Flow In Turbomachines Research Articles

Supercritical Carbon Dioxide Mixing Loss Characteristics Near the Critical Point

Abstract This paper aims to study the mixing loss characteristic of sCO2 near the critical point and shed light on the non-ideal fluid effect on the mixing losses. As a simplified model of the mixing process in the trailing edge or tip leakage flow in turbomachines, a case of mixing in a constant area adiabatic duct involving two streams of parallel flows is studied by control volume analysis. Both perfect gas and non-ideal fluid calculations are carried out for comparisons. The isolated and coupled effects of the two mixing streams' temperature, velocity, and pressure differences on the loss are investigated. The study shows that non-ideal fluid mixing produces significantly higher losses when compared to the equivalent perfect gas estimation. It also reveals that the non-ideal fluid mixing loss is sensitive to the average thermodynamic states of the mixing streams. A significant variation of the mixing loss coefficient is reported if the static state of any of the two streams is near the critical point, which is mainly caused by the significant variation of the temperature gradient regarding the entropy along the isobar. The results also show that the change in the mixing loss due to the variation of one property difference between mixing streams is almost independent of the other property difference. The results from this simple case contribute to understanding the mixing loss behaviour differences between perfect gas and non-ideal fluids in sCO2 turbomachinery and hold significance for the development of mean-line models.

Quasi-three-dimensional modeling as an effective tool for studying flows in turbomachines

Quasi-three-dimensional modeling as an effective tool for studying flows in turbomachines

A Free Streamline Model for Fully Separated Turbomachine Flows

Abstract This paper describes the use of the free streamline approach to determine flows in fully separated cascades, and to thereby provide a rational basis for estimating the performance of axial turbomachinery at severely off-design conditions. On the assumption of two-dimensional, incompressible flow, the free streamlines that bound the separated region are determined using the hodograph method, in which the methods of complex analysis are used to define a tractable numerical problem. The resulting jet-wake flow profile at cascade exit is then presumed to undergo mixing at constant-area, from which performance parameters for the cascade can be determined. The proposed method is verified by comparison against flat plate cascade solutions and by checks against momentum conservation. Validation of the free streamline approach is carried out by comparison against several cascade configurations for which experimental data and simulation results are available. In general, the predictions based upon the present theoretical model are found to be good, especially for negative incidences such as those that occur in the compressor section of an engine operating at sub-idle conditions. This suggests that the present approach can be used in the estimation of turbomachinery performance maps far from the design point, especially during early design phases.

Application of immersed boundary method in turbomachines

Simulating unsteady turbulent flow in turbomachines is still challenging due to the complexity of blade geometry and relative motion between rotor and stator. This study presents an Immersed Boundary Method (IBM) for high-Reynolds turbomachinery internal flows, and shows the advantage of the automatic grid generation techniques and flexible moving boundary treatments. The wall functions are used in the present method to alleviate the wall resolution restriction of turbulence simulation. The Two-Dimensional (2-D) IBM solver, which was previously developed and tested for a low-speed compressor, is further validated for a well-documented Low-Pressure Turbine (LPT) cascade. Both the blade loading and the total pressure losses in the wake are well captured by the present 2-D solver. The complex Three-Dimensional (3-D) effects in turbomachines motivate the further development of an extended 3-D IBM solver by using a curvilinear-coordinate system that facilitates the hub and casing boundary treatment. The good performance of the 3-D solver is demonstrated through comparison with CFX solver solutions for the rotor configuration of Advanced Noise Control Fan (ANCF). Further effects of the grid resolution on capturing the blade wake are discussed. The results indicate that the present 3-D solver is capable of reproducing the evolution of the blade wake with suitable computational grid.

