Predictions of Unsteady Interactions Between Closely Coupled HP and LP Turbines With Co- and Counter-Rotation

Abstract

Here we report on an analytical study of the unsteady aerodynamic interactions of a closely coupled, co-rotating, high- and low-pressure turbine configuration. The effort was focused on the prediction of unsteady surface pressures imparted on the first blade of the low-pressure turbine (LPT). As a first step, a baseline three-row time-accurate prediction was carried out for the first three rows of the low-pressure turbine (vane-blade-vane). In contrast to the three-row results, a four-row analysis, which included the blade of the high-pressure turbine, revealed that the temporally varying tangential load on the LPT blade was increased in amplitude by a factor of five compared to the three-row case with a shift in primary unsteady energy to unexpected frequencies. In the four-row analysis, a region of unusually high unsteadiness near the tip of the LPT blade was also characterized by an increase in the amplitude of the fluctuating surface pressure by a factor of nearly seven, again, with unexpected attendant frequencies. A model is presented which explains the unexpected frequencies realized in the four-row results and allows the redetermination of these frequencies without the use of CFD. In an effort to better understand the complex interactions between the high- and low-pressure turbines, the first vane of the low-pressure turbine was redesigned, and the remaining airfoils were reoriented, to establish a counter-rotating turbine configuration. While substantial reductions in unsteady surface-pressure amplitudes were realized near the tip of the LPT blade with the switch to counter rotation, the amplitude of the temporally varying tangential load on the blade remained notably higher than that from the three-low analysis. The precise physical cause for the high levels of local unsteadiness near the tip of the first LPT blade in the co-rotating configuration remains unclear.

Prediction of Effects of Potential Field Interaction and Wake Interaction on Unsteady Force for Buckets

“…The gradients of these lines of the phase are almost the same in each case, then they do not depend on FT and FA, and this agrees with values obtained by Eqs. (1) and (2). Therefore the line of the phase is expressed by Eq.…”

“…Many examples of a direct solution for the unsteady force by CFD have been published (see e.g., [1,2]). Lan et al [1] carried out a 3-dimensional numerical simulation of the unsteady flow across the one-and-a-half stage axial turbine with different mass flow cases.…”

“…They clearly saw that the unsteady aerodynamic force on the blade was changed at different mass flows from the design case. Praisner et al [2] presented 3-dimensional computational results of closely coupled high and low pressure turbines in both co-and counter-rotating configurations with a focus on the prediction of the unsteady loading for the first blade of the low pressure turbine. The peak-to-peak amplitude of the tangential load was varied compared to the co-rotating 3-row and 4-row results.…”