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

Abstract

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.