Low Pressure Compressor Research Articles

A Perspective on the Process and Turbomachinery Design of Compressed Air Energy Storage Systems

Abstract The present paper will describe the Baker Hughes experience in the development of the turbomachinery equipment for Hydrostor's advanced compressed air energy storage (A-CAES) system. At the core of a compressed air energy storage (CAES) plant, there is an air compressing system, followed by an air expander used to recover the stored energy. To achieve a reliable and effective solution, the expander is obtained from the architecture of Baker Hughes steam turbines, which was adapted to match the specific process needs. The criticalities that were addressed and solved to derive the expanders from the original steam turbine are presented. Specifically, the paper describes the activities performed to optimize the inlet and exhaust sections of each segment, the development of the blades for high atmospheric volume flow, and the implications that thermal transients have on the machine reliability. The inlet and exhaust sections were arranged according to the layout constraints, which were set to mitigate the effects of the thermal stresses and to reduce the weight to facilitate the machine transportation, while maintaining high aero-performance. As for the expander, a single-body configuration was selected to optimize capital expenditures and reduce leakage to atmosphere. A new set of blades derived from Baker Hughes's Steam Turbine stages were developed. This new stage is characterized by rotating blades with high radius ratio (HRR); therefore, an optimization strategy was adopted to include the mechanical constraints from the beginning of the design cycle and obtain a final geometry that can be used with different flow path and operating conditions. Finally, the three-dimensional full Navier Stokes Computational fluid dynamics analysis was used to assess the performance of the new stages in nominal and off-design conditions. A detailed analysis of the thermal transient of the expander parts with finite element analysis methods was performed to assess the life expectations of the equipment. The finite element analysis results are discussed in the paper to show the capability of the machine to sustain an extremely fast start up sequence. Compressor train is characterized by a multiple body configuration. A detailed optimization was performed to improve the efficiency and the availability of the selected solution. The low-pressure axial compressor discharge section has been optimized to reduce losses and a detailed study of rotor has been performed to evaluate the capability to withstand a high number of start and stop cycles.

Validation of the Numerical Simulation of Rotor/Stator Interactions in Aircraft Engine Low-Pressure Compressors

Validation of the Numerical Simulation of Rotor/Stator Interactions in Aircraft Engine Low-Pressure Compressors

High-temperature fretting wear behavior and microstructure stability of a laser-cladding Ti-Al-C-N composite coating meditated by variable cycle conditions

Ti-6Al-4 V titanium alloy is used for low-pressure compressor blades of aero-engine due to its high specific strength, excellent corrosion resistance and high-temperature stability. Low-pressure compressor blades are subject to fretting wear in service, which may lead to fracture failure of the blades. The preparation of wear-resistant coatings is currently an effective solution in solving the problem of fretting wear on Ti-6Al-4 V titanium alloy. However, the fretting wear resistance and microstructure stability of the coating under variable cycle fretting conditions require more attention. In this work, the fretting wear performance and microstructure stability of the Ti-Al-C-N composite coating under variable cycle fretting conditions are investigated. The results show that the parameters of variable cycle operating conditions have a notable effect on the coefficient of friction (COF). The variable frequency and variable temperature working conditions result in the alteration of the fretting operation mechanism. Whereas the variable stroke amplitude and the variable load conditions do not cause changes in the fretting operation mechanism, and both are in the gross slip region (GSR). The lowest wear rate occurs under variable stroke amplitude conditions. The wear rate is the maximum under the variable cycle temperature condition. According to the dissipated energy calculation, the difference between the two different stroke amplitudes is the biggest among all the conditions. EBSD and TEM analyses show that the subsurface of variable stroke amplitude worn scar has a more uniform strain distribution, and reveals a unique subsurface structure with a transformation from crystalline to amorphous, which contributes to improving the wear resistance. This work provides insights into coating microstructure stability and wear mechanisms under variable cycle conditions.

The influence of heat treatment on AlSi-polymer abradable coating

Abstract Abradable coatings, as one of the most important technologies for aero and industry gas turbine engines, have an extremely significant influence on engine performance. The macro-hardness of AlSi-Polyester for low-pressure compressor and as-sprayed coating degradation mechanism has been investigated by heat treatment process in imitation service condition. SEM (Scanning Electron Microscope) and OM (Optical Microscope) were used to observe the morphology of powder and coating microstructure in cross-section, respectively. Rockwell hardness tester was used to measure the macrohardness of coatings before and after heat treatment. It shows that AlSi-Polyester powder has an approximately spherical shape and uniform microstructure with alloy phase and polyester phase distribution in the coating prepared by air plasma spraying. Oxidation of the alloy phase and reduced polyester, as well as macrohardness in the as-sprayed coating, occur after the heat treatment process.

