Separating Loss in Axial-Flow-Turbine Cascades
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 4, July, 2020
SEPARATING LOSS IN AXIAL-FLOW-TURBINE CASCADES Yu. A. Grishina and V. N. Bakulinb
UDC 629.7.036:629.438
The use of a system of basic hydrogasdynamic equations, which was written for the portion of contraction of the flow at entry into a cascade of axial-flow turbine profiles at large angles of reattachment to the walls of a blade passage, has enabled the authors to obtain analytical dependences (characteristics) of the loss on the angles of entry of the flow into the cascade. The obtained procedure of calculating the loss permits taking account of the radius of rounding of the profiles at entry, on which the region of insensitivity to the angle of attack is dependent, and also determining relevant flow-entry angles limiting the insensitivity region. The authors have proposed variants of the procedure for both an incompressible-fluid model and a compressible gas, where use is made of the gasdynamic functions of the reduced velocity. The calculated characteristics of the turbine cascades are in good agreement with the existing experimental data in a wide range of angles of attack and radii of rounding of the profiles′ leading edges. The influence of the flow compressibility on the loss has been shown. To test the procedure under the conditions of pulsed flow through the turbine where the angles of attack are particularly large, the authors carried out investigations whose results are also in good agreement with the results of corresponding calculations. Keywords: turbine, angle of attack, separating loss, system of equations, characteristics, gasdynamic functions. In numerical investigations of flows in turbine-driven sets in off-design operating modes when at entry into turbine cascades, there is considerable profile loss of separating flow, the procedure of determining this loss is of significance. This is pressing when the interaction between gasdynamic pulses and turbine-driven sets in turbocharged piston engines is modeled [1–9]. Separating loss is usually calculated using procedures that are based on experimental data [10–13] or are developed with the aid of basic hydrodynamic dependences [14, 15]. As a rule, these procedures have a limited field of application as far as the range of angles of attack is concerned and, which is significant, disregard the great influence of the thickness of the leading edge of the profiles. The influence lies in the fact that the loss begins to grow not immediately with the appearance of an angle of attack, but only when it exceeds a certain limiting value. That is, there is a "region of insensitivity to the angle of attack," which expands with increase in the thickness of the profiles′ leading edge. In a number of cases, taking account of this factor is very important, e.g., in piston-engine turbosupercharge sets with pulsating flow and a relevant change in the direction of entry of the flow into operating cascades, where cascades with an increased radius of rounding of the profiles at entry may be u
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