Numerical Analysis of Flow Characteristics of the Low-Speed Centrifugal Compressor Stage Comprising Wedge Diffuser

Wedge type diffusers are used generally in the highly loaded stages of smaller jet engines as it is compact in size. Low speed centrifugal compressor (LSCC) is selected for this present study as experimental details are available. A centrifugal compressor stage comprising wedge type diffuser is used in this numerical investigation in which studies are carried out at design mass flow rate (30kg/s) and off-design mass flow rates (23.64kg/s and 36.36kg/s) at constant rotational speed of 1920 rpm. Single passage approach is chosen to model the computational domain which is meshed with unstructured grid. Turbulence model chosen is kω-SST. The investigation revealed the jet-wake structure along the pressure side and suction side of the impeller and its subsequently mixing at impeller exit vaneless diffuser region. Diffusion process in the LSCC is observed to be effective as the outlet values of absolute velocity are lesser compared to its inlet values. Highest static pressure rise is observed for design mass flow rate and followed by below and above off-design mass flow rates.

Effects of geometric and operational uncertainties on aerodynamic performance of centrifugal compressor stage

No real centrifugal compressor can exactly conform to its design geometry and expected operating conditions due to the uncertainties existing in the manufacturing and operational processes. Such uncertainties have been increasingly recognised to be detrimental to compressor performance. However, quite few studies have focused on the combined effects of geometric and operational uncertainties on compressor performance, and the underlying flow mechanism even remains unclear. In this context, we here present an uncertainty analysis of a centrifugal compressor stage, with both geometric and operational uncertainties taken into account. With the combination of CFD simulation and nonintrusive sparse grid based stochastic collocation methods, the combined and individual effects of total inlet temperature, total inlet pressure, outlet mass flow, impeller tip clearance and hub fillet radius on the stage/impeller performance are quantified and analysed. Particular attention is paid to elucidating the compressor performance variations through flow field and energy decomposition analyses. Results show that the considered uncertainties exert more influence on the compressor stage performance rather than on the impeller performance. Amongst the examined uncertainties, the impeller tip clearance contributes the most to the stage performance. The underlying mechanism lies in that the wake of impeller tip clearance produces distorted flow downstream towards the diffuser, which causes complicated vortex structures and less conversion of kinetic energy to pressure rise in the diffuser passage. The present study lays a theoretical foundation for the further uncertainty quantification and robust design of centrifugal compressors against various sources of uncertainties.

Numerical Study of the Improvement in Stability and Performance by Use of a Partial Vaned Diffuser for a Centrifugal Compressor Stage

The influence of different diffuser configurations on the flow stability and aerodynamic performance of a centrifugal compressor stage with a mass flow coefficient of 0.196 is numerically investigated. Research results show that the performance of a traditional full-height vaned diffuser (TVD) deteriorates rapidly, and a shroud-side partial vaned diffuser (SVD) displays better adaptability in off-design conditions. SVD can suppress the development of vortices generating at the diffuser leading-edge. Therefore, it can reduce the flow loss inside the stage and improve the flow stability of the stage at low mass flow rates. The unsteady analysis for TVD and SVD shows that the stall cell propagates at about 35.7% of impeller rotational speed in the semi-vaneless space and diffuser passages. Furthermore, the internal flow in TVD and SVD is studied by employing the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods. The flow loss and instability mechanism in the stage are consequently revealed more comprehensively.

Aerodynamic Design and Rig Test Validation of a High Flow Coefficient Process Centrifugal Compressor Stage

Efficient and diametrically compact very high flow coefficient stages with wide operability are desirable for economic reasons in many process multistage centrifugal compressor applications. Such stages present special aerodynamic and mechanical design challenges. There is often a sizeable efficiency lapse rate as well as substantial reduction in useable operating range for traditional stages having design flow coefficients greater than 0.15 and moderate to high machine Mach numbers. This paper describes aerodynamic design and rig test validation of a very high flow coefficient (φ0 = 0.237) process centrifugal compressor stage. Some useful experience of the detailed design work required to navigate certain technical challenges and its rig test validation are reflected in the manuscript. A relatively high machine Mach number (MU ∼ 0.878) mixed-flow shrouded impeller matched with a curved radial vaneless diffuser and return channel was developed. Test results confirmed that the principal aerodynamic design intents were met or exceeded. A sensible design strategy guided by a well-anchored design method is shown to successfully extend an existing stage portfolio to very high-flow coefficients for multistage process centrifugal compressor applications.

