Vapor-water two phase flow separation in pressure vessel of nuclear power plants is accomplished with swirl motion using vanes. In order to reduce separation pressure loss and to make it economic, a new type of low cost simplified innovative separator using lattice core configuration is proposed where swirling is caused by the orthogonal driving flow. The performance of the separator has been assessed numerically with the commercial CFD code FLUENT 14.0. The numerical analysis is compared with the experiment. The geometry and flow conditions are chosen according to the experiment. In the analysis, standard k – e and realizable k – e turbulence models are implemented. The prediction of maximum air void fraction with realizable k – e model was almost the same as input air void fraction but the void fraction computed by standard k – e model was compared better with the experimental results than the realizable k – e model. Some discrepancies in flow pattern between the experimental and simulation results are observed which might be due to the difference of nozzle shape. However, a more detailed model is necessary to arrive at the final conclusion.
Boiling water reactors (BWRs) are equipped with steam separators for splitting a two-phase mixture into steam and water before feeding the steam to dryers and turbines [
Hirai and Yokobori have proposed a new two phase flow separation system using swirling fluidics [
These performances have been estimated in such mock-up tests, however, such tests are of long duration and costly. Therefore, it would be better to develop a numerical method to estimate separator performances so as to develop an innovative separator. In the present study, an idea of low cost simplified innovative separator using lattice core configuration is proposed where swirling is caused by the orthogonal driving flow. The four fuel assemblies around the control rod of BWR, the conventional separator with swirler and the proposed concept of the separator are shown in Figures 1-3, respectively. But the performance of the separator is still needed to be assed. Hence, the objective of the present study is to develop a numerical model to estimate the performance of the proposed separator and to validate the developed model with the experimental results. A three dimensional analysis of steam-water separation has been performed with the commercial numerical analysis code ANSYS FLUENT 14.0 which consists of a three-dimensional two-fluid model [
The computational fluid dynamic code, FLUENT 14.0 was used to investigate the 3-D turbulent flow in the cylinder using the standard k − e and the realizable k − e turbulence models. In the calculation a phase–coupled SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm for the pressure-velocity coupling is adopted. The second order upwind schemes were used for the momentum while first order upwind schemes were used for the turbulence kinetic energy, turbulence dissipation rate, and volume fraction. Convergence was assumed when the residual of the equations dropped more than three orders of magnitude. The under relaxation parameters for pressure, momentum, volume fraction, turbulence kinetic energy and turbulence dissipationrate were selected as 0.3, 0.7, 0.5, 0.8 and 0.8,
respecttively. Since the flow simulation involved the combined effects of turbulence and two phase flow, the standard k − e and realizable k − e turbulence models are implemented in this study.
The standard k − e model [
The turbulence kinetic energy, k, and its rate of dissipation, e, are obtained from the following transport equations:
and
In these equations, represents the generation of turbulence kinetic energy due to the mean velocity gradients, is the generation of turbulence kinetic energy due to buoyancy, represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, , and are constants. and are the turbulent Prandtl numbers for k and, respectively and are user-defined source terms.
The turbulent (or eddy) viscosity, , is computed by combining k and as follows:
where is a constant.
The model constants for the standard k − e model are .
The realizable k − e model proposed by Shih et al. [
• A new eddy-viscosity formula involving a variable originally proposed by Reynolds [
• A new model equation for dissipation (e) based on the dynamic equation of the mean-square vorticity fluctuation.
The modeled transport equations for k andin the realizable k − e model are
and
where
It is to be noted that the k equation in the realizable k–e model is the same as that in the standard k − e model except for the model constants. However, the form of the equation is quite different from that in the standard k − e model.
The turbulence viscosity is computed from
where
and, where is the mean rate of rotation tensor viewed in a rotating reference frame with the angular velocity.
The model constants and are given by
,
where
The model constants for the realizable k − e model are
Water and air velocity of mail flow was 0.3 m/s and
the swirling driving flow velocity was 1.2 m/s. Inlet void fraction of air in the main flow was 68% where as inlet void fraction of swirling driving flow was 0%. The calculation was performed under the gravitational acceleration of 9.81 m/s2. The time step size, number of time steps and maximum number of iterations per time steps were 0.01s, 2000 and 1000, respectively. The angle of rotation of swirler was 90˚.
The results of the three dimensional numerical computations of vapor-water two phase flow separation using swirl has been presented in this section. In order to gain confidence of the modeling methodology that is required to adequately simulate the separator flow, the analysis results were compared with the experimental results performed by Hirai [
about 69% which was almost the same as input air void fraction (68%) but the void fraction computed by standard k − e model compared well with the experimental results rather than the realizable k − e model. Figures 8 and 9 represent the velocity vector computed by standard k–e model and realizable k − e model, respectively. The velocity vectors clearly indicate the generation of swirl flow for both of the cases.
swirling flow of high velocity around the center and weak swirling flow of low velocity near the wall. The strong swirling flow in the center results in centrifugal forces that propel the liquid phase (water) to the outer free vortex region, where water is collected and reside in the weak swirling intensity region (
The numerical methodology presented herein establishes that the proposed innovative separator has the potentially significant value in effectively separating the vapor-water two phase flow in BWR. The standard k − e turbulence model and realizable k − e turbulence models are able to predict the flow features inside the separator such as void fraction and velocity distribution. The velocity vectors clearly indicate the generation of swirl flow for both of the cases. The results of the numerical simulation were compared with the experimental data and were found in reasonable agreement. The analysis revels that the maximum air void fraction with realizable k − e model was almost the same as input air void fraction but the void fraction computed by standard k − e model compared better with the experimental results than the realizable k − e model. Some discrepancies in flow pattern between the experimental and simulation results are observed which might be due to the difference of nozzle shape. However, a more detailed model is necessary to reach in the final conclusion.