Gas-dynamic Stability Research Articles

Аеродинамічне вдосконалення вентилятора авіаційного газотурбінного двигуна

This work is concerned with the development of approaches to the aerodynamic improvement of axial-flow compressors for gas-turbine engines. The aim of this work is the aerodynamic improvement of an aircraft gas-turbine engine two-stage fan by numerical simulation of 3D turbulent gas flows. The approach used in this study features: varying the spatial shape of the fan blades for the first- and the second-stage impeller by varying the profile angle along the blade height; formulating quality criteria as the mean integral values of the power characteristics of each impeller of the fan over the operating range of the air flow rate through the impeller; and searching for advisable values of the impeller blade parameters by scanning the independent variable range at points that form a uniformly distributed sequence of small length. The basic tool is a numerical method developed at the Institute of Technical Mechanics of the National Academy of Sciences of Ukraine and the State Space Agency of Ukraine, which simulates 3D turbulent gas flows using the complete averaged Navier¬–Stokes equations and a two-parameter turbulence model. It is shown that varying the profile angle along the blade height for the fan second-stage impeller allows one to increase the air compression ratio in the fan by about 2 percent throughout the operating range of the fan air flow rate without affecting the adiabatic efficiency of the fan. On the whole, by the example of the fan under study, the paper considers the assumption that the aerodynamic improvement of compressors at the initial stage can be made on an impeller by impeller basis. It is shown that in further analysis providing the gas-dynamic stability of the compressor should be accounted for. The results obtained are intended to be used in the aerodynamic improvement of multistage compressors for aircraft gas-turbine engines and various power plant.

Дослідження цифровий адаптивної системи повороту лопаток направляючого апаратів осьового компресора

In the simplest cases, P-, PI- and PID-controllers with rigid (unchanged) parameter settings are used to control technological processes. If the mathematical model of the control object contains unknown disturbing influences and parameters that change during the control process, then a digital control system with a learning model should be used. The tasks of control, analysis and modeling of various processes and systems are solved using their mathematical models. The choice of a model is dictated by the conditions of implementation and the requirement of adequacy. The problem of developing control algorithms under uncertainty occupies one of the central places in modern control theory. To solve the arising problems of structural and parametric identification, as a rule, methods, and algorithms of the theory of adaptive control systems are used. The application of the principles of adaptation allows to ensure high accuracy of modeling with a significant change in the dynamic properties of the system under study, to unify individual subsystems and their blocks; reduce the development and debugging time of the system. This article discusses the problem of studying a digital adaptive system for turning the blades of guide apparatus (GA) of an axial compressor of gas turbine engines (GTE). The aim of the study is to increase the efficiency of the control system for turning the blades of the GA using a digital adaptive system. The gas-dynamic stability of the GTE axial compressor under changes in external conditions and engine throttling is provided by air bypass and turning the blades of the first and last compressor stages. The statement of the problem of synthesis of the system of adaptive control of the rotation of the blades of the GA compressor of the gas turbine engine is carried out. A mathematical model of the ACS in the canonical form “model of the system in the state space” and an algorithm for turning the blades of the GA using an adaptive PI-controller have been developed. Simulation modeling of ACS was carried out using the Matlab / Simulink software package. When implementing an adaptive PI-controller, the pressure deviation downstream of the compressor stage from the required value is reduced to 0.4 %.

