Numerical Simulation of Flow through a Centrifugal Impeller

The paper reports computational results of the flow through an impeller (backward curved) of a centrifugal compressor. 3D steady state investigations are performed at off-design and design mass flow rates. The static pressure as well as stagnation pressure distribution contours and velocity vector reveals the behavior of flow through the impeller at different flow coefficients. The flow pattern observed within the impeller passage is complex and influenced by several factors. Flow through the impeller is distorted due to presence of jet and wake. As flow happens through the impeller, energy is transferred from the impeller’s blades to the fluid. This creates the jet at pressure side region and wake at suction side region of the impeller. This fluid with different energy level gets mixed at the exit of the impeller causing mixing losses as well as secondary flows. These loss leads to a considerable fall in static pressure rise in the compressor and thereby affecting the overall efficiency of the centrifugal compressor. Also, existence of vortices in the flow field as flow turned from axial direction to radial is seen.

Unified low-pressure compressor concept for engines of future high-speed micro-unmanned aerial vehicles

The vast majority of unmanned aerial vehiches are propeller-driven with low speed. For higher speeds and longer ranges, new cost-effective microjets, which operate efficiently in both “fly-fast” and “loiter” modes are required. As a solution, a novel variable-cycle geared micro-turbofan architecture without the typical components of booster and low-pressure turbine is considered. This study discusses a key element, the low-pressure compression system. Instead of a separate and complicated booster to extract more power from the basic turbine, it is proposed to incorporate its positive functionality in the fan root. By preliminary and detailed fluid models, and structural concerns, systematic comparisons are made on demonstrative and representative cases to explore the feasibility of the proposal. Beyond the required very wide-chord design, the concept yields to a significantly increased pressurization and axial velocity at the fan root and exact opposite at the rest, causing extreme twist. The corresponding transonic stator root greatly increases downstream mixing losses. Moreover, a limitation is found to be the downstream compressor duct due to a notable increase in the diffusion requirements. Findings present that the concept is dramatically different from typical highly-loaded fans and this paper attempts to present new design guidelines.

Mixing Losses in Steady and Unsteady Simulations of Turbomachinery Flows

The aim of the present work is to facilitate insight into the modeling errors in the context of blade row coupling approaches which capture unsteady flow phenomena at different levels of detail. The focus is on RANS-based steady mixing plane computations as well as time domain and frequency domain unsteady computations. The concept of mixing loss is revisited to quantify the amount of unsteadiness in a flow field. Following an idea by Fritsch and Giles, we compute a second order approximation of the mixing losses which are generated at blade row interfaces. The resulting formula decomposes the entropy jump into contributions of circumferential and temporal fluctuations. The mathematical derivation, however, is based upon simpler arguments. It is shown that Fritsch and Giles’ main result can be extended to non-ideal gases. Moreover, the second order mixing loss formula is applied to time and frequency domain unsteady simulations. It is shown that an additional term has to be computed which accounts for the interaction of evanescent acoustic modes if the method is applied to unsteady flows. The methodology decomposes the overall mixing entropy into contributions of single perturbation types and harmonics. This may be used to assess whether unsteady flow phenomena of interest are adequately resolved and, in particular, to quantify the unsteadiness contained in the unresolved harmonics. A detailed investigation of the transonic IGV-rotor configuration of DLR’s Rig 250 compressor demonstrates the approach.

Profile and Mixing Losses of a Turbine Cascade Under the Condition of Low Reynolds Number Flows

Systematic flow measurements of profile and mixing losses were made across the blade pitches of a linear turbine cascade at low and moderate Reynolds numbers. The effect of Reynolds number on mixing behavior and development of the wake downstream of the trailing edge were investigated by means of traverse results at different planes downstream of the cascade. The experimental loss data were compared with predictions of theoretical boundary layer flow methods and of computational fluid dynamics (CFD) analyses. Several turbulence models available in commercial CFD tools and in the open CFD code OpenFOAM were considered for the present case study. It was found that substantial deviations between the numerical predictions and the experimental data occurred. These deviations may be attributed to the fact that at low Reynolds numbers, laminar and transitional boundary layer flows and flow separation phenomena became relevant. Based on the experimental two-dimensional loss data, the validity of simple Reynolds number scaling rules was assessed.

Influence of a Shroud Axisymmetric Slot Injection on a High Pressure Turbine Flow

This paper focuses on an axisymmetric slot injecting cooling air at the casing between the stator and the rotor in a one-stage unshrouded transonic high pressure turbine. This configuration has been studied with the help of unsteady RANS computations with and without the slot. Special care has been taken to model and describe the interaction induced unsteady mechanisms. It has been found that the cooling air is ejected from the axisymmetric slot at a fixed position with respect to the stator vanes, with a much lower incidence angle than the main stream. The flow through the rotor passage is highly modified and reveals an unsteady behaviour which highlights the necessity of using unsteady simulations in order to accurately model such a configuration. The effect on the efficiency and on the repartition of loss generation has been determined. As several different definitions of the efficiency can be used for cooled turbine cases, this choice is discussed. In particular, Young & Horlock’s “Weighted Pressure” definition, which takes into account some unavoidable mixing losses in the definition of the ideal process, is evaluated. With this definition, the slot does not yield any significant decrease in overall efficiency.

