Sensitivity analysis of low-pressure gland seal deformation behavior during vacuuming for a 1000 MW nuclear steam turbine based on thermo-mechanical coupling method

The performance of the axial-radial diffuser downstream of the last low-pressure steam turbine stages and the losses occurring subsequently within the exhaust hood directly influences the overall efficiency of a steam power plant. It is estimated that an improvement of the pressure recovery in the diffuser and exhaust hood by 10% translates into 1% of last stage efficiency [11]. While the design of axial-radial diffusers has been the object of quite many studies, the flow phenomena occurring within the exhaust hood have not received much attention in recent years. However, major losses occur due to dissipation within vortices and inability of the hood to properly diffuse the flow. Flow turning from radial to downward flow towards the condenser, especially at the upper part of the hood is essentially the main cause for this. This paper presents a detailed analysis of the losses within the exhaust hood flow for two operating conditions based on numerical results. In order to identify the underlying mechanisms and the locations where dissipation mainly occurs, an approach was followed, whereby the diffuser inflow is divided into different sectors and pressure recovery, dissipation and finally residual kinetic energy of the flow originating from these sectors is calculated at different locations within the hood. Based on this method, the flow from the topmost sectors at the diffuser inlet is found to cause the highest dissipation for both investigated cases. Upon hitting the exhaust hood walls, the flow on the upper part of the diffuser is deflected, forming complex vortices which are stretching into the condenser and interacting with flow originating from other sectors, thereby causing further swirling and generating additional losses. The detailed study of the flow behavior in the exhaust hood and the associated dissipation presents an opportunity for future investigations of efficient geometrical features to be introduced within the hood to improve the flow and hence the overall pressure recovery coefficient.

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Performance Prediction and Optimization of Low Pressure Steam Turbine Radial Diffuser at Design and Off-Design Conditions Using Streamline Curvature Method

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4035527 ◽

2017 ◽

Vol 139 (7) ◽

Author(s):

Luying Zhang ◽

Francesco Congiu ◽

Xiaopeng Gan ◽

David Karunakara

Keyword(s):

Steam Turbine ◽

Flow Separation ◽

Large Scale ◽

Current Method ◽

Low Pressure ◽

Numerical Approach ◽

Optimization Process ◽

Exhaust Hood ◽

Streamline Curvature ◽

Wide Range

The performance of the radial diffuser of a low pressure (LP) steam turbine is important to the power output of the turbine. A reliable and robust prediction and optimization tool is desirable in industry for preliminary design and performance evaluation. This is particularly critical during the tendering phase of retrofit projects, which typically cover a wide range of original equipment manufacturer and other original equipment manufacturers designs. This work describes a fast and reliable numerical approach for the simulation of flow in the last stage and radial diffuser coupled with the exhaust hood. The numerical solver is based on a streamline curvature throughflow method and a geometry-modification treatment has been developed for off-design conditions, at which large-scale flow separation may occur in the diffuser domain causing convergence difficulty. To take into account the effect of tip leakage jet flow, a boundary layer solver is coupled with the throughflow calculation to predict flow separation on the diffuser lip. The performance of the downstream exhaust hood is modeled by a hood loss model (HLM) that accounts for various loss generations along the flow paths. Furthermore, the solver is implemented in an optimization process. Both the diffuser lip and hub profiles can be quickly optimized, together or separately, to improve the design in the early tender phase. 3D computational fluid dynamics (CFD) simulations are used to validate the solver and the optimization process. The results show that the current method predicts the diffuser/exhaust hood performance within good agreement with the CFD calculation and the optimized diffuser profile improves the diffuser recovery over the datum design. The tool provides General Electric the capability to rapidly optimize and customize retrofit diffusers for each customer considering different constraints.

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Design Optimization of a Low Pressure Steam Turbine Radial Diffuser Using an Evolutionary Algorithm and 3D CFD

Volume 6: Oil and Gas Applications; Concentrating Solar Power Plants; Steam Turbines; Wind Energy ◽

10.1115/gt2012-69515 ◽

2012 ◽

Cited By ~ 6

Author(s):

Tom Verstraete ◽

Johan Prinsier ◽

Alberto Di Sante ◽

Stefania Della Gatta ◽

Lorenzo Cosi

Keyword(s):

Design Optimization ◽

Steam Turbine ◽

Differential Evolution Algorithm ◽

Low Pressure ◽

Pressure Recovery ◽

Strong Impact ◽

Optimized Design ◽

Steam Turbines ◽

Recovery Coefficient ◽

Lp Turbine

The design of the radial exhaust hood of a low pressure (LP) steam turbine has a strong impact on the overall performance of the LP turbine. A higher pressure recovery of the diffuser will lead to a substantial higher power output of the turbine. One of the most critical aspects in the diffuser design is the steam guide, which guides the flow near the shroud from axial to radial direction and has a high impact on the pressure recovery. This paper presents a method for the design optimization of the steam guide of a steam turbine for industrial power generation and mechanical drive of centrifugal compressors. This development is in the frame of a continuous effort in GE Oil and Gas to develop more efficient steam turbines. An existing baseline exhaust and steam guide design is first analyzed together with the last LP turbine stage with a frozen rotor full 3D Computational Fluid Dynamics (CFD) calculation. The numerical prediction is compared to available steam test turbine data. The new exhaust box and a first attempt new steam guide design are then first analyzed by a CFD computation. The diffuser inlet boundary conditions are extracted from this simulation and used for improving the design of the steam guide. The maximization of the pressure recovery is achieved by means of a numerical optimization method that uses a metamodel assisted differential evolution algorithm in combination with a 3D CFD solver. The profile of the steam guide is parameterized by a Bezier curve. This allows for a wide variety of shapes, respecting the manufacturability constraints of the design. In the design phase it is mandatory to achieve accurate results in terms of performance differences in a reasonable time. The pressure recovery coefficient is therefore computed through the 3D CFD solver excluding the last stage, to reduce the computational burden. Steam tables are used for the accurate prediction of the steam properties. Finally, the optimized design is analyzed by a frozen rotor computation to validate the approach. Also off-design characteristics of the optimized diffuser are shown.

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