Design of Aircraft Air Conditioning System Turbomachine with Electric Compressor and Use of Vapor Compression and Water Evaporation Cooling

BACKGROUND: Traditionally, air is taken from engine compressors to address the needs of aviation air conditioning systems (ACSs) because the air in the environment is rarefied, and maintainining a high pressure that is comfortable for passengers is required in the cabins [1–3]. This solution consumes a considerable amount of energy because the air is extracted from compressors at significantly high pressure. Therefore, the ACS works at all flight altitudes.
 The use of ACS without air extraction from airplane engines with an autonomous electric compressor is proposed [5–7]. As a cooling unit, the utilization of a combined scheme using a traditional air cycle with a turbo cooler and a vapor compression refrigeration machine with the possibility of operation in heat pump mode and additional water evaporative cooling is proposed.
 This work aims to calculate and justify the energy efficiency of the proposed scheme of aircraft ACS without air extraction from the marching engines and auxiliary propulsion system using an electric compressor, a steam compression refrigeration machine with heat pump mode, and water vapor cooling. The paper also aims to design and compare the turbo compressor impeller using traditional methods and modern modeling tools to conclude its prospects.
 METHODS: Computational comparative study of traditional and proposed schemes of ACS. Calculation of compressor wheel flow part using traditional methods and CFX modeling.
 RESULTS: A reduction in power consumption of ACS during summer parking on the ground by 10 times and that in cruising flight by 16.6 times is observed when using the proposed scheme. The comparison result of calculations of the turbine unit using the classical method and CFX modeling revealed that the same initial data yielded a slightly different geometry of the impeller. This difference is due to modeling, which considers the reality of the properties of the working gas and internal overflows.
 CONCLUSION: The proposed scheme of ACS has considerable energy efficiency. Thus, conducting a comprehensive comparative analysis with traditional solutions on all indicators is reasonable. Overall, 3D modeling of the turbomachine flow part shows a real illustration of parameter changes. However, using 2D modeling for approximate and preliminary calculations is reasonable because it solves the problem quite accurately.

Improving the antifriction properties of the housing-rotor coupling in axial turbomachines

The article discusses the frictional interaction of carbon-carbon composite material on steel. Antifriction properties and wear resistance of the material under dry friction at speeds up to 100 m/s are investigated. The effect of velocity and contact pressure on the friction coefficient and wear resistance of a carbon composite is analyzed. The regularities of the change in the coefficient of friction are determined depending on the load-speed factors. The assumption is substantiated that the high thermal stability of the carbon composite, its low hardness, good antifriction properties and high wear resistance allow the material to be used in friction units operating at very high speeds and temperatures. In order to improve the reliability and reduce gas flows in axial turbomachines, on the basis of the study carried out, it is recommended that instead of heat-resistant coatings, a lining of the gas duct casing should be made of carbon composite.

Assessment of MULTALL as Computational Fluid Dynamics Code for the Analysis of Tube-Axial Fans

Abstract This work deals with the application of the open-source computational fluid dynamics (CFD) code MULTALL to the analysis of tube-axial fans. The code has been widely validated in the literature for high-speed turbomachine flows but not applied yet to low-speed tutbomachines. The aim of this work is to assess the degree of reliability of MULTALL as a tool for simulating the internal flow in industrial axial-flow fan rotors. To this end, the predictions of the steady-state air-flow field in the annular sector of a 315 mm tube-axial fan obtained by MULTALL 18.3 are compared with those obtained by two state-of-the-art CFD codes and experimental data of the global aerodynamic performance of the fan and the pitch-wise averaged velocity distribution downstream of the rotor. All the steady-state Reynolds-averaged Navier–Stokes (RANS) calculations were performed on either fully structured hexahedron or hexa-dominant grids using classical formulations of algebraic turbulence models. The pressure curve and the trend of the aeraulic efficiency in the stable operation range of the fan predicted by MULTALL show very good agreement with both the experimental data and the other CFD results. Although the estimation of the fan efficiency predicted by MULTALL can be noticeably improved by the more sophisticated state-of-the-art CFD codes, the analysis of the velocity distribution at the rotor exit supports the use of MULTALL as a reliable CFD analysis tool for designers of low-speed axial fans.

