Loss Generation Mechanisms Research Articles

Effects of variable blowouts on aerodynamic loss reduction performance of compressor bionic blades

Abstract A passive flow control method, inspired by blowouts, is employed to eliminate flow separation and reduced losses associated with higher compressor loads. By implementing bionic-blowout variant dimples on the blade suction surface, the flow characteristics of the dimple-shaped cascade were investigated. The loss generation mechanism was also analysed. Firstly, the reliability of the numerical method was confirmed through experimental validation. Subsequently, biomimetic principles were applied to arrange dimples on the suction surface of the cascade blades, and the effects of various blowouts were analysed The result revealed that vortices within the dimples induce external fluid into the dimples, enhancing the turbulent kinetic energy of the external fluid and improving wall-adjacent flow adherence. The bionic-blowout variant dimples creates a "rolling bearing" effect that reduces frictional losses, which control flow separation. Within a certain blowout range, the bionic-blowout variant dimples significantly improve the flow conditions. At -6° angle of attack, the total pressure loss of the dimple-structured cascade decreased by 35.85%, and the pressure ratio increased by 2.35%. The bionic-blowout variant dimples on the blade suction surface exhibit a three-dimensional disturbance effect. The induced vortex structures regulates the boundary layer transition and suppress the formation of laminar separation bubbles, effectively improving the flow conditions near the corner region.

A Comparative Analysis of Distributor and Rotor Single Regulation Strategies for Low Head Mini Hydraulic Turbines

Tubular axial turbines (TATs) play a crucial role in mini and micro hydropower setups that require simplified yet reliable solutions. In very low head scenarios, single regulation in TATs is common, due to economic impracticality of the sophisticated mechanisms involved in the conjugate distributor–rotor regulation typical of the Kaplan turbines. Distributor or rotor single regulation strategies offer operation flexibility, each with distinct advantages and disadvantages. Stator regulation is simpler, while rotor regulation is more complex but offers potential efficiency gains. The present paper analyzes energy losses associated with these regulation strategies using two approaches: 1D mean line turbomachinery equations and 3D Computational Fluid Dynamics (CFD). The 1D mean line approach is used for understanding the conceptual fluid dynamic aspects involved in the two different regulation approaches, thereby identifying the loss-generation mechanisms in off-design operation. Fully 3D CFD simulations allow for quantifying and deeply explaining the differences in the hydraulic efficiencies of the two regulation strategies. Attention is focused on the two main loss contributions: residual tangential kinetic energy at the rotor outlet and entropy generation. Rotor regulation, even if more complex, provides better results than distributor regulation in terms of both effectiveness (larger flow rate sensitivity to stagger angle variation) and turbine operating efficiency (lower off-design losses).

Exergy destruction within a centrifugal water pump

Understanding the loss generation mechanisms in water pumps is a vital step in decarbonising our built environment, and achieve sustainable cities and communities. In this paper, loss generation mechanisms in a centrifugal pump are quantified by performing exergy analysis with unsteady Reynold Averaged Navier Stokes simulations (uRANS). Exergy analyses are performed at various operational conditions for a commercially available pump and its ideal version that has zero surface roughness. Numerical results are used to derive mathematical expressions to describe exergy destruction rates as functions of normalized flow rates. These expressions provide insight on how and where losses are generated within a centrifugal pump, and how loss generation mechanisms are affected by the flow rate. Results show that 80% of the losses are generated within the impeller, intersection and volute, whereas secondary flows through the deadzone and leakage paths have insignificant contribution to the total losses even though mass flow rate through these paths are considerable. The exergy destruction rate equations derived here, have the potential to replace the semi-empirical estimations of losses in traditional turbomachinery design methodologies and serve as a tool to develop a novel knowledge-based turbomachinery design methodology.

