Michael Cave

Numerical study and experimental validation of the performance of two different volutes with the same compressor impeller

Abstract Volute scroll, conic diffuser, and sudden expansion discharge losses account for 4–6 points of efficiency decrement in a typical centrifugal compressor stage. The flow in a volute is highly complex. It is strongly believed that an understanding of the detailed flow structure in a volute will provide insights on minimizing the losses by isolating the mechanisms that contribute to entropy generation. The result will be a more efficient centrifugal compressor product for customers. This article presents the experimental and numerical investigation on the matching of two different overhung volutes with the same centrifugal compressor impeller. The experimental data were measured from flange to flange initially, and then three Kiel probes were installed at different circumferential pinch locations. At the same time, a detailed numerical simulation of the performance of the two volutes has been carried out. A computational model, using the k-ε turbulence model and wall function, has been used to predict the internal flows of both volutes. Good agreement between experimental data and numerical simulation results is found, except at high mass flowrate. The overall performance of the two volutes was also compared thoroughly.

Improved centrifugal compressor impeller optimization with a radial basis function network and principle component analysis

Abstract The development of a fast and reliable computer-aided design and optimization procedure for centrifugal compressors has attracted a great deal of attention both in the industry and in academia. Artificial neural networks (ANNs) have been widely used to create an approximate performance map to substitute the direct application of flow solvers in the optimization procedure. Although ANNs greatly decrease the computational time for the optimization, their accuracies still limit their applications. Furthermore, ANNs also bring errors to the final results. In this study, principal component analysis (PCA) or independent component analysis (ICA) is applied to transform the training database and make a radial basis function network (RBFN), a type of ANN, trained in a new coordinate system. The present study compares the accuracies of three different trained ANNs: RBFN, RBFN with PCA, and RBFN with ICA. Furthermore, the total performances of the centrifugal compressor impeller optimization procedures using these three different trained ANNs are compared. Genetic algorithm (GA) is used as an optimization method in the optimization procedure and influences of GA parameters on the optimization procedure performances are also studied. All results demonstrate that the application of PCA significantly increases the accuracy of trained ANN as well as the total performance of the centrifugal compressor impeller optimization procedure.

Prediction of impeller rotating stall onset using numerical simulations of a centrifugal compressor. Part 1: Detection of rotating stall using fixed-flow transient simulations

Rotating stall is an unsteady flow phenomenon that appears in both axial and centrifugal compressors. It is detrimental to the performance of the compressor, significantly narrowing its operating range. Numerical modeling of this phenomenon has been a major area of investigation for axial compressors with some success. While stall occurs less often in centrifugal compressors than axial ones, it can be much harder to predict. Some preventive measures are known but are mostly rules of thumb developed through experimental experience. This work focuses on the detection of impeller rotating stall using computational fluid dynamics. A compressor was chosen that has demonstrated rotating stall instabilities with different diffuser lengths and return vanes. Unsteady numerical simulations were performed on full 360° models of this geometry. The transient simulations were conducted using distributed processing on high-performance servers. The objective was to determine the accuracy in which rotating stall can be captured in simulations. For this purpose, simulation results were compared to experimental results for the same compressor and show a good correlation between the experimental and numerical tests.

A CFD Study on the Dynamic Coefficients of Labyrinth Seals

Labyrinth seals are widely used in gas compressors to reduce internal leakage and increase the compressor efficiency. Due to the eccentricity between the rotating impeller and the stationary part as *well as the shaft whirling motion, forces are generated when the leakage flow passing through the cavities and the seals. For a lot of applications with high speed and pressure, these forces can drive the system unstable. Thus, predicting the forces accurately become a very important for compressor rotordynamic designs.A lot of research and studies has been done to the seals itself, including bulk flow method, computational fluid dynamic (CFD) and test measurement. The seal and leakage flow interaction forces can be predicted relatively accurate. But very few research treat the seal and cavities as one component interacting with the leakage flow and produce the forces. This paper presents results of CFD investigations on the dynamic coefficients of one typical impeller eye seal and front cavity. The CFD results show that large forces are generated in the front cavity due to circumferential uniform pressure distribution, which caused by the downstream labyrinth seal. The forces generated in the front cavity are more than in the front seal.It was found that the inertia, damping, and stiffness are proportional to average pressure. The cross-coupling stiffness increases with speed with power of 2 while the direct stiffness increases with speed with power of about 1.7.Copyright

A flow field study of the interaction between a centrifugal compressor impeller and two different volutes

Abstract The interaction between an impeller with one large and one small overhung volute was investigated by both computational fluid dynamics simulation and experimental methods. The large volute was mainly generated by increasing the small volute axial length. The computational model, with k—∊ turbulence model and wall function, has been used to predict the internal flow of both volutes. The effect of volute tongue on the flow in the impeller was analysed at off-design conditions. The flow structure in the volutes was also investigated in detail. The performance test of the two configurations was carried out in the aero test facility at Solar Turbines Inc. A good agreement between experimental data and numerical simulation results was found on both the whole compressor stage performance and the impeller performance. The largest deviation is close to 10 per cent for the compressor stage performance prediction at high mass flowrate only.

