M. De Lucia

Experimental investigation and characterization of the rotating stall in a high pressure centrifugal compressor. Part I: Influence of diffuser geometry on stall inception

Extensive research on centrifugal compressors has been planned. The main task of the research is to improve present prediction criteria coming from the literature with particular attention to low flow coefficient impellers (low width to radius ratios) where they are no more valid. Very little data has been published for this kind of stages, especially for the last stage configuration (with discharge volute). Many experimental tests have been planned to investigate different configurations. A simulated stage with a backward channel upstream, a 2D impeller with a vaneless diffuser and a constant cross section volute downstream constitute the basic configuration. Several diffuser types with different widths, pinch shapes and diffusion ratios were tested. The effect of geometric parameters on stage stability has been discussed inside part I of the present work; the purpose of this part of the work is to illustrate the effect of the same geometric parameters on stage performance and to quantify the impact of stability improvements on stage losses.Copyright

A Vision-Based Particle Tracking Velocimetry

Particle Image Velocimetry (PIV) is a non-intrusive optical technique to measure velocity of flows. It provides the simultaneous visualization of the streamline pattern in unsteady flows and the quantification of the velocity field over the image plane. To reveal the flow motion, the flow is seeded by small scattering particles. The instantaneous fluid velocities are evaluated by recording the images of tracers, suspended in the fluid and traversing a light sheet. A PIV system consists of seeding particles, illumination unit, image acquisition system, and a computer for image processing. For industrial applications a classical PIV system is not suitable for its cost, sizes and needs of specialised users and work areas. Moreover, classical PIV are unable to work in real-time for the huge amount of data and expensive algorithms adopted. In this paper, a study and the implementation of a new PIV system is described and compared against classical PIV solutions. The solution proposed is capable of working in real-time and is a Continuous PIV, CPIV, system: with respect to classical PIV, it is composed of a continuous laser light source and a CCD camera. A specifically new image-processing algorithm for velocity estimation and recognition of correct traces has been developed. It is based on the grey level distribution in the particle trace image, and indicates those particles moving with an out-of-plane velocity vector component and provides the measure with a limited error.

Temperature Measurements in a Heavy Duty Gas Turbine Using Radiation Thermometry Technique: Error Evaluation

This paper describes a technique for the analysis of temperature measurement data obtained by using a radiation thermometer in a heavy duty gas turbine (PGT2 from Nuovo Pignone GE). The main sources of error are shown and analyzed and their influence on measurements and techniques used to correct data read. Success in significant error elimination in acquisition results in a more realistic and correct description of target temperature distribution.

Application of Low Solidity Vaned Diffusers to Prevent Rotating Stall in Centrifugal Compressors: Experimental Investigation

Rotating stall is a key problem in centrifugal compressors especially for high-pressure applications. Among the several solutions proposed to prevent this problem, low solidity vaned diffusers (LSDs) have been successfully applied. The aim of this research is to find an optimized LSD configuration in terms of stall inception and performance for the tested compressor. The influence of the vane leading and trailing edge radii, maintaining the same solidity and null deflection were investigated. The geometrical parameters of tested configurations were methodically chosen in order to achieve a design of experiment (DOE) analysis. In view of the above, a series of tests on five low solidity vaned diffuser configurations were carried out. Tests allowed determination of 2 different LSDs that showed the best behavior in terms of working range and performance. These LSDs were then compared with the standard and the reduced-width vaneless diffuser to better understand the best solution to the stall problem.Copyright

Synchronized LDV Measurement in Centrifugal Impeller: Seeding Insemination Set Up and CFD Comparison

Investigation of flow is necessary for turbomachinery performance analysis.The development of high performance centrifugal impellers leads to use of higher and higher rotational speeds and smaller and smaller impellers.The use of intrusive probe for flow evaluation is often inadequate since the probe size can be comparable with the flow passage area and can cause flow distortion so large as to invalidate measurements.Therefore the use of non-intrusive measurement techniques, which will not alter the flow, is required.Laser Doppler Velocimetry (LDV) provides an accurate velocity valuation through non-intrusive measurements even in the small hub to shroud area passage.The right set up of the LDV system is fundamental for accurate measurement; investigated area and working conditions are often completely unknown, so erroneous data, due to system set up, are not easily recognizable. The definition of lens, seeding, insemination technique and LDV parameters can be a source of errors not detectable in measurement phases. Special equipment (seeding inseminating test system and a dedicated wind tunnel) is developed to overcome these problems and to define the right parameters and working conditions to be used in turbomachinery applications. Details of LDV set up are reported in this paper.In real turbomachinery applications standard LDV measurement (time averaged velocity) could be effected by a non-homogeneous distribution of seeding: e.g. in an impeller in the presence of jet and wake, so it is necessary to link seeding particle passages to their position through the time history in a non-ambiguous way. A customized synchronization technique is presented. Validation of measurements has been achieved by comparing LDV data with results of 3D fully viscous computations.Copyright