Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters1

Abstract

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses and construction costs increase drastically with diffuser length, it is strongly preferred to design shorter diffusers with steeper opening angles. However, these designs are more susceptible to boundary layer separation. In this paper, the stabilizing properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time, a correlation between the pressure recovery of the diffuser and the integral rotor parameters of the last stage, namely, the loading coefficient, flow coefficient, and reduced frequency, is established. Experimental data and scale-resolving simulations, carried out with the shear stress transport scale-adaptive simulation (SST-SAS) method, both show excellent agreement with the correlation. Blade tip vortex strength predominantly depends on the amount of work exchanged between fluid and rotor, which in turn is described by the nondimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even preventing separation locally.

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

“…We present a range of operating points differing from each other in Ψ, Φ, and f red by varying the blade count n, rotor speed N, and mass flow rate _ m. The test cases comprise numerical (NUM) and experimental (EXP) test cases, which are listed in Table 2. The test cases have been presented in Mimic et al (2018). We averaged between identical operating points of EXP that were measured multiple times.…”

“…The object is to reduce numerical dissipation in the former and to enhance stability in the latter. Further details concerning the numerical setup are explained in Mimic et al (2018).…”

“…Examples for such calculations are given by Denton (1993) as well as Rose and Harvey (1999). (Mimic et al, 2018 The correlation Figure 6 shows for the numerical simulations performed that the rectified diffuser pressure loss coefficient ϖ decreases in a linear way for increasing values of the stabilisation number Σ. The following correlation can be given from the numerical samples:…”

“…The stabilising properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics have been examined in Mimic et al (2018). A correlation between the pressure recovery of the diffuser and integral rotor parameters of the last stage has been established, based on analytical considerations, numerical simulations, and experimental data.…”

“…The number represents a measure for the prediction of the diffuser effectiveness ϵ. With the definition of the stabilisation number Σ presented in Mimic et al (2018), the correlation then gives…”