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

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Zoe Burton ◽  
Grant Ingram ◽  
Simon Hogg
Keyword(s):  
Steam Turbine ◽  
Static Pressure ◽  
Pressure Recovery ◽  
Exhaust Hood ◽  
Current Standard ◽  
Industrial Practice ◽  
Flow Asymmetry ◽  
Pressure Steam ◽  
Last Stage ◽  
Turbine Exhaust

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.

Numerical Investigation on the Aerodynamic Performance of a Low-Pressure Steam Turbine Exhaust Hood Using Design of Experiment Analysis

2020 ◽  
Vol 142 (11) ◽  
Author(s):  
Tommaso Diurno ◽  
Tommaso Fondelli ◽  
Leonardo Nettis ◽  
Nicola Maceli ◽  
Lorenzo Arcangeli ◽  
...  
Keyword(s):  
Flow Field ◽  
Steam Turbine ◽  
Design Of Experiment ◽  
Low Pressure ◽  
Pressure Recovery ◽  
Exhaust Hood ◽  
Recovery Coefficient ◽  
Pressure Steam ◽  
Pressure Recovery Coefficient ◽  
Turbine Exhaust

Abstract Nowadays, the rising interest in using renewable energy for thermal power generation has led to radical changes in steam turbine design practice and operability. Modern steam turbines are required to operate with greater flexibility due to rapid load changes, fast start-up, and frequent shutdowns. This has given rise to great challenges to the exhaust hood system design, which has a great influence on the overall turbine performance converting the kinetic energy leaving the last stage of low-pressure turbine into static pressure. The radial hoods are characterized by a complex aerodynamic behavior since the flow turns by 90 deg in a very short distance and this generates a highly rotational flow structure within the diffuser and exhaust hood outer casing, moreover, the adverse pressure gradient can promote the flow separation drastically reducing the hood recovery performance. For these reasons, it is fundamental to design the exhaust system in order to ensure a good pressure recovery under all the machine operating conditions. This paper presents a design of experiment (DOE) analysis on a low-pressure steam turbine exhaust hood through computational fluid dynamics (CFD) simulations. A parametric model of an axial-radial exhaust hood was developed, and a sensitivity of exhaust hood performance as a function of key geometrical parameters was carried out, with the aim of optimizing the pressure recovery coefficient and minimizing the overall dimensions of the exhaust casing. Since hood performance strongly depends on a proper coupling with the turbine rear stage, such a stage was modeled using the so-called mixing-plane approach to couple both stator–rotor and rotor-diffuser interfaces. A detailed analysis of the flow field in the exhaust hood in the different configurations was performed, detecting the swirling structures responsible for the energy dissipation in each simulation, as well as correlating the flow field with the pressure recovery coefficient.

Numerical Investigation of the Influence of Hood Height Variation on Performance of Low Pressure Steam Turbine Exhaust Hoods

2018 ◽  
Author(s):  
Dickson Munyoki ◽  
Markus Schatz ◽  
Damian M. Vogt
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Pressure Recovery ◽  
Reference Configuration ◽  
Maximum Efficiency ◽  
Parameter Study ◽  
Exhaust Hood ◽  
Pressure Steam ◽  
The Impact ◽  
Turbine Exhaust

Performance optimization of low pressure steam turbine exhaust hood has been a subject of a number of both numerical and experimental studies. This is driven by the understanding that improving the diffuser and exhaust hood outer casing performance results in a lower turbine back pressure and hence an increased plant overall output. The performance of the exhaust hood is greatly influenced by many structural factors such as the size of its outer casing, design of the diffuser parts and the arrangement of the internal supports. A number of studies have shown that a decrease of the hood height is detrimental to the exhaust hood performance [1, 2], however, up to now the impact of increased hood height has not been researched. In the present study, a scaled axial-radial diffuser test rig operated by ITSM is used as reference configuration for a parameter study. A total of fourteen different configurations with both increased and reduced hood height are investigated numerically. Design load at three different tip jet Mach numbers (no tip jet, tip jet Mach number of 0.4 and 1.2) is chosen as operating condition. Numerical and experimental data is available for the reference configuration and the numerical results have already been validated in a previous paper by the authors [3]. While a decrease of hood height shows the expected deterioration of efficiency, an increase of the hood height only initially results in an improved performance. After reaching a maximum efficiency, which is dependent on the tip leakage, the exhaust hood performance decreases noticeably again. Apart from the variation of pressure recovery, the results allow a better understanding of the loss mechanisms and flow phenomena in exhaust hoods, showing that the deflection of the flow coming out of the diffuser in the top part of the hood has a major impact on exhaust hood pressure recovery.

