Low Pressure Turbine Research Articles

Effects of Reynolds number on the flow field in a low-pressure turbine with incoming wakes

Effects of Reynolds number on the flow field in a low-pressure turbine with incoming wakes

RANS Prediction of Losses and Transition Onset in a High-Speed Low-Pressure Turbine Cascade

Current trends in aero-engine design are oriented at designing high-lift low-pressure turbine blades to reduce engine weight and dimensions. Therefore, the validation of numerical methods able to correctly capture the boundary layer transition at cruise conditions with a steady inflow for high-speed blades is of great relevance for turbine designers. The present paper details numerical simulations of a novel open-access high-speed low-pressure turbine test case that are performed using RANS-based transition models. The test case is the SPLEEN C1 cascade, tested in transonic conditions at the von Karman Institute for Fluid Dynamics. Both physics-based and correlation-based transition models are employed to predict blade loading, boundary layer characteristics, and wake development. 2D simulations are run for a wide range of operating conditions ranging from low to high transonic Mach numbers (0.7–0.95) and from low to moderate Reynolds numbers (70,000–120,000). The γ-Re˜θt transition model shows a good performance over the whole range of simulated operating conditions, thus demonstrating a good capability in both reproducing blade loading and average losses, although the wake’s width is underestimated. This leads to an overestimation of the total pressure deficit in the center of the wake which can exceed experimental measurements by more than 50%. On the other hand, the k-ν2-ω model achieves satisfactory results at Ma6,is = 0.95, where the boundary layer state is affected by the presence of a weak shock impinging on the blade suction side which thickens the boundary layer, leading to a predicted shape factor equal to five, downstream of the shock. However, at low and moderate Mach numbers, the k-ν2-ω model predicts long or open separation bubbles contrary to the experimental findings, thus indicating insufficient turbulence production downstream of the boundary layer separation. The slow boundary layer transition in the aft region of the suction side that is exhibited by the k-ν2-ω model also affects the prediction of the outlet flow, featuring large peaks of a total pressure deficit if compared to both the experimental measurements and the γ-Re˜θt predictions. For the k-ν2-ω model, the maximum overestimation of the total pressure deficit is approximately 60%.

Large Eddy Simulation of a Low-Pressure Turbine Cascade with Turbulent End Wall Boundary Layers

We present results of implicit large eddy simulation (LES) and different Reynolds-averaged Navier–Stokes (RANS) models of the MTU 161 low pressure turbine at an exit Reynolds number of 90000\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$90\\,000$$\\end{document} and exit Mach number of 0.6. The LES results are based on a high-order discontinuous Galerkin method and the RANS is computed using a classical finite-volume approach. The paper discusses the steps taken to create realistic inflow boundary conditions in terms of end wall boundary layer thickness and freestream turbulence intensity. This is achieved by tailoring the input distribution of total pressure and temperature, Reynolds stresses and turbulence length scale to a Fourier series based synthetic turbulence generator. With this procedure, excellent agreement with the experiment can be achieved in terms of blade loading at midspan and wake total pressure losses at midspan and over the channel height. Based on the validated setup, we focus on the discussion of secondary flow structures emerging due to the interaction of the incoming boundary layer and the turbine blade and compare the LES to two commonly used RANS models. Since we are able to create consistent setups for both LES and RANS, all discrepancies can be directly attributed to physical modelling problems. We show that both a linear eddy viscosity model and a differential Reynolds stress model coupled with a state-of-the-art correlation-based transition model fail, in this case, to predict the separation induced transition process around midspan. Moreover, their prediction of secondary flow losses leaves room for improvement as shown by a detailed discussion of turbulence kinetic energy and anisotropy fields.

Influence of swirl and ingress on windage losses in a low-pressure turbine stator-well cavity

As part of the drive towards achieving net-zero flight by 2050, the development of ultra-high bypass ratio engines will place greater importance on the efficiency of the low-pressure turbine. Stator-well cavities can be a significant source of loss within this system, primarily due to the ingestion of annulus air, which can rotate at a speed less than that of the disc. This paper presents the results of a joint experimental/numerical campaign to relate windage torque to the flow physics in a scaled, engine-realistic stator-well geometry with superposed flows. The primary variables were the pressure ratio across the stator row and the swirl ratio at inlet to the cavity. This is the first paper to relate simultaneous torque and swirl measurements in a stator-well cavity. The numerical simulations have also provided insight into the fluid dynamic behaviour. A reduction in cavity windage torque with superposed flow rate was shown, consistent with the increase in swirl measured by experiments and predicted by computations. The pre-swirled superposed flows reduce the ingestion of negatively swirling fluid from the annulus, effectively increasing the swirl ratio of the flow in the cavity towards that of the rotor. This reduces windage losses; at the design condition with superposed flow, the cavity windage torque reduced by 60% of that of the reference case. Sealing effectiveness increased with superposed flow rate. This was demonstrated through gas concentration measurement/simulations, a reduced radial flow velocity into the cavity and a reduced mass flow rate across the interstage seal. The application of a turbulent transport model showed that ingestion could be explained by shear-driven diffusion. Ingestion increased as inlet swirl ratio reduced, while the pressure ratio was found to have a negligible influence on windage moment coefficient, swirl ratio and sealing effectiveness. In addition to providing fluid dynamic insight, the gas concentration results can be used for validating thermomechanical models. The results in this paper will be of practical interest to the engine designer who wishes to scale information to engine-operating conditions.

