Experimental and Computational Study of Hybrid Diffusers for Gas Turbine Combustors

Abstract

The increasing radial depth of modern combustors poses a particularly difficult aerodynamic challenge for the pre-diffuser. Conventional diffuser systems have a finite limit to the diffusion that can be achieved in a given length and it is, therefore, necessary for designers to consider more radical and unconventional diffuser configurations. This paper will report on one such unconventional diffuser; the hybrid diffuser which, under the action of bleed, has been shown to achieve high rates of diffusion in relatively short lengths. However, previous studies have not been conducted under representative conditions and have failed to provide a complete description of the relevant flow mechanisms making optimization difficult. Utilizing an isothermal representation of a modern gas turbine combustor an experimental investigation was undertaken to study the performance of a hybrid diffuser compared to that of a conventional, single-passage, dump diffuser system. The hybrid diffuser achieved a 53% increase in area ratio within the same axial length generating a 13% increase in the pre-diffuser static pressure recovery coefficient which, in turn, produced a 25% reduction in the combustor feed annulus total pressure loss coefficient. A computational investigation was also undertaken in order to investigate the governing flow mechanisms. A detailed examination of the flow field, including an analysis of the terms within the momentum equation, demonstrated that the controlling flow mechanisms were not simply a boundary layer bleed but involve a more complex interaction between the accelerating bleed flow and the diffusing mainstream flow. A greater understanding of these mechanisms enabled a more practical design of hybrid diffuser to be developed that not only simplified the geometry but also improved the quality of the bleed air making it more attractive for use in component cooling.

Impact of a Cooled Cooling Air System on the External Aerodynamics of a Gas Turbine Combustion System

“…11 show that at 4% bleed the diffuser flow separates. This was surprising as the previous work of Walker et al [21,22] and some preliminary 2D axisymmetric predictions suggested that in this configuration the flow would remain attached. Flow separation occurs in line with the OGV wake as the low-momentum wake fluid is unable to overcome the increased adverse pressure gradient.…”

“…Consequently, a hybrid bled diffuser was designed based on the work of Walker et al [21,22]. This hybrid diffuser (Fig.…”

“…9. Firstly, the bleed flow gains streamwise momentum as it accelerates An initial design of hybrid diffuser was specified from the guidelines presented by Walker et al [21,22]. The aim was to return the strutted area ratio to that of the datum clean pre-diffuser (1.67) which meant that without struts the hybrid diffuser would have a clean area ratio of approximately 1.93.…”

“…Without bleed the flow has arguably separated from the outer wall, with 2% bleed the flow is now fully attached and with 4% bleed the condition of the outer wall flow is much improved. It is likely that some further increases in bleed may give additional improvements to the outer wall flow but Walker et al [21,22] suggested that there are diminishing returns. For this investigation the amount of bleed was limited to 4% as a practical upper limit for currently proposed cooled cooling air systems.…”

Annular Diffusers with Large Downstream Blockage Effects for Gas Turbine Combustion Applications

“…Britchford et al [20] demonstrated the superiority of the Reynolds stress model over a k-" model in capturing the combined effects of strong pressure gradients and streamline curvature in an annular S-shaped duct. Similarly, Walker [21] also showed that a Reynolds stress model gave better agreement with measured data in a dump diffuser combustion system.…”