Aerodynamic impact of hub and shroud leakage flow on an axial turbine stage

This paper presents a study analyzing the aerodynamic effect and loss mechanism of hub and shroud leakage flow for an axial turbine stage. A series of computational fluid dynamics computations were performed to investigate the effect of various complexity of leakage configurations on the flow field. It is found that a non-linear relationship between different flow systems emerges in the vicinity of both the shroud and hub leakage flow exhibiting a deviation of flow direction and a radial shift of flow pattern, while the efficiency drop caused by the shroud and hub leakage configurations can be added linearly. By analyzing the expansion process in the cooled turbine and the spatial distribution of viscous dissipation term, the loss sources which can be directly traced back to the flow phenomena were indentified. Based on the aerodynamic feature of the turbine, an analytical approach to separate and quantify loss sources was proposed and applied to analyze the turbine. Four kinds of loss mechanisms, referred to as cavity losses, mixing losses, extra losses, and the leakage work reduction, were observed and their contributions were eventually represented by efficiency penalty.

Full 3-D Unsteady Numerical Simulation of Control Stage in Partial Admission Turbine

The unsteady flow parameters in control stage of partial admission are analyzed in details with full 3-D numerical simulation. The full annulus structure of air turbine in partial admission is modeled due to the unsymmetrical geometry. The partial admission is accomplished through the inlet blocked using segmental arc. The unsteady surface pressure changes of eight blades in the transition regions which demonstrate the power output ability are presented. That the entropy rise associated with the losses at different cross mainly caused by mixing losses and flow separation in partial admission is analyzed to estimate the efficiency distribution.

Probabilistic and Numerical Modelling of a Lobed Mixer at Windmilling Conditions

Aero-gas turbine engines with a mixed exhaust configuration offer significant benefits to the cycle efficiency relative to separate exhaust systems, such as increase in gross thrust and a reduction in fan pressure ratio required. A number of military and civil engines have a single mixed exhaust system designed to mix out the bypass and core streams. To reduce mixing losses, the two streams are designed to have similar total pressures. In design point whole engine performance solvers, a mixed exhaust is modelled using simple assumptions; momentum balance and a percentage total pressure loss. However at far off-design conditions such as windmilling and altitude relights, the bypass and core streams have very dissimilar total pressures and momentum, with the flow preferring to pass through the bypass duct, increasing drastically the bypass ratio. Mixing of highly dissimilar coaxial streams leads to complex turbulent flow fields for which the simple assumptions and models used in current performance solvers cease to be valid. The effect on simulation results is significant since the nozzle pressure affects critical aspects such as the fan operating point, and therefore the windmilling shaft speeds and air mass flow rates. This paper presents a numerical study on the performance of a lobed mixer under windmilling conditions. An analysis of the flow field is carried out at various total mixer pressure ratios, identifying the onset and nature of recirculation, the flow field characteristics, and the total pressure loss along the mixer as a function of the operating conditions. The data generated from the numerical simulations is used together with a probabilistic approach to generate a response surface in terms of the mass averaged percentage total pressure loss across the mixer, as a function of the engine operating point. This study offers an improved understanding on the complex flows that arise from mixing of highly dissimilar coaxial flows within an aero-gas turbine mixer environment. The total pressure response surface generated using this approach can be used as look-up data for the engine performance solver to include the effects of such turbulent mixing losses.

Investigation of Trailing Edge Cooling Concepts in a High Pressure Turbine Cascade—Aerodynamic Experiments and Loss Analysis

As part of a European research project, the aerodynamic and thermodynamic performance of a high pressure turbine cascade with different trailing edge cooling configurations was investigated in the wind tunnel for linear cascades at DLR in Go¨ttingen. A transonic rotor profile with a relative thick trailing edge was chosen for the experiments. Three trailing edge cooling configurations were applied, first central trailing edge ejection, second a trailing edge shape with a pressure side cut-back and slot equipped with a diffuser rib array, and third pressure side film cooling through a row of cylindrical holes. For comparison, aerodynamic investigations on a reference cascade with solid blades (no cooling holes or slots) were performed. The experiments covered the subsonic, transonic and supersonic exit Mach number range of the cascade while varying cooling mass flow ratios up to 2 %. This paper analyzes the effect of coolant ejection on the airfoil losses. Emphasis was given on separating the different loss contributions due to shocks, pressure, and suction side boundary layer, trailing edge, and mixing of the coolant flow. Employed measurement techniques are schlieren visualization, blade surface pressure measurements, and traverses by pneumatic probes in the cascade exit flow field and around the trailing edge. The results show that central trailing edge ejection significantly reduces the mixing losses and therefore decreases the overall loss. Higher loss levels are obtained when applying the configurations with pressure side blowing. In particular, the cut-back geometry reveals strong mixing losses due to the low momentum coolant fluid, which is decelerated by the diffuser rib array inside the slot. The influence of coolant flow rate on the trailing edge loss is tremendous, too. Shock and boundary layer losses are major contributions to the overall loss but are less affected by the coolant. Finally a parameter variation changing the temperature ratio of coolant to main flow was performed, resulting in increasing losses with decreasing coolant temperature.

Control of shroud leakage flows to reduce mixing losses in a shrouded axial turbine

The losses caused by the leakage flows through the rotor tip clearance, and the mixing losses by the re-entering leakage into the main flow are considerable parts of the total losses in turbines. The main reason for the mixing losses is the different velocity components of main and leakage flows. This leads to shear stresses which cause increased turbulence and losses. This article presents a numerical investigation on three different configurations to control the leakage flows: (a) turning vanes are fixed onto the casing between the fins to turn the shroud leakage flow into the main flow direction in order to reduce the circumferential mixing losses; (b) honeycomb bands are inserted into the casing to weaken the leakage flow in the circumferential direction and reduce the circumferential mixing losses due to the special hexagon structure; and (c) downstream edge of the cavity is chamfered to reduce the radial velocity component of the leakage jet and the separation at the downstream edge, and also to reduce the streamwise mixing losses. A 1.5-stage axial turbine with high-aspect ratio blading was used in this study to investigate the sealing designs as mentioned. The flow simulation results of the three configurations were analysed and compared in this article.