Інтелектуальний регулятор запасу газодинамічної стійкості компресора авіаційного ГТД

The gas-dynamic stability of compressors of aircraft gas turbine engines is one of the most important conditions that determine their reliability and level of flight safety. Unstable operation of the compressor in the engine system (surge) leads to loss of thrust accompanied by an increase in gas temperature in front of the turbine and increased vibration because of large amplitudes of pressure pulsations and mass flow through the engine path. To improve the parameters of ACS aviation gas turbine engines are increasingly using regulators built using fuzzy logic algorithms. The implementation of fuzzy control algorithms differs from classical algorithms, which are based on the concept of feedback and reproduce a given functional dependence or differential equation. These functions are related to the qualitative assessment of system behavior, analysis of the current changing situation, and the selection of the most appropriate for the situation supervision of the gas turbine engine. This concept is called advanced management. ACS gas turbine engines with fuzzy regulators are nonlinear systems in which stable self-oscillations are possible. Approximate methods are used to solve the problems of analysis of periodic oscillations in nonlinear systems. Among them, the most developed in theoretical and methodological aspects is the method of harmonic linearization. The scientific problem is solved in the work – methods of synthesis of intelligent control system with the fuzzy regulator as a separate subsystem based on the method of harmonic linearization and design on its basis of fuzzy ACS reserve of gas-dynamic stability of aviation gas turbine engine. Based on the analysis of the principles of construction of fuzzy control systems, it is shown that the use of fuzzy logic provides a new approach to the design of control systems for aviation gas turbine engines in contrast to traditional control methods. It is shown that the fuzzy controller, as the only essentially nonlinear element when using numerical integration methods, can be harmonically linearized. Harmonic linearization allows using the oscillation index to assess the quality in the separate channels of fuzzy ACS aviation gas turbine engines. A fuzzy expert system has been developed for optimal adjustment of the functions of belonging of typical fuzzy regulators according to quality criteria to transients.

ОПРЕДЕЛЕНИЕ ПОЛОЖЕНИЯ ГРАНИЦЫ ОБЛАСТИ УСТОЙЧИВОЙ РАБОТЫ ОСЕВОГО МНОГОСТУПЕНЧАТОГО КОМПРЕССОРА ПРИ ВЫПОЛНЕНИИ РАСЧЕТНЫХ ИССЛЕДОВАНИЙ

The implementation of the high requirements for modern gas turbine engines depends on the perfection of their components. The reliability of the engine operation is largely determined by the compressor gas-dynamic stability margin. In this paper, a review and analysis of some of the most common approaches to determining the boundary of the compressor gas-dynamic stability region calculation are performed. Based on the analysis, it is shown that the diffuser factor and the equivalent diffuser coefficient are the most suitable for the practice of computational studies at the stage of designing and adjusting the compressor. However, different literary sources present different ranges of the limiting values of these criteria, corresponding to the boundary of gas-dynamic stability, which, in turn, can significantly affect the compressor stability margin. In this regard, it becomes necessary to determine the limiting values of the diffusivity criteria, which can be used both in the design of a compressor and in the construction of a model of a gas turbine engine using the proprietary description of blade machines. To analyze these criteria, we calculated a two-dimensional flow in an axial multistage compressor of a modern by-pass engine using the AxSym software package developed at the Department of Theory of Aircraft Engines of the National Aerospace University "KhAI". A comparison of the calculated data with the results of a physical experiment is carried out. The features of the change in the diffuser factor and the equivalent diffusion ratio depending on the compressor operating mode are established. Dependences which allow approximating the values of these criteria for different rotational speeds are proposed. Recommendations which specify the limiting values of the diffuser factor and the equivalent diffusion ratio for various compressor operating modes are presented.

Shear layer on a ventilated supercavity wall

The shear layer on a supercavity wall is the gas entrainment channel to wake flow for ventilated supercavitating flows with various tail closure modes. To thoroughly understand the gas loss mechanism and the relationship between the gas loss mechanism and dynamic stability of the flows, the flow structures and associated characteristics in the shear layer must be studied. The multi-fluid model used in our previous work is further applied to consider the turbulence of each phase in the flows and then validated again via effectiveness analyses by simulating the ventilated supercavitating flows, including an obvious bubbly wake around a body and the shear-layer characteristic scale of the ventilated supercavity around a single disk cavitator at a low speed. Based on this model, the critical transition criterion is inspected from two vortex tubes to the re-entrant flow closure modes corresponding to different supercavity gas loss mechanisms. The gas volume fraction used to define the supercavity wall and the potential flow solution of the flow velocity on the wall are also examined via simulations. The shear flows are analyzed to reveal the distribution laws of gas velocity in the shear layer under different gravity effects. The shear-layer thickness is calculated and the influence of gravity and water viscous force acting on the outer wall of the supercavity is assessed by analyzing the gaseous velocity distributions in the supercavity cross sections. The results also show that the thickness is sensitive to the inner body shape.