On the Application of Endoscopic Background-Oriented Schlieren Technique for Near-Field Investigation of Density Field

Background-Oriented Schlieren (BOS) is a non-intrusive and non-destructive technique for measuring density gradient in transparent media. Given the dependency of the refractive index and density of a medium, its static density field can be determined as a secondary measurand from BOS measurements. For an ideal gas, if the pressure variation is negligible, the static temperature field can be determined as a tertiary measurand. While the BOS technique for far-field measurements has been extensively researched, the implementation of this technique for near-field measurements still needs to be investigated. Near-field BOS is especially important for the measurement and visualization of internal flows, such as turbomachine flows, in which access to flow passage is limited and might be feasible using endoscopes (borescopes). This paper presents the features and challenges of the endoscopic BOS technique and presents methods and solutions for the application of this method in near-field measurements. The features of endoscopic BOS records as well as the comparison of endoscopic BOS and BOS measurements with camera are presented.

Effects of Input Voltage and Freestream Velocity on Active Flow Control of Passage Vortex in a Linear Turbine Cascade Using Dielectric Barrier Discharge Plasma Actuator

Passage vortex exists as one of the typical secondary flows in turbomachines and generates a significant total pressure loss and degrades the aerodynamic performance. Herein, a dielectric barrier discharge (DBD) plasma actuator was utilized for an active flow control of the passage vortex in a linear turbine cascade. The plasma actuator was installed on the endwall, 10 mm upstream from the leading edge of the turbine cascade. The freestream velocity at the outlet of the linear turbine cascade was set to range from UFS,out = 2.4 m/s to 25.2 m/s, which corresponded to the Reynolds number ranging from Reout = 1.0 × 104 to 9.9 × 104. The two-dimensional velocity field at the outlet of the linear turbine cascade was experimentally analyzed by particle image velocimetry (PIV). At lower freestream velocity conditions, the passage vortex was almost negligible as a result of the plasma actuator operation (UPA,max/UFS,out = 1.17). Although the effect of the jet induced by the plasma actuator weakened as the freestream velocity increased, the magnitude of the peak vorticity was reduced under all freestream velocity conditions. Even at the highest freestream velocity condition of UFS,out = 25.2 m/s, the peak value of the vorticity was reduced approximately 17% by the plasma actuator operation at VAC = 15 kVp-p (UPA,max/UFS,out = 0.18).

A hybrid RANS model of wing-body junction flow

Abstract The three-dimensional flow separation over the Rood wing-body junction is an exemplar application of separation affecting many important flows in turbomachinery and aerodynamics. Conventional Reynolds Averaged Navier Stokes (RANS) methods struggle to reproduce the complexity of this flow. In this paper, an unconventional use is made of a hybrid Reynolds Averaged Navier Stokes (RANS) model to tackle this challenge. The hybridization technique combines the Menter k − ω − S S T model with the one equation sub-grid-scale (SGS) model by Yoshizawa through a blending function, based on the wall-normal distance. The hybrid RANS turbulence closure captured most of the flow features reported in past experiments with reasonable accuracy. The model captured also small secondary vortices at the corner ahead of the wing nose and at the wing trailing edge. This feature is scarcely documented in the literature. The study highlights the importance of the spatial resolution near the wing leading edge, where this localized secondary recirculation was observed by the hybrid RANS model. It also provides evidence on the applicability of the hybrid Menter and Yoshizawa turbulence closure to the wing-body junction flows in aircraft and turbomachines, where these flows are characterized by a substantially time-invariant three-dimensional separation.

The method of averaging the gas flow parameters in turbomachines to evaluate their efficiency considering the velocity-field helicity

The methodology of averaging the nonuniform swirling flow in turbomachines has been developed. It is based on preserving the mass flow rate, the fluxes of total enthalpy and momentum (or entropy) with the addition of new invariant into the algorithm, i.e., the integral of the gas flow helicity. The dependence has been obtained and validated to specify the turbine stage efficiency calculation and to take into account the presence of large-scale vortex structures in the inter-blade channel of the turbomachine (the uncertainty of the influence of secondary flows on the value of losses does not exceed 0.2%). The methods of averaging the gas flow parameters, taking into account the contribution of secondary flows to the kinetic energy loss are presented. They show a 2.4% lower efficiency compared to the calculation previously used for the turbine stage of the Leningrad Polytechnic Institute.