AC losses calculation of parallel multi‐strand flat wire windings for automotive drive motor

AbstractAutomobile drive motor with flat wire winding has improved the slot fill factor and efficiency. However, under the current environment of drive motors development to high frequency and voltage, the increase in winding AC loss has weakened those advantages. Accurate analysis of AC loss and the design of high‐performance windings have become the key issues and difficulties in the design process of drive motors. Based on the winding AC loss generation mechanism, the authors propose a winding AC loss analysis method, which can effectively separate DC loss, eddy current loss, and circulating current loss. The method is used to analyse winding AC loss of 60 kW flat wire winding permanent magnet synchronous motor (PMSM). In addition, parallel multi‐strand flat wire windings are proposed to reduce the winding AC loss and eliminate the limitation of the magnetic load of the motor on the selection of winding layers. The AC loss of the parallel multi‐strand flat wire windings at different frequencies is calculated, which demonstrates the effectiveness of the proposed winding topology in reducing the winding AC loss. In addition, the correctness of the simulation model was verified through experiments, and the loss calculation method was verified through finite element analysis.

Flow irreversibility and heat transfer effects on turbine efficiency

The quantification of the flow irreversibilities is crucial to developing future turbomachinery. Traditionally, design correlations and efficiencies are based on comparisons to isentropic relations to quantify the deviation from an “ideal” case. However, isentropic relations typically assume one-dimensional adiabatic flow, representing a significant departure from the actual situation in turbines operating with large levels of heat transfer. In this paper, the aerothermal losses and power generation in reversible processes are derived for three-dimensional compressible flow considering heat transfer effects. A new definition of the power potential in a reversible process is proposed that can be used to perform detailed budgeting of the losses. The proposed new loss framework can be adopted locally, which enables the precise identification of the loss source. New aerodynamic and aerothermal efficiency expressions are proposed to assess shaft power extraction in combination with heat exchange. The new method enables designers to identify the regions with high loss generation in steady and also in unsteady flow conditions, offering new avenues to evaluate the loss generation mechanisms during the design phase. Therefore, our new tool is essential to developing reduced-order models for the design of future fluid machinery.

Incorporating Harmonic-Analysis-Based Loss Minimization Into MPTC for Efficiency Improvement of FCFMPM Motor

According to air-gap field-modulation theory, multiple field harmonics are responsible for high torque density of field modulation (FM) motors. However, abundant field harmonics will undoubtedly increase motor losses, especially iron loss and PM eddy-current loss. Therefore, a harmonic-analysis-based loss minimization (HLM) method incorporated into model predictive torque control (MPTC) for efficiency improvement of FM motors is proposed in this article. First, by analyzing the loss generation mechanism, a harmonic-based online loss calculation model is built to serve for loss minimization control. Then, the reference flux for MPTC can be derived from the proposed HLM method, which avoids complexity of equivalent resistance tuning and improves accuracy of loss calculation compared to the conventional model-based loss minimization method which lacks attention on the harmonic-based loss analysis. Consequently, the HLM-MPTC strategy is finally determined and then applied in a flux-concentrating field-modulation permanent-magnet prototype to verify its validity. The experimental results evidence that the proposed HLM-MPTC can effectively achieve loss reduction and efficiency improvement.

AC Loss Analysis and Measurement of a Hybrid Transposed Hairpin Winding for EV Traction Machines

To address the problem of high-frequency AC loss of hairpin winding, a hybrid transposed hairpin winding (HTHW) scheme, which combines flat wire and litz wire, is proposed. The HTHW can effectively reduce the high-frequency AC loss, while maintaining a relatively high slot fill factor. In this paper, the AC loss generation mechanism under high-frequency excitation is firstly analyzed. Then, based on the field-circuit-coupled finite element method, the current density distribution and AC loss of hairpin winding with different transposed topologies are calculated and compared. Finally, an experimental platform enabling direct measurement of hairpin winding loss is built to verify the simulation. The simulation and experimental results show that the AC loss of the proposed HTHW is much lower than that of the traditional hairpin winding, for medium to high running frequency. In our case, the AC and DC resistance ratio of the presented HTHW is 2.12 at the frequency of 1200 Hz, contrasting that of the ratio of traditional hairpin winding, which is 10.2.