Heat Transfer and Film Cooling in an Aggressive Turbine Center Frame

Abstract This article investigates the heat transfer coefficient and the film cooling effectiveness in a turbine center frame (TCF). The TCF is a duct connecting the high-pressure turbine (HPT) to the low-pressure turbine (LPT) and is equipped with nonturning airfoils (struts). The TCF is operated in a product-representative 1.5-stage test turbine setup working under Mach number similarity. Upstream of the TCF, an unshrouded HPT is operated with four individually adjustable purge flow injections through the forward and aft cavities on the hub and tip of the rotor. The heat transfer coefficient and the purge film cooling effectiveness are measured on the hub and the nonturning struts of the aerodynamically aggressive TCF using infrared thermography and tailor-made heating foils. To further extend the film cooling investigation, the seed gas concentration technique, in conjunction with the heat-mass transfer analogy, is used as a second film cooling measurement technique. Seeding the HPT cavities with different foreign gases reveals every individual purge flow's contribution to the global film cooling effectiveness in the TCF. In addition, the seed gas technique extends the investigated area for film cooling to the optically inaccessible shroud of the TCF. The heat transfer in the TCF was found to be dominated by secondary flow features of the upstream HPT. Longitudinal streaks of alternating high and low heat transfer were found on the hub connected to the number and the position of the upstream HPT vanes. A similar pattern was found in the film cooling effectiveness, where the film cooling streaks were situated between the high heat transfer streaks. The film cooling coverage on the shroud was found to be even, symmetric, and superior to the hub cooling performance, with around 10% less usage of purge mass flow.

Fractographic Investigations of the Failure Low Pressure Turbine Blade in a Coal-Based Thermal Power Plant

Fractographic Investigations of the Failure Low Pressure Turbine Blade in a Coal-Based Thermal Power Plant

Controlling secondary flow in high-lift low-pressure turbine using boundary-layer slot suction

The design of high-lift Low-Pressure Turbines (LPTs) causes the separation of the boundary layer on the suction side of the blade and leads to a strong secondary flow. This present study aims to minimize secondary losses through endwall slot suction and incoming wakes in a front-loaded high-lift LPT cascade with Zweifel of 1.58 under low Reynolds number of 25000. Two slotted schemes for the boundary layer of the endwall were designed (Plan A and Plan B), and the effects of suction mass flow on secondary flow were studied. The underlying physics of the endwall boundary layer of the suction and secondary flow under unsteady wakes was discussed. The results show that slot suction at the endwall boundary layer can significantly suppress the secondary flow by removing low-momentum fluids. Plans A and B significantly reduced the secondary kinetic energy by 44.2% and 36.9%, respectively, compared with the baseline cascade at the suction mass flow ratios of 1%. With an increase in the mass flow ratio of suction, the secondary flow was gradually reduced in both Plans A and B. It is more beneficial to control the secondary flow to destroy the intersection of the pressure side and suction side of the horseshoe vortex before it develops into a passage vortex. Under unsteady wakes, the combined effects of incoming wakes and endwall boundary layer suction can further suppress the secondary flow at the suction mass flow ratios of 2% for Plan A, because the positive and negative vorticity inside upstream wakes accelerated the mixing of the main flow and secondary flow and thus increased the energy of secondary vortices.