Numerical Investigation on the Aerodynamic Performance of a Low-Pressure Steam Turbine Exhaust Hood Using DOE Analysis

2020 ◽  
Author(s):  
Tommaso Diurno ◽  
Tommaso Fondelli ◽  
Leonardo Nettis ◽  
Nicola Maceli ◽  
Lorenzo Arcangeli ◽  
...  
Keyword(s):  
Flow Field ◽  
Steam Turbine ◽  
Low Pressure ◽  
Operating Conditions ◽  
Pressure Recovery ◽  
Exhaust Hood ◽  
Recovery Coefficient ◽  
Pressure Steam ◽  
Pressure Recovery Coefficient ◽  
Turbine Exhaust

Abstract Nowadays, the rising interest in using renewable energy for thermal power generation has led to radical changes in steam turbine design practice and operability. Modern steam turbines are required to operate with greater flexibility due to rapid load changes, fast start-up, and frequent shutdowns. This has given rise to great challenges to the exhaust hood system design, which has a great influence on the overall turbine performance converting the kinetic energy leaving the last stage of LP turbine into static pressure. The radial hoods are characterized by a complex aerodynamic behavior since the flow turns by 90° in a very short distance and this generates a highly rotational flow structure within the diffuser and exhaust hood outer casing, moreover, the adverse pressure gradient can promote the flow separation drastically reducing the hood recovery performance. For these reasons it is fundamental to design the exhaust system in order to ensure a good pressure recovery under all the machine operating conditions. This paper presents a Design of Experiment analysis on a low-pressure steam turbine exhaust hood through CFD simulations. A parametric model of an axial-radial exhaust hood was developed and a sensitivity of exhaust hood performance as a function of key geometrical parameters was carried out, with the aim of optimizing the pressure recovery coefficient and minimizing the overall dimensions of the exhaust casing. Since hood performance strongly depends on a proper coupling with the turbine rear stage, such a stage was modeled using the so-called mixing-plane approach to couple both stator-rotor and rotor-diffuser interfaces. A detailed analysis of the flow field in the exhaust hood in the different configurations was performed, detecting the swirling structures responsible for the energy dissipation in each simulation, as well as correlating the flow field with the pressure recovery coefficient.

Detailed Numerical Study of the Main Sources of Loss and Flow Behavior in Low Pressure Steam Turbine Exhaust Hoods

2017 ◽  
Author(s):  
Dickson Munyoki ◽  
Markus Schatz ◽  
Damian M. Vogt
Keyword(s):  
Steam Turbine ◽  
Numerical Study ◽  
Flow Behavior ◽  
Low Pressure ◽  
Operating Conditions ◽  
Pressure Recovery ◽  
Exhaust Hood ◽  
Recovery Coefficient ◽  
Pressure Steam ◽  
Underlying Mechanisms

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.

The Pressure Field at the Output From a Low Pressure Exhaust Hood and Condenser Neck of the 1090 MW Steam Turbine: Experimental and Numerical Research

2018 ◽  
Author(s):  
Michal Hoznedl ◽  
Antonín Živný ◽  
Aleš Macálka ◽  
Robert Kalista ◽  
Kamil Sedlák ◽  
...  
Keyword(s):  
Boundary Condition ◽  
Steam Turbine ◽  
Cross Section ◽  
Static Pressure ◽  
Cooling Water ◽  
Low Pressure ◽  
Maximum Temperature ◽  
Angle Distribution ◽  
Exhaust Hood ◽  
Last Stage

