Compressor Surge Margin Research Articles

Integrated modeling and coupling characteristics analysis of helicopter/engine/infrared suppressor

Abstract In order to explore the influence of the infrared stealth technology on the performance of the integrated helicopter/engine system, an integrated modeling method of helicopter/engine/infrared suppressor is proposed. Firstly, based on the power calculation model of the helicopter, combined with the high-precision turboshaft engine component-level model, an integrated simulation platform is built, which takes into account the nonlinear characteristics. Then, the aerodynamic characteristics of infrared suppressors under different engine operation states are studied by CFD numerical computation method, and the infrared radiation characteristics are obtained through combining the positive and negative ray tracing method and narrow band model method. Ultimately, the utilization of the power turbine outlet stagnation pressure is employed as the pivotal interface linking the turboshaft engine and the infrared suppressor in order to formulate an integrated model encompassing the helicopter, engine, and infrared suppressor subsystems. The simulation results demonstrate that compared with the conventional exhaust system, the application of the infrared suppressor greatly enhances the stealth performance of the helicopter, but also results in the unexpected decrease in engine output power. Moreover, the specific fuel consumption of the turboshaft engine increase and the compressor surge margin decreases in case of the consistent flight condition.

Experimental and numerical study on aerodynamic stability of compressor L-inlet duct

Total pressure loss and vortex pair are the two independent flow phenomena that affect the flow quality at the compressor L-inlet duct (CLID). Both flow phenomena do affect the compressor efficiency, aerodynamic stability and surge margin. However, no general report on the control of the two flow phenomena is found in the open literature. Therefore, in this study, a wind tunnel test bench is independently built to undertake the optical diagnostic test research of the CLID internal flow. Then, the internal flow in CLID is investigated with both the Particle Image Velocimetry (PIV) test and the numerical simulation. Furthermore, the effects of typical parameters on the CLID aerodynamic stability are investigated. Finally, based on the parametric sensitivity analysis, an orthogonal design method with array L16(45) is adopted to achieve the optimized CLID aerodynamic design. Results show that the designed wind tunnel test bench can successfully carry out the PIV test, and the test error is controlled below 1.5%. PIV test results show that two counter rotating vortex pairs are generated at compressor inlet case annular section P-a4 (φ=180°) and P-a5 (φ=225°). They are believed to affect the compressor surge margin. SC60 is validated to capture the vortex pairs, DC60 is validated to capture the total pressure loss. The larger the width (a/D4), depth (b/D4) and cone diameter (D0/D4), the better the CLID aerodynamic stability. The optimized CLID aerodynamic design is achieved by the L16(45) orthogonal design.

Improved Thrust Performance Optimization Method for UAVs Based on the Adaptive Margin Control Approach

This study proposes a strategy for improving the thrust performance of fixed-wing UAV turbine engines from the perspective of aircraft/engine integration. In the UAV engine control process, the inlet distortion caused by the angle of attack change is taken into account, the inlet distortion index is calculated in real time by predicting the angle of attack, and the influence of the inlet distortion on the engine model is analyzed mechanically. Then, the pressure ratio command is adjusted according to the new compressor surge margin requirement caused by the inlet distortion to finally improve the engine thrust performance. To verify the effectiveness of the algorithm, an adaptive disturbance rejection controller is designed for the flight control of a fixed-wing UAV to complete the simulation of horizontal acceleration. The simulation results show that, with this strategy, the UAV turbofan engine can improve the turbofan engine thrust performance by more than 8% under the safety conditions.

On the Effect of Side Clearance in the Vaned Diffuser of a Centrifugal Compressor

