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

Abstract

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, stabilise 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 parameters, we propose a semi-empiric correlation to predict and evaluate the change in total pressure losses with regards to design operating conditions.

Total Pressure Loss Reduction in Annular Diffusers

“…The influence of the dynamic pressure on wake mixing is characterised by II,r l . Details on the definition of Λ can be found in [14]. It was demonstrated that the rectified total pressure loss coefficient correlates well with Σ for numerical and experimental data, as seen in Fig.…”

“…The ratio r l incorporates a simple Fig. 2 Absolute difference in rectified diffuser total pressure loss from reference Δ against stabilisation number Σ (adapted from [14]) estimate for the velocity deficit. The influence of the dynamic pressure on wake mixing is characterised by II,r l .…”

“…Since the experimental validation of the Σ model has already been presented in [12][13][14], the test rig shall be addressed just briefly in this paper.…”

“…Fig. 3 Computational domain (coarse mesh and multiple rotor pitches for display, flow direction from right to left) [13] Regarding the total pressure losses in the diffuser, it was demonstrated in [14] that the model is likewise able to predict the reduction in total pressure losses for a 15° diffuser, as seen in experiment and simulations. For the time being, no total pressure loss data for the 20° diffuser is available to us.…”

Estimation of Pressure Loss Derivatives for Turbine Center Frames via L1-regularized Regression Models

“…Despite their greatly simplified geometries, their correlations still provide valuable predictions and the TCFs of many modern aero engine architectures cluster in a parametric region, which was identified as optimal by their work (Göttlich 2011;Gräsel et al 2006). Notwithstanding, other less crucial geometrical parameters still influence the performance of TCFs (Gräsel et al 2006;Wallin and Eriksson 2008;Mimic et al 2018).…”