Challenges in Modeling of Turbulence in the Tip Region of Axial Turbomachines

The flow and turbulence in turbomachines are characterized by spatially and temporally nonuniform strain rate fields and tendency to develop partially understood large-scale instabilities. Consequently, the applicability of popular Reynolds stress models in Reynolds-averaged Navier-Stokes simulations of turbomachinery flows is questionable and requires validation under relevant conditions. Experimental studies performed in the Johns Hopkins University optical refractive index-matched facility have investigated the complex flow and turbulence in the blade tip region of several axial turbomachines, including two waterjet pumps and an aviation compressor. The comprehensive databases obtained for different flow conditions and tip geometry using high-resolution particle image velocimetry are used in this article to examine the characteristics of turbulence in the vicinity of the tip leakage vortex (TLV). The results reveal several common features in the spatial distribution of the extremely anisotropic Reynolds stress tensor, suggesting that there are similarities in the mechanisms involved among the different machines. Of the quantities that could be measured accurately, both the local production and mean-flow advection rates appear to govern the evolution of turbulence in the passage. The dissipation rate also appears to be a major factor, but it cannot be measured accurately because of its dependence on motions at small scales that are not resolved by the measurements. The turbulent transport terms are important only in limited cases and locations. The strain rate tensor components, which appear to play primary roles in the production of turbulent kinetic energy (TKE), display common features in the spatial distributions of regions with high contraction, extension, and shear under different conditions. Some but not all of the inhomogeneity and anisotropy in the distributions of Reynolds stresses can be related to the corresponding local production rates. There is disagreement near the TLV core, where the persistent presence of multiple interacting vortices contributes significantly to the elevated TKE there. Lack of a clear functional correlation between the Reynolds stresses and mean strain rates is ubiquitous in all cases, resulting in widely spatially varying positive and negative values for the eddy viscosity. However, surprisingly, results from different machines and conditions have similar spatial distributions of individual stress and strain rate components and, accordingly, the eddy viscosity.

Past Developments and Current Advancements in Unsteady Pressure Measurements in Turbomachines

The aim of this paper is to review, summarize, and record long-term experience with development and application of aerodynamic probes with built-in miniature pressure transducers for unsteady pressure measurement and industrial research in turbomachine components. The focus of the first half of the paper is on the work performed at VZLU Prague, Czech Republic (Secs. 3–8). The latest development in unsteady pressure measurement techniques and data reduction methodology suitable for future research in highly loaded, high-speed turbine engine components performed at NASA GRC Cleveland, OH, is reported in Secs. 8–15 of this paper. Excellent reviews of similar activities at ETH Zürich, Switzerland by Kupferschmied, et al. (2000, “Time-Resolved Flow Measurements With Fast-Response Aerodynamic Probes in Turbomachines,” Meas. Sci. Technol., 11(7), pp. 1036–1054) and at VKI Rhode-Sain-Genèse, Belgium by Sieverding, et al. (2000, “Measurement Techniques for Unsteady Flows in Turbomachines,” Exp. Fluids, 28(4), pp. 285–321) were already reported and are acknowledged here. A short list of reported accomplishments achieved by other researchers at various laboratories is also reported for completeness. The authors apologize to those whose contributions are not reported here. It is just an unfortunate oversight, not an intentional omission.

