Performance Of An Opposed-Setting-Two-Stage Centrifugal Compressor Under Water-Laden Conditions

Developing marine gas turbines vigorously is a trend in recent decades.The humidity of the air in the ocean environment is high,and the intake duct will inevitably inhale water during the high-speed and maneuvering operation of ships.These two aspects will affect the performance of gas turbines and their components.Based on the research and development of a marine micro gas turbine,the influence of inflow water-laden conditions on the characteristics and flow field of an opposed-setting-two-stage centrifugal compressor is studied in this project.Referring to the inlet conditions in the ocean,the current conditions are divided into "passive water"(humidity),and "active water"(water injection)."Passive water" refers to the humidity of the environment.Humid air is a mixture of vapor and dry air,of which thermodynamic properties are quite different from a pure one,probably exceeding the measurement uncertainty,and leading to the failure of repeatability of a test.With the increase of humidity,the specific heat at constant pressure and gas constant of the gas increase,the specific heat ratio and the compressor efficiency decrease.On this basis,the modified factors of air humidity on general characteristics of a compressor are deduced,which are closely related to the modular design,and the modified coefficient of similar design under wet air condition is deduced as well.6 ambient conditions with water-air ratios varying from 0 to 0.01 are calculated by ANSYS CFX to show the humidity effects on the compressor performance.As the results show with the increment of water-air ratio,the specific heat at constant pressure and gas constant rise,the specific heat ratio decreases.At the same mass flow rate,the pressure ratio,temperature ratio,and polytropic efficiency deteriorate slightly as well as the torque of the compressor.The static pressure along the surface of HP diffuser blade,the loading at the leading edge of the vane and the attack angle of HP diffuser inlet decrease with the augment of humidity,which reduces the flow separation along the suction side.Meanwhile,as the humidity is regarded,the stall margin gets increased under all moisture conditions compared with the dry one.The larger the humidity is,the smaller the stall margin improvement is."Active water" refers to water injection which is quite close to the practical processes including splashing,wet compression,and online washing.The Euler-Lagrangian method is used to calculate the continuous flow field and discrete particle terms.At 7 inlet conditions with dry air and different combinations of water spraying rate and particle size,the variation of compressor total performance,component performance and the trajectory of water droplets are analyzed and compared.The results show that wet compression can improve the pressure ratio and efficiency of the whole machine,but the higher the pressure ratio and efficiency are,the less the stall margin is.In the low-pressure stage,liquid evaporation decreases the outlet temperature.The high-pressure stage benefits from the cooling effect of the former stage so that pressure ratio and temperature ratio increase,but the enhancement by wet compression is less obvious.Droplets mainly accumulate at the root of the pressure side of the impeller blades.Large losses occur near the trailing edge in both the diffusers and the return channel.The leading edge of the diffuser vane is attacked by the water droplets,and there will be a large entropy generation due to droplet breakup and mixing.