Total Pressure Loss Coefficient Research Articles

Hybrid RANS/LES study of complex turbulence characteristics and flow mechanisms on the highly-loaded turbine endwall

Abstract The strong secondary flow and the interaction between multiple vortices result in substantial aerodynamic losses in the endwall region of the highly-loaded turbine. Accurate predictions of complex turbulence characteristics propose severe challenges to the widely used Reynolds-Averaged Navier-Stokes (RANS) approach. This work adopts a high accuracy hybrid RANS/Large Eddy Simulation (LES) method on a highly-loaded turbine blade for which the Reynolds number is 611,000 and the exit Mach number is 0.8. Results verify that, in contrast with the RANS method, the hybrid method can provide more accurate predictions and detailed small-scale flow structures than the RANS simulation. Analysis of the Reynolds stresses predicted by the hybrid method reveals the complex turbulence characteristics in the endwall region and highlights shortcomings of Reynolds stresses predictions existing in the RANS method in the highly-loaded turbine blade environment. To analyze turbulence characteristics near the blade endwall, the Lumley triangle method is applied to quantify the turbulence anisotropy degree. Based on fine flow field structures, the proper orthogonal decomposition (POD) method is used to extract the dominant flow structures from the unsteady hybrid method results. The turbulence kinetic energy budget is employed to study dominant terms in turbulent flow. The total pressure loss coefficient based on the first law of thermodynamics and the entropy generation rate (EGR) based on the second law of thermodynamics are both used to study the loss generation mechanism in the endwall region. Predicted turbulence characteristics and flow mechanism can provide guide towards low loss design and flow control of the turbine blade.

Influence of aerodynamic characteristics optimization of exhaust passage on heat transfer of condenser in steam turbine

In the previous research, static pressure recovery coefficient (SPRC) and total pressure loss coefficient (TPLC) were used to evaluate the aerodynamic characteristics of exhaust passage of steam turbine. In this paper, the heat transfer performances of condenser are used to evaluate the aerodynamic characteristics of exhaust passage. The aerodynamic characteristics of exhaust passage are optimized through installing guide devices considering the influence of the whole last stage. The computational fluid dynamics software is applied to numerically solve N–S equation and k-ε equation in three dimensions. Then the flow field and the heat transfer performances of condenser were simulated, which using porous medium model and UDF condensation program. The results show that the heat transfer performances of condenser are improved obviously after the flow field of exhaust passage is optimized. The heat transfer coefficient of condenser increases by about 37.5% and the vacuum (VAC) increases by about 0.9 kPa.

Study on the behavior of streamwise vortices formed between leading edge tubercles in a compressor cascade

A study has been carried out to investigate the formation mechanism and development of streamwise vortices induced by leading edge tubercles in a high speed compressor cascade. The preliminary assessment of the cascade performance in terms of the total pressure loss coefficient shows that the loss reduction is achieved at high incidence angles. A smaller wavelength leads to higher additional losses at the design point, but gives rise to a greater loss reduction at high incidence angles. The W4 cascade achieves the maximum loss reduction of 36.1% at i = 10°, as well as the stall angle improvement of 27.6%. The formation mechanism of streamwise vortices is elaborated on the basis of the streamwise vorticity equation, in which the streamwise turning terms may be responsible for the generation of streamwise vortices. Slices of streamwise vorticity at various streamwise locations, combined with vorticity strength distributions, have been presented to study the development of streamwise vortices. The counter-rotating vortices are divided into the crest-induced streamwise vortices (CSVs) and trough-induced streamwise vortices (TSVs). A streamwise vortex pair formed from part of the CSV sheets behind troughs, is gradually entrained by the TSV pair along the streamwise direction. In addition, the tubercles with a smaller wavelength result in higher streamwise vorticity strength, with which the streamwise vortices interact with the flow separation more sufficiently and delay the separation to a greater extent.