AbstractEnvironmental and regulatory pressures have prompted original equipment manufacturers (OEMs) to explore highly boosted internal combustion (IC) engines as a means to minimize toxic air pollutants and meet strict legislative targets. The exhaust gas turbocharger is a key technology used to achieve the boost pressures necessary for advanced engines of the future. Centrifugal compressors with vaneless diffusers have been widely implemented in turbochargers for IC engines, particularly passenger vehicles, primarily due to their compact size and ability to deliver high-pressure ratios over a wide range of flowrates. Methods to improve compressor efficiency near surge without reducing the surge margin have received considerable interest in the past. In many situations, changes to the impeller blade design and/or diffuser are limited by the need to meet the strict engine-rated power target. Such competing demands have encouraged designers to develop variable geometry compressors with the aim of improving near surge performance without sacrificing rated power performance. The on/off-type vaned diffuser is a variable geometry compressor capable of delivering a pressure ratio benefit near surge by utilizing a vaned diffuser rather than a vaneless diffuser. The vanes are retracted prior to the diffuser choking to form a vaneless diffuser in order to reach the engine-rated power condition. Diffuser vane-shaped clearances in the hub or the shroud end wall, termed side clearances, enable axial sliding variability of the vanes into and out of the diffuser passage. Unfortunately, the clearances manifest themselves as a discontinuity in the flow field which gives rise to an efficiency penalty. It is not yet clear how the side clearance geometry and position affect the compressor performance i.e., efficiency and stability. Hence, a numerical and experimental investigation was carried out to determine the impact of side clearance geometry and end wall position on compressor performance. It was found that a side clearance positioned at the shroud end wall reduced the surge margin of the investigated compressor for both types of clearance geometry considered. In contrast to this, a clearance positioned at the hub end wall improved the compressor surge margin but had the lowest efficiency benefit near surge. An experimentally validated Computational Fluid Dynamics (CFD) model was used to inspect the flow field and to elucidate the reasons behind the changes in compressor performance.

Experimental investigations on effects of the self-circulation casing treatment on acoustic and surge characteristics in a centrifugal compressor

Self-recirculation casing treatment is an effective technique to improve centrifugal compressor surge margin. This paper experimentally investigates the impacts of casing treatment on a centrifugal compressor using experimental methods and focuses on aerodynamic performance, surge trigger, surge cycles, and inlet acoustic characteristics. The measuring data were obtained from fast dynamic pressure transducer and microphones, and temporarily and spectrally analyzed to show surge inception and noise variation. The experimental results lead to the conclusions that (1) two separate phenomena trigger compressor to surge with different surge frequencies in the compressor with and without the casing treatment, and (2) the casing treatment suppresses the broadband noise sources but enhances the discrete frequency noise sources during surge cycles.

Optimization of ACE mode transition control schedule considering geometric adjustment speed

Abstract Aiming to optimize the mode transition control schedule of the adaptive cycle engine (ACE), an optimization method based on a gradient algorithm was proposed. During the mode transition, the compressor surge margin, the total turbine inlet temperature and the combustion chamber fuel-air ratio are not exceeded. The integration of thrust by time was selected as the optimization objective. The geometric parameters were updated via the optimization target derivative. The speed limit of the geometric adjustment mechanism was also considered in the optimization process. The control schedules of two mode transition processes, from triple to double bypass mode and from double to triple-bypass mode were optimized. The optimization method presented in this paper can be applied to various mode transitions and the geometric adjustment speed meets all the constraints.

Optimization of ACE mode transition control schedule considering geometric adjustment speed

Abstract Aiming to optimize the mode transition control schedule of the adaptive cycle engine (ACE), an optimization method based on a gradient algorithm was proposed. During the mode transition, the compressor surge margin, the total turbine inlet temperature and the combustion chamber fuel-air ratio are not exceeded. The integration of thrust by time was selected as the optimization objective. The geometric parameters were updated via the optimization target derivative. The speed limit of the geometric adjustment mechanism was also considered in the optimization process. The control schedules of two mode transition processes, from triple to double bypass mode and from double to triple-bypass mode were optimized. The optimization method presented in this paper can be applied to various mode transitions and the geometric adjustment speed meets all the constraints.

Investigation on Broadening Compressor Surge Margin by Using Active Magnetic Bearing

The impeller design usually expects to achieve high efficiency and a wide working range while they are influenced by tip clearance in different ways. In this study, a blower with a controllable magnetic suspension bearing was studied. The magnetic bearing enables the impeller to adjust the tip clearance during operation to optimize the efficiency of the blower at the design point and significantly expand the surge margin when it deviates from the design point. The clearance ratio (CR) is defined as the ratio of tip clearance over impeller outlet width. The computational fluid dynamics (CFD) method is used to study the performance of a centrifugal impeller under CR = 2.61%∼7.83% and verified by experiments. The results show that the pressure ratio and efficiency of the impeller at the design point decreased linearly with the increase of the gap. For each 1% increment in CR, the pressure ratio reduces by 0.02 and the polytropic compression efficiency declines by 0.5%. Conversely, the surge margin increases with the expansion of CR. For every 1% rise in CR, the surge margin would improve by 7.8%. A control method of changing the reference suspension position according to the optimal efficiency fitting model is proposed. It realizes the real-time control of the axial suspension position of the impeller according to the changes of pressure and flow rate signals to obtain the optimal work efficiency with a broad surge margin. Experimental results show that using an impeller equipped with a controllable magnetic suspension bearing could minimize the gap stably in the nonsurge area to obtain high efficiency, and when necessary, it could enlarge the gap to meet the low flow rate condition.

