Ivestigation On Deposition Characteristics Of Salt-out Particles From Steam On Cascade Surfaces In Steam Turbine

With the development of high parameter and large capacity of the thermal power unit,the damage inside the steam turbine caused by salt deposition has become considerable.When the salt deposition occurs in the flow passage of steam turbine,it may result in an increase of the bending stress,centrifugal stress and the axial thrust of the blades,threatening steam turbine safety;it may also change blade shape and then adversely affect the aerodynamic characteristics of the blade,leading to a reduction of the steam turbine efficiency.However,this problem becomes so complicated and hard to study since the steam-particle flow system dramatically differs from the traditional gas-solid two-phase flow system,which makes people have no deep insight in the salt deposition process in the steam turbine stages,so that the theoretical support about the online cleaning couldn’t be provided for the steam turbine.In this study,molecular dynamics method is applied to simulate the agglomeration characteristics and their effect parameters for certain typical kinds of salts in the flow passage of steam turbine;population balance model is loaded based on Euler-Euler two fluid model to study the microscopic behaviors of salt-out particles in the turbine stages;the effects of different kinds of salt deposits and different loads of the unit on the distribution characteristics of salt-out particles are analyzed.The simulation results can provide useful theory information for related online cleaning of the flow passage in the steam turbine.The main research contents are as follows:（1）The model of Na⁺and Cl^-dissolved in steam is built and simulated under the condition of the intermediate pressure cylinder of a 600MW steam turbine.The ionic agglomeration process and the energy variation under different working conditions are analyzed and the specific positions of the ions in the agglomeration process are recorded to obtain various information such as the cluster number,the change of cluster size with time and radial distribution functions between different kinds of salt-out particles.The influences of pressure and temperature on the agglomeration process of Na⁺and Cl^-are also studied.The results show that the larger the degree of pressure drop is,the faster the ions agglomerate process is and the more closely between particles in the agglomerate process,while the degree of the temperature reduction fails to correspond with the ion agglomeration rate.（2）The molecular dynamics models of Na⁺,CO₃^2-and Na⁺,CO₃^2-,Na⁺,Cl^-dissolved in steam are respectively established to study the agglomeration characteristics of salt-out particles under different pressure and temperature.The diameter and number of clusters and the time that agglomeration process takes are analyzed.The radial distribution functions of salt particles under various operating conditions,the collision rate among salt-mixture particles and the effects of pressure and temperature on the ion agglomeration process are investigated.The results show that temperature is the dominated influencing factor to collision and agglomeration process of Na⁺and CO₃^2-,and it is beneficial for the formation of the initial clusters as the temperature goes higher;in the late period of Na₂CO₃ agglomeration process,the pressure begins to play a dominated role,and Na₂CO₃ particles aggregate more closely as the pressure decreases.The agglomeration characteristic of salt-mixture particles,to some extent,is the superposition of single salt particles’;mixing two kinds of salts can accelerate the agglomeration process of salt particles.（3）Population balance model is loaded based on Euler-Euler two fluid model and considering the microscopic behaviors of salt-out particles,the nucleation,growing up,agglomeration and breakage micro-behaviors are conducted numerically.After that the distribution characteristics of particle number,particle diameter and volume fraction of salt-out particles in the turbine stages are presented.The results show that Sauter mean diameter keeps continually growing at an increasing rate in the processes of nucleation and growth;in the process of agglomeration,the Sauter mean diameter also increases with time,but its increasing rate gradually decreases;while during the breakage process,Sauter mean diameter will decrease with time at a constant decreasing rate.The number of the salt-out particles on the suction surface of the stator blades decreases slower than that of the rotor blades.The diameter of particles at the leading edge of stator blades remains the same with time,while that of rotor blades varies drastically with time.The particle volume fraction decreases at the entrance and fluctuates at the middle section of the stator blades,but the peak position of the particle volume fraction on the suction surface of rotor blades is consistent with the position of the maximum turbulence intensity.（4）The distribution characteristics of different kinds of salt-out particles under different loads are studied.The results show that the distribution area at the leading edge of stator blades where the number of sodium chloride particles reaches 6.5?10¹66 decreases gradually with the reduction of the load;the particle diameter on the suction surface of stator blades also decreases as the decrease of the load,while on the pressure surface of rotor blades the situation is opposite.Na₂CO₃ particles have the fastest deposit process on the blades then followed by SiO₂ particles,while Fe₂O₃ particles have the slowest deposit process.Moreover,Na₂SO₄ particles deposit far slower than other sodium salts.