Understanding loss generation mechanisms in a centrifugal pump using large eddy simulation

Water pumps are amongst the most frequently used turbomachines, which consume about 9% of the global energy production. This paper provides insight on the loss generation mechanisms in inline centrifugal pumps operating under realistic conditions through a detailed analysis based on large eddy simulation (LES). The equations for the resolved, sub-grid and wall entropy generation terms are derived using the LES filtering and Smagorinsky-Lilly formalism. Specific exergy is calculated at the grid points. Entropy generation and exergy transfer within the flow domain are visualised to highlight the loss generation mechanisms within the pump. Results show that there is a strong component interaction. The double-bended inlet pipe initiates Dean-vortices at the eye of the impeller. This asymmetric inflow coupled with the disturbances at the cut-off cause a non-uniform flow through the blade passages. Other drivers of the asymmetrical loading of the blade passages are high leakage and unsteady and asymmetrical mass and exergy transfer between the dead zone. The paper also investigates the effect of the grid resolution on the first and second order statistics of the flow field and its turbulent characteristics. A thorough evaluation of the discretisation error is presented to provide a measure for the required grid density and demonstrate the extent and impact of the temporal and spatial discretisation error in centrifugal turbomachinery.

Analysis of the Loss Production Mechanism Due to Cavity–Main Flow Interaction in a Low-Pressure Turbine Stage

Abstract In the present work, URANS simulations are presented to describe the unsteady interaction process between the flow ingested/ejected from a cavity system and the main flow evolving into a low-pressure turbine stage. Particular care is posed on the analysis of the loss generation mechanisms acting outside the stator row and in the rear part of the axial gap separating the cavity flow ejection section and the leading edge plane of the downstream rotor row. The simulated geometry reproduces a typical engine cavity configuration, with upstream and downstream rotor rows reproduced by means of moving bars. Experimental results have been used to validate the simulations. These experimental data cannot explain and quantify alone the overall interaction process between the cavity flows and the main flow. The results of a simulation made by removing the domain of the cavity have been employed in order to better highlight and quantify the effects due to main flow and cavity flows interaction on total pressure loss. A deep inspection of the loss amount along the axial direction makes evident that losses generated in the vane row are basically increased prior to entering into the downstream rotor bars, due to cavity main flow interaction.

Loss prediction of axial compressors using genetic algorithm–back propagation neural network in throughflow method

While the consistent advance in computational power has enabled the Computational Fluid Dynamics (CFD) an effective tool for compressor performance characterization, the need for quick performance estimates at initial design phase of compressor still requires the use of low order models. Thus, the throughflow method remains the backbone of compressors design process. The accuracy of the throughflow calculation mainly depends on the adopted empirical correlations. However, the traditional empirical models are just accurate for the conventionally loaded compressor at normal working conditions. In this article, the mechanism of blade profile loss generation and the formation mode of existing empirical correlation are studied, and the reason why the traditional diffusion factor based empirical models are not applicable for modern high-loading compressors or conventional-loading blades at negative incidence is also discussed. Then, the Genetic Algorithm assisted Back Propagation Neural Network (GA-BPNN) is used to train the surrogate model for the design and off-design loss prediction along the blade span of the compressor. Based on the test data of four transonic compressor stages, a database containing 72 sets of blade element geometry and about 1400 sets of blade element performance data is established. Considering the different mechanisms of rotor and stator losses at different working conditions, the entire database and surrogate model are divided into four components according to the rotor and stator at positive incidence and negative incidence. Comparing the prediction results of the surrogate model with the traditional empirical correlations and experimental data, the results show that the GA-BPNN is an alternative solution for developing the empirical model.