An Optimal Speed Control Method of Multiple Turboshaft Engines Based on Sequence Shifting Control Algorithm

Abstract In order to overcome the problem of significant drop in operational efficiency remarkably while power turbine speed varies among a large range, an optimal speed control method of multiple turboshaft engines based on sequential shifting control (SSC) algorithm is proposed. Firstly, combined with multispeed gearboxes, a sequential shifting control algorithm of multiple turboshaft engines is proposed and designed to accomplish continuously variable speed control. Then, selecting the minimum engine fuel flow as the optimization objective, an integrated optimization method of optimal speed based on multiple engines and multispeed gearboxes is proposed to promote the operational economy. Finally, the simulation tests of the optimal speed control method of twin and triple turboshaft engines are conducted separately. The results demonstrate that the optimal speed control method of multiple turboshaft engines based on SSC algorithm can change the power turbine speeds by no more than 7% and the main rotor speed by over 8% simultaneously. In addition, compared with the fixed-ratio transmission (FRT), engine fuel flows decrease by more than 2% under different cruise states. It proves that the optimal speed control method is beneficial to obtain more superior overall performances of the integrated helicopter/multi-engine system without considerable loss of compressor surge margin.

Modelling and performance analysis of a recuperated gas turbine with low-Btu fuel

• A gas turbine model was estabilished considering recuperator, fuel, and generator. • The recuperator is the dominant component for thermal inertia of smallgas turbine. • 1-D heat soakage model was more proper for radial rotor than 0-D lumped model. • The thermal inertia and volume effect are good for gas turbine with sudden load change. • Overspeed in load shedding and surge in deceleration need to be studied case by case. Low-Btu fuels such as land fill gas and bio-gas are good alternatives to high priced natural gas. Due to the limited production of low-Btu fuel, small gas turbines are widely applied in such applications. For higher power efficiency at full and part load conditions, the small gas turbine is always equipped with a recuperator and a high-speed generator. Due to low shaft inertia, a small gas turbine’s speed response is quick. Therefore, for standard simple cycle gas turbine, control lags from instruments, actuators, thermal inertia, or volume effect may induce control instabilities during transient loading, and resulting in over speed, flow instabilities, and even mechanical failure. However, for recuperated engine burning low BTU fuel, other factors including the large volume of the combustor and recuperator, the thermal inertia of the rotor and recuperator, lags in the fuel valve lags, and efficiency changes caused by rectifiers of the high-speed generators could all induce visible changes and lags in the engine’s transient operations. This paper summarizes the influences of the gas turbine engine itself, the recuperator, and the high-speed generator in one model. It is believed that this is the first time that all these effects and interactions have been analysed together. Heat soakage, volume dynamics, recuperator transient characteristics, and the rectifier model were combined and tuned in detail to understand the separate effects. From the analysis of transient cases such as, load shedding and acceleration, conclusions are drawn made regarding overspeed and recovery time. The recuperator thermal inertia, low-Btu fuel, and combustor and recuperator volume effect interact with each other to produce more than three times larger overspeed or nearly five times more recovery time compared to a natural gas engine with recuperator. The fuel valve with a 0.02 s-time constant was found to be adequate even for the most severe case described in this paper. The recuperated gas turbine did not necessarily lead to lower compressor surge margin and should be investigated on a case-by-case basis. Rectifier system efficiency variation should be taken into account especially for the acceleration process. To convert a natural gas turbine engine into low-Btu gas turbine, the matching point of the gas turbine components needs to be carefully selected for efficiency, surge margin, and transient manoeuvre behaviour.

