FLUID-FLOW AND HEAT-TRANSFER CHARACTERISTICS IN AXISYMMETRICAL ANNULAR DIFFUSERS

The present paper provides and validates a numerical procedure for the calculation of turbulent separated Bow and heat transfer characterist ics in axisymmetric expanding ducts, with emphasis on the annular diff user geometry. The method is based on the fully-conserved control-volu me representation of fully elliptic Navier-Stokes and energy equations in body-fitted orthogonal curvilinear coordinate systems. Turbulence is simulated via the two-equation (k-epsilon) model. The presented res ults consist of computed velocity and streamline distributions, the ki netic energy of turbulence and local and average Nusselt number distri butions. Systematic variations are made in the Reynolds number (6 x 10 (3)-6 x 10(5)) and the outer wall half angles (7 degrees-20 degrees, 9 0 degrees). The study was further extended to flows with a range (0.0- 0.9) of inlet swirl number. Comparison with available experimental dat a shows that the method with the utilized turbulence closure model and the discretization scheme reproduces the essential features of variou s diffuser heat transfer and fluid flow effects observed in the experi ments. The degree of heat transfer coefficient enhancement, both maxim um and average, increases strongly as the wall cant angle increases. T he peak, average and exit Nusselt numbers exhibit clear dependence on the Reynolds number and were well correlated with similar to Re-2/3, a s was previously encountered in the literature for other types of sepa rated regions. Although there is some indication that the exponent inc reases to similar to 0.8 for Re > 50,000. Local heat transfer rates ha ve been shown to increase with the increase of swirl number and to pea k near the reattachment point.