Effects of the squealer winglet structures on the heat transfer characteristics and aerodynamic performance of turbine blade tip

The heat transfer characteristics and aerodynamic performance of three different squealer winglet structures based on GE-E3 turbine rotor squealer tip are numerically investigated, which is compared with the conventional squealer tip. The Reynolds-Averaged Navier-Stokes (RANS) solutions and standard k-ω turbulence model are solved to analyze the aerothermal performance of rotor tip with different squealer winglet structures. The numerical method is verified with experimental data. The numerical results show that the area-averaged heat transfer coefficient of squealer winglet structure at the pressure side, suction side and both the pressure and suction side are respectively reduced by 12.2%, 17.1% and 19.8% compared to the conventional squealer tip. The squealer winglet structure on the blade tip mostly influence the flow structures inside the cavity with the decreased strength of the pressure side corner vortex and the scraping vortex, which reduce the heat transfer coefficient and the leakage flow rate of the blade tip. The squealer winglet structure at the pressure side is found to increase the total pressure loss coefficient of the rotor blade by 8.5% in contrast to the conventional squealer tip. However, the squealer winglet structure at the suction side and both sides effectively reduce the total pressure loss coefficient by 8.5% and 2.5% respectively. The results reveal that squealer winglet structure at the suction side has the tradeoff of the heat transfer characteristics and aerodynamic performance among four different turbine rotor squealer tip profiles.

Accurate 2-D Modelling of Transonic Compressor Cascade Aerodynamics

Modern aeronautic fans are characterised by a transonic flow regime near the blade tip. Transonic cascades enable higher pressure ratios by a complex system of shockwaves arising across the blade passage, which has to be correctly reproduced in order to predict the performance and the operative range. In this paper, we present an accurate two-dimensional numerical modelling of the ARL-SL19 transonic compressor cascade. A large series of data from experimental tests in supersonic wind tunnel facilities has been used to validate a computational fluid dynamic model, in which the choice of turbulence closure resulted critical for an accurate reproduction of shockwave-boundary layer interaction. The model has been subsequently employed to carry out a parametric study in order to assess the influence of main flow variables (inlet Mach number, static pressure ratio) and geometric parameters (solidity) on the shockwave pattern and exit status. The main objectives of the present work are to perform a parametric study for investigating the effects of the abovementioned variables on the cascade performance, in terms of total-pressure loss coefficient, and on the shockwave pattern and to provide a quite large series of data useful for a preliminary design of a transonic compressor rotor section. After deriving the relation between inlet and exit quantities, peculiar to transonic compressors, exit Mach number, mean exit flow angle and total-pressure loss coefficient have been examined for a variety of boundary conditions and parametrically linked to inlet variables. Flow visualisation has been used to describe the shock-wave pattern as a function of the static pressure ratio. Finally, the influence of cascade solidity has been examined, showing a potential reduction of total-pressure loss coefficient by employing a higher solidity, due to a significant modification of shockwave system across the cascade.

CFD Analysis of Flow and Performance Characteristics of a 90°curved Rectangular Diffuser: Effects of Aspect Ratio and Reynolds Number

Abstract The effect of aspect ratio (AS = 0.5, 1.0, 2.0) and Reynolds number (Re = 1, 2, 4, 8 × 105) on the flow and performance characteristics of a 90° curved rectangular diffuser was examined with the aid of the commercial CFD code ANSYS FLUENT. The results from computational analysis revealed the presence of stream vorticity indicative of secondary flow brought about by the curvature of the diffusing duct. As the fluid flow developed, the inertial core flow was pushed to the outer concave wall by overbearing centrifugal forces. The fluid flow sustained marginal changes with increasing aspect ratio and Reynolds number. However, the flow uniformity at the exit was enhanced. The rise in coefficient of static pressure (Cp) recovery and the drop in coefficient of total pressure loss (Closs) with increasing Reynolds number suggested improved diffuser performance. The flow uniformity index was found to be the highest for the diffuser of aspect ratio 1.