СИСТЕМАТИЗАЦІЯ І УЗАГАЛЬНЕННЯ ТЕОРЕТИЧНИХ ТА ЕКСПЕРИМЕНТАЛЬНИХ ДАНИХ ПО ЕФЕКТИВНОСТІ НАДРОТОРНОГО ПРИСТРОЮ В ОСЬОВОМУ КОМПРЕСОРІ

The systematization of physical flow models at the peripheral region of the rotors of axial compressor is carried out. Based on the experimental and numerical studies, the flow features in subsonic and transonic rotors are analyzed. Similar features of the flow near the wall at the periphery of subsonic and transonic rotors are formulated. The characteristic areas and individual features of the near-wall flow in them, which are obtained in experimental studies of the flow structure at the periphery of the blade rows, are reflected. The analysis of the influence of annular grooves in the axial compressor case on the flow in the airfoil channel of the subsonic and transonic rotors is presented. The hypothetical mechanism of the flow effect in the cavity of the annular groove on the main flow at the tip region of the airfoil channel of axial compressor rotor is described. An approach to generalize the experimental data of the axial compressor stages with the casing treatment based on the selected fundamental system of dimensionless parameters characterizing the main features of the flow at the rotor tip region are proposed. Using the approach, the dependences of the casing treatment effect on the gas-dynamic stability limit and efficiency are obtained. It was found, when Reynolds numbers ReΔr> 400 increase, the efficiency of annular groove casing treatment in the axial compressor wall on the gas-dynamic stability limit of the compressor decreases. The existence of the region of aerodynamic efficiency modes of the annular groove casing treatment in the case is shown. In this area, there is an optimal mode when the maximum effect of efficiency from install annular groove casing treatment is achieved. The obtained generalization al-lows us, at the step of making design decisions, to evaluate the effectiveness of the annular groove casing treatment in the case when it is used in subsonic and transonic rotors of the axial compressor stages.

AUTOMATION CONTROL SYSTEM OF TECHNICAL CONDITION OF GAS TURBINE ENGINE COMPRESSOR

The article is devoted to one of the approaches to the construction of an automated system for solving the problems of diagnostics and monitoring of the flow duct of aircraft gas turbine engines and gas turbine plants. Timely detection of faults and subsequent monitoring of their development in operation are possible thanks to automated systems for assessing the technical condition of engines. This is particularly relevant in operating conditions as the knowledge of the technical condition of the engine is necessary in any engine maintenance system allows to choose the content and timing of maintenance, repair of the flow duct of gas turbine engines and gas turbine plants, as well as commissioning. The engineering technique, which can be applied at performance of maintenance and at stages of tests and debugging of aircraft engines, is considered. The automated system implements a method of measuring the air flow through the compressor and a technique for assessing the technical condition of the compressor by the relative change in air flow. To determine the air flow rate through the gas turbine engine, it is sufficient to measure only static pressure values in the flow part. The static pressure receivers are not located in the flow part and do not obscure it, and thus do not affect the compressor gas dynamic stability margin. The inspection area is selected for measuring in the flow duct of the air intake. Static pressure in the maximum and minimum cross sections of the chosen area is measured; the maximum cross-section area of the flow duct, the total temperature of the air flow is measured outside the air intake. To determine the air flow rate, the functional dependence of the air flow rate on the static pressure is used. The algorithm for monitoring and diagnosing the operating condition of the engine is based on a comparison of the actual values of air flow rate with the air flow rate determined during the control tests or when using a mathematical model adapted for this gas turbine engine. The positive effect of the using of the proposed automated control system of technical condition is that the air flow rate measured under operating conditions will significantly increase the objectivity of the control of the operation and technical condition of the gas turbine engine.

