The Effect of Water Injection on Multi-Spool Gas Turbine Behaviour

The injection of water droplets into industrial gas turbines is now common place 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 aero-derivative gas turbine cycle. The paper first describes the development of a computer program to study the effects of water injection in multi-spool industrial gas turbines. The program can operate in two modes: the first uses pre-determined non-dimensional 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.