Modeling of Variable Inlet Guide Vanes Affects on a One Shaft Industrial Gas Turbine Used in a Combined Cycle Application

It is well known that gas turbine simulation involves satisfying the conditions of compatibility between its components. At design point, the components are all well matched and working at high efficiency regions. However, at steady state off-design, due to the compatibility issues and changes in operating parameters, basically turbine entry temperature and pressure ratio to attain a certain load, it is possible that the components may be working within regions of low efficiency. A reason for this phenomenon is that the flow areas at the various sections of the engine correspond to that at design point, such that operation at off-design is restricted. One way to widen the operational envelope of an engine is varying these flow areas, providing a good match between the gas turbine components. A widely used type of variable geometry which has attracted a great amount of interest is the use of compressor variable geometry, the so called variable inlet guide vanes (VIGVs), as a power control strategy, which involves the control of the air flow rate entering the compressor and the power output modulation at constant rotational speed. The purpose of the air flow rate modulation is to enhance the heat recovery performance and thus increase the combined cycle efficiency by maintaining high turbine exhaust temperature. One methodology used to model a variable geometry compressor, in the absence of its geometric data involves the use of correction factors, as functions of the VIGV change. Fundamentally, this methodology assumes that each new position of the VIGVs represents a new machine, i.e., a new design point, such that its original map of characteristics is displaced in order to describe this “new” compressor. The purpose of this work is to analyze the influence of the use of different functions for these correction factors on a W501F (one shaft, industrial) gas turbine simulation. An in-house computer program developed for performance modeling of gas turbines was utilized to carry out the simulations. The results provided by this computer code show good agreement with operational data, indicating that, although more tests must be conducted, the methodology seems to be reliable enough for the aims of the project for which it has been developed.