About improvement of the scheme of correction of frequency characteristics of probes for measurement of pulsations of pressure in gte

It is known that pressure pulsations affect the performance of power plants, including a gas turbine engine, reducing the reserve of gas-dynamic stability of the compressor, creating conditions for oscillatory combustion in the combustion chamber. When fine-tuning the engine in the context of the pressure pulsations there is need for measuring means working at extreme temperatures and vibrations. The most acceptable of these means is a probe consisting of a supply waveguide, a pressure pulsation sensor and a corrective element in the form of a long line. However, in the low frequency region there is an uneven amplitude-frequency characteristic of the probe, due to the influence of the reflected pressure waves from the closed end of the long line. It is proposed to install an acoustic RC-filter with resistance equal to the wave resistance of the long line at the output of a long line. The paper theoretically shows that such an improvement of the probe allows to expand the frequency range of measurement of pressure pulsations. The material of the article will be useful for specialists who develop and use tools for measuring pressure fluctuations in extreme conditions, for example, when developing gas turbine engines.

Gas-Dynamic Stability and the Distribution of Gas Flow Rates over the Blast-Furnace Height

Abstract—The distribution of the gas flow rates over the blast-furnace height is studied. The best blast-furnace gas dynamics is determined by the range of gas-flow rates in the batch column—in particular, at the tuyere exit and in the batch-free charging zone. The velocity of the blast and the blast-furnace gas may be used to assess the stability and optimality of the gas-dynamic conditions in smelting.

ДІАГНОСТИКА ПЕРЕДПОМПАЖНОГО СТАНУ ОСЬОВОГО КОМПРЕСОРА З НАДРОТОРНИМ ПРИЛАДОМ

Ensuring the gas-dynamic stability of gas turbine engine, still remains one of the actual problem of modern aircraft engine building. To date, a significant number of studies have been conducted. This studies are the foundation for different techniques of diagnosing the pre-surge condition condition. Also known that one of the ways to solve the problem of increasing gas-dynamic stability is the use of casing treatment. However, there are no clearly formulated techniques that allow diagnosing the surge line in an axial compressor with casing treatment. Consequently, the problem of their development is relevant. The paper addresses the results of experimental and computational studies of unsteady flows in an axial compressor with the casing treatment in the form of annular grooves. Specific features of the transition to the regime of rotating stall (with break of the compressor characteristic) by an axial compressor with the annular grooves are formulated. The temporary conditions are provided for the formation of the reliable diagnostic signal of the axial compressor pre-surge operating mode with the annular grooves casing treatment by stabilization of the partial rotating stall condition were displayed. Calculation techniques for estimating informative criteria for detecting of the compressor pre-surge condition were proposed. One of them is the appearance of a discrete component of the amplitude-frequency characteristic of the total pressure pulsations (in the direction of rotation of the rotor) in the cavity of the groove of the casing treatment located above the leading edge of the blades. This discrete component appears at the frequency of a single-zone rotating stall, so it can serve as a trigger pulse for surge prevention automatic control system. The second informative criterion is the appearance of the gradientless region in the distribution of pressure on the bounding wall of the airgas channel above the blade. The position of the gradientless section in the pressure distribution on the bounding wall of the airgas channel above the blade with respect to the meridional projection of the chord of the tip end of the blade has been determined experimentally and by calculation according to the developed technique. Comparison of the proposed mathematical models with experimental data was performed. Their qualitative and quantitative conformity to the experiment were shown.