Analysis and design of an inverted oscillating water column for energy storage under choked flow conditions

A novel idealised system based on an inverted, oscillating water column (iOWC) immersed in a mass of water as a potential mechanism for energy storage has been investigated. The system consists of a cylindrical tube separated by a moving disc, dividing the device into two chambers. One chamber contains air and the other water. During the discharge, water enters and contributes to the compression of the air enclosed in the lower chamber. Simultaneously, the compressed air exits the iOWC, ideally at atmospheric choked conditions. The device has been conceived for the energy storage during low-price periods, and for the recovery (e.g. through a low-pressure gas turbine) during peak-pricing hours. The performance the iOWC has been examined for two main design parameters, namely the aspect ratio Λ and of the nondimensional size of the outlet orifice δ. It has been concluded that Λ=16.5 and δ=0.0044 might be a realistic configuration for the practical implementation because (i) the gas is choked at the exit orifice, and (ii) the iOWC’s performance is stable (with relatively small oscillations). Additionally, a thermal study analysing the gas temperature has been conducted. Approximate theoretical solutions for the variables in study have been provided.

Electricity and water cogeneration using a small PWR of 75 MW(th) coupled to a DCMD desalination plant with heat recovery

The use of a small PWR of 75 MW(th) coupled to a Direct Contact Membrane Distillation (DCMD) desalination plant with heat recovery, for cogeneration of electricity and water, is investigated here. Part of the heat used for desalination is waste heat from the power plant, as the desalination plant collects warm seawater from the power plant condenser instead of taking water directly from the sea. Two distinct coupling strategies are considered. In the first one, the additional heating required for the desalination process is obtained from steam extractions, taken at the low pressure turbine, after the extracted steam has already done a substantial amount of mechanical work. The second strategy is based on splitting the steam produced in the steam generator into two parallel Rankine cycles. The condenser at one of those cycles is set to operate just below atmospheric pressure in order to provide extra heating, at a higher temperature, for the desalination plant. Both strategies analysed in this work, relying on the use of waste heat and employing a DCMD desalination plant with efficient heat recovery, are shown to be competitive with other well established thermal desalination technologies currently used in nuclear cogeneration plants, such as MED and MSF.

Numerical study of flow in low-pressure turbines with undulated suction surface

The low-pressure turbine is an important component of the aero engine, and its efficiency greatly influences on the performance of the aero-engine. In order to improve the efficiency of low-pressure turbines, flow control methods can be adopted to overcome the large adverse pressure gradient in the passage of high-load blades, suppress the flow separation at the rear of the suction surface, thereby reducing losses in low-pressure turbines. Based on the understanding of the separated flow mechanism on the suction surface, a new passive control method using undulations on the suction surface to suppress the separated boundary layer is proposed in the current study. It is found that the counter rotating vortices generated by the undulation is attributed to the suppression of the separation bubble, reductions in the loss coefficient and the deviation in the exit flow angle by 15% and 23%, respectively.

Rotating Instabilities in Shrouded Low Pressure Turbine at Design and Off-Design Conditions

Abstract The present study aims to analyze the rotating instabilities that may occur inside shroud cavities above rotors of low-pressure turbine configurations. Unsteady simulations have been carried out at design and off-design conditions on two configurations, one being a multi-stage configuration. Unsteady flow structures, uncorrelated from blade passing frequencies and depending on operating points, are identified in every rotor tip shroud cavity. Similarities regarding flow patterns and interactions with the main flow are observed: hot spots of gas having different azimuthal extend come from the tip shroud cavity and rotate at the interface with the main flow path. The influence on the main flow and the origin of these instabilities are discussed. The study at off-design operating points deepens the analysis and allows the identification of physical parameters which drive the instability.

High-temperature microstructure evolution of an advanced intermetallic nano-lamellar γ-TiAl-based alloy and associated diffusion processes

Nano-lamellar advanced γ-TiAl based alloys doped with small amounts of C and Si are being developed to improve the creep resistance in order to increase the performances of this kind of alloys applied in the low-pressure turbine of aircraft engines. In order to extend the service temperature up to 1073 K or even above, the control of the microstructure stability is key. In this work, a complete study of the microstructure evolution during high-temperature exposure up to 1153 K has been approached through different electron microscopy techniques including HRTEM and HRSTEM with microanalysis. The nucleation and growth of the ordered βo precipitates and the ζ silicides inside the α2 lamellae has been carefully characterized and new orientation relationships and the misfit between all crystalline lattices has been determined, as well as the chemical concentration of the different atomic species on each phase. The electron microscopy study shows that βo and ζ precipitates inside α2 prevents or retards the dissolution of the α2 lamellae and its final disintegration in favor of the γ lamellae. This phenomenon has been discussed in terms of the phase coherence and diffusion processes. These important results allow conclude that the coarsening of the γ lamellae is delayed because of the βo and ζ precipitation, allowing to explain the observed enhancement of the creep resistance in this γ-TiAl based alloy exhibiting a nano-lamellar microstructure.

