Physical Interpretation of Flow and Heat Transfer in Pre-Swirl Systems

This paper compares heat transfer measurements from a pre-swirl rotor-stator experiment with 3D steady state results from a commercial 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 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 (Re_φ), pre-swirl ratio (β_p) and the turbulent flow parameter (λ_T). For this study rotational Reynolds numbers are in the range 0.8×10⁶ < Re_φ < 1.2×10⁶. The turbulent flow parameter and pre-swirl ratios varied between 0.12 < λ_T < 0.38 and 0.5 < β_p < 1.5, which are comparable to values that occur in industrial gas turbines. At high coolant flow rates, computations have predicted peaks in heat transfer at the radius of the pre-swirl nozzles. These were discovered during earlier experiments and are associated with the impingement of the pre-swirl flow on the rotor disc. At lower flow rates, the heat transfer is controlled by boundary-layer effects. The Nusselt number on the rotating disc increases as either Reφ or λ_T 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. Two performance parameters have been calculated: the adiabatic effectiveness for the system, Θ_b,ab, and the discharge coefficient for the receiver holes, C_D. The computations show that, although Θ_b,ab increases monotonically as β_p increases, there is a critical value of β_p at which C_D is a maximum.