NUMERICAL INVESTIGATION ON FLOW AND HEAT TRANSFER CHARACTERISTICS OF STEAM AND MIST/STEAM IN INTERNAL COOLING CHANNELS WITH DIFFERENT RIB ANGLES

In order to further increase the gas turbine efficiency by increasing the turbine inlet temperature, an advanced cooling technology needs to be developed. Recently, mist /steam (air) cooling is considered as a promising technology to effectively cool the hot components such as gas turbine vanes and blades. A series of experimental investigations and numerical simulations conducted in the past proved the feasibility and superiority of mist cooling technology in elevated gas turbine working condition. The aim of this study is to numerically analyze the secondary flow structure and the influence of secondary flow distribution on heat transfer in steam and mist/steam cooling channels with different rib angles by using vortex core interaction. In addition, the heat transfer characteristics of steam and mist/steam in gas turbine cooling channels with rib angles of 30, 45, 60, 90, duct aspect ratio 2:1, Reynolds number ranging from 10000 to 60000 and mist ratio increasing from 2% to 8% are also investigated. The commercial software ANSYS CFX 14.5 is used to solve the 3-D steady Reynolds-averaged Navier-Stokes equations with a SST turbulent model. The numerical results of Nusselt number (Nu) distribution along the centerline of each channel with steam-only are validated with the experimental values. Numerical results indicate that the predicted results are in good agreement with the experimental data. The distribution and strength of longitudinal secondary flows in 30, 45, 60 ribbed channels and transverse secondary flows in 90 ribbed channel have a great influence on the distribution of Nusselt number. The averaged Nu in 30, 45, 60 ribbed channels is higher than that in 90 ribbed channel due to longitudinal secondary flow having a better heat transfer performance than transverse secondary flow. The decrease of averaged Nu between two neighbored ribs along inclined ribs is mainly induced by the decreased strength of longitudinal secondary flow along the same direction in 30, 45, 60 ribbed channels. The averaged Nu of mist/steam with 5% mist injection in the four channels increases by 97.98%-151.9% compared with steam at Re=60000. Furthermore, the averaged Nu increases by about 11.08% to 213.6% compared with steam, when the mist ratio increases from 2% to 8%. The 60 ribbed channel achieves the best heat transfer performance in mist/steam cooling channels.