Numerical Investigation of Equilibrium Wet Steam Flow Property Based on S2 Calculation Code
Authors: Li Deying , Song Yanping , Fu Yunfeng , Chen Huanlong , Ooyama Hiroharu 
Published: September 2014

Though it is non-equilibrium wet steam flow in steam turbine, the droplets with micron dimension are usually not considered in the primary design of the steam turbine in the industrial application because the global properties of steam are concerned mostly. Two calculation models of equilibrium wet steam property are proposed and applied to the performance prediction of a one stage low pressure (LP) steam turbine with superheated vapor and a 1.5-stage LP steam turbine with superheated and saturation steam by the using of the central S2 stream surface calculation code developed in this paper.

Different from the ordinary S2 stream surface calculation code, the three-dimensional Euler equations are adopted to count the influence of the cascade spatial structure on the flow field parameters in this S2 calculation method. The non-conservative variables are used to avoid the non-uniqueness of density. The implicit time marching method and the upwind-diagonal implicit approximate factored format are applied to improve the calculation stability. The high-order TVD scheme is applied to increase the calculation accuracy. The Riemann problem is utilized to solve the discontinuity of variables transition at grid interface.

Two models of wet steam property computation are developed to adapt the numerical calculation in this paper. The first one is the “Ideal Steam” model, using the ideal gas properties formulations to calculate the steam parameters with the given cp(T)-T relationship obtained from the evaluated enthalpy-entropy curve and the IAPWS-IF97 formula. The second one is the “IF97 Table” model, calculating the steam properties with the inerratic multi-variables combination tables generated by the IAPWS-IF97 formulations and the fast interpolation method.

Furthermore, both the models are added to the S2 calculation code to predict the performance of the steam turbine respectively. The results show that the S2 code developed in this paper can predict the performance of steam turbine with high precision. The results of the one stage LP steam turbine imply that the two models are almost the same and are similar to the 3D results, indicating that both the models can be applied to the superheated vapor flow with high precision. Compared to the full 3D results, the error of the mass flow rate and efficiency with the “IF97 Table” model is 0.83% and 0.46% respectively, indicating a higher accuracy than the “Ideal Gas” model with 1.00% and 1.54%. The distribution of primary parameters shows that both the steam calculation models are well applied to the aerodynamic performance prediction. The distribution of temperature and moisture imply that the “IF97 Table” model can predict the aerodynamic parameters better than the “Ideal Gas” model. The S2 calculation method with the “IF97 Table” method or the “Ideal Gas” saves most of the time compared to the full 3D calculation, supplying an effective and fast calculation method in the primary design and the performance prediction of steam turbine.