Experimental and Numerical Assessment of the Impact of an Integrated Active Pre-Swirl Generator on Turbocharger Compressor Performance and Operating Range

The implementation of increasingly stringent emissions and efficiency targets has seen engine downsizing and other complementary technologies increase in prevalence throughout the automotive sector. In order to facilitate ongoing improvements associated with the use of these strategies, delivering enhancements to the performance and stability of the turbocharger compressor when operating at low mass flow rates is of paramount importance. In spite of this, a few concepts (either active or passive) targeting such aims have successfully transitioned into use in automotive turbochargers, due primarily to the requirement for a very wide compressor-operating range. In order to overcome the operational limitations associated with existing pre-swirl generation devices such as inlet guide vanes, this study developed a concept comprising of an electrically driven axial fan mounted upstream of a standard automotive turbocharger centrifugal compressor. Rather than targeting a direct contribution to compressor boost pressure, the fan was designed to act as a variable pre-swirl generation device capable of being operated completely independently of the centrifugal impeller. It was envisioned that this architecture would allow efficient generation of the large pre-swirl angles needed for compressor surge margin extension and efficiency enhancement at low mass flow rate-operating points, while also facilitating the delivery of zero pre-swirl at higher mass flow rates to ensure no detrimental impact on performance at the rated power point of the engine. Having progressed through 1-D and 3-D aerodynamic modelling phases to understand the potential of the system, detailed component design and hardware manufacture were completed to enable an extensive experimental test campaign to be conducted. The experimental results were scrutinized to validate the numerical findings and to test the surge margin extension potential of the device. Compressor efficiency improvements of up to 3.0% pts were witnessed at the target-operating conditions.

Designing method of acceleration and deceleration control schedule for variable cycle engine

Studies show that different geometries of a Variable Cycle Engine (VCE) can be adjusted during the transient stage of the engine operation to improve the engine performance. However, this improvement increases the complexity of the acceleration and deceleration control schedule. In order to resolve this problem, the Transient-state Reverse Method (TRM) is established in the present study based on the Steady-state Reverse Method (SRM) and the Virtual Power Extraction Method (VPEM). The state factors in the component-based engine performance models are replaced by variable geometry parameters to establish the TRM for a double bypass VCE. Obtained results are compared with the conventional component-based model from different aspects, including the accuracy and the convergence rate. The TRM is then employed to optimize the control schedule of a VCE. Obtained results show that the accuracy and the convergence rate of the proposed method are consistent with that of the conventional model. On the other hand, it is found that the new-model-optimized control schedules reduce the acceleration and deceleration time by 45% and 54%, respectively. Meanwhile, the surge margin of compressors, fuel–air ratio and the turbine inlet temperature maintained are within the acceptable criteria. It is concluded that the proposed TRM is a powerful method to design the acceleration and deceleration control schedule of the VCE.

The modeling design and analysis of centrifugal compressor

Objectives As a hot topic in the field of turbomachinery, centrifugal compressor has a huge impact on gas turbine general performance. In order to achieve the new compressor design with excellent performance under relatively few test conditions, the high efficiency and low cost modeling design method can be adopted. Methods In this paper, a new centrifugal compressor based on similarity principles with pressure ratio of 4.42 and efficiency of 79.19% was designed by modeling a prototype of a high-performance centrifugal compressor. NUMECA software was used to mesh and simulate the three-dimensional flow field of the centrifugal compressor, and the new compressor's main performance parameters and those of the prototype were compared at 80%, 90%, 100% and 110% rotation speeds. Results The results showed that at rotation speed settings of under 100%, the new compressor's surge margin is 5.48% higher than the prototype, but reduced by 1% at other speed settings. The new compressor's pressure ratio and efficiency are slightly improved at low flow conditions when compared to the prototype. However, at high flow conditions, the pressure ratio and efficiency of both compressors are significantly reduced. Conclusions The numerical simulation results show that the general performance of the new compressor has reached the performance level of the prototype compressor. The research results can be used as a reference for the modeling design of a centrifugal compressor.

