Thermo-Fluid Dynamic Analysis on the Compression Process of Liquid-Gas Two-Phase

Abstract

In a typical compressor - turbine cycle, the majority of the output power from the turbine is consumed for the operation of the compressor. In order to obtain higher output from turbines, certain techniques are employed to reduce compressor work. It is known that the compressor work increases continually with the increase in temperature of the operating fluid. One of the techniques to achieve reduction of temperature of the fluid is through the inter-cooling between the compressor stages. But this may lead to the decrease in the overall efficiency of the cycle. Another concept is to utilize the high enthalpy of vaporization of water. One of these techniques is known as wet compression. Here water droplets are introduced into the compressor and the fluid mixture is compressed. The droplets absorb the heat from the surroundings and evaporate, and thus reduces the temperature of the operating fluid. This in turn decreases the compression work. Also in order to maintain the O/F ratio of gas turbine, the mass flow rate of the fuel will also increase. All the above mentioned factors thus increase the net power output of the turbine engine. For the current study, a cylinder-piston system containing air involving fine droplets of water is modeled as a simple representation of the wet compression process. The compression process is achieved by the movement of the piston. Thermodynamic properties such as pressure and temperature are investigated in detail for different parameters such as rates of compression, droplet mass and sizes. Analytical equations are derived and validated using the classical D-square law. These equations are used in order to track the change in fluid properties during the compression process and its deviation from dry air compression. The results thus obtained are discussed in terms of the rates of compression, absolute and relative humidities. Corresponding thermodynamic curves are generated which are seen to deviate significantly from the dry isentropic curves. It is observed that smaller diameter droplets, slower speeds of compression and higher amount of overspray percentages lead to lower compressor work.