CFD analysis of the influence of guide ribs/vanes on the heat transfer enhancement of a trapezoidal channel

Abstract

Gas turbines are used to generate mechanical power using the thermal energies of working fluid. Turbine thermal efficiency and power output both increases by letting high temperature combustion gasses at the turbine inlet. Combustion technology has advanced to such an extent that it can create hot gases having temperatures greater than the melting points of turbine built materials. Gas turbine blades are an important turbine part and they come in direct contact with combustion gases. To have a longer safe operation life of gas turbine blades, different configurations are designed to bring blade surface temperatures down to safe working limits. One way of accomplishing this is by the internal cooling of the gas turbine blades. Gas turbine blade trailing edge is designed to be thin and sharp. Usually pin slots are provided on its surface to eject a cooler air out for its cooling purposes, but if an internal cooling serpentine channel has to be placed inside it, it will have a shape of trapezoid. The channel inside can be modeled as a two-pass trapezoidal channel with a 180° turn. Gas turbine blade tip is an important region to be cooled. Clearance gap between blade and casing causes secondary leakage flow with high tip wall heat fluxes and local high blade temperatures. To increase the blade tip region heat extracting capacity of the internal cooling trapezoidal channel, different geometry designed ribs and vanes are placed at the channel turn. Together they make up six different cases to be analyzed for their heat transfer enhancement characteristics. Numerical based CFD technique is utilized to investigate different channels heat transfer enhancements. Channels pressure drops and overall performance is evaluated and compared. It is found that by placing ribs/vanes at the trapezoidal channel turn region, we can increase channel walls heat transfers up to 40%. A vane type design suggested that overall thermal performance for the channel tip wall region can be improved up to 21% as compared to a channel with no rib or vane attached. Rib/vane cases affect local flow fields to gain improved heat transfers at different regions of the trapezoidal channel. It is also concluded that with increase in Reynolds number, heat transfer decreases while pressure drop increases in a trapezoidal channel, regardless of presence or absence of vane/ribs in the turn region. This eventually results in decrease in the thermal performance of the channel with increase in Reynolds number. Finally, it is found that for same thermal and flow conditions, heat transfer is more at the tip wall for a trapezoidal channel compared to a rectangular channel.