Effect of arbitrary blade tip design on tip leakage flow

Tip geometry modification is frequently used to suppress the tip leakage flow in the turbine cascade however a universally beneficial tip geometry modification design has not been fully discovered. In this paper, the two-surface coupling arbitrary blade tip design method in three-dimensional physical space which satisfies the simple trigonometric function law is proposed and the mathematical parametric description is presented. The effects of different arbitrary blade tips on tip leakage flow have been studied numerically in a highly loaded axial turbine cascade. The aerodynamic performance of different tips is assessed by the tip leakage mass flow rate and the total pressure loss coefficient at the exit section. The Kriging model and genetic optimization algorithm are used to optimize the arbitrary blade tips to obtain the optimal arbitrary blade tip. Compared with the flat tip, the tip leakage mass flow rate is decreased by 10.57% and the area-average total pressure loss coefficient at the exit section is reduced by 8.91% in the optimal arbitrary blade tip.

Cooling effectiveness and aerodynamic performance in a 2D-Contoured endwall passage with different mass flow ratios

Endwall 2D-contouring is a presently-employed design feature for reducing the strength of secondary flows within gas turbine passages. Such contouring can lead to significant changes of passage flow, making the flow field different from that of the widely-studied flat-endwall passage. Also, there is an interface, or leakage slot, on the endwall between the combustor and the turbine affecting the passage flow. Though introduced for eliminating ingression of passage gas into the cavity, leakage flow through it, which has bypassed the combustor, can be used to cool the endwall and vane surfaces. Moreover, this leakage flow interacts with the main flow resulting in a change of aerodynamic losses. In this study, a 3D RANS method using an SST γ-θ transition model was employed to investigate endwall adiabatic cooling effectiveness values, η, and passage total pressure loss coefficients, TPLC, in a nozzle guide vane passage with a 2D-contoured endwall. Cases of differing mass flow ratios, MFRs, for flow through the slot were evaluated and compared. The numerical method employed was validated by comparing its computed results with experimental data. The results show how cooling effectiveness, aerodynamic performance and vortex structures near the endwall are significantly affected by passage flows of various MFR values. An appropriate range of MFR was found for gaining high adiabatic cooling effectiveness values and low aerodynamic losses while avoiding hot gas ingression into the slot.

Influence of endwall 2D contouring on endwall adiabatic cooling effectiveness and aerodynamic performance

Endwall 2D contouring is a presently-employed design method for reducing the strength of secondary flows within gas turbine airfoil passages. Such contouring can lead to significant changes of passage flow. Also affecting passage flow is flow through an interface, or leakage slot, between the combustor and the turbine. Though introduced for eliminating ingression of passage gas into the cavity, leakage flow through the leakage slot, which has bypassed the combustor, can be used to cool the endwall and vane surfaces. Moreover, the leakage flow interacts with the main flow resulting in a change of aerodynamic losses. In this study, a 3D numerical method was used to compare endwall adiabatic cooling effectiveness values, η, and passage Total Pressure Loss Coefficients (TPLC) between a 2D contoured-endwall passage and a flat-endwall passage in a Nozzle Guide Vane (NGV) cascade using several Mass Flow Rate (MFR) values and several momentum flux ratios, I, of slot leakage flow. The numerical method was validated by comparing its computed results with experimental data, then the shape of endwall contouring was designed using an aerodynamic optimization system. The results indicate that with endwall contouring, adiabatic cooling effectiveness values on the endwall are higher than with the flat endwall for all MFR values investigated. Moreover, with the same average momentum flux ratio, I, value of leakage flow, contoured endwall and flat endwall passages show similar adiabatic cooling effectiveness distributions, especially at the regions over the upstream part of the passage, which means that η scales well with I. Over the downstream part of the passage, the contoured endwall cases show higher adiabatic cooling effectiveness values on the contoured endwall with the same average I value as used with the flat endwall passage. Over the MFR range of this study, the contoured-endwall passage shows better aerodynamic performance than that of the flat-endwall passage.