Effects of Acetic Acid on the Initiation of Pitting Corrosion and Stress Corrosion Cracking on Low-Alloy Steel in pH Buffer Solutions

Applying organic chemicals as an oxygen scavenger or a corrosion inhibitor to the water treatment of steam-water circuits of power plants possibly results in the generation of organic acids, such as acetic acid. This necessitates assessing the effects of the generated organic acids on the corrosion of the steam-water circuits prior to applying the organic chemicals. In this research, the effects of acetic acid on pitting corrosion and stress corrosion cracking (SCC) on low-pressure turbine materials of low-alloy steel were examined by electrochemical measurements and U-bend tests. Buffer solutions of various pHs from 4.5 to 10 were used as test solutions to investigate the effects of acidification by acetic acid on corrosion. Electrochemical measurements indicated that the steel was passivated at pHs above 8 and was the most susceptible to pitting corrosion at pH 8. In the U-bend test, SCC was initiated readily at a pH of around 8. Because SCC occurred at sites of pitting corrosion, pitting corrosion was suggested to be a precursor of SCC. Acetate ions did not trigger pitting corrosion or SCC on the steel. Rather, the addition of acetate ions to a Cl−-containing solution mitigated the initiation of pitting corrosion, resulting in the inhibition of SCC initiation.

Numerical Investigation of Friction Induced Interaction of Flutter Modes in a Realistic Low Pressure Turbine Rotor

Abstract Flutter response of bladed disks is a quite an important problem in modern designs of low pressure turbines, but, even for tuned configurations, it is not fully understood yet. Some research work suggests that, in a tuned rotor, the flutter induced vibration state that sets in consists of just a single traveling wave mode, while others show flutter states composed of several traveling waves active at the same time. Many of these studies were performed using conceptual models, such as mass-spring systems or asymptotically reduced models. In this work, a high fidelity bladed disk model is integrated into the time domain for the case of a realistic aerodynamically unstable low pressure turbine. In this configuration, the only nonlinear effect is the friction present at the blade disk attachment, which is responsible for the saturation of the flutter growth, and it is also the only mechanism that allows the interaction among the different unstable traveling wave modes. A case with multiple unstable modes of the same modal family is studied first. After that, the study is extended to a case where modes of different modal families are unstable and have similar aerodynamic instability levels. The interaction of unstable modes from different modal families is studied, we believe, for the first time, as previous flutter analyses were restricted to the interaction of unstable modes from the same family.

Compressibility effects on the flow past a T106A low-pressure turbine cascade

The present numerical investigation delves into the intricate interplay between Mach number (Ms), flow characteristics, and vorticity dynamics within a T106A low-pressure turbine (LPT) blade passage. The two-dimensional (2D) compressible Navier–Stokes equations are solved using a high-accuracy, dispersion relation preserving methodology, which is validated against benchmark direct numerical simulations. Four Ms ranging from 0.15 to 0.30 are computed in order to display the intricate response of compressibility on the separation-induced transition process. The emergence and evolution of unsteady separation bubbles along the suction surface of the T106A blade are explored, revealing a growing trend with Ms. The time-averaged boundary layer parameters evaluated along the suction surface display a delayed separation with a smaller streamwise extent with increasing Ms. However, an overall increase in the blade profile loss and a decrease in turbulent mixing are observed with increasing Ms, suggesting a detrimental effect on LPT performance. Applying the compressible enstrophy transport equation (CETE) to the flow in a T106A blade passage reveals that while a linear relationship exists between Ms and certain CETE budget terms, other terms have a nuanced dependency, which paves the way for future investigations into the role of compressibility on enstrophy dynamics.

Matching Characteristics of CTHP Circulation Systems

Cogeneration can not only reduce the use of primary energy, but also reduce the pollution to the environment; the article selects a 1200kW low-pressure stage turbine as an example to give an analysis, calculates the variation of parameters such as heat production, heat production coefficient, flow rate of cold water and hot water of this turbine, and proposes and analyzes the circulation characteristics of the CTHP system, determines the matching relationship between the steam engine driven heat pump that the system needs to meet, and fits the relationship between the steam engine efficiency and power of the heat pump. The correlation equation of steam engine efficiency and power is regressed, and the relationship of circulating cooling water flow rate with turbine discharge temperature is fitted. It is found that the heat production performance coefficients of the high and low pressure stages of the heat pump increase by 27.6% and 29.3% respectively when the discharge temperature increases from 160°C to 220°C, and the system can realize better economic and social benefits compared with the conventional CHP system.