Increase the Efficiency of a Gas Turbine Unit for Gas Turbine Locomotives by Means of Steam Injection into the Flow Section

In the modern world of railways, electrification is given great importance. Currently, more than 50% of all freight traffic carried out by electric traction. However, today, about half of the railways are not electrified, so it is necessary to use thermal engines to drive the locomotive. One of the possible variant is use gas turbine unit. The power of the gas turbine unit is given to the electric generator, and the electric motors drive the locomotive. In the present paper, as a power plant of a gas turbine locomotive, considered gas turbine unit with a twin -shaft gas generator of two schemes: 1- with steam supply to the inlet of the high-pressure turbine (into the combustion chamber) and 2- with steam supply to inlet of the free turbine. By CAE system of ASTRA, Collaboration operation lines calculated for different variants of steam injection. When the steam injected into the inlet of a free turbine and a high-pressure turbine. in the case of steam supply to the input of the free turbine and the high-pressure turbine there is a significant shift in Collaboration operation lines, which can lead to a decrease in the gas-dynamical stability of the compressors, and efficiency. To maintain the position of Collaboration operation lines, was applied the correction of the throughput capacity of free turbine nozzle vanes (by 15%). In the case of steam supply to the inlet of a free turbine, to ensure gas-dynamic stability of the compressors, a change in the throughput capacity of its nozzle vanes is required.

ВЛИЯНИЕ ПЫЛЕВОЙ ЭРОЗИИ НА ГАЗОДИНАМИЧЕСКИЕ ХАРАКТЕРИСТИКИ ОСЕВОГО КОМПРЕССОРА ГТД

The results of flow simulation of an axial multi-stage compressor of a gas turbine engine are presented. The relation between the main gas-dynamic characteristics of the compressor and operating time in dusty atmosphere are established. The estimation of the maximum operating time of the engine in terms of loss of the gas-dynamic stability of the compressor is performed.

FLOW MODELLING AT COMPRESSOR CASCADE WITH TURBULATORS ON BLADE SURFACE

The research of boundary layer control at compressor blade row is actually. It is practical interest to solve the problem of providing of gas-dynamic stability of compressors of gas turbine engines. Aim of work is to investigate the effect of the placement density of turbulators on blade surface at the loss level at the critical flow mode. Flow at airfoil cascade with different placement densities is studied in work. Roughness elements were placed at the entrance of the blade witch was 30% of the surface area of the blade. The characteristics of airfoil cascades at placement density coefficient 0.36, 0.24, 0.18 were investigated. Shape of roughness element is hemisphere. Using of roughness elements at the entrance of the blade with a placement density coefficient 0.18…0.36 lead to a decreasing in value of total pressure loss coefficient at critical flow mode from 0.084…0.12 (for cascade with smooth blades) to 0.03…0.041

Pilot study of frequency characteristics of an acoustic probe for measuring pressure pulsations in the gas turbine engine inlet device

Special tests are carried out to determine the effect of heterogeneity of the air flow at the engine intake to the gas-dynamic stability margin of the compressor. There are acoustic probes mounted in the engine air flow meter manifold to measure pressure pulsations for this purpose. The probes are installed around the circumference of the collector at an angle of 90° relative to its axis. There is a differential pressure sensor installation in the acoustic probe due to the smallness of the amplitudes of velocity head pulsations in the manifold. Stagnated pulsed-flow of air is directed to the main input of the sensor and its damped component to the discharge chamber. In the paper on the basis of electrodynamics’ analogies the method of calculation of acoustic RC-damper to eliminate the constant and low frequency components of the flow speed pulsations is presented. To confirm compliance of the measuring probes characteristics to the design specification requirements acoustic probes’ frequency tests are conducted. Test results show that the developed acoustic probe can measure pressure fluctuations in the gas turbine engine inlet flow meter manifold with dynamic accuracy of ± 10% in the frequency range of 2-300 Hz.

