The Influence of Inlet Asymmetry on Steam Turbine Exhaust Hood Flows

Abstract

It has been widely recognized for some decades that it is essential to accurately represent the strong coupling between the last stage blades (LSB) and the diffuser inlet, in order to correctly capture the flow through the exhaust hoods of steam turbine low pressure cylinders. This applies to any form of simulation of the flow, i.e., numerical or experimental. The exhaust hood flow structure is highly three-dimensional and appropriate coupling will enable the important influence of this asymmetry to be transferred to the rotor. This, however, presents challenges as the calculation size grows rapidly when the full annulus is calculated. The size of the simulation means researchers are constantly searching for methods to reduce the computational effort without compromising solution accuracy. However, this can result in excessive computational demands in numerical simulations. Unsteady full-annulus CFD calculation will remain infeasible for routine design calculations for the foreseeable future. More computationally efficient methods for coupling the unsteady rotor flow to the hood flow are required that bring computational expense within realizable limits while still maintaining sufficient accuracy for meaningful design calculations. Research activity in this area is focused on developing new methods and techniques to improve accuracy and reduce computational expense. A novel approach for coupling the turbine last stage to the exhaust hood employing the nonlinear harmonic (NLH) method is presented in this paper. The generic, IP free, exhaust hood and last stage blade geometries from Burton et al. (2012. "A Generic Low Pressure Exhaust Diffuser for Steam Turbine Research,"Proceedings of the ASME Turbo Expo, Copenhagen, Denmark, Paper No. GT2012-68485) that are representative of modern designs, are used to demonstrate the effectiveness of the method. This is achieved by comparing results obtained with the NLH to those obtained with a more conventional mixing-plane approach. The results show that the circumferential asymmetry can be successfully transferred in both directions between the exhaust hood flow and that through the LSB, by using the NLH. This paper also suggests that for exhaust hoods of generous axial length, little change in C is observed when the circumferential asymmetry is captured. However, the predicted flow structure is significantly different, which will influence the design and placement of the exhaust hood internal "furniture."

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

“…All have shown that the unsteady flow field can be reproduced with good accuracy with the NLH approach provided sufficient number of harmonics are modeled. The harmonic study carried out in Burton et al [9] has shown the number of harmonics used in the calculations presented in this paper is sufficient.…”

“…A balance must therefore be found between accuracy and computation time. Results from Burton et al [9] showed that three harmonics are sufficient to accurately model the LP exhaust hood interfaces.…”

“…The computation was rerun at nominal load (a mass flow rate of 88.6 kg/s), and then repeated with each coupling method (mixing plane, NLH and frozen rotor (full annulus)) at 70 kg/s, 65 kg/s, and finally at 50 kg/s. Further details of the use of the mixing plane method in exhaust hood calculations can be found in the work of Burton et al [9]. Figure 11 shows the pressure recovery for the full exhaust hood system, including both the exhaust diffuser and hood.…”

“…The stator and rotor meshes were the same as those used previously in the work of Burton et al [9]. The stator mesh consisted of 0.48 Â 10 6 cells and the rotor of 0.86 Â 10 6 with a y þ maintained below 10 in all regions of the flow.…”

“…Recent work by Burton et al [9] has shown that the circumferential asymmetry, which forms due to the rotation of the turbine blades and the nonaxisymmetric hood geometry, can be captured when modeling only a single blade passage by using the NLH approach. However, at present this has only been compared to mixing plane calculations by the authors and not to other methods which also capture the circumferential variations at the interface plane.…”