Centrifugal Compressor Stage Efficiency and Rotor Stiffness Augmentation via Artificial Neural Networks

Centrifugal compressor stages with high rotor stiffness (i.e. impeller hub-to-outer-diameter ratio) may represent a crucial element to cope with tight rotordynamic requirements and constraints that are needed for certain applications. On the other hand, high-stiffness has a detrimental effect on the aerodynamic performance. Thus, an accurate design and optimization are required to minimize the performance gap with respect to low-stiffness stages. This paper shows a redesign and optimization procedure of a centrifugal compressor stage aimed at increasing the impeller stiffness while keeping high aerodynamic performance. Two different optimization steps are employed to consider a wide design space while keeping the computational cost as low as possible. At first the attention is focused on the impeller only, then the diffuser and the return channel are taken into account. The multi-objective and multi-operating point optimization makes use of artificial neural networks (ANNs) as a surrogate model to obtain the response surfaces. RANS calculations are carried out using the TRAF code and are employed to create the training dataset. Once the ANN has been trained, an optimization strategy is used to find the constrained optimum geometries for the impeller and the static components. The optimized high-stiffness stage is finally compared to the low-stiffness one to assess its applicability.

Experimental and Numerical Investigation of Vaned Hub and Shroud Wall Contoured Diffusers Designed to Improve Flexibility and Efficiency of an Open Impeller Centrifugal Compressor Stage

State-of-the-art centrifugal compressor stages are required to provide both a flexible and a highly efficient operation. To extend the stable operating range and to improve the design-point polytropic total-to-total efficiency of an open impeller centrifugal compressor stage, three vaned contoured diffusers characterized by geometric modifications of the hub and shroud wall in the vaneless space upstream the diffuser vanes and within the diffuser passages were designed. In this paper, a shroud wall, a hub wall and a hub and shroud wall contoured diffuser and a state-of-the-art baseline diffuser are experimentally examined. For the hub-contoured diffuser an operating range extension of 5.3% was measured at design stage Mach number. For the shroud-contoured diffuser an improvement of polytropic total-to-total efficiency by up to 0.3% is observed. The experimental data including normalized total-to-static pressure ratio and 5-hole-probe data is utilized to validate the numerical setup. By means of the CFD simulations the hub- and the shroud-contoured diffuser designs are analyzed and the hub-contoured diffuser’s effect on the local flow at diffuser vane leading edge is investigated. The results illustrate the local effect of the hub-contoured diffuser design on the flow field in the examined centrifugal compressor stage.

CFD-researches of centrifugal compressor stage vane diffusers

The paper presents result of CFD simulations of a series of centrifugal compressor stage vane diffusers in the Ansys CFX. Objects of research are vane diffusers with external relative diameter (relative to the diameter of the impeller) equal to 1.5, vane inlet angle of 20 degrees, relative vane heights of 0.025, 0.034, 0.045, 0.06, 0.08, vane profile curvature angles of 10, 15, 20 degrees. The characteristics of polytrophic efficiency, loss coefficient, recovery coefficient, ratio of inlet and outlet velocities, flow deviation angle versus incidence angle are set. The analysis of the flow structure in the vane diffuser channels is presented. Unlike with a straight vane cascade, the deviation angle in the circular rows of vane diffusers tends to increase with increasing row density. This may be due to the complex nature of the interaction of the active part of the flow with separation zones. In rows with almost straight vanes at a lower density, the separation zone on the pressure side decreases, and even shifts to the very end of the suction side.