Unsteady flows of a highly loaded turbine blade with flat endwall and contoured endwall

Strong secondary flow results in substantial aerodynamic loss for highly-loaded turbine. Accurate prediction of the complicated flow structures proposes challenges to the widely used Reynolds Average Navier-Stokes (RANS) approach. This work employs the hybrid RANS/Large Eddy Simulation (LES) method to study the unsteady flows for a highly-loaded turbine blade, with both flat endwall and optimized contoured endwall. Evolution of the unsteady flows in the endwall region is analyzed, with emphasis on the loss generation mechanism. Results show that for the flat endwall, the horseshoe vortex system is highly unsteady and is a significant source of unsteadiness in both the passage and the wake region. It contributes to the unsteady passage vortex, also the earlier breakdown of the trailing edge shedding vortex. The Probability Density Function (PDF) histogram of velocity in the wake region is bimodal, implying the perturbations from two mechanisms. For the contoured endwall, the unsteady evolution of the horseshoe vortex is blocked, which results in significantly reduced unsteadiness, also the merge between the horseshoe vortex with the passage vortex is prevented. Effect of the flow unsteadiness on the loss generation is assessed based on the entropy generation rates contributed by the time-averaged flow and the fluctuations. The effect of unsteadiness is two-fold: unsteady perturbations trigger the vortex breakdown into small-scale structures and thus weakened wake velocity deficit and loss generation; however, in both the passage and the wake region the entropy generation by the fluctuations are remarkable. For the contoured endwall, due to the reduced flow unsteadiness, loss generation contributed by the fluctuations are much smaller compared to the flat endwall. The results highlight the importance of including the loss generation by the fluctuations, also a possible mechanism to reduce the secondary loss by attenuating the flow unsteadiness with the contoured endwall.

Numerical investigations of the separated transitional flow over compressor blades with different loading distributions

Large eddy simulations (LESs) were conducted to investigate the separated flow transition process over two compressor blades with different loading distributions at Reynolds numbers (Re) of 1.5×105 and 0.8×105. The baseline airfoil (V103-B) was redesigned to obtain a new front-loaded airfoil (V103-F). At Re=1.5×105, a time-averaged laminar separation bubble (LSB) formed on the suction surface of V103-B and V103-F. The two-dimensional spanwise vortices shed periodically at approximately the same frequency and were further twisted through their interaction with the streamwise evolving vortices. Then, small vortex structures were generated in the vortex pairing, which was related to the onset of transition. The distorted hairpin vortices finally broke down into small turbulent eddies near the reattachment, along with an ejection-sweeping process of the near-wall flow. As Re decreased to 0.8×105, the separated shear layer failed to reattach on the blade surfaces of the two airfoils. The near-wall flow ejection-sweeping disappeared, and there was no distinct periodicity for the two-dimensional spanwise vortex shedding. The spectral analysis indicated that the transition processes in the LSBs of the two airfoils at Re=1.5×105 were both dominated by the Kelvin-Helmholtz (K-H) mechanism; however, the transition onset on the suction surface of V103-F was promoted, and the size of LSB was consequently smaller than that of V103-B. The loss generation mechanism in the LSB was analyzed by comparing the deformation work terms due to the viscous and turbulent dissipation effects, with the results indicating that the largest amount of loss was determined by the Reynolds shear stresses. Due to the suppression of the LSB on the suction surface of V103-F, the profile loss was decreased distinctly by 32.3% at Re=1.5×105 compared with that of V103-B.

Trailing edge loss analysis of high-speed cryogenic microturbines used in helium applications

The trailing edge flow characteristics have significant impact on the turbine performance. To improve the performance of the turbine, a detailed analysis of the turbine flow field and the loss generation mechanisms associated with trailing edge is called for. This is of much importance in cryogenic helium turbines as the flow characteristics would be different from other turbines due to its characteristics like higher speed and smaller dimensions. In this paper, the trailing edge loss mechanism and the effect of trailing edge profile on the performance of a cryogenic helium turbine was analysed using CFD. The effect of flow separation, base pressure and trailing edge blockage were analysed. The study observed that the trailing edge loss became significant only when its thickness was increased beyond 3% of the blade pitch. This will provide some leeway for the manufactures to have thicker trailing edges without compromising on the turbine performance.

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.