The paper presents the results of measurements of flow parameters behind the last stage of a 1090 MW nominal power steam turbine in a nuclear power plant. The results were obtained by traversing a pneumatic probe at a distance of about 100 mm from the trailing edges of the LSB (Last Stage Blade). Furthermore, both side walls as well as the front wall of one flow of the LP (Low Pressure) exhaust hood were fitted with a dense net of static pressure taps at the level of the flange of the turbine. A total of 26 static pressures were measured on the wall at the output from the LP exhaust hood. Another 14 pressures were measured at the output from the condenser neck. The distribution of static pressures in both cross sections for full power and 600 and 800 MW power is shown. Another experiment was measured pressure and angle distribution using a ball pneumatic probe in the condenser neck area in a total of four holes at a distance up to 5 metres from the neck wall. The turbine condenser is two-flow design. In one direction perpendicular to the axis of the turbine cold cooling water comes, it heats partially. It then reverses and it heats to the maximum temperature again. The different temperature of cooling water in the different parts of the output cross section should influence the distribution of the output static pressure. Differences in pressures may cause problems with uneven load of the tube bundles of the condenser as well as problems with defining the influential edge output condition in CFD simulations of the flow of the cold end of the steam turbine Due to these reasons an extensive 3D CFD computation, which includes one stator blade as well as all moving blades of the last stage, a complete diffuser, the exhaust hood and the condenser neck, has been carried out. Geometry includes all reinforcing elements, pipes and heaters which could influence the flow behaviour in the exhaust hood and its pressure loss. Inlet boundary conditions were assumed for the case of both computations from the measurement of the flow field behind the penultimate stage. The outlet boundary condition was defined in the first case by an uneven value of the static pressure determined by the change of the temperature of cooling water. In the second case the boundary condition in accordance with the measurement was defined by a constant value of the static pressure along all the cross section of the output from the condenser neck. Results of both CFD computations are compared with experimental measurement by the distribution of pressures and other parameters behind the last stage.

Numerical Investigation of the Influence of Flow Deflection at the Upper Hood on Performance of Low Pressure Steam Turbine Exhaust Hoods

2019 ◽  
Author(s):  
Dickson Munyoki ◽  
Markus Schatz ◽  
Damian M. Vogt
Keyword(s):  
Mach Number ◽  
Steam Turbine ◽  
Swirling Flows ◽  
Steam Turbines ◽  
Exhaust Hood ◽  
Design Load ◽  
Last Stage ◽  
Flow Deflection ◽  
Turbine Exhaust ◽  
Loss Mechanisms

Abstract Most of the world’s power is produced by large steam turbines using fossil fuel, nuclear and geothermal energy. The LP exhaust hoods of these turbines are known to contribute significantly to the losses within the turbine, hence a minor improvement in their performance, which results in a lower backpressure and thus higher enthalpy drop for the steam turbine, will give a considerable benefit in terms of fuel efficiency. Understanding the flow field and the loss mechanisms within the exhaust hood of LP steam turbines is key to developing better optimized exhaust hood systems. A detailed analysis of loss generation within the exhaust hood was done by the authors [1]. It was found that most losses occur at the upper hood and are caused by the swirling flows, which mostly start at the diffuser outlet, especially for the top diffuser inlet sector flows that have a complex path to the condenser. The authors further numerically investigated the influence of hood height variation on performance of an LP turbine exhaust hood [2], which further contributed to the knowledge of the loss mechanisms. With the loss mechanisms in exhaust hoods reasonably well understood, flow deflection at the upper hood is investigated in the current paper. The deflection is aimed at minimizing the intensity of the vortices formed thus reducing the exhaust losses. The deflector configurations analyzed are modifications of the walls of the reference configuration’s outer casing. The numerical models of the reference configuration which are based on a scaled axial-radial diffuser test rig operated by ITSM have already been validated by the authors at design and overload operating conditions and three tip jet Mach numbers (0, 0.4 and 1.2)[1]. Deflector configurations investigated are found to re-direct the flow at the upper hood and minimize the intensity of the swirling flows hence leading to improvement in performance of LP steam turbine exhaust hoods. The best performing deflector configuration is found to give a considerable improvement in performance of 20% at design load and 40% at overload both at tip jet Mach number of 0.4 (corresponding to shrouded last stage blades). At design load and tip jet Mach number of 1.2 (corresponding to unshrouded last stage blades), the improvement is found to be moderate. About 7% performance increase is observed.

