SPRAY CHARACTERISATION DOWNSTREAM OF THE SWIRL PRESSURE NOZZLES IN GAS TURBINE FOGGING AND HIGH FOGGING APPLICATIONS

摘要

Injection of water into the intake of a GT compressor is becoming a standard technique for gas turbine power augmentation. Injection of water to humidify the inlet air up to a 95% relative humidity and therefore cool it close to the prevailing wet bulb temperature is known as conventional inlet fogging or under spray application. High fogging, or over spray application, is characterized by its injection of up to 2% water relative to the inlet air mass flow. The high fogging technique allows water droplets to enter GT compressors, thus creating different conditions for the compressor operation with a design operating point. This technique is still being extensively developed to optimize integrated gas turbine performance. Depending on ambient conditions (temperature and humidity) a 5-10% power boost for fogging applications can be achieved. A typical target when using high fogging systems is a power boost of 5% per 1% of injected water. This paper reports the findings of the spray characterization downstream of swirl pressure nozzles, typically used in fogging and high fogging systems. Results are gathered from numerous experimental studies in the laboratory and in a scaled down heavy-duty gas turbine air intake. The droplet size and water mass flux within the spray plume are measured by varying the following parameters: Distance downstream of the nozzle orifice and as well as ambient air temperature and relative humidity. The main criterion for characterization of nozzles is the droplet size distribution of produced sprays. Droplet size distribution is required to be within 20-30μm, in order to enable quick evaporation for the inlet fogging application due to a very short residence time between the filter house and the compressor inlet. For the high fogging application droplets have to be able to follow the air stream when entering the compressor thus reducing the danger of their impact on compressor blades and causing blade erosion. This is only achieved if they are sufficiently small. Droplet size distribution is measured by using laser diffraction and imaging based spray diagnostics, under conditions similar to those appearing in GT intakes. Therefore, all measurements reported here are performed in wind tunnels with air velocities from 5 to 25m/s and on-site. When measuring droplet size distribution far away from a nozzle, evaporation of droplets can occur. This evaporation process of the droplets changes the measured statistics of the droplet size distribution, shifting the measured spectrum towards the larger end of distribution. In order to avoid this biasing effect during measurements, the air in the wind tunnel is humidified up to 99% rH., preventing evaporation taking place downstream of the nozzle.