Efficient Methods for Predicting Low Pressure Steam Turbine Exhaust Hood and Diffuser Flows at Design and Off-Design Conditions

Abstract

The exhaust hood of a steam turbine is an important area of turbomachinery research as its performance strongly influences the power output of the last stage blades (LSB). This paper compares results from 3D simulations using a novel application of the nonlinear harmonic (NLH) method with more computationally demanding predictions obtained using frozen rotor techniques. Accurate simulation of exhausts is only achieved when simulations of LSB are coupled to the exhaust hood to capture the strong interaction. One such method is the NLH method. In this paper, the NLH approach is compared against the current standard for capturing the inlet circumferential asymmetry, the frozen rotor approach. The NLH method is shown to predict a similar exhaust hood static pressure recovery and flow asymmetry compared with the frozen rotor approach using less than half the memory requirement of a full annulus calculation. A second option for reducing the computational demand of the full annulus frozen rotor method is explored where a single stator passage is modeled coupled to the full annulus rotor by a mixing plane. Provided the stage is choked, this was shown to produce very similar results to the full annulus frozen rotor approach but with a computational demand similar to that of the NLH method. In terms of industrial practice, the results show that for a typical well designed exhaust hood at nominal load conditions, the pressure recovery predicted by all methods (including those which do not account for circumferential uniformities) is similar. However, this is not the case at off-design conditions where more complex interfacing methods are required to capture circumferential asymmetry.

Advances in coupled axial turbine and nonaxisymmetric exhaust volute aerodynamics for turbomachinery

“…Recently, Burton et al 61,62 have adopted a novel approach, by coupling the axial turbine and the exhaust volute by the Non-Linear Harmonic Method, a technique widely used to reduce calculation size in turbomachinery. The results showed that the nonuniform exhaust volute inlet flow can be captured using the Non-Linear Harmonic Method when modeling only a single blade passage.…”

An Alternative Numerical Model for Investigating Three- Dimensional Steam Turbine Exhaust Hood

“…The specified numerical boundary has been generated by the mesh interface treatment in the three domains. The energy transfer happening in the real turbine stage is represented here by specifying the pressure drop across the actuator disc as a function of flow velocity and those values are calculated from the turbine blade specifications reported in Burton et al (2015). The representative profile of velocity distribution was applied at the disc boundary as represented in Fig.…”