Evaluation of an algebraic model for laminar-to-turbulent transition on secondary flow loss in a low-pressure turbine cascade with an endwall

The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier–Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A one-equation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit.

Effect of the nonaxisymmetric endwall on wet steam condensation flow in a stator cascade

AbstractSteam turbines are critical pieces of power equipment in the electric power industry, and the study of wet steam condensation flow is important for improving the efficiency and safety of steam turbines. Wet steam nucleation, which is the main reason for thermodynamic losses, usually occurs downstream of the stator cascade. The mechanism of secondary flow loss control by the nonaxisymmetric endwall is the change of pressure distribution in the cascade channel. This mechanism also affects the wet steam condensation phenomenon. The positive/cosine function was proposed for endwall modification in the nucleation stage of a steam turbine. The design method presented in this paper can be used to produce endwall protrusions with different heights at different axial chords. To analyze the influence of the endwall protrusion design parameters on the condensation flow, nine endwall protrusion models were set up at varied axial positions. In comparison with the original cascade, the total pressure loss coefficient and outlet wetness were found to be ideal when the endwall protrusion maximum height was located at the 50% axial chord length of the stator and the protrusion was 3% of the blade height. The ideal incidence angle of this modified cascade was from −2° to + 10°.

Comprehensive Geometric Description of Manufacturing Scatter of High-Pressure Turbine Nozzle Guide Vanes for Probabilistic CFD Analysis

The increasing demands on jet engines require progressive thermodynamic process parameters, which typically lead to higher aerothermal loadings and accordingly to designs with high complexity. State-of-the-art high-pressure turbine (HPT) nozzle guide vane (NGV) design involves vane profiles with three-dimensional features including a high amount of film cooling and profiled endwalls (PEWs). Typically, the specific mass flow, also called capacity, which governs the engine's operation, is set by the HPT NGV. Hence, geometric variations due to manufacturing scatter of the HPT NGV's passage can affect relevant aerodynamic quantities and the entire engine behavior. Within the traditional deterministic design approach, the influences of those geometric variations are covered by conservative assumptions and engineering experience. This paper addresses the consideration of variability due to the manufacturing of HPT NGVs through probabilistic CFD investigations. To establish a statistical database, 80 HPT NGVs are digitized with a high precision optical 3D scanning system to record the outer geometry. The vane profiles are parametrized by a section-based approach. For this purpose, traditional profile theory is combined with a novel method that enables the description of NGV profile variability taking the particular leading edge (LE) shape into account. Furthermore, the geometric variability of PEWs is incorporated by means of principle component analysis (PCA). On this basis, a probabilistic system assessment including a sensitivity analysis in terms of capacity and total pressure loss coefficient is realized. Sampling-based methods are applied to conduct a variety of 3D CFD simulations for a typical population of profile and endwall geometries. This probabilistic investigation using realistic input parameter distributions and correlations contributes to a robust NGV design in terms of relevant aerodynamic quantities.

Numerical investigation on heat transfer performance and flow characteristics in a roughened vortex chamber

In this study, an investigation of a vortex chamber was carried out to gain a full understanding of the nature of the vortex flow and the cooling capability inside the chamber. The paper discusses the effects on flow and heat transfer rates when the inside surface of the vortex chamber was roughened by adding flow turbulators to its wall. The turbulators took the shape of a rib with a square cross-section, the dimension of which varied between 0.25 mm and 2.00 mm. The paper also presents the results of a comparative investigation of jet impingement and vortex cooling on a concave wall using different parameters, such as the total pressure loss coefficient, Nusselt number and thermal performance factor, to evaluate the cooling effectiveness and flow dynamics. Furthermore, the entropy generation in swirl flow with the roughened wall was assessed over a wide range of Reynolds numbers.The results show that surface roughness considerably influences the velocity distribution, heat transfer patterns and pressure drop in the vortex chamber. The highest thermal performance factor takes place at rib heights of 0.25 mm and 0.50 mm with a low Re number. Further increase in rib height has an adverse impact on thermal performance. At a Reynolds number lower than 50,000, it is highly recommended to use roughened vortex cooling to obtain the best thermal performance.

