Abstract
The aim of this work is the decomposition, quantification, and analysis of losses related to the axial-gap size effect. Both experimental data and unsteady RANS calculations are investigated for axial gaps equal to 20%, 50% and 80% of the stator axial chord. A framework for identifying sources of loss typical in turbomachinery is derived and utilized for the low-pressure turbine presented. The analysis focuses on the dependency of these losses on the axial-gap variation. It is found that two-dimensional profile losses increase for smaller gaps due to higher wake-mixing losses and unsteady wake-blade interaction. Losses in the end-wall regions, however, decrease for smaller gaps. The total system efficiency can be described by a superposition of individual loss contributions, the optimum of which is found for the smallest gap investigated. It is concluded that these loss contributions are characteristic for the medium aspect-ratio airfoils and operating conditions investigated. This establishes a deeper physical understanding for future investigations into the axial-gap size effect and its interdependency with other design parameters.
Highlights
The axial spacing between moving and stationary rows has been a major focus of research over the past years
Praisner et al (2006) argued that a larger axial gap would be beneficial regarding system loss, as the wakes mix out to a higher degree before they are affected by dilation resulting in higher loss
The wake impinging upon the leading edge of a downstream blade can result in increased incidence and can induce bypass transition, see, e.g., Coull and Hodson (2011); the intensity of both effects is inversely proportional to the axialgap size
Summary
The axial spacing between moving and stationary rows has been a major focus of research over the past years. The axial-gap size affects the intensity of intra-row secondary flow, wake and potential interactions. One reason for conflicting results is the general interdependency of the loss contributions as well as the axial-gap size effect correlating with secondary design parameters. Accounting separately for individual loss contributions in a quantitative manner forms the basis for future investigations into the interdependency of the axial-gap size effect with other design parameters and operating conditions To this end, the present paper will answer the following three questions: 1. The blading itself has been equipped with pressure taps (time-averaged and time-accurate wall taps) on the suction and pressure sides and in the leading and trailing edges This allows determining the wake-flow properties and unsteady flow characteristics for all three axial-gap configurations. Total losses at the rotor trailing edge are substantially lower than for the other gaps
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