A vibration analysis used in the methods of detecting unsteady operational modes of axial compressors

We have grounded the necessity to devise a system of a compressor surge prevention at the stage of its inception and analysed possible informative parameters characterizing an unsteady flow in a compressor and the possibility of a surge. We have presented the results of numerical simulation of the flow and dynamics of the loss of gas-dynamic stability in the inter-blade channel of the axial compressor impeller.It has been determined that the process of the loss of dynamic stability begins with the appearance of the initial vortex zone on the back of the blade profile. We have carried out an experimental research with measuring the pressure pulsation and vibration, revealed that the levels of pressure fluctuations on the impeller with a frequency of rotor blades can signal of the compressor stall, and pointed out the problems of technical use of this criterion. We have experimentally researched the connection of unstable flows in the flow part and vibration levels in the compressor housing. It is proved that the harmonics of the rotating stall that is determined by the level of vibrations is stable and significantly stands out from the background noise, so it can be used to diagnose the rotating stall as a signal of the surge.The paper contains an experimentally verified dependence for determining the frequency of the rotating stall. We have studied the possibility of using the median and band filters in processing the vibrations and suggested an algorithm for identifying a pre-surge compressor state by the rate of vibrations in the compressor housing, which is aimed at devising of automatic warning systems for the surge prevention at the stage of its inception. Devising a system of an anti-surge protection with the use of the vibration signal would increase the reliability of its work and raise the operational reliability of a gas turbine engine by eliminating false positives.

Influence research of profile chords ratio on aerodynamic characteristics of tandem compressor cascade

Efficiency improvement of gas turbine compressors is ensured by the extensive use of tandem blade rows in straightener blades of the last stages of multistage axial compressors. Their installation provides large flow deflection angles in the unstalled flow of blade rows and, consequently, a low level of losses in the design mode of the compressor. The problem of influence research of the first- and second-row blade chords ratio in tandem blade rows in a wide range of angles of attack is relevant and is of practical interest in developing recommendations for ensuring the gas-dynamic stability of gas turbine compressors. To solve the problem, the method of the computational experiment was used and characteristics of tandem compressor cascade with different first- and second-row blade chords ratios were calculated in the paper. The calculation results of the characteristics of tandem cascade were compared with the characteristics of an equivalent single compressor cascade. For the computational domain of the studied tandem and equivalent single cascades, small adaptive unstructured grid and the second-order design scheme with the local use of the first-order design scheme (High resolution) was applied. The Menter's SST model was used to solve the problems. The results showed that the aerodynamic characteristics of the tandem compressor cascades essentially depend on the first- and second-row blade chords ratio. With positive angles of attack, the quality parameter of tandem cascades is higher than that of single, with the first- and second-row blade chords ratio, corresponding to the position of the slot x щ =(0,3 ... 0,4)b ∑ . With negative angles of attack, the quality parameter of tandem cascades is higher than that of single, with the first- and second-row blade chords ratio, corresponding to the position of the slot x щ =(0,5 ... 0,7)b ∑ .

Early detection of vibration behaviour of aircraft engine multistage axial compressor in pre-surge states during rig testing

Surge is a dangerous aero-elastic phenomenon. When surge occurs a shock wave originates in the gas path of the compressor, which damages individual elements of the compressor, as well as the test beds for measuring the parameters of the workflow. Therefore, early diagnostics of pre-surge states of multistage axial compressors of modern aircraft engines characterized by extremely high aerodynamic loading and small radial clearances between the rotor and the stator is an urgent task. The paper presents the results of the analysis of dynamic processes that take place during throttling of aircraft engine multistage compressors in test bed conditions. Diagnostic features of the pre-surge state of the compressor were revealed in the signals from strain gauges mounted on the blades, and the signals from sensors of static pressure pulsations installed in the compressor housing. The research was carried out using up-to-date methods of processing and analysis of the signals of dynamic processes based on the fast Fourier transform and wavelet analysis using a 3D representation of the results of the study. The results of the study indicate the possibility of early diagnosis of surge, and also show that several forms of axial vibrations and their respective acoustic frequencies depending on the size of connected masses may occur in the case of loss of gas dynamic stability. The developed methods and means are proposed to be used for the creation of automated systems for preventing surge in compressor rig testing to determine the gas dynamic stability margin.

