The preliminary design of new centrifugal stages often relies on one-dimensional codes implementing the concept of slip factor. This parameter plays a primary role in the stage design process since it directly affects the calculation of the impeller work coefficient and hence of the components situated downstream. Classical slip factor correlations may not always provide a satisfactory accuracy and generally they fail while attempting at covering a design space in a wide range of flow coefficients and peripheral Mach numbers. In that case the preliminary design has to be refined with more advanced tools, such as computational fluid dynamics (CFD). Often this process needs to be repeated several times before the design cycle ends. In order to predict more effectively the work coefficient as well as to reduce the number of iterations between 1D/CFD codes during the design activity, a new correlation has been developed, which is based on a large number of historical data from both CFD and experimental results. Accurate statistical analyses have shown that slip factor can be strongly linked to significant flow and geometry parameters by means of the outlet deviation angle. As the available calibration dataset gets more and more populated, the presence of specific constants in the structure of the correlation allows the designer to improve the accuracy of predictions.
In modern industrial practice, the quality of a new product is achieved by identifying characteristics critical to quality (CTQs) and minimizing deviation from targets, rather than merely optimizing CTQs in the absence of variation. Estimating the variation of CTQs is thus critical to understand and correctly manage risk caused by different interacting sources of uncertainty. We have developed a method to estimate performance variability for centrifugal compressor stages. In this paper, we quantify the performance variation due to impeller manufacturing variability of two stage families. The stages studied are 2D stages designed for multi-purpose applications and 2D stages for high head applications. In a related paper, the stage performance variability here determined is considered together with other sources of variation to compute the variation in flange-to-flange performance for a full compressor. The proposed approach propagates the uncertainty of the design parameters to the stage aerodynamic performance through a Monte Carlo method. In order to keep a low computational budget, calibrated 1D/2D aerodynamic models have been run in parallel to compute the performance of the stages with randomly modified geometry. The results allow the quantification of the stage performance variability and the identification of the main sources of variation. As a step towards the corresponding analysis for a complete centrifugal compressor, results from this study have been used as input for a method where all factors affecting the flange-to-flange performance are considered. The method and the results are discussed in a companion paper.
A full-annulus cascade for radial compressor stator development has been designed and commissioned. The cascade has been developed for the rapid screening of novel stator concepts to facilitate risk mitigation in the early design phase and the validation/calibration of numerical predictions. The rig consists of two main parts. The first part is comprised of an exchangeable set of stationary preswirl vanes that have been designed to mimic discrete points on the operating characteristic of the impeller. The second part consists of a diffuser, bend and return channel with return channel vanes that can also be quickly exchanged. All exchangeable parts are manufactured by rapid prototyping, allowing rapid turnaround times from aerodynamic design to full validation. This is achieved at a significantly lower cost than that of a full rotating test. This investigation summarizes the experimental results and numerical predictions of two test rigs that were designed to study the effect of diffusion ratio, i.e. the ratio of the maximum outer diameter at the top of the bend to the exit of the impeller, on stator performance. To further investigate the sensitivity of the aerodynamic performance to different flow conditions, metal gauzes were positioned immediately downstream of the trailing edges of the preswirl vanes. This allowed the modification of angle and pressure distributions in the diffuser and bend as well as the setting of different turbulence conditions (intensity and length scale) in the downstream sections.
In the last few years wet compression has received special attention from the oil and gas industry. Here, the development and implementation of new subsea solutions are important focus areas to increase production and recovery from existing fields. This new technology will contribute to exploitation of small and remote fields and access in very deep water. In this regard liquid tolerance represents a viable option to reduce the cost of a subsea compression station bringing considerable simplification to the subsea process itself. However, the industry may experience some drawbacks: the various levels of liquid presence may create operational risk for traditional compressors; the liquid may cause mechanical damage because of erosion and corrosion of the internal units and the compressor performance might be affected too. The experimental investigation conducted in the study considers dry and wet conditions in a laboratory setup to understand how the presence of liquid influences the stage performance. The test campaign has been carried out at the Norwegian University of Science and Technology, NTNU, in Trondheim, to assess the performance and operating range of a tridimensional impeller when processing a mixture of gas and liquid phases. Experimental results allowed validating the OEM internal prediction code for compressors’ performance in wet conditions. Finally, the effect of liquid on machine operability has been assessed through a left-limit investigation by means of dynamic pressure probes readings in order to evaluate the stall/surge behaviour for different values of liquid mass fraction.
This paper presents a methodology to control flange to flange performance prediction of centrifugal compressors using a probabilistic approach. In order to have reliable prediction for the performance of centrifugal compressors, a thorough knowledge of critical parameters contributing to the deviation and an efficient way to control the variation of these parameters becomes necessary. This paper discusses about a robust methodology for identifying and controlling the variation of these parameters and hence the predicted performance. This probabilistic technique involves a Design of Experiments (DoE) study to handle large number of input parameters, sensitivity study to identify critical ones and a Monte-Carlo based approach to identify the uncertainty in flange to flange performance. This approach takes into consideration the compressor stage performance variability driven by impeller manufacturing tolerances, statoric component losses variability and leakages variability in order to compute overall performance variation in a compressor. An in-house developed probabilistic optimization code (PEZ) is interfaced with a well-validated & calibrated thermodynamic tool to analyse large sets of possible combinations and to provide best possible solution for a given design space. This concept is successfully applied for different compressor configurations by varying the stage numbers and process conditions. The results give an insight on the main sources and magnitude of variations on compressor performance, thus enabling to control the predictions in an efficient way. This methodology will provide a novel and an efficient way to generate robust compressor performance, where it will be possible to take into account design and manufacturing uncertainty. The use of this methodology can thus drastically improve the performance predictability and risk associated with each compressor selection.
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