A Dual-Channel-Enhanced Power Generation Architecture With Back-to-Back Converter for MEA Application

This article proposes an improved power generation architecture (PGA) for future more electric aircraft applications. In the proposed architecture, a starter/generator is connected to the high-pressure (HP) shaft, and a generator is connected to the low-pressure (LP) shaft. Their outputs supply a common dc bus via active power converters. A back-to-back (B2B) converter is deployed to link the ac terminals of the two generators. The proposed topology embraces three main advantages. First, with the B2B converter, the HP generator can operate at a high speed without flux weakening; thus, the magnitude of stator current will be decreased when output same active power. This will lead to the reduction of power losses on the generators and the active rectifiers. Second, the proposed PGA allows power transfer from the LP and to the HP shafts. This can potentially reduce the fuel consumption and increase aircraft engine compressor surge margins when the engine is at low-speed setting. Third, the B2B converter provides an additional power flow path to the generators under converter fault scenarios, hence improving the postfault operation ability. For the proposed PGA, engine benefits, modeling, control design, and efficiency improvements are illustrated in detail. The control performances of the proposed PGA, engine performance improvement by transferring power from the LP to HP shaft, and power loss reduction are verified via simulations and experimental results collected from a twin-shaft aircraft power generation test rig.

Investigation of variable orifice plate design for centrifugal compressor low-end performance improvement

In the present paper, a compressor with variable inlet configuration upstream of the compressor wheel was investigated to improve the compressor low-end performance. As a concept study, the variable inlet configuration was assumed to be a variable orifice plate which can reduce the compressor inlet flow area when it is activated. The compressor performance with and without the initial orifice plate was assessed quantitatively using both numerical and experimental methods. The results indicated that the initial orifice plate design can improve the compressor efficiency at low speed and flow rate condition. Different orifice plate designs with different setting angles to the compressor inlet main flow direction and different radial heights were also numerically studied. It was found that: (1) changing the orifice plate setting angle from 90° for the initial design to 67° can improve compressor efficiency about 0.56% at low flow rate condition; (2) increasing the orifice plate radial height from 16% of the compressor inlet radius for the initial design to 27% can improve the compressor efficiency by 1.78% at near surge condition and extend surge margin by 9%. The results indicated that the radial height has larger effects on compressor performance than the setting angle at near surge flow rate. While increasing the orifice plate height variation can further improve compressor low-end efficiency and surge margin, there is an increased complexity of the actuation and control system.

Off-design performance improvement of twin-shaft gas turbine by variable geometry turbine and compressor besides fuel control

A novel procedure for finding the optimum values of design parameters of industrial twin-shaft gas turbines at various ambient temperatures is presented here. This paper focuses on being off design due to various ambient temperatures. The gas turbine modeling is performed by applying compressor and turbine characteristic maps and using thermodynamic matching method. The gas turbine power output is selected as an objective function in optimization procedure with genetic algorithm. Design parameters are compressor inlet guide vane angle, turbine exit temperature, and power turbine inlet nozzle guide vane angle. The novel constrains in optimization are compressor surge margin and turbine blade life cycle. A trained neural network is used for life cycle estimation of high pressure (gas generator) turbine blades. Results for optimum values for nozzle guide vane/inlet guide vane (23°/27°–27°/6°) in ambient temperature range of 25–45 ℃ provided higher net power output (3–4.3%) and more secured compressor surge margin in comparison with that for gas turbines control by turbine exit temperature. Gas turbines thermal efficiency also increased from 0.09 to 0.34% (while the gas generator turbine first rotor blade creep life cycle was kept almost constant about 40,000 h). Meanwhile, the averaged values for turbine exit temperature/turbine inlet temperature changed from 831.2/1475 to 823/1471°K, respectively, which shows about 1% decrease in turbine exit temperature and 0.3% decrease in turbine inlet temperature.

Performance analysis and optimized control strategy for a three-shaft, recuperated gas turbine with power turbine variable area nozzle

Performance analysis and optimization for a three-shaft, recuperated gas turbine with power turbine variable area nozzle (VAN) is conducted in this paper. A modified recuperator simulation model is obtained from recuperator experiment. A gas turbine performance simulation program is established. The influence of power turbine shaft speed on engine performance is studied and optimized control of power turbine shaft speed is proposed. Comparative performance analysis proves VAN angle control improves engine performance, but leads to less compressor surge margin and increased risk of over temperature. The optimized control of VAN angle under different safety operation temperature limits and ambient temperature is investigated. Power turbine shaft speed barely influences the optimized control of VAN angle, but temperature limits and ambient temperature greatly do. By decoupling power turbine shaft speed and VAN angle, an optimized control strategy for power turbine shaft speed and VAN angle is proposed, which brings 6.37%, 15.88%, 47.80% increases in output power, and 10.84%, 25.59%, 64.97% increase in thermal efficiency when relative high-pressure shaft speed is 0.95, 0.90 and 0.85, respectively. Maximum thermal efficiency could be achieved at part-load conditions rather than design-point if temperature limit is relaxed.

