Introduction

As an illustrative problem in this area, Figure 13.1 shows the exhaust diffuser, designed by the author, for an existing commercial turboprop engine. Such a diffuser is a critical component of any gas turbine engine in both propulsion and power-system applications. Exhaust diffusers, in general, are of primary impact on the total-to-static efficiency of the turbine/exhaust-diffuser system assembly because they raise the static pressure up to the ambient magnitude, making use of existing kinetic energy in the flow stream. Note that the exit pressure value of the last turbine stage is normally less than the ambient pressure. Exit diffusing passages are also common in such power systems as auxiliary power units, which are frequently used in aircraft and various ground applications. In all cases, the demands for maximum static pressure recovery and minimum total pressure loss across the diffuser are often difficult design tasks. This is particularly true for the engine off-design operation due to the excessive amount of turbine exit swirl angle, which may lead to flow separation, normally over the diffuser inner wall.

A literature survey reveals that a considerable amount of experimental research has been focused on annular-diffuser flows [12, 15–17]. Although several crucial design aspects have, in these and other publications, been addressed, most of this experimental work has been centered predominantly around straight-wall diffusers with modest area ratios. Nevertheless, the empirical relationships formulated in these studies and the charts produced in the process have long been appreciated and used extensively in the gas turbine industry.