Characterization of the Unsteady Aerodynamics of Optimized Turbine Blade Tips through Modal Decomposition Analysis

The present research focused on the analysis of the leakage flows developing from advanced blade tip geometries. The aerodynamic field of a contoured blade tip and of a high-performance rimmed blade were investigated against a baseline squealer rotor. Time-resolved numerical predictions were combined with high-frequency pressure measurements to characterize the tip leakage flow of each tip design. High spatial and temporal resolution measurements provided a detailed representation of the unsteady flow in the near-tip region and at the stage outlet. Numerical computations, based on the nonlinear harmonic method, were employed to assess the unsteady blade row interactions and identify the loss generation mechanisms depending on the tip design. The space- and time-resolved flow field was analysed by modal decomposition to identify the main periodicities of the near-tip and outlet flow and classify the most relevant sources of aerodynamic unsteadiness and entropy generation across the stage.

On the Influence of an Acoustically Optimized Turbine Exit Casing Onto the Unsteady Flow Field Downstream of a Low Pressure Turbine Rotor

Modern low pressure turbines (LPT) are designed in order to fulfil a various number of requirements such as high endurance, low noise, high efficiency, low weight, and low fuel consumption. Regarding the reduction of the emitted noise, different designs of LPT exit guide vanes (aerodynamically and/or acoustically optimized) of the turbine exit casing (TEC) were tested, and their noise reduction capabilities and aerodynamic performance were evaluated. In particular, measurements of TEC-losses were performed, and differences in the losses were reported. Measurements were carried out in a one and a half stage subsonic turbine test facility at the engine relevant operating point approach. This work focuses on the study of the unsteady flow field downstream of an unshrouded LPT rotor. The influence on the upstream flow field of a TEC design including acoustically optimized vanes (inverse cut-off TEC) is investigated and compared with a second TEC configuration without vanes (Vaneless TEC), by means of fast response aerodynamic pressure probe (FRAPP) measurements. The second configuration served as a reference concerning the influence of turbine exit guide vanes (TEGVs) onto the upstream located LPT rotor. The interactions between the stator and rotor wakes, secondary flows, and the TEGVs potential effect are identified via modal decomposition according to the theory of Tyler and Sofrin. The main structures constituting the unsteady flow field are detected, and the role of the major interaction effects in the loss generation mechanism and in the acoustic emission is analyzed. This study based on the modal analysis of the unsteady flow field offers new insight into the main interaction mechanisms and their importance in the assessment of the aerodynamic and aeroelastic performance of modern LPT exit casings.

Rotor work characteristics and loss generation mechanism of an axial flow compressor at windmill operation

Rotor work characteristics and loss generation mechanism of an axial flow compressor at windmill operation

Entropy Analysis of the Flat Tip Leakage Flow with Delayed Detached Eddy Simulation.

In unshrouded turbine rotors, the tip leakage vortices develop and interact with the passage vortices. Such complex leakage flow causes the major loss in the turbine stage. Due to the complex turbulence characteristics of the tip leakage flow, the widely used Reynolds Averaged Navier–Stokes (RANS) approach may fail to accurately predict the multi-scale turbulent flow and the related loss. In order to effectively improve the turbine efficiency, more insights into the loss mechanism are required. In this work, a Delayed Detached Eddy Simulation (DDES) study is conducted to simulate the flow inside a high pressure turbine blade, with emphasis on the tip region. DDES results are in good agreement with the experiment, and the comparison with RANS results verifies the advantages of DDES in resolving detailed flow structures of leakage flow, and also in capturing the complex turbulence characteristics. The snapshot Proper Orthogonal Decomposition (POD) method is used to extract the dominant flow features. The flow structures and the distribution of turbulent kinetic energy reveal the development of leakage flow and its interaction with the secondary flow. Meanwhile, it is found that the separation bubble (SB) is formed in tip clearance. The strong interactions between tip leakage vortex (TLV) and the up passage vortex (UPV) are the main source of unsteady effects which significantly enhance the turbulence intensity. Based on the DDES results, loss analysis of tip leakage flow is conducted based on entropy generation rates. It is found that the viscous dissipation loss is much stronger than heat transfer loss. The largest local loss occurs in the tip clearance, and the interaction between the leakage vortex and up passage vortex promotes the loss generation. The tip leakage flow vortex weakens the strength of up passage vortex, and loss of up passage flow is reduced. Comparing steady and unsteady effects to flow field, we found that unsteady effects of tip leakage flow have a large influence on flow loss distribution which cannot be ignored. To sum up, the current DDES study about the tip leakage flow provides helpful information about the loss generation mechanism and may guide the design of low-loss blade tip.