Off-design performance analysis of hybridised aircraft gas turbine

ABSTRACTAn advanced geared turbofan with year 2035 technology level assumptions was established and used for the hybridisation study in this paper. By boosting the low-speed shaft of the turbofan with electrical power through the accessory gearbox, a parallel hybrid concept was set up. Focusing on the off-design performance of the hybridised gas turbine, electrical power input to the shaft, defined as positive hybridisation in this context, generally moves the compressor operation towards surge. On the other hand, the negative hybridisation, which is to reverse the power flow direction can improve the part-load operations of the turbofan and minimise the use of compressor handling bleeds. For the pre-defined mission given in the paper, negative hybridisation of descent, approach and landing, and taxi operations with 580 kW, 240 kW and 650 kW, respectively was found sufficient to keep a minimum compressor surge margin requirement without handling bleed.Looking at the hybridisation of key operating points, boosting the cruise operation of the baseline geared turbofan is, however, detrimental to the engine efficiency as it is pushing the cruise operation further away from the energy optimal design point. Without major modifications to the engine design, the benefit of the hybridisation appears primarily at the thermomechanical design point, the hot-day take-off. With the constraint of the turbine blade metal temperature in mind, a 500kW positive hybridisation at hot-day take-off gave cruise specific fuel consumption (SFC) reduction up to 0.5%, mainly because of reduced cooling flow requirement. Through the introduction of typical electrical power system performance characteristics and engine performance exchange rates, a first principles assessment is illustrated. By applying the strategies discussed in the paper, a 3% reduction in block fuel burn can be expected, if a higher power density electrical power system can be achieved.

Effect of fuel composition of LNG on the performances and operating characteristics of a gas turbine cycle

The effect of natural gas compositions on the thermodynamic performance characteristics of the single-shaft gas turbine cycle is analyzed in this study. The fuel characteristics are signified by H/C ratio and heating value, then the system are analyzed, and the full off-design analysis is performed by using TIT, TET, and power control. As the heating value of natural gas decreases, power output increases due to the increased mass flow passing through the turbine for all control methods. Additionally, regardless of the control method, the compressor surge margin is reduced and the turbine blade temperature is raised, making the gas turbine operation extremely vulnerable. Most parameters are sensitively affected by the HC ratio of fuel with TIT control. When the power control is used, gas turbine efficiency is reduced due to the increased operation of the compressor. The results are expressed by polynomial equations relative to each design value. In terms of the performance curves of the gas turbine cycle, the trends of most parameters accord with high H/C ratio and low heating value.

Compressor Instability Analysis Within a Hybrid System Subject to Cycle Uncertainties

The dynamic modeling of energy systems can be used for different purposes, obtaining important information both for the design phase and control system strategies, increasing the confidence during experimental phase. Such analysis in dynamic conditions is generally performed considering fixed values for both geometrical and operational parameters such as volumes, orifices, but also initial temperatures, pressure. However, such characteristics are often subject to uncertainty, either because they are not known accurately or because they may depend on the operating conditions at the beginning of the relevant transient. With focus on a gas turbine fuel cell hybrid system (HS), compressor surge may or may not occur during transients, depending on the aforementioned cycle characteristics; hence, compressor surge events are affected by uncertainty. In this paper, a stochastic analysis was performed taking into account an emergency shut-down (ESD) in a fuel cell gas turbine HS, modeled with TRANSEO, a deterministic tool for the dynamic simulations. The aim of the paper is to identify the main parameters that impact on compressor surge margin. The stochastic analysis was performed through the response sensitivity analysis (RSA) method, a sensitivity-based approximation approach that overcomes the computational burden of sampling methods. The results show that the minimum surge margin occurs in two different ranges of rotational speed: a high-speed range and a low-speed range. The temperature and geometrical characteristics of the pressure vessel, where the fuel cell is installed, are the two main parameters that affect the surge margin during an emergency shut down.