Interactions of row blades of a marine compressor based on POD analysis

The complex internal flow of the marine low-pressure compressor (LPC) is characterized by a series of unsteady flow structures, presenting extensive temporal and spatial features that pose challenges to direct data analysis. This paper employs the Unsteady Reynolds-averaged Navier Stokes (URANS) method to simulate a marine 1.5-stage LPC with full-channel configuration. Validation of the compressor’s overall characteristics and the unsteady pressure is achieved through comparison with experimental data. Additionally, the Proper Orthogonal Decomposition (POD) method is applied to decompose the velocity and pressure fields in various computational regions. The results demonstrate that the combined use of URANS and POD facilitates detailed insights into blade interactions. The consistency between time-averaged variables and POD modes underscores the practical physical significance of the POD modes. Furthermore, the study reveals that the Rotor-Stator interaction significantly outweighs the Inlet Guide Vane (IGV)-Rotor interaction. The coherent modal pairs generated by different interferences exhibit diverse characteristics within the computational domain, with distinct frequencies observed for the same interference reaction in the upstream and downstream regions. Notably, the POD modes of the rotor pressure field unveil a separation bubble structure.

Investigation of an Inter-Compressor S-Duct Using the Lattice Boltzmann Method

Abstract This article presents the study of a subsonic inter-compressor S-duct. Numerical simulations are performed using large-eddy simulation (LES) based on a compressible hybrid thermal lattice Boltzmann method (LBM) implemented within the ProLB solver. Comparisons are made between the LES–LBM results, Reynolds-averaged Navier–Stokes (RANS) computations, and experimental measurements on a representative S-duct taken from the European project AIDA. Several cases with increasing complexity are addressed where the different rows surrounding the duct are gradually included in the computations. The effects of each row on the flow field development and loss levels are studied. The goal is to evaluate the ability of the LES–LBM to recover the aerodynamic behavior and the total pressure loss evolution within the duct. Results show that the LES–LBM retrieves the correct flow evolution inside the S-duct compared to the experiment and previous RANS results. The case where the upstream stator row or the low-pressure compressor stage is integrated shows an increase in total pressure loss, as previously observed in the literature, and a more developed flow field with complex flow features contributing to the loss generation. To further analyze the loss mechanism, an entropy-based approach is presented and highlights that most losses are generated close to the hub wall due to the migration of the upstream stator wakes.

Loss model of low pressure compressor for mechanical vapor recompression system

Abstract Low pressure compressor (LPC) serves the role of enhancing pressure levels of secondary steam at nominal pressure ratios (1.5), procured from the evaporator as a primary thermal energy source. A loss model has been proficiently proposed to rapidly predict the performance of the LPC. Instead of conventional lump sum loss equations, the model adopts well-established one-dimensional (1D) loss formulations scattered across scholarly publications. Given an explicit configuration of the LPC, the environmental conditions, rotational velocity, and capacity, static and total temperatures, static and total pressures, pressure reductions, polytropic head, and efficiencies of each individual segment of the compressor can be calculated. Validation of this model was done using computational fluid dynamics (CFD) analysis. This model renders it feasible to conduct a parametric examination of the impact of each loss apparatus on the LPC’s capability, thereby aiding designers in substantiating design choices that minimize these losses and consequently positively influencing the comprehensive efficiency.

Modelling the nonlinear system performance of hybrid-electric propulsion systems with aerothermodynamic interdependencies

The development of commercial aircraft with hybrid-electric propulsion systems is currently a subject of extensive research in order to improve local air quality and reduce combustion emissions. Among the various types of engines being studied, the two-spool parallel hybrid-electric turbofan engine is particularly challenging due to the low-pressure compressor (LPC). The hybridisation process tends to throttle the LPC, accentuating its significance in the propulsion system. For reliable operation of such systems, accurate predictions of the LPC performance during time-sensitive manoeuvres such as a go-around are important. These manoeuvres are heavily influenced by time-dependent effects that govern the propulsion system’s performance. Often, the aerothermodynamic interplay between these effects is overlooked in propulsion models. In this study, the influence of these aerothermodynamic interdependencies on the modelling results is investigated. To investigate these aerothermodynamic interactions, a dynamic model is developed to simulate the performance of the hybrid-electric turbofan engine. In comparison, a constant mass flow model is used, which is not able to simulate these interdependencies. The results show that the aerothermodynamic interdependencies significantly affect the modelled time-resolved performance, especially for surge margin of the LPC, with this effect becoming more pronounced at higher levels of hybridisation. Therefore, the study recommends the adoption of dynamic simulation methodologies for hybrid-electric engines to guarantee high simulation precision, enhance reliability, and satisfy safety standards.