Numerical analysis of the aerodynamic performance & heat transfer of a transonic turbine with a partial squealer tip

The purpose of this study is to evaluate thermal load and aerodynamic performance by applying different shapes of tips depending on the velocity region in a transonic turbine where subsonic and supersonic regions coexist. The RS1S, which is the transonic turbine of SNECMA, is used. To understand the tendencies, geometries A, B and C were created by varying the length of the pressure surface rim and the suction surface rim, and they were compared with a flat tip, squealer tip, pressure surface rim and suction surface rim. RANS computational analysis was performed using the commercial code ANSYS CFX 18.0, and aerodynamic performance was evaluated by using the mass-averaged total pressure loss coefficient at the 133% Cax and the mass flow rate of tip leakage. Thermal load was also evaluated by using the area-averaged heat transfer coefficient at the tip. Although the mass flow rate of leakage was smallest with the squealer tip, geometry C, the 75% rim of the pressure surface and the suction surface showed the lowest total pressure loss coefficient, which was 6.5% lower than that of the flat tip. In addition, when only the suction surface rim was applied, the area-averaged heat transfer coefficient was 10% lower than that of the flat tip.

Treatment and optimization of casing and blade tip for aerodynamic control of tip leakage flow in a turbine cascade

Various shape designs for tip clearance contribute to leakage control in turbomachinery whereas few studies so far focus on the combination treatments of casing and blade tip, their smoothness and continuousness as well. A novel circumferential casing and blade tip treatments based on the bimodal Gaussian function curve for suppression of tip leakage flow have been numerically studied in a linear turbine cascade. This configuration is then optimized by applying the Kriging model and orthogonal design for further loss reduction. As the main aerodynamic parameters, leakage flow and total pressure loss coefficient are evaluated with three configurations: flat-tip without casing treatment (BASE), casing and blade tip treatments (CBTT) and its optimization result (OPT). Compared with BASE, the results in CBTT and OPT indicate that under the effect of blocking vortex (BV), the tip leakage vortex (TLV) is divided into two parts. One becomes involved in mixing process with BV, and then joins to the upper passage vortex (UPV) downstream, the other rebuilds behind bimodal peaks. The further optimization would lead to a sharp decline of loss in CBTT and a better result in OPT. Compared with the baseline case, a reduction of 5.39% in relative total pressure loss could be noticed in the CBTT case while a decreasing of 10.87% found in the OPT case, as a result of the earlier blocking effect.

Computational investigation on secondary flows in a linear turbine cascade with tapered dual fence

The focus of the present work is to minimize the secondary flow losses inside a linear turbine cascade by means of a novel design of streamwise dual fence. The leading edge and trailing edge of the fences have been modified so as to reduce the total pressure loss coefficient in the passage. The study has been carried out computationally based on RANS simulations with SST turbulence model. Numerous simulations have been undertaken with single fence and dual fence models and compared with the base case model. The dual fence model with tapered trailing edge exhibits significant loss reduction compared to the base case. A suitable fence height ratio (FHR) has been identified for the dual fence model. The FHR = 2 configuration reduces the secondary flow kinetic energy by 78 % within the blade passage and it reduces the exit angle deviation significantly throughout the span. Detailed flow field analysis has been carried out to understand the physical mechanism behind the loss reduction with dual fence models. It is observed that fence-1 breaks the pressure side leg of the horse shoe vortex at the beginning of their formation itself. The radial penetration of the suction side leg of the horse shoe vortex is restrained by fence-2. These combined effects prevent the formation and mixing of two prominent loss core regions thereby avoiding the accumulation of low energy fluid near the suction side of blade.

