Physical Interpretation of Flow and Heat Transfer in Preswirl Systems

This paper compares heat transfer measurements from a preswirl rotor–stator experiment with three-dimensional (3D) steady-state results from a commercial computational fluid dynamics (CFD) code. The measured distribution of Nusselt number on the rotor surface was obtained from a scaled model of a gas turbine rotor–stator system, where the flow structure is representative of that found in an engine. Computations were carried out using a coupled multigrid Reynolds-averaged Navier-Stokes (RANS) solver with a high Reynolds number $k-ε∕k-ω$ turbulence model. Previous work has identified three parameters governing heat transfer: rotational Reynolds number $(Reϕ)$⁠, preswirl ratio $(βp)$⁠, and the turbulent flow parameter $(λT)$⁠. For this study rotational Reynolds numbers are in the range $0.8×106<Reϕ<1.2×106$⁠. The turbulent flow parameter and preswirl ratios varied between $0.12<λT<0.38$ and $0.5<βp<1.5$⁠, which are comparable to values that occur in industrial gas turbines. Two performance parameters have been calculated: the adiabatic effectiveness for the system, $Θb,ad$⁠, and the discharge coefficient for the receiver holes, $CD$⁠. The computations show that, although $Θb,ad$ increases monotonically as $βp$ increases, there is a critical value of $βp$ at which $CD$ is a maximum. At high coolant flow rates, computations have predicted peaks in heat transfer at the radius of the preswirl nozzles. These were discovered during earlier experiments and are associated with the impingement of the preswirl flow on the rotor disk. At lower flow rates, the heat transfer is controlled by boundary-layer effects. The Nusselt number on the rotating disk increases as either $Reϕ$ or $λT$ increases, and is axisymmetric except in the region of the receiver holes, where significant two-dimensional variations are observed. The computed velocity field is used to explain the heat transfer distributions observed in the experiments. The regions of peak heat transfer around the receiver holes are a consequence of the route taken by the flow. Two routes have been identified: “direct,” whereby flow forms a stream tube between the inlet and outlet; and “indirect,” whereby flow mixes with the rotating core of fluid.