Stochastic axial compressor variable geometry schedule optimisation

The design of axial compressors is dictated by the maximisation of flow efficiency at on design conditions whereas at part speed the requirement for operation stability prevails. Among other stability aids, compressor variable geometry is employed to rise the surge line for the provision of an adequate surge margin. The schedule of the variable vanes is in turn typically obtained from expensive and time consuming rig tests that go through a vast combination of possible settings. The present paper explores the suitability of stochastic approaches to derive the most flow efficient schedule of an axial compressor for a minimum variable user defined value of the surge margin. A genetic algorithm has been purposely developed and its satisfactory performance validated against four representative benchmark functions. The work carries on with the necessary thorough investigation of the impact of the different genetic operators employed on the ability of the algorithm to find the global extremities in an effective and efficient manner. This deems fundamental to guarantee that the algorithm is not trapped in local extremities. The algorithm is then coupled with a compressor performance prediction tool that evaluates each individual's performance through a user defined fitness function. The most flow efficient schedule that conforms to a prescribed surge margin can be obtained thereby fast and inexpensively. Results are produced for a modern eight stage high bypass ratio compressor and compared with experimental data available to the research. The study concludes with the analysis of the existent relationship between surge margin and flow efficiency for the particular compressor under scrutiny. The study concludes with the analysis of the existent relationship between surge margin and flow efficiency for the particular compressor under scrutiny

Using Magnetic Bearing Orbit Information to Maximize Centrifugal Compressor Efficiency at Off-Design Conditions

Active magnetic bearings used on oil-free centrifugal refrigeration compressors have lower stiffness than conventional oil-lubricated journal or rolling element bearings. The lower stiffness of these bearings makes them sensitive to internal flow instabilities that are precursors of rotating stall or compressor surge. At operating conditions far away from surge the internal flow is very stable and the magnetic bearings keep the shaft centered, resulting in a minimal bearing orbit. The internal flow instabilities that arise when the compressor approaches the surge limit result in some radially fluctuating forces on the shaft. The active magnetic bearings correct for these fluctuating radial forces on the shaft. The bearing orbit increases with the size of these radial forces. Optimum compressor efficiency occurs close to surge at incipient stall conditions when maximum internal flow pressure recovery occurs. The positional feedback system of the active magnetic bearing control loop system indicates the bearing orbit which relates to compressor efficiency. At high flow conditions capacity is controlled by compressor speed for the imposed pressure ratio. At low flow conditions a combination of variable speed and a means of range extension (e.g. inlet guide vanes, variable geometry diffuser, or flow recirculation) are required to control capacity at the imposed pressure ratio and guarantee stable compressor operation. The bearing orbit signal can be used to determine which speed/geometry combination gives the highest compressor efficiency. If the bearing orbit is below a minimum value the compressor runs too close to choke and a reduction in speed combined with an opening of the diffuser throat area would increase compressor efficiency. If, on the other hand, the bearing orbit is above a maximum value the compressor runs too close to surge and an increase in speed combined with a closing of the diffuser throat area is required to increase compressor efficiency and guarantee surge-free compressor. Experience with this control scheme will be illustrated for a newly developed 350 ton two-stage centrifugal compressor where the variable geometry hardware is replaced with a controlled internal flow recirculation

Effect of slotted casing treatment on performance of a multistage compressor

A J85-13 engine was equipped with a compressor case that allowed changes to the case wall over the rotor tips of six of its eight stages. The engine was tested with four inlet configurations: uniform inlet flow, 180 degree circumferential distortion, hub radial distortion, and tip radial distortion. Slotted inserts were installed in the first three stages, and the compressor was mapped under similar conditions. Overall compressor performance obtained with tip treatment was inferior to the performance for the compressor's normal operating range. Pumping capacity with the slotted inserts was reduced. Overall compressor efficiency was reduced 1 to 2 percent with the slotted rings installed for 90 and 100 percent corrected engine speeds

An Iterative Method to Derive the Equivalent Centrifugal Compressor Performance at Various Operating Conditions: Part II: Modeling of Gas Properties Impact

This is the second part of a study conducted to model the aerothermodynamic impact of suction parameters and gas properties on a multi-stage centrifugal compressor’s performance. A new iterative method has been developed in the first part to derive the equivalent performance at various operating conditions. This approach has been validated to predict the compressor map at different suction pressures and temperatures using the design characteristics as reference values. A further case is included in this paper in order to emphasize the validity of the developed approach to obtain the performance characteristics at various gas compositions. The provided example shows that the performance parameters at different gas mixtures can be predicted to within ±1.34%. Furthermore, the conducted optimization in this paper reveals that the proposed method can be applied for the compressor design evaluation corresponding to the expected variation in suction conditions. Moreover, the examined case study demonstrates the effect of gas properties’ variation on the operating point and aerodynamic stability of the entire compression system. In order to achieve that, a simple approach has been established to assess the contribution of gas properties’ variation to the inefficient and unstable compressor performance based on the available operational data

