Fluid flow and heat transfer characteristics in axisymmetric annular diffusers

The present paper provides and validates a numerical procedure for the calculation of turbulent separated flow and heat transfer characteristics in axisymmetric expanding ducts, with emphasis on the annular diffuser geometry. The method is based on the fully-conserved control-volume representation of fully elliptic Navier-Stokes and energy equations in body-fitted orthogonal curvilinear coordinate systems. Turbulence is simulated via the two-equation (k-ε) model. The presented results consist of computed velocity and streamline distributions, the kinetic energy of turbulence and local and average Nusselt number distributions. Systematic variations are made in the Reynolds number (6 × 10³-6 × 10⁵) and the outer wall half angles (7°-20°, 90°). The study was further extended to flows with a range (0.0-0.9) of inlet swirl number. Comparison with available experimental data shows that the method with the utilized turbulence closure model and the discretization scheme reproduces the essential features of various diffuser heat transfer and fluid flow effects observed in the experiments. The degree of heat transfer coefficient enhancement, both maximum and average, increases strongly as the wall cant angle increases. The peak, average and exit Nusselt numbers exhibit clear dependence on the Reynolds number and were well correlated with ∼Re^2/3, as was previously encountered in the literature for other types of separated regions. Although there is some indication that the exponent increases to ∼0.8 for Re > 50,000. Local heat transfer rates have been shown to increase with the increase of swirl number and to peak near the reattachment point.