Numerical Investigations on the Aerodynamic Performance of Last Stage – Exhaust Hood for Steam Turbines

2011 ◽  
Author(s):  
Rui Yang ◽  
Jiandao Yang ◽  
Zeying Peng ◽  
Liqun Shi ◽  
Aping He ◽  
...  
Keyword(s):  
Working Conditions ◽  
Static Pressure ◽  
Aerodynamic Performance ◽  
Pressure Recovery ◽  
Computational Domain ◽  
Steam Turbines ◽  
Exhaust Hood ◽  
Recovery Coefficient ◽  
Last Stage ◽  
Pressure Recovery Coefficient

The aerodynamic performance and internal flow characteristics of the last stage and exhaust hood for steam turbines is numerically investigated using the Reynolds-Averaged Navier-Stokes (RANS) solutions based on the commercial CFD software ANSYS CFX. The full last stage including 66 stator blades and 64 rotor blades coupling with the exhaust hood is selected as the computational domain. The aerodynamic performance of last stage and static pressure recovery coefficient of exhaust hood at five different working conditions is conducted. The interaction between the last stage and exhaust hood is considered in this work. The effects of the non-uniform aerodynamic parameters along the rotor blade span on the static pressure recovery coefficient of the non-symmetric geometry of the exhaust hood are studied. The numerical results show that the efficiency of the last stage has the similar values ranges from 89.8% to 92.6% at different working conditions. In addition, the similar static pressure recovery coefficient of the exhaust hood was observed at five working conditions. The excellent aerodynamic performance of the exhaust hood was illustrated in this work.

scholarly journals Investigation on low-pressure steam turbine exhaust hood modelling through computational fluid dynamic simulations

2019 ◽  
Author(s):  
Tommaso Fondelli ◽  
Tommaso Diurno ◽  
Lorenzo Palanti ◽  
Antonio Andreini ◽  
Bruno Facchini ◽  
...  
Keyword(s):  
Steam Turbine ◽  
Computational Fluid Dynamic ◽  
Fluid Dynamic ◽  
Low Pressure ◽  
Exhaust Hood ◽  
Dynamic Simulations ◽  
Pressure Steam ◽  
Turbine Exhaust

Effects of the Last Stage Rotor Tip Leakage Flow on the Aerodynamic Performance of the Exhaust Hood for Steam Turbines

2013 ◽  
Author(s):  
Jun Li ◽  
Zhigang Li ◽  
Zhenping Feng
Keyword(s):  
Static Pressure ◽  
Aerodynamic Performance ◽  
Pressure Recovery ◽  
Leakage Flow ◽  
Steam Turbines ◽  
Exhaust Hood ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Last Stage ◽  
Recovery Performance

The static pressure recovery coefficient of the exhaust hood has significant impact on the aerodynamic performance of the low pressure cylinder for steam turbines. Numerical investigations on the aerodynamic performance of the exhaust hood and full last stage with consideration of the rotor tip leakage were presented in this paper. Three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions and k–ε turbulent model were utilized to analyze the static pressure recovery performance of the exhaust hood using the commercial CFD software ANSYS-CFX. Effect of the last stage rotor tip leakage flow on the aerodynamic performance of the downstream exhaust hood was conducted by comparison of the computational domains for the exhaust hood and full last stage with and without tip clearance. The numerical results show that the last stage rotor tip leakage jet can suppress the flow separation near the diffuser wall of the exhaust hood and improve its static pressure recovery performance. The detailed flow fields of the exhaust hood with and without consideration of the rotor tip leakage flow were also illustrated and corresponding flow mechanism was discussed.

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