Experimental Investigation of Boundary Layer Relaminarization in Accelerated Flow

Flow in turbomachines is generally highly turbulent. Nonetheless, boundary layers may exhibit laminar-to-turbulent transition, and relaminarization of the turbulent flow may also occur. The state of flow of the boundary layer is important since it influences transport phenomena like skin friction and heat transfer. In this paper, relaminarization in accelerated flat-plate boundary-layer flows is experimentally investigated, measuring flow velocities with laser Doppler anemometry (LDA). Besides the mean values, statistical properties of the velocity fluctuations are discussed in order to understand the processes in relaminarization. It is shown that strong acceleration leads to a suppression of turbulence production. The velocity fluctuations in the accelerated boundary layer flow “freeze,” while the mean velocity increases, thus reducing the turbulence intensity. This leads to a laminar-like velocity profile close to the wall, resulting in a decrease of the local skin friction coefficient. Downstream from the section with enforced relaminarization, a rapid retransition to turbulent flow is observed. The findings of this work also describe the mechanism of retransition.

Development and Implementation of a Technique for Fast Five-Hole Probe Measurements Downstream of a Linear Cascade

Flow measurement using a linear compressor or turbine cascade is a well-established technique to characterize the flow in turbomachines with a certain degree of abstraction. A common way to obtain a general characterization of the flow is to measure the flow downstream of the cascade with a five-hole probe, obtaining, e.g., total pressure losses and flow turning. Pneumatic five-hole probes are used to capture steady or time-averaged flow quantities, if not specified otherwise. In dependency of probe geometry, measurement set-up and flow properties, such measurements can be very time-consuming. Various techniques, in order to decrease the measurement time, are proposed in literature but for certain applications the efforts required to implement such techniques can outweigh the enhanced measurement speed. In this paper, methods proposed by other authors are combined and extended to allow for fast or transient five-hole probe measurements at strongly varying flow conditions. The effectiveness of this method is presented for flow measurements downstream of a compressor cascade with attached and stalled flow (by varying the Reynolds number) as well as with steady and periodically unsteady inflow. The new method allows to reduce the measurement time by up to 90 percent without compromising measurement accuracy. In fact, due to higher spatial resolution, the flow downstream of the cascade can be better resolved with the new method.

Optimization of the non-axisymmetric stator casing of a 1.5 stage axial turbine

The interaction of secondary flows with the main passage flow in turbomachines results in entropy generation and in aerodynamic loss. This loss source is most relevant to low aspect ratio blades. One approach for reducing this flow energy loss is by endwall contouring. However, limited work has been reported on using non-axisymmetric endwalls at the stator casing and on its interaction with the tip leakage flow. In this paper, a non-axisymmetric endwall design method for the stator casing is implemented through a novel surface definition, towards mitigating secondary flow losses. This design is tested on a three-dimensional axial turbine RANS model built in OpenFOAM 3.2 Extend, with k−ω SST turbulence closure. The flow analysis confirms the foundations of the new surface definition approach, which is implemented using Alstom Process and Optimization Workbench (APOW) software. Computer-based optimization of the surface topology is demonstrated towards automating the design process of axial turbines in an industrial design workflow. The design is optimized using the total pressure loss across the first stator and across the full stage, as the target function. Numerical predictions of the 1.5 stage axial turbine show the positive impact of the optimized casing design on the efficiency that increases by 0.69% against the benchmark axisymmetric stage from RWTH Aachen, which is validated against experiment. The new non-axisymmetric casing is also beneficial at off-design condition. The effective mitigation of the secondary flows is predicted to give a 0.73% efficiency gain off-design.

Imaging Flow Velocimetry with Laser Mie Scattering

Imaging flow velocity measurements are essential for the investigation of unsteady complex flow phenomena, e.g., in turbomachines, injectors and combustors. The direct optical measurement on fluid molecules is possible with laser Rayleigh scattering and the Doppler effect. However, the small scattering cross-section results in a low signal to noise ratio, which hinders time-resolved measurements of the flow field. For this reason, the signal to noise ratio is increased by using laser Mie scattering on micrometer-sized particles that follow the flow with negligible slip. Finally, the ongoing development of powerful lasers and fast, sensitive cameras has boosted the performance of several imaging methods for flow velocimetry. The article describes the different flow measurement principles, as well as the fundamental physical measurement limits. Furthermore, the evolution to an imaging technique is outlined for each measurement principle by reviewing recent advances and applications. As a result, the progress, the challenges and the perspectives for high-speed imaging flow velocimetry are considered.

