Numerical prediction on mist/steam cooling in a square ribbed channel at real gas turbine operational conditions

Flow and heat transfer characteristics of mist/steam cooling in a square channel with 60° rib angle at elevated gas turbine operational conditions featured by high temperature, pressure and Reynolds number are numerically investigated. The heat transfer enhancement of mist/steam at elevated conditions is compared with that at laboratory conditions with the identical mist mass ratio and droplet size. The 3D steady Reynolds-averaged Navier-Stokes equations with a standard k-ω turbulent model are solved by using commercial software ANSYS CFX. The CFD model has been validated by experimental data for both steam-only and mist/steam cases with a good agreement. Distribution and evolution of secondary flow in the ribbed channel as well as its effect on heat transfer are analyzed by vortex core technology. The results show that the strength of longitudinal secondary flow has a great influence on Nusselt number distribution on the ribbed surface. When injecting 2-8% mist into steam coolant, the heat transfer enhancement of 6.8-25.7% with average wall temperature reduction of 21-71.2 K is achieved at elevated conditions, while that of 11.2-34.3% with average wall temperature reduction of 23-71 K is obtained at laboratory conditions. The heat transfer enhancement and thermal performance of mist/steam at elevated conditions increase with Reynolds number, mist mass ratio and droplet size, but the thermal performance of steam-only gradually decreases with Reynolds number. The maximum heat transfer enhancement of mist/steam is 19.8% with 20 μm mist at elevated conditions, while that is 41.6% with 15 μm mist at laboratory conditions. The key of heat transfer performance of mist/steam is the survivability of droplets in the ribbed channel.