Operational Strategies of Two-Spool Micro Gas Turbine With Alternative Fuels: A Performance Assessment

Abstract Micro gas turbines (mGT) have not yet succeeded in conquering the small-scale combined heat and power (CHP) market. One reason is that their electrical efficiency is not high enough to maintain a cost-effective operation. A two-shaft intercooled mGT has the potential to meet the current market demand. This technology maintains a high electrical efficiency even at part-load and coupled with its fuel-flexible combustion chamber, it is an ideal candidate for CHP concepts in a renewable future. In this paper, performance analysis on two-spool mGT is carried out with various fuel blends. Attention is given to the low-pressure and high-pressure compressors and the variation of surge margin by adding hydrogen and syngas. Two control strategies for the mGT are adopted. In the first scenario, the two shafts have equal rotational speeds while in the second, the speeds are controlled independently. As the engine is operated at equal speeds, the maximum performance with 100 vol. % of syngas is observed at 85% of the nominal load while 100 vol. % of hydrogen shows maximum efficiency at a load of 63.7%. At electric power lower than 60% and for high amounts of syngas in natural gas, the low-pressure compressor (LPC) operates closely to surge line. In the second scenario, the efficiency increases as the load decreases and the LPC runs in an efficient and safe operating region. Additionally, the amount of nitrogen in syngas affects the part-load performance of the two-spool mGT.

Integrated Design of a Variable Cycle Engine and Aircraft Thermal Management System

Abstract The integrated design of a variable cycle engine (VCE) and an aircraft thermal management system (TMS) is investigated. The integrated system is designed using the multiple design point approach in order to achieve required performance metrics at points other than the cycle design condition. The VCE architecture is a three stream design where the third stream is split off after the fan, exhausting through a separate third-stream nozzle. The primary air stream passes through a low-pressure compressor before splitting into an inner bypass stream and a core stream. The inner streams mix aft of the low-pressure turbine and exhaust through a core nozzle. The variable cycle engine utilizes variable compressor inlet guide vanes, a variable area bypass injector at the core stream mixing plane, and variable throats in the two exhaust nozzles. The TMS architecture is an air cycle system using air bled from the high-pressure compressor. The effect of integrating the TMS into the engine design loop is investigated. A comparison is made to prior studies where the same TMS architecture was connected to a low bypass ratio turbofan engine. The comparison shows that the variable cycle engine is able to improve heat dissipation capability versus a ram air cooled system, while eliminating the airframe integration impact that comes with a separate ram-air stream. Lastly, the impact of modulating the variable geometry features on overall cooling capability is investigated. Results are presented for individual operating points as well as at the aircraft mission level.

Modeling and optimization of two-stage compression heat pump system for cold climate applications

When an air source heat pump runs at very low ambient temperatures, several issues could arise and thus limit its applications. These issues include lower coefficient of performance (COP), deteriorated heat output and higher compressor discharge temperature resulting from an increased pressure ratio. Therefore, there is a need to develop more efficient heat pump systems for low ambient temperature (∼-30 °C) applications. Under this circumstance, it becomes advantageous to use devices in which the compression of refrigerant vapor occurs in two or more stages. Two-stage vapor compression heat pumps can operate more efficiently at a greater difference in temperatures between evaporator and condenser and improve their performance under cold climate conditions. In this study, a model for a two-stage compression heat pump system is developed with the system configuration containing a closed economizer cycle and inter-stage refrigerant injection. In the two-stage heat pump, the intermediate temperature and injection pressure (or displacement ratio) between low-pressure compressor and high-pressure compressor are important parameters that influence the operating performance. In this paper, the optimal intermediate parameters corresponding to the maximum possible COP have been determined. The performance of the two-stage heat pump using R410A and R290 are modeled and compared. It is shown that the two-stage heat pump achieves a COP of 2.85 and a COP of 2.7 at an ambient temperature of −30 °C using R290 and R410A, respectively. In addition, the optimized total fin surface areas of the evaporator and the condenser heat exchangers are calculated from the developed model.

