Endwall Blockage in Axial Compressors

Abstract

This paper presents a new methodology for quantifying compressor endwall blockage and an approach, using this quantification, for defining the links between design parameters, flow conditions, and the growth of blockage due to tip clearance flow. Numerical simulations, measurements in a low-speed compressor, and measurements in a wind tunnel designed to simulate a compressor clearance flow are used to assess the approach. The analysis thus developed allows predictions of endwall blockage associated with variations in tip clearance, blade stagger angle, inlet boundary layer thickness, loading level, loading profile, solidity, and clearance jet total pressure. The estimates provided by this simplified method capture the trends in blockage with changes in design parameters to within 10 percent. More importantly, however, the method provides physical insight into, and thus guidance for control of, the flow features and phenomena responsible for compressor endwall blockage generation.

Aerospace Turbomachinery Flow Physics

“…We then detail the procedure for choosing a mixing model and present expressions for mixing losses in the impeller. A blockage estimate, developed for axial compressors [Khalid et al, 1999] is also adapted for use in centrifugal impellers to provide estimates of performance sensitivity to tip clearance as function of different design parameters. We can estimate the magnitude of viscous forces by comparing the stagnation pressure coefficient for the shrouded and 0% tip clearance case; these differ by less than 1%.…”

“…These compressor designs are known to exhibit spike stall inception and thus this tip clearance flow behavior is hypothesized to be limiting the useful compressor flow range. Figure 5.1 shows the trajectory of the interface between the incoming and tip clearance flows (Khalid et al, 1999). As the flow coefficient decreases, the axial momentum of the incoming flow is reduced while the axial momentum of the tip clearance flow increases (from increased blade loading).…”

“…The computed total-to-static pressure rise characteristics depicted in figure 5.4 shows both the steady state flow regime (points 1 through 4) and transient regime (points 5 through 8) for a compressor rotor tip clearance size of 1.8% chord. The corresponding variation with respect to flow coefficient of total blockage and its individual contribution (tip clearance flow blockage and passage-blade/hub-boundary layer blockage) (Khalid et al, 1999) are also indicated. The results show that the flow solution limit (point 4) marks the onset of rapid growth in blockage.…”

“…Moreover, tip clearance backflow is also responsible for the increase in blockage from local tip clearance fluid. This can be elucidated based on the blockage generation mechanism of tip clearance flow in compressors put forward by Khalid et al (1999); he modeled the flow exiting the tip clearance gap as series of wakes along the blade chord because its relative stagnation pressure is lower than that of the incoming fluid. As these wakes convect up the passage pressure gradient towards to exit plane, their blockage increases.…”

A Model-Based Mixed Data Approach for Optimizing the Performance of an Offshore Gas Turbine Compressor

“…The correct construction and operation of the components of the gas turbine (GT) plant are also necessary for proper understanding and monitoring (Khalid et al 1998;Kurz and brun, 2001). Furthermore, the function of a GT is the result of the fine-tuned co-operation of the many different components of the plant.…”