Measurements and Characterization of Turbulence in the Tip Region of an Axial Compressor Rotor

Modeling of turbulent flows in axial turbomachines is challenging due to the high spatial and temporal variability in the distribution of the strain rate components, especially in the tip region of rotor blades. High-resolution stereo-particle image velocimetry (SPIV) measurements performed in a refractive index-matched facility in a series of closely spaced planes provide a comprehensive database for determining all the terms in the Reynolds stress and strain rate tensors. Results are also used for calculating the turbulent kinetic energy (TKE) production rate and transport terms by mean flow and turbulence. They elucidate some but not all of the observed phenomena, such as the high anisotropy, high turbulence levels in the vicinity of the tip leakage vortex (TLV) center, and in the shear layer connecting it to the blade suction side (SS) tip corner. The applicability of popular Reynolds stress models based on eddy viscosity is also evaluated by calculating it from the ratio between stress and strain rate components. Results vary substantially, depending on which components are involved, ranging from very large positive to negative values. In some areas, e.g., in the tip gap and around the TLV, the local stresses and strain rates do not appear to be correlated at all. In terms of effect on the mean flow, for most of the tip region, the mean advection terms are much higher than the Reynolds stress spatial gradients, i.e., the flow dynamics is dominated by pressure-driven transport. However, they are of similar magnitude in the shear layer, where modeling would be particularly challenging.

Choice of measure source terms in interface coupling for a model problem in gas dynamics

This paper is devoted to the mathematical and numerical analysis of a coupling procedure for one-dimensional Euler systems. The two systems have different closure laws and are coupled through a thin fixed interface. Following the work of [5], we propose to couple these systems by a bounded vector-valued Dirac measure, concentrated at the coupling interface, which in the applications may have a physical meaning. We show that the proposed framework allows to control the coupling conditions and we propose an approximate Riemann solver based on a relaxation approach preserving equilibrium solutions of the coupled problem. Numerical experiments in constrained optimization problems are then presented to assess the performances of the present method. 1. Introduction The study of large-scale and complex problems exhibiting a wide range of physical space and time scales (see for instance [62, 35, 14]), usually requires separate solvers adapted to the resolution of specific scales. This is the case of many industrial flows. Let us quote, for example, the numerical simulation of two-phase flows applied to the burning liquid oxygen-hydrogen gas in rocket engines [58]. This kind of flow contains both separated and dispersed two-phase flows, due to atomization and evaporation phenomena. This requires appropriate models and solvers for separated and dispersed phases that have to be appropriately coupled. Another example concerns turbomachine flows which can be modeled by the Euler equations of gas dynamics with different closure laws between the stages of the turbine, where the conditions of temperature and pressure are strongly heterogeneous. The coupling of these different systems is thus necessary to give a complete description of the flow inside the whole turbine. The method of interface coupling allows to represent the evolution of such flows, where different models are separated by fixed interfaces. First, coupling conditions are specified at the interface to exchange information between the systems. The definition of transmission conditions generally results from physical consideration, e.g. the conservation or the continuity of given variables. Then, the transmission conditions are represented at the discrete level. The study of interface coupling for nonlinear hyperbolic systems has received attention for several years. In [43], the authors study the scalar case from both mathematical and numerical points of view.

ВЛИЯНИЕ ПАРАМЕТРОВ ТУРБУЛЕНТНОСТИ НА РЕЗУЛЬТАТЫ ЧИСЛЕННОГО ИССЛЕДОВАНИЯ КОНФУЗОРНЫХ ТЕЧЕНИЙ В ТУРБОМАШИНАХ

ВЛИЯНИЕ ПАРАМЕТРОВ ТУРБУЛЕНТНОСТИ НА РЕЗУЛЬТАТЫ ЧИСЛЕННОГО ИССЛЕДОВАНИЯ КОНФУЗОРНЫХ ТЕЧЕНИЙ В ТУРБОМАШИНАХ