Equivalent diffusion ratio distribution in the UGT15000 low pressure compressor

With spatial and temporal periodicity approach the results of steady state CFD simulation of compressible air flow in each blade-to-blade row of low pressure compressor (LPC) of the UGT15000 gas turbine engine (Ukrainian SSR constr. and prod.) are presented. Near the nominal rotation speed compressor map and adiabatic efficiency of each stage and the flow angles at leading and trailing blade edges are predicted and calculated. At the average radius stage flow and stage load coefficients are determined. It has been established that the reason of the low efficiency of the last stage is associated with the low stage reaction (≈0.5) leading to high airflow swirling at outlet. For LPC optimization a calculation of the radial distributions of the equivalent diffuser coefficient was performed. It is shown that the greatest total pressure losses are in the stator rows of stages 9, 8, 5 and in the rotor rows of stage 9. High losses are also shown on the upper half of the 4th, 3rd, 2nd stator blades and the lower half of the 1st rotor blades. The identified potential for increasing the efficiency of the LPC is up to 2% without significant construction changes.

Experimental Investigation of the Effect of Bleed on the Aerodynamics of a Low-Pressure Compressor Stage in a Turbofan Engine

Abstract The compression system in modern turbofan engines is split into stages linked by transition ducts. Downstream of the low-pressure system, a handling bleed is often required and, in conjunction with structural vanes, can introduce component interactions, which compromise aerodynamic performance. In this paper, a fully annular, low-speed test facility incorporating a 1½ stage axial compressor is used to examine the flow in the last stage of a low-pressure compressor and the downstream transition duct. The transition duct incorporated load-bearing struts, including a so-called king strut with twice the thickness of the regular struts. The bleed utilized a 360-deg annular slot located on the casing immediately downstream of the low-pressure rotor and upstream of the outlet guide vane. The results showed that both the regular and king strut caused a similar flow distortion in the vane row but overall imposed a negligible effect on overall performance. The addition of bleed had a larger effect, generating an increasing outboard bias at the rotor exit as the bleed flow migrated toward the offtake. At the design operating point, the outlet guide vanes were relatively insensitive to this until the highest bleed rate (18%) where evidence of stall was observed. At a lower operating point, a modification to the rotor swirl caused additional incidence onto the vanes, resulting in an earlier onset of the stall; a full stall was observed above 10% bleed. Increasing bleed caused a gradual increase in duct loss up to stall when losses increased rapidly.

Model-based safety analysis with time resolution (MBSA-TR) method for complex aerothermal–mechanical systems of aero-engines

Current aero-engine safety assessments mostly rely on experience-based safety analysis methods such as Fault Tree Analysis (FTA) or Failure Modes and Effects Analysis (FMEA). The progressively significant dynamic characteristics of aero-engines during their transition between states render these methods incapable of providing accurate quantitative safety guidance, which is vital for complex aerothermal–mechanical systems. To solve this problem, a Model-Based Safety Analysis with Time Resolution (MBSA-TR) method was established, which included system modeling, fault modeling, and Safety Critical Parameter (SCP) identification. An integrated system model combining the main gas path and air system was built based on dynamics principles, and it was capable of reflecting the dynamic characteristics of aero-engines. The fault model included several time-resolved sub-modules and interacted bidirectionally with the system model at each time step to realize time-sequential fault propagation. SCPs were defined as parametric analysis objectives, and they were identified using an FTA-like identification framework. Finally, a case study of a typical turbofan engine with low-pressure compressor blade fractures as the primary fault was analyzed. The results showed that some hazard risks are incapable of detection without time resolution. Thus, time resolution is indispensable for the safety evaluation of complex aerothermal–mechanical systems of aero-engines.

Centrifugal compressor design for high-temperature heat pumps

Heat pumps are identified as crucial technology for decarbonising the heat production. Especially for large-scale, high-temperature heat pumps the use of centrifugal compressors has been seen as an attractive technological option. However, there are very few detailed studies available discussing the compressor designs with different refrigerants for high-temperature heat pumps. This study combines the analyses of a 500 kW two-stage heat pump cycle and centrifugal compressor designs. The study qualitatively assesses the feasibility of compressor designs and provides information on how the dimensions, efficiency, and rotational speed are influenced by cycle conditions and the refrigerant. Temperature lift and evaporation temperature were varied with eight working fluids, including hydrocarbons and hydrofluoro-olefins. The estimated COPs were 2.0–2.55 with the highest values observed with pentane and isopentane. The compressor rotational speeds were between 20 – 50 krpm with hydrocarbons requiring higher values than fluids containing fluorine. The predicted compressor efficiencies were between 77% to 84%, with low pressure compressor reaching higher efficiencies than the high-pressure compressor. However, high compressor Mach numbers were observed, and the Mach number seems to be the main obstacle for the increase of the heat pump temperature lift when centrifugal compressors are used.

