Non-Equilibrium Condensation

Condensation is a phase-change phenomenon in steam flow that occurs when expansion takes place during non-equilibrium states. The non-equilibrium state is a result of sudden expansion in the flow and vapor reaching a significant level of supercooling.

Condensing steam flows can be observed in transonic/supersonic nozzle-type applications and steam turbines. When steam is used in fluid machinery, such as steam turbines, condensation occurs inside the low-pressure stages of the steam turbine. At this stage, the nucleation of droplets creates wet steam. The nucleation starts at a given level of supercooling. Then, nucleation stops at the point where the condensation shock occurs. The shock occurs due to the droplet growth/condensation and the associated latent heat release. These phenomena are known to affect machine performance by degrading turbine performance, efficiency, and by causing erosion of the blade walls due to impingement of the formed (and growing) droplets.

The non-equilibrium condensation in high speed nozzle flows is described briefly in the following.

In high-speed nozzle flows, pressure and temperature increase locally in the middle of the nozzle. At some point, due to expansion, the supercooled vapor in the nozzle might depart from the equilibrium distribution which changes the main flow field, and a steady condensation shock is generated in the nucleation zone. This phenomenon interrupts the nucleation process, supercooled vapor starts to condensate, and droplet size and wetness will start to increase.

Generally, the condensation phenomenon is modeled in two steps, nucleation and droplet growth. Classical nucleation theory is used to define the minimum energy barrier that is, the level of super-saturation that the vapor phase has to overcome to initiate the transition. Due to the expansion and rapid cooling, the vapor phase can depart from the equilibrium distribution into a metastable state and reach a large degree of supersaturation before condensation actually occurs. Droplet growth is determined by a steady influx of vapor molecules finally decreasing the vapor pressure and correspondingly the level of supersaturation leading to the formation of a stable liquid phase.

The Non-Equilibrium Condensation model available with the Dispersed Multiphase (DMP) approach supports the non-equilibrium condensation modeling described above.