Experimental and Numerical Investigation of Secondary Flow Structures in an Annular Low Pressure Turbine Cascade Under Periodic Wake Impact—Part 2: Numerical Results

In this work, we present the results of the numerical investigations of periodic wake–secondary flow interaction carried out on a low pressure turbine (LPT) equipped with modified T106-profile blades. The numerical predictions obtained by means of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations using a k-ω-model have been compared with measurements conducted in the same configuration and showed a good agreement. Based on the verified numerical data, the Q-criterion has been employed to characterize the secondary flow structures and accurately identify their origin. An analysis of the fundamental wake kinematics and the unsteady vortex migration revealed dominant interaction mechanisms such as the circumferential fluctuation of the pressure side horseshoe vortex (HSV) and its direct interaction with the passage vortex (PV) and the concentrated shed vortex (CSV). Finally, a correlation with the total pressure loss coefficient is provided and a link to the incoming wake structures is given.

Loss Reduction in a 1.5 Stage Axial Turbine by Computer-Driven Stator Hub Contouring

Improvements in stage isentropic efficiency and reductions in total pressure loss are sought in a 1.5 stage axial turbine. This is representative of power generation equipment used in thermal power cycles, which delivers about 80% of the 20 × 1012 kWh world-wide electricity. Component-level improvements are therefore timely and important toward achieving carbon dioxide global emission targets. Secondary flow loss reduction is sought by applying a nonaxisymmetric endwall design to the turbine stator hub. A guide groove directs the pressure side branch of the horseshoe vortex away from the airfoil suction side, using a parametric endwall hub surface, which is defined as to obtain first-order smooth boundary connections to the remainder of the passage geometry. This delays the onset of the passage vortex and reduces its associated loss. The Automatic Process and Optimization Workbench (apow) generates a Kriging surrogate model from a set of Reynolds-averaged Navier–Stokes simulations, which is used to optimize the hub surface. The three-dimensional steady Reynolds-averaged Navier–Stokes model with an axisymmetric hub is validated against reference experimental measurements from the Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen. Comparative computational fluid dynamics (CFD) predictions with an optimized nonaxisymmetric hub show a decrease in the total pressure loss coefficient and an increase in the isentropic stage efficiency at and off design conditions.

Influence of solid particle erosion (SPE) on safety and economy of steam turbines

At present, the economic deterioration of the unit due to solid particle erosion (SPE) at the governing stage has attracted widespread attention. Based on the Euler-Lagrange equation and the Finnie erosion model, the influence of SPE on the safety and the efficiency of the unit are numerically analyzed for different types of notches in the governing stage of a 1000 MW ultra-supercritical steam turbine in this article. The results indicate that the total pressure loss coefficient and the outlet steam angle increase as the depth of the notch goes up. And with the rising of the notch depth, the erosion at the rear edge of the rotor blade increases and the isentropic efficiency decreases rapidly. Furthermore, the decline trend of efficiency differs when notch type is different. When the notch depth is 20 mm, the maximum decline of entropy efficiency is 4.06%.

Clocking effect of vaned diffuser on hydraulic performance of high-power pump by using the numerical flow loss visualization method

The high-power mechanical pumps used in thermal power plants consume a considerable amount of electrical energy. Thus it is of great importance to improve the efficiency from every potential feature of such pumps. In this study, the clocking position of the pump, defined as the circumferential position of the vaned diffuser relative to the circular casing, is investigated to improve the hydraulic performance. Thereafter, the flow loss visualization method is employed to depict the loss caused by clocking positions with the help of entropy production theory based on computational fluid dynamics (CFD). The numerical results are then validated by experiments. At the design condition, the numerical maximum difference in the hydraulic efficiency is 2.61% while the head of each scheme meets the design requirement. The clocking position influences the total pressure loss coefficient more in the circular casing than other components, and the largest difference in the circular casing is 1.91%. Furthermore, the proven visualization method reveals that the clocking position introduces additional flow losses in the circular casing of the worst scheme, and even increases the loss in the vaned diffuser. This study can also provide a new perspective to investigate the optimal assembly strategy for other hydraulic machinery.