Hub Extension in an Axial Gas Turbine Diffuser

Abstract

Diffusers downstream of axial gas turbines typically support a hub carrying the turbine rotor bearing. The blunt end of the hub is one of the main causes of loss production. However, a long hub can theoretically reduce the losses and improve the pressure recovery of the diffuser and thus the overall performance of the gas turbine. At the end of the hub the flow experiences a sudden expansion, which can be described by the Borda Carnot equation for incompressible fluid. The pressure loss at the end of the hub depends on the dynamic head and the area ratio of the expansion. With a long hub the blunt end moves further downstream into the diffuser and therefore the flow velocity and the area ratio of the step are reduced, resulting in a lower pressure loss. Two different casing geometries, each with a short and a long hub configuration are examined experimentally and numerically using CFD (Computational Fluid Dynamics). The geometrical configurations lead to a separation at the casing for a uniform total pressure profile at the inlet which shows the highly loaded state of the diffuser configuration. Experiments have been conducted at a high subsonic inlet Mach number comparable to real turbine exit flows and a low Mach number representing incompressible flow. It is shown that in some cases the long hub has a benefit to pressure recovery. However, the Mach number at the inlet of the diffuser influences the intensity of this effect. A high inlet Mach number increases the losses in the inlet section of the diffuser and at the struts supporting the hub and hence decreases the diffuser performance. Consequently the performance depending on the particular diffuser design can be decreased with an extended hub configuration.