Corrections to inter-blade-row flow measurements in axial compressors

Abstract

For industrial multistage axial compressors, flow measurements in axial gaps between the blade rows are limited by mechanical constraints and the high uncertainty of the gathered data. The main factors affecting measurement uncertainties are the confined space between the blade rows, large relative dimensions of the probe and the holding stem, high flow velocities and complex flow structures. In this thesis, it has been shown that the uncertainties become large when a cylindrical probe is placed in proximity to the blades or the large vortical structures, such as blade wakes or separation zones, or when the flow Mach number exceeds the probe critical values of 0.7. In these cases, the uncertainties may exceed 10° in the flow angle and over 30% of dynamic head in measured static pressure. A set of boundaries are defined for geometrical and flow conditions, where the deviations from the true value of the undisturbed flow are quasi-linear and can be estimated. These boundaries are based on the physical principles of the internal turbomachinery flows and can be universally applied to other multistage compressors. The corrections were applied first to a CFD-simulated probe placed at representative locations along the span and circumference of the passage both upstream and downstream of stator blade rows at representative flow conditions. When the corrections were applied, the measurement uncertainties caused by the probe's flow field distortion were reduced to ±1° in the flow angle and to ±5% of the dynamic head. Finally, the corrections were applied to the experimental spanwise measurements made on an axial compressor as a part of an industrial gas turbine during its operation on site. A procedure was developed to control the consistency of the measurement corrections based on fundamental physical principles such as mass and energy conservation and radial equilibrium. The corrected data have shown improved consistencies: the variation of the integrated mass flow values along the compressor axial locations was reduced from ±15% to ±4% from the inlet value, and the static pressure distribution along the span became within 5% of dynamic head away from the radial equilibrium condition compared to more than 30% initially. The corrections presented in this thesis have fixed the first-order errors relating to the use of finite-size probes in multistage axial compressors, namely, consistently identifying and correcting for the blockage from the probe itself and the potential field of the upstream and downstream blade rows. Before the corrections, these errors meant spanwise measurements of flow angle and pressure had large uncertainties and errors; therefore, they were rarely performed in multistage industrial compressor environments. After the corrections, the results of spanwise traversing have shown improved consistency and can now be used as feedback in the design and development of multistage axial compressors.