Pilot study of frequency characteristics of an acoustic probe for measuring pressure pulsations in the gas turbine engine inlet device

Special tests are carried out to determine the effect of heterogeneity of the air flow at the engine intake to the gas-dynamic stability margin of the compressor. There are acoustic probes mounted in the engine air flow meter manifold to measure pressure pulsations for this purpose. The probes are installed around the circumference of the collector at an angle of 90° relative to its axis. There is a differential pressure sensor installation in the acoustic probe due to the smallness of the amplitudes of velocity head pulsations in the manifold. Stagnated pulsed-flow of air is directed to the main input of the sensor and its damped component to the discharge chamber. In the paper on the basis of electrodynamics’ analogies the method of calculation of acoustic RC-damper to eliminate the constant and low frequency components of the flow speed pulsations is presented. To confirm compliance of the measuring probes characteristics to the design specification requirements acoustic probes’ frequency tests are conducted. Test results show that the developed acoustic probe can measure pressure fluctuations in the gas turbine engine inlet flow meter manifold with dynamic accuracy of ± 10% in the frequency range of 2-300 Hz.

Research of gas turbine engine operation in dust-filled atmosphere

The article describes the experimental data on the effect of erosion the turbomachines elements under theinfluence solids on the engine performance. The analysis was performed for the generator of compressed air forthe aggregate of cleaning pipelines and the gas jet treatment plants temporary technological railways. The aggregateof cleaning pipelines was operated under conditions of high concentrations of dust in the air - in the semidesert.Both plants are based on the converted aircraft twin-shaft turbojet engines. According to the model oferosive wear was set, that the depth of erosion depends on the dimension of the engines. Also shown is the abilityto detect impending failure of the engine working in a dusty atmosphere, based on trend analysis of its parametersusing various statistical smoothing options: simple linear trend, exponential trend, damped trend. Revealedthat the first of the considered parameters controlled workflow approach to engine failure reacts highpressure rotor speed. Timely detection of trends in parameters due to the development of a number of irreversibleprocesses will predict current and stable operation of the critical state of gas turbine engines, the correspondingloss of gasdynamic stability of the engine, so as to use the work on modes near with the marginal stability ofthe compressor is unacceptable.

Influence of setting angle of the airfoil cascade on the flow «choking» regimes in the blade channel

The emergence of “choking” regimes of last stages leads to the stall in the blade rows of first stages, appearance of rotating stall and surge, and is one of the main causes of reduced GTE effectiveness on the off-design operating conditions. The paper deals with investigating the impact of setting angle in the airfoil cascade at the critical flow regime. To solve this problem, a series of gas-dynamic calculations for compressor airfoil cascade with different setting angles γ=30º, γ=40º, γ=50º, γ=60º, γ=70º was carried out. The computational domain for this problem consists of one blade and one blade channel. To solve this problem, irregular adaptive grid was selected. The turbulent gas flow calculation was performed by numerical solution of the averaged Navier-Stokes equations (Reynolds equations). To close the Navier-Stokes equations, the Menter's SST turbulence model was chosen. The second-order design scheme with the local use of the first-order design scheme was used for the calculation. Comparison of theoretical and experimental research results have shown that the flow nature in the blade channels and boundary layer formation depend strongly on the airfoil setting angle. With a decrease in the airfoil setting angle there is a significant decrease in the relative value of the minimum actual flow area of blade channels (described in aerodynamics as free area), which leads to a decrease in the Mach number value at the cascade entrance, at which the flow “choking” regime in blade channels by air flow occurs. These features should be taken into account in “choking” regime calculations. The obtained generalized characteristics of “choking” regimes of compressor cascades can be used in calculating “choking” regimes of the axial flow compressor stages and determining the boundaries of gas-dynamic stability of multistage axial flow compressors.