Thermo-Fluid Dynamics of the Effects of Water Spray on Air Compression Process

Abstract

In a conventional gas turbine setup, a majority of the output power (more than 60%) from the turbine is spent as compressor work. In order to increase the effective output power of the system, it is necessary to cut down the input power requirement of the compressor. In industries techniques such as 'Wet Compression'are employed to achieve this reduction.The process takes advantage of the high enthalpy of vaporization of water droplets to achieve reduction in the overall temperature of the compressed medium. It is established that the thermodynamic work required for the compression of a fluid increases monotonically with the increase in fluid temperature. In wet compression process, during the compression of the fluid (i.e. air), fine droplets of water are injected into the medium. The droplets absorb heat from the surrounding medium and start evaporating; thereby reducing the temperature of air which in turn decreases the compressor work. In addition to this the mass flow rate of the fuel required by the gas turbine system increases in order to maintain its oxidizer-fuel (O/F) ratio, thereby generating extra power output. To study the fundamental thermodynamic process behind wet compression, a cylinder-piston system containing fine droplets of water suspended in air is considered. The axial inward movement of piston enables compression of the fluid mixture. Properties such as pressure, temperature and relative humidity of compressed air are studied in detail for different parameters such as compression rates, droplet diameter and droplet mass. Thermodynamic curves are generated and power savings achieved via wet compression process are calculated. From the results it is seen that smaller sized droplets, slower speeds of compression and higher percentages of overspray lead towards a higher reduction in compressor work.