One dimensional approach to predict the performance of high pressure multi-stage axial turbine considering air cooling and diffuser performance

This paper provides an improved 1D mid-span program to quickly grasp the off-design characteristic of modern axial turbines. The improvements are mainly focused on cooling, diffuser and the calculation of choke condition. Disk cooling and blade cooling air mixing are considered in detail. A 1D diffuser model that considers the effects of area change, heat transfer, and friction is added so that a realistic boundary condition at the turbine outlet can be obtained. A choke routine that enables the assessment of choked flow up to limit load conditions is introduced to avoid the shortcoming that the calculation method fails to converge near or at the initial choking mass flow rate. Flexible modules for losses, deviation, and mixing models are built-in to increase the accuracy and versatility of the program. This work is of great value because few programs take cooling and diffuser into account in such detail when calculating turbine performance in the form of maps. The salient issues presented here deal first with the construction of the turbine performance prediction program. The validation of the code is first performed on public data for a five-stage low pressure turbine (LPT). The results show that the efficiency relative errors with the experimental value is in the range of −(0.11%−1.64%), which is within acceptable limits. Then, the performance of GE PG9351FA transonic axial turbine is estimated using this program. This industrial axial turbine is predicted for the first time in a form of turbine performance maps in open literature and the obtained performance map is very useful for the simulation of gas turbine.

LES MODELING OF HIGH-LIFT HIGH-WORK LPT BLADES: PART II—VALIDATION AND APPLICATION

Abstract Over the years, computational fluids dynamics (CFD) has matured to such a state so as to be indispensable in turbine design. In the past two decades, significant advances in turbine design have been made through the use of CFD—in particular, through the use of Reynolds-Averaged Navier-Stokes (RANS) simulations. Currently, turbine design is RANS-driven; however, significant gains in performance and efficiency are becoming more difficult to achieve using RANS. For this reason, the turbomachinery CFD community is moving toward Large-Eddy Simulations (LES). In the design of low-pressure turbine (LPT) blades, LES is particularly beneficial owing to its ability to capture accurately both transition and separation. In this paper, LES is used to characterize a new family of high-lift high-work LPT blades—designated the LXFHW-LS family—designed at the U.S. Air Force Research Laboratory (AFRL). LES simulations are conducted in accordance with the methodology outlined in Part I of this paper. The purpose of this paper is twofold: 1) to use LES to predict the performance of the LXFHW-LS family and compare to measurements in a low-speed linear cascade and, in doing so, 2) to illustrate how LES may be used in LPT design as it evolves from RANS-driven to LES-driven. For each blade in the family, the loading distribution and loss coefficient are computed for sixteen separate Reynolds numbers. Computational results are validated using detailed experimental measurements from a low-speed linear cascade wind tunnel.

Impact of Unsteady Wakes on the Secondary Flows of a High-Speed Low-Pressure Turbine Cascade

The aerodynamics of a high-speed low-pressure turbine (LPT) cascade were investigated under steady and unsteady inlet flows. The tests were performed at outlet Mach (M) and Reynolds numbers (Re) of 0.90 and 70k, respectively. Unsteady wakes were simulated by means of a wake generator equipped with bars. A bar reduced frequency (f+) of ∼0.95 was used for the unsteady case. The inlet flow field was characterized in terms of the total pressure profile and incidence. The blade aerodynamics at midspan and the secondary flow region were investigated by means of pneumatic taps and hot-film sensors. The latter provided a novel view into the impact of the secondary flows on the heat transfer topology on the blade suction side (SS). The cascade performance was quantified in terms of the outlet flow angle and losses by means of a directional multi-hole probe. The results report the phase-averaged impact of unsteady wakes on the secondary flow structures in an open test case high-speed LPT geometry.

Implementation of hot steam injection in steam turbine design: A novel mean-line method coupled with multi-objective optimization and neural network

Low-pressure turbines usually work in wet conditions which causes both lifetime and efficiency reduction. Hot steam injection (HSI) which has received great interest recently, is a suggested solution to reduce wetness. There is a research gap in implementing HSI in the mean-line procedure, which is a conventional method of designing and analyzing turbines. In this paper for the first time, an object-oriented in-house mean-line code is developed with the ability of HSI calculation for steam turbines. The validation of code is performed with the 3D simulation of a 2-stage axial steam turbine. In addition, considering that HSI may reduce efficiency of the turbine due to mixing entropy generation, a multi-objective genetic algorithm optimization and a neural network is used to redesign the steam turbine. Mass fraction and total temperature of injected flow from the trailing edge of blade rows are the decision variables and, liquid mass fraction and efficiency of turbine are the objective functions. The comparison of Pareto front points reveals that the maximum possible improvement of quality and power relative to baseline is 5% and 10%, respectively. Furthermore, if efficiency is the desired objective function, by enhancing 1% of steam quality, it can be increased by 0.1%.