Features of Gas Flow in the Axial Stage of a Turbine with an Adjustable Nozzle Vane Assembly

Abstract

The method of changing the gas flow rate through the turbine by adjusting (turning) the nozzle vanes has been known as the most economical one for more than 50 years. It is most widely used in transport gas turbines and internal combustion engine pressurization units. Recently, many articles have appeared in the technical literature on the advisability of using adjustable nozzle vane assemblies (ANVA) in advanced aircraft engines, power generating units, and in turbo-expander units of natural gas preparation and low-temperature separation systems. In all cases of using ANVAs, their developers are attracted by the possibility of economical and reliable provision of variable operating modes of installations. The article shows by calculation that a decrease in the stage efficiency due to the presence of end gaps in the ANVA, related to the flow-over the nozzle vane ends on the profile pressure side to the suction side and subsequent interaction with the main flow through the interblade channel, can be so significant, especially with low-height vanes, that it can lead to vanishing the advantages of this flowrate changing method. Design measures for tightening the gaps in the form of peripheral and root cylindrical bosses at the profile ends, which eliminate overflows in the vane end major part and partially screening the gap are proposed and studied. A favorable effect of the proposed measures on the losses in the ANVA and in the turbine stage has been confirmed.