and dimple surfaces, there are limited studies on the hybrid structures consisting of both single rib and dimples despite their potential to enhance flow and heat transfer performance. In particular, miniature V rib-dimple hybrid structures have been deemed a potential candidate for enhancing heat transfer performance owing to the strong vortex flow interactions.
On this account, Ms. Shuangye Ran, Dr. Peng Zhang and Professor Yu Rao from Shanghai Jiao Tong University studied the heat transfer performance of the miniature V-shaped rib and dimple hybrid structures over the internal cooling channel surfaces of gas turbine blades. The heat transfer enhancement mechanism as well as the effects of critical configuration parameters, including rib height, rib pitch, dimple depth and rib-to-dimple pitch, on flow characteristics and heat transfer, were also investigated. The numerical study was carried out at Reynolds number Re = 50,500 using the Abe Kondoh Nagano (AKN) k – ε turbulence model. The main objective was to enhance the heat transfer of gas turbine blades. Experimental results from transient liquid crystal thermography indicated local heat transfer enhancement characteristics. Their work is currently published in the International Journal of Thermal Sciences.
The authors showed that the miniature V-shaped rib-dimple hybrid structure exhibited significant improvement in the heat transfer augmentation performance. The V rib upstream triggered a strong downwashing flow that destroyed the recirculation region within the dimple. Moreover, the interactions between the longitudinal vortices generated by the dimple and V rib promoted the turbulent mixing between the mainstream and near-wall flow, thereby improving the uniformity and intensity of heat transfer. As a result, smaller streamwise rib pitch brought about superior heat transfer augmentation due to the larger area exposed to the cooling air and increased turbulent mixing.
An increase in both the dimple depth and rib height significantly increased the globally averaged heat transfer augmentation initially before it stabilized when approaching the peak. The maximum heat transfer enhancement was achieved when the dimple depth-to-diameter ratio and rib height-to-channel hydraulic diameter ratio were 0.2 and 0.075, respectively. The optimal hybrid structure reported an overall thermal performance factor of 1.73 at a friction ratio of 11.6 and a total Nusselt number ratio of 3.9. For the studied parameters, the total heat transfer enhancement of the hybrid structure was increased by 43.2% and 95.3% compared to that of V-rib-only and dimple-only structures, respectively.
In summary, the study reported a study of heat transfer and flow structure over cooling channel surfaces consisting of miniature V rib-dimple hybrid structures. While the larger spacing between the downstream dimple and the rib increased the heat transfer, densely fabricated hybrid structures induced the highest heat transfer. In a statement to Advances in Engineering, Professor Yu Rao stated that their findings provided a better understanding of the heat transfer and flow structure and would contribute to designing highly effective cooling technologies for gas turbine engines.
Ran, S., Zhang, P., & Rao, Y. (2022). Numerical study of heat transfer and flow structure over channel surfaces featuring miniature V rib-dimples with various configurations. International Journal of Thermal Sciences, 172, 107342.