Steady and Unsteady Numerical Characterization of the Secondary Flow Structures of a Highly Loaded Low-Pressure Compressor Stage

This paper presents the numerical characterization of a highly loaded compressor by means of 3D unsteady RANS simulations. The focus is on critical flow structures and their evolution at different operating points of the machine. First, the numerical setup and mesh quality are presented to support the reliability of the provided results. The comparison against experiments is then described for this purpose. Later, a full description of the unsteady behavior of the machine is provided, giving special attention to the two regions where the most critical features are expected: the rotor hub wall and the casing. Rotor–stator interactions are then investigated and the role of the inlet guide vane (IGV) is finally discussed. Results are analyzed at design and near-stall conditions, with a focus on the behavior close to the stability limit at 100% speed.

Crack cause analysis of a third stage stator vane of the low-pressure compressor in an aeroengine

A cracked third stage stator vane of the low-pressure compressor was found during a major overhaul of an aeroengine. The crack originated from a corrosion pit located on the arc transition between the leading edge and the upper edge plate. The finite element analysis results demonstrated the arc transition was the region with the large von-Mises stress under the wake excitation. The cleavage-like fracture and fatigue striations predominated throughout the fracture surface, indicating it was a pit-related fatigue crack. Meanwhile, Metallographic examination discovered most of corrosion pits beneath the bulging or peeling of the paint layer were distributed on the leading edge and the concave side. Moreover, the composition and phase identification results showed that the corrosion products included sulfate and alumina. Considering the pitting corrosion products and outdoor environment of the aircraft downtime, it was inferred that the cracked stator vane suffered from atmospheric corrosion.

The heat transfer potential of compressor vanes on a hydrogen fueled turbofan engine

Hydrogen is a promising fuel for future aviation due to its CO2-free combustion. In addition, its excellent cooling properties as it is heated from cryogenic conditions to the appropriate combustion temperatures provides a multitude of opportunities. This paper investigates the heat transfer potential of stator surfaces in a modern high-speed low-pressure compressor by incorporating cooling channels within the stator vane surfaces, where hydrogen is allowed to flow and cool the engine core air. Computational Fluid Dynamics simulations were carried out to assess the aerothermal performance of this cooled compressor and were compared to heat transfer correlations. A core air temperature drop of 9.5 K was observed for this cooling channel design while being relatively insensitive to the thermal conductivity of the vane and cooling channel wall thickness. The thermal resistance was dominated by the air-side convective heat transfer, and more surface area on the air-side would therefore be required in order to increase overall heat flow. While good agreement with established heat transfer correlations was found for both turbulent and transitional flow, the correlation for the transitional case yielded decent accuracy only as long as the flow remains attached, and while transition was dominated by the bypass mode. A system level analysis, indicated a limited but favorable impact at engine performance level, amounting to a specific fuel consumption improvement of up to 0.8 % in cruise and an estimated reduction of 3.6 % in cruise NOx. The results clearly show that, although it is possible to achieve high heat transfer rate per unit area in compressor vanes, the impact on cycle performance is constrained by the limited available wetted area in the low-pressure compressor.

Presentation of some constructive solutions for a rotating machine with profiled rotors

The purpose of the paper is to present an original solution for conveying some liquids with the help of a rotating volumetric pump. The pump can be equipped with 1 to 4 specially processed profiled rotors and can transport viscous fluids, pure fluids, or polyphase fluids. It was concluded that the performance of this machine, which can also be used as a fan or low-pressure compressor, is superior to other similar machines.

Elasto-plastic bending behavior of Ti–6Al–4V alloy under coupling conditions of elevated temperature and combined tension-bending

Overload failure under coupling conditions of elevated temperature and combined tension-bending (CTB) is one of the typical failure modes of aero-engine low-pressure compressor blade materials, which is commonly occurring in fighter engines performing low-altitude high-speed penetration missions. In this study, the elasto-plastic deformation behavior and fracture mechanism of Ti–6Al–4V titanium alloy under coupling conditions of elevated temperature and CTB were investigated by experiments and finite element analysis (FEA). CTB tests were conducted on Ti–6Al–4V alloy in the pre-tension ratio range of 0.60–0.90 and in the temperature range of 298 K–573 K. The results show that the maximum bending load, effective bending modulus, and total energy dissipated for the Ti–6Al–4V alloy at room temperature increase with the increase of the pre-tension ratio. The maximum bending load and the energy dissipated are significantly higher at elevated temperatures compared to that of room temperature, due to the elevated temperature induced deformation mode change from bending-dominated to tensile-dominated. The FEA results are in good agreement with the experimental results. SEM fractography indicated that the Ti–6Al–4V alloy fracture at elevated temperature showed only equiaxed dimples, which exhibited a tensile fracture mode. This study is valuable for the in-depth understanding of the deformation behavior and fracture mechanism of blade materials under elevated temperature and complex loads.