Free energy dominates nucleation and condensation

Improving the efficiency and performance of compressors and turbines have drawn the attention of most scientists and researchers. Injection of water droplets has been a common method for improving the functionality of compressors. It aids the cooling process by increasing the pressure of the droplets and air passing through the compressor passage thereby resulting in evaporation. The formed vapor absorbs heat from the surroundings to cool the compressor. Generally, water vapor in air-flows and steam-flows condenses to small water droplets. The condensation process is by nucleation and nonequilibrium condensation. For instance, heterogeneous nucleation starts when the saturated pressure and temperature fall below their saturated values. Despite the significant effects of the inlet wetness on the condensation and evaporation of water droplets in turbines and compressors, the area has not been fully explored.

Recently, a team of researchers at Tohoku University in Japan led by Professor Satoru Yamamoto investigated various flow problems to determine the effects of inlet wetness on transonic wet-steam and moist-air flows in turbines and compressors. Their work is published in the research journal, International Journal of Heat and Mass Transfer.

The research team commenced their work by simulating transonic wet-steam flows through a Moses-Stein nozzle and Bakhtar’s turbine cascade channel. In these cases, they assumed a homogeneous nucleation. Consequently, the wet-steam flows at the dry and wet inlets of the turbine were simulated and compared to each other. Also, they applied their numerical method to the simulations through a transonic compressor cascade channel with different inlet flow conditions like that in aircraft engines.

The authors observed that the inlet wetness affected the evaporation and growth rates of the water droplets in both the wet-steams and moist-air flows. Also, it influenced the shock location. For instance, in moist-air flows, a decrease in the temperature above the shock was as a result of the large number density of small water droplets. Furthermore, they noticed that 1% wetness in the inlet of the wet-steam flow and a slightly lower number of density of the water droplets (1^*10¹⁴ /kg) resulted in secondary condensation. Consequently, a higher density number (1^*10¹⁶ /kg) resulted in no secondary condensation. This was due to the difference of maximum free energy between the two cases. The free energy deeply dominates nucleation and condensation.

Since the growth of water droplets can take place even in wetted flows depending in the velocity, the Tohoku University researchers realized the importance of resolving the growth rate in supersonic flows with shock as a breakthrough in accurate prediction of transonic moist-air and wet-steam flows. For a moist air with 0.1% wetness, the water droplets expanded and evaporated after the normal shock since it was inadequate to facilitate the cooling process after the shock. From the experimental results that were extended to the industrial gas turbine, the authors concluded that after the shock, the air can be effectively cooled by increasing the number density of smaller water droplets.