جستجوی مقالات مرتبط با کلیدواژه « condensation shock » در نشریات گروه « مکانیک »

Condensation phenomena in steam flow, can cause droplets with different sizes to form. For an exact prediction of two phase flow behavior, it is necessary to consider the effects of all droplets with different sizes on the steam. In this paper, nucleation equation in an Eulerian–Lagrangian framework has been used to analyze one–dimensional flow of wet steam in a supersonic convergent–divergent nozzle. Polydispersed and monodispersed radius methods for modeling the formed droplets are compared. In polydispersed method, all the formed droplets in the spontaneous condensation zone, are retained in the calculations, without being merged with other groups; but in monodispersed method, all groups are merged, and only one group with averaged radius is retained in the calculations. The polydispersed method has an advantage and can predict the complete droplet spectra. Results of comparing the two methods with experimental data indicates that the predicted radius in the polydispersed method, in every four investigated cases, is closer to experimental data, than that of monodispersed method.

Wetness loss is a thermodynamic loss، which is due to irreversible heat transfer between the liquid and vapour phase during condensation. Formation and subsequent behavior of the liquid decreases the performance of turbine last stages. This phenomenon has a major effect on the efficiency of steam turbines. In this study، the steady compressible condensation flow field with heat transfer in a two dimensional of convergent divergent nozzle has been studied. The ANSYS FLUENT V14. 5 software is used to calculate flow properties. The CFD coded 2D- approximate Eulerian- Eulerian multi- phase method for non- equilibrium condensation in transonic steam flow is used to capture nucleation phenomenon in the wet steam flow. Density based finite- volume discritization is employed and flow is considered inviscid. To examine heat transfer effects on the condensation flow in the convergent section of nozzle، a thermal source is used. The perfect gas equation of state is used to calculate the pressure and the properties of steam condensation. The pressure distribution over the nozzle and the average radius of droplets in adiabatic case are compared with both experimental and analytical data and show a good agreement. After verifying the model، the condensation flow with volumetric heat transfer is studied. The results show that by adding volumetric heating in convergent section of the nozzle the condensation shock strength decreases and even is omitted which cause to reduce droplets produced during the condensation flow.

The prediction of distillation zone is very important in steam turbine blades and steam nozzles. In identification of distillery with equilibrium method, as the steam flow contacts the two-phase dome, the second phase formes and flow properties will pass the distillery without any jumping, therefor after crossing the saturation curve, the droplet formation transpires, but in non-equilibrium method by a sudden increase in pressure, called “condensation shock” a discontinuity in the flow characteristics is seen and after crossing the saturation curve, the formation of droplets starts. In this paper, numerical analysis of a vapor-liquid two-phase transonic flow in a convergent-divergent nozzle with and without shock is investigated. Effects of stagnation temperature at nozzle inlet, viscosity and geometry is studied using thermodynamic equilibrium and non-equilibrium methods and results compared with experimental datas. Roe numerical method is used for vapor-liquid two-phase flow numerical solution. The main properties of the flow at the boundary of elements is extrapolated by MUSCL third order acuracy and time discretization is performed using Lax-Wendroff explicit two-step method of second order accuracy. It is observed that the results of non-equilibrium solution, has more correspondence to experimental results and Condensation starts earlier in the nozzle with further expansion rate. By increasing the temperature at nozzle inlet, the place at which condensation starts goes forward. Also in comparision with non-viscous flow, the shock location in viscous flow comes closed to the throat.

Steam flow at blade passages of low pressure steam turbines becomes a supercooled drysteam due to rapid expansion and crossing the saturation line. But after nucleation and condensation shock, phase change from vapor to liquid droplets occurs which is called two-phase or wet steam flow. In this paper, the aim of developing finite-volume flow of wet Jameson is considered for the first time in two-dimensional study by using the advantages of CUSP's numerical method. In this paper, equations governing the formation of liquid phase are combined with equations of survival and by using CUSP's numerical approach in Jameson's finite-volume method (the integrated method) the positive features of both of these methods can be simultaneously used in the modeling of two-phase flow. To calculate nucleation, the classical equation of nucleation with appropriate correction and also Lagrangian solution for growth of droplets are used in integrated method. Additionally, condensation shock effect on the pressure distribution and the droplet size has been calculated and compared with experimental data. Given the importance of areas of shocks on the suction surface of the blade, the focus of integrated method is shifted to this area. The results of integrated two-phase model are examined in subsonic and supersonic flow output. In the shock area on suction surface blade, using the CUSP's method (the integrated method) demonstrates better coverage in predicting attributes of flow in target area in comparison with the experimental data by a reduction of 20 percent in numerical errors.

During the course of expansion of steam in LP steam turbines، the vapor becomes supersaturated، and subsequently nucleates. Then condensation occurs، that causes some damages. Exact prediction of wetness terms، and formed droplets radius has extensive effects on accurate estimation of the flow properties، and damages due to vapor condensation. Nucleation rate is one of the governing equations of liquid phase which finally determines number of generated droplets. Considering exchange of mass and energy between two phases، different models of radius averaging at each cross-section of nozzle have been used. In the present investigation، various nucleation models have been combined with different droplet radius averaging methods in a semi-analytical Eulerian-Lagrangian model. The pressure distribution and droplets radius، in several cases، have been compared with experimental data. According to the results، using Girshik-Chiu’s refinement on Hale’s nucleation model with simultaneous application of Sauter averaging approach، provide the best prediction of the flow properties. In addition، classical nucleation equation with Girshick-Chiu’s refinement and without any refinement in combination with Sauter averaging approach; are standing on the second and third ranks and specially focusing on the pressure distributions.

During the course of expansion of steam in a Laval nozzle and a cascade of turbine, the state path crosses the saturation line; the steam first supercools and then reverts to equilibrium through the spontaneous formation of droplets or condensation shock, which causes aerodynamics and thermodynamics losses. In this way by formation of droplets and so reducing Gibbs energy, equilibrium is reached. This paper describes a two-phase model and provides an approach for including spontaneous homogeneous nucleation. In order to solve conservation equations, coupled with the equations of formation and growth of the droplets, a 2-D time-marching solution scheme with Baldwin-Lomax turbulence model, was used in this study. Pressure distribution and droplets size in a Laval nozzle and a turbine cascade are predicted and compared with empirical results. In the strength of validation, the effect of injection of water droplets into the steam flow in order to control the intensity and location of condensation shock is considered theoretically. A converging nozzle is used to producing droplets at inlet of turbine cascade. The results illustrate that wet steam at inlet of a turbine cascade weakens or delays the condensation shock in the passage of the blades.