Experimental and Numerical Investigation of Optimized Blade Tip Shapes—Part II: Tip Flow Analysis and Loss Mechanisms

The leakage flows within the gap between the tips of unshrouded rotor blades and the stationary casing of high-speed turbines are the source of significant aerodynamic losses and thermal stresses. In the pursuit for higher component performance and reliability, shaping the tip geometry offers a considerable potential to modulate the rotor tip flows and to weaken the heat transfer onto the blade and casing. Nevertheless, a critical shortage of combined experimental and numerical studies addressing the flow and loss generation mechanisms of advanced tip profiles persists in the open literature. A comprehensive study is presented in this two-part paper that investigates the influence of blade tip geometry on the aerothermodynamics of a high-speed turbine. An experimental and numerical campaign has been performed on a high-pressure turbine stage adopting three different blade tip profiles. The aerothermal performance of two optimized tip geometries (one with a full three-dimensional contoured shape and the other featuring a multicavity squealer-like tip) is compared against that of a regular squealer geometry. In the second part of this paper, we report a detailed analysis on the aerodynamics of the turbine as a function of the blade tip geometry. Reynolds-averaged Navier-Stokes (RANS) simulations, adopting the Spalart–Allmaras turbulence model and experimental boundary conditions, were run on high-density unstructured meshes using the numecafine/open solver. The simulations were validated against time-averaged and time-resolved experimental data collected in an instrumented turbine stage specifically setup for the simultaneous testing of multiple blade tips at scaled engine-representative conditions. The tip flow physics is explored to explain variations in turbine performance as a function of the tip geometry. Denton's mixing loss model is applied to the predicted tip gap aerodynamic field to identify and quantify the loss reduction mechanisms of the alternative tip designs. An advanced method based on the local triple decomposition of relative motion is used to track the location, size and intensity of the vortical flow structures arising from the interaction between the tip leakage flow and the main gas path. Ultimately, the comparison between the unconventional tip profiles and the baseline squealer tip highlights distinct aerodynamic features in the associated gap flow field. The flow analysis provides guidelines for the designer to assess the impact of specific tip design strategies on the turbine aerodynamics and rotor heat transfer.

An Investigation on the Loss Generation Mechanisms Inside Different Centrifugal Compressor Volutes for Turbochargers

In centrifugal compressor design, the volute plays a key role in defining the overall efficiency and operating range of the stage. The flow at the impeller outlet is indeed characterized by a high kinetic energy content, which is first converted to potential energy in the diffuser downstream. The compressed gas is then collected by the volute at the cylindrical outlet section of the diffuser and directed to the intake piping, possibly with a further pressure recovery to enhance the stage performance. Due to the high flow speed at the volute inlet, the capability of ensuring the lowest amount of total pressure loss is pivotal to prevent a detriment of the machine efficiency. Moreover, the flow conditions change when the volute operates far from its design point: at mass flow rates lower than the design one, the flow becomes diffusive, while at higher mass flow rates the fluid is accelerated, thus leading to different loss-generation mechanisms. These phenomena are particularly relevant in turbocharger applications, where the compressor needs to cover a wide functioning range; moreover, in these applications, the definition of the volute shape is often driven also by space limitations imposed by the vehicle layout, leading to a variety of volute types. The present paper reports an analysis on the sources of thermodynamic irreversibilities occurring inside different volutes applied to a centrifugal compressor for turbocharging applications. Three demonstrative geometrical configurations are analyzed by means of three-dimensional (3D) numerical simulations using common boundary conditions to assess the overall volute performance and different loss mechanisms, which are evaluated in terms of the local entropy generation rate. The modification of the loss mechanisms in off-design conditions is also accounted for by investigating different mass flow rates. It is finally shown that the use of the entropy generation rate for the assessment of the irreversibilities is helpful to understand and localize the sources of loss in relation to the various flow structures.