Control of wetness fraction and liquid droplet size in wet steam two phase flows with hot steam injection

Abstract

The sudden expansion of steam in the last stages of turbines causes a nucleation process to take place and liquid droplets to appear. Parts of these droplets, which grow collide with the blades and seriously damage them. This mechanical destruction also coincides with a thermodynamic shock which suddenly increases the pressure and reduces the performance of the steam turbine. In the present study, an attempt is made to control the size of droplets and liquid mass fraction by injecting hot steam to reduce the level of the damage. This process is similar to extracting steam from upper stages and injecting it into downstream stages. In order to prove the concept of this method, a one-dimensional in-house, two-phase flow code with steam injection is developed. The conservation equations are formulated for the vapor/liquid mixture and the liquid phase is modeled by two additional transport equations. Size and number of droplets together with liquid mass fraction are calculated in different convergent-divergent nozzles and the results are compared with experimental values. Then, the steam with higher pressure and temperature is injected into various locations in the nozzle. The results show that injecting hot steam in the divergent point of the nozzle downstream of the throat and following the Wilson point reduces the size of droplets but increases their number, which can ultimately, degrade the erosion of the blades. It is also concluded that in such a situation, the injection mass flow rate of nearly 5% of main stream flow with a temperature of about 30% higher than the main stream temperature is efficient in controlling wet steam parameters.