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Diffusers convert kinetic flow energy into a rise in static pressure. This pressure recovery is the primary aerodynamic design objective for exhaust gas diffusers in power-generating steam and gas turbines. The total pressure loss is an equally important diffuser design parameter. It is strongly linked to the pressure recovery and the residual kinetic energy of the diffuser outlet flow. A reduction benefits the overall thermodynamic cycle, which requires the adjacent components of a diffuser to be included in the design process. This paper focuses on the total pressure losses in the boundary layer of a highly loaded annular diffuser. Due to its large opening angle the diffuser is susceptible to flow separation under uniform inlet conditions, which is a major source for total pressure losses. However, the unsteady tip leakage vortices of the upstream rotor, which are a source of losses, stabilize the boundary layer and prevent separation. Experiments and unsteady numerical simulation conducted show that the total pressure loss reduction caused by the delayed boundary layer separation exceed the vortex-induced losses by far. This flow interaction between the rotor and diffuser consequently decreases the overall total pressure losses. The intensity of the tip leakage vortex is linked to three rotor design parameters, namely work coefficient, flow coefficient and reduced blade-passing frequency. Based on these paramaters, we propose a semi-empiric correlation to predict and evaluate the change in total pressure losses with regards to design operating conditions. INTRODUCTION Diffusers make an essential contribution to increase the efficiency of power-generating steam and gas turbines. They harvest the otherwise wasted kinetic energy of the turbine exhaust flow and convert it into pressure-volume work, i.e. an increase in static pressure. For a given static outlet pressure a diffuser enables a higher turbine pressure ratio, resulting in an increase in power output. As the heat input of the thermodynamic cycle remains constant the thermal efficiency increases as well. The primary aerodynamic design goal of an exhaust diffuser is to maximise the kinetic energy conversion of the turbine outlet flow. The effectiveness of this process can be evaluated using the dimensionless pressure recovery cp defined as cp = pout− pin ptot,in− pin . (1) Assuming an ideal flow, it can be shown that an infinite area ratio AR of outlet to inlet area is required to convert all kinetic energy of the flow into static pressure. For such an ideal diffuser, the pressure recovery coefficient is defined as cp,ideal = 1− 1 AR2 + tan2α ( rin rout )2 1 + tan2α , (2) where α = tan−1 (cθ/cm) is the swirl angle at diffuser inlet and rin and rout are the Euler radii at inlet and outlet, respectively. It represents the theoretical limit for the pressure recovery of a certain diffuser geometry for a given swirl angle. The effectiveness of the diffuser is defined as

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters