The Effect of Water Injection on Multispool Gas Turbine Behavior

Abstract

Background: The injection of water droplets into industrial gas turbines is now commonplace and is central to several proposed advanced cycles. These cycles benefit from the subsequent reduction in compressor work, the increase in turbine work, and (in the case of recuperated cycles) reduction in compressor delivery temperature, which all act to increase the efficiency and power output. An investigation is presented here into the effect such water droplets will have on the operating point and flow characteristics of an aeroderivative gas turbine cycle. Method of Approach: The paper first describes the development of a computer program to study the effects of water injection in multispool industrial gas turbines. The program can operate in two modes: the first uses pre-determined nondimensional wet compressor maps to match the components and is instructive and fast but limited in scope; the second uses the compressor geometries as input and calculates the wet compressor operating conditions as and when required. As a result, it is more computationally demanding, but can cope with a wider range of circumstances. In both cases the compressor characteristics are calculated from a mean-line analysis using suitable loss, deviation and blockage models, coupled with Lagrangian-style droplet evaporation calculations. The program has been applied to a three-spool machine to address issues such as the effects of water injection on power output and overall efficiency, and the off-design nature of the compressor operation. Results: Preliminary results calculated on this basis show similar trends to predictions for single-shaft machines, namely that air mass flow rates and pressure ratios are increased by water injection, and that early stages of the compressor are shifted towards choke and rear stages towards stall. The LP compressor in particular operates at severely off-design conditions. Conclusions: The predicted overall performance of the three-spool machine shows a substantial power boost and a marginal increase in thermal efficiency.