Operation Of Centrifugal Compressors In Choke Conditions

Centrifugal compressors are at times required to operate in or near the choke region. Various limits of the degree of allowable operation in choke have been established. Based on test data and numerical data, the behavior of centrifugal compressors in the choke region is studied. Changes in aerodynamic performance, thrust load, volute behavior and radial loading are considered. The issue of excitation of impeller vanes is addressed. Particular consideration is given to multistage machines, as well as dual compartment machines, in particular regarding the effects of impeller mismatch during operating conditions at flows significantly higher than the design flow. Limitations in the overload operating range of a compressor not only impact the operational flexibility , but also can require more complicated control systems. The paper addresses aerodynamic, structural as well as rotordynamic issues related to the operation in choke.

Fully Three-Dimensional Viscous Semi-Inverse Method for Subsonic Mixed-Flow and Radial Impeller Design

A new semi-inverse design method for turbomachinery blading is proposed in this paper. Built on a time-marching Reynolds-Averaged Navier-Stokes solver, the proposed design method takes pressure loading, blade tangential thickness, blade stacking line, and flow path contour as prescribed quantities and computes the corresponding three-dimensional blade camber surface. In order to have the option of imposing geometrical constraints on the designed blade shapes, a new algorithm is developed to solve the camber surface at specified spanwise grid-lines, after which the blade geometry is constructed through ruling (e.g. straight-line element) at the remaining spanwise stations. The new semi-inverse algorithm involves re-formulating the boundary condition on the blade surfaces as a hybrid inverse/analysis boundary condition while preserving the fully three-dimensional nature of the flow field. The new design method can be interpreted as a fully three-dimensional viscous semi-inverse method. The ruled camber surface design procedure ensures blade surface smoothness and some control of mechanical integrity, and results in cost reduction for the manufacturing process. The proposed fully three-dimensional semi-inverse method is demonstrated through design modifications of generic industrial mixed-flow and radial impellers which are typically used for gas process applications.Copyright

Effects of Blade Stack on Pipeline Compressor Impeller Performance

Back to back testing of two centrifugal gas compressors used for natural gas pipeline transmission applications were completed. Blade geometry is very similar with the exception of stack of the impeller mean camber-lines. This resulted in significantly different lean angle distributions and rake at the exit of the two impellers. Both impellers performed exceptionally well in terms of efficiency and range, but with a marked difference in head making capability. CFD analysis using CFX-TASCflow was conducted to identify the underlying cause for the change in head. The CFD analysis was then validated using an experimental rig impeller that included traverse probe measurements at the pinch exit. Once the CFD was validated, a discussion is included on how to apply the knowledge learned to future impeller designs.Copyright

Frictional Effects on a Base and a Flow-Trimmed Impeller of a Low Specific Speed Industrial Compressor

Flow-trimming base designs for lower flow applications are widely used in the turbo-machinery industry to minimize development costs. This process requires the use of the same casing as the base design, along with a decrease in the flow path width of the impeller, diffuser, crossover, and return vane. Two stages: a baseline and a flow-trimmed impeller, diffuser and return vane are studied in this paper to review impacts on performance and their underlying causes. To measure the performance changes of the two rigs, a traverse survey for flow angle and total pressure was done at the locations of diffuser inlet, return vane leading edge and stage discharge. It was found that the performance difference between the base impeller and trimmed one is very little. However, diffuser performance of the base rig is better than the flow-trimmed rig. Diffuser data shows frictional losses increased in the flow-trimmed rig. Rig data also suggests that increased friction in the trimmed stage changes the flow angle at the return vane leading edge compared to the base stage. This increases the loss further for the flow-trimmed rig because of resulting negative incidence on the return vane relative to the base rig. Data suggests that while flow-trimming may be a valid design approach for impellers, additional issues arise when downstream stationary components are trimmed.Copyright

Investigation of a Centrifugal Compressor Stage With Two Volutes and the Same Impeller

Volute scroll, conic diffuser and sudden expansion discharge loss account for 4–6 points of efficiency decrement in a typical centrifugal compressor stage. The flow in a volute is highly complex. It is strongly believed that understanding of the detailed flow structure in a volute will provide insights on minimizing the losses by isolating the mechanisms that contributes to entropy generation. The result will be a more efficient centrifugal compressor product for customers and users and a product at higher profitability levels for manufacturers. This paper presents the experimental and numerical investigation on the matching of two different overhung volutes to the same centrifugal compressor impeller. The experimental data were measured from flange to flange firstly, then three Kiel probes were installed on pinch position circumferentially. At the same time, a detailed numerical simulation of the performance of the two volutes has been carried out. A computational model, using the k-e turbulence model and the wall function, has been used to predict the internal flow of the both volutes. A good agreement between experimental data and numerical simulation results is found. The overall performance of the two volutes was also discussed in detail.Copyright