Preliminary design study of advanced multistage axial flow core compressors

A preliminary design study was conducted to identify an advanced core compressor for use in new high-bypass-ratio turbofan engines to be introduced into commercial service in the 1980's. An evaluation of anticipated compressor and related component 1985 state-of-the-art technology was conducted. A parametric screening study covering a large number of compressor designs was conducted to determine the influence of the major compressor design features on efficiency, weight, cost, blade life, aircraft direct operating cost, and fuel usage. The trends observed in the parametric screening study were used to develop three high-efficiency, high-economic-payoff compressor designs. These three compressors were studied in greater detail to better evaluate their aerodynamic and mechanical feasibility

Iterative Method to Derive the Equivalent Centrifugal Compressor Performance at Various Operating Conditions: Part I: Modelling of Suction Parameters Impact

This paper introduces a new iterative method to predict the equivalent centrifugal compressor performance at various operating conditions. The presented theoretical analysis and empirical correlations provide a novel approach to derive the entire compressor map corresponding to various suction conditions without a prior knowledge of the detailed geometry. The efficiency model was derived to reflect the impact of physical gas properties, Mach number, and flow and work coefficients. One of the main features of the developed technique is the fact that it considers the variation in the gas properties and stage efficiency which makes it appropriate with hydrocarbons. This method has been tested to predict the performance of two multistage centrifugal compressors and the estimated characteristics are compared with the measured data. The carried comparison revealed a good matching with the actual values, including the stable operation region limits. Furthermore, an optimization study was conducted to investigate the influences of suction conditions on the stage efficiency and surge margin. Moreover, a new sort of presentation has been generated to obtain the equivalent performance characteristics for a constant discharge pressure operation at variable suction pressure and temperature working conditions. A further validation is included in part two of this study in order to evaluate the prediction capability of the derived model at various gas compositions

Performance of a low-pressure-ratio centrifugal compressor with four diffuser designs

A low-pressure-ratio centrifugal compressor was tested with four different diffuser configurations. One diffuser had airfoil vanes. Two were pipe diffusers. One pipe diffuser had 7.5 deg cone diffusing passages. The other had trumpet-shaped passages designed for linear static-pressure rise from throat to exit. The fourth configuration had flat vanes with elliptical leading edges similar to those of pipe diffusers. The side walls were contoured to produce a linear pressure rise. Peak compressor efficiencies were 0.82 with the airfoil vane and conical pipe diffusers, 0.80 with the trumpet, and 0.74 with the flat-vane design. Surge margin and useful range were greater for the airfoil-vane diffuser than for the other three

CF6 High Pressure Compressor and Turbine Clearance Evaluations

In the CF6 Jet Engine Diagnostics Program the causes of performance degradation were determined for each component of revenue service engines. It was found that a significant contribution to performance degradation was caused by increased airfoil tip radial clearances in the high pressure compressor and turbine areas. Since the influence of these clearances on engine performance and fuel consumption is significant, it is important to accurately establish these relatonships. It is equally important to understand the causes of clearance deterioration so that they can be reduced or eliminated. The results of factory engine tests run to enhance the understanding of the high pressure compressor and turbine clearance effects on performance are described. The causes of clearance deterioration are indicated and potential improvements in clearance control are discussed

Compressor Efficiency Improvement By Reducing Heat Transfer

Many ways is possible to improve the compressor efficiency, like to reduce the mechanical losses through valve or bearing which represents from 20% to 30% of total electric consumption power on the compressor. Another ways is to decrease the pressure drop through the compression circuit of the compressor, which can reduce the temperature variation through the compressor…etc. In our case of study, we will be interested in the heat transfer and pressure drop through the suction circuit. In this paper, we present the challenge done on the heat transfer reduction through the suction line of the CAJ4519Z compressor, in reducing from 10°C to 20°C the flow temperature through the suction line and, thus improve the cooling capacity in increasing density of the flow. Indeed, according to the mechanical conception of the compressor, the suction line, the fresh part of the flow, is in contact with the discharge line, the hot part of the flow, at the cylinder head part. Thus, thermal conduction phenomena at this level heat up the fresh flow of the suction line therefore decrease the density of the flow which impacts on the global cooling capacity of the compressor. In this case of study, the cylinder head is on cast iron material, and separates the suction line (fresh flow at 64°C) with the discharge line (hot flow at 115°C) by a thickness wall of 0.8cm. Consequently, the fresh flow of suction line reheats by the conductivity and loses between 10°C and 20°C according to the operating condition. To reduce the conductivity phenomenon on the cylinder head, a judicious choice of material is done according to the Ashby methodology based on the conductivity coefficient, and a new concept of cylinder head is done. Before, testing the new concept of the cylinder head, CFD (Computational Fluid Dynamics) simulation allows understanding the improvement through the standard and new cylinder head concept, and allows defining the position of the sensor. The laboratory test result confirms the simulation results. Thus, thanks to the thermal characteristic of the material and re design the cylinder head part, we can reduce the temperature and improve by 2% ( average value) the cooling capacity of the compressor

Preliminary compressor design study for an advanced multistage axial flow compressor

An optimum, axial flow, high pressure ratio compressor for a turbofan engine was defined for commercial subsonic transport service starting in the late 1980's. Projected 1985 technologies were used and applied to compressors with an 18:1 pressure ratio having 6 to 12 stages. A matrix of 49 compressors was developed by statistical techniques. The compressors were evaluated by means of computer programs in terms of various airline economic figures of merit such as return on investment and direct-operating cost. The optimum configuration was determined to be a high speed, 8-stage compressor with an average blading aspect ratio of 1.15