Constantine P. Tzanos

A Movable Boundary Model for Once-Through Steam Generator Analysis

A method was developed for the analysis of a once-through steam generator that is based on a multinode movable boundary formulation and an accurate description of the departure from nucleate boilin...

HIGHER-ORDER DIFFERENCING METHOD WITH A MULTIGRID APPROACH FOR THE SOLUTION OF THE INCOMPRESSIBLE FLOW EQUATIONS AT HIGH REYNOLDS NUMBERS

A higher-order differencing method was recently proposed for the convection-diffusion equation, which even with a coarse mesh gives oscillation-free solutions that are far more accurate than those of the upwind scheme. In this subsequent work, the performance of this method was investigated in conjunction with the performance of different iterative solvers for the solution of the Navier-Stokes equations in the vorticUy-streamfunetion formulation for incompressible flow at high Reynolds numbers. Flow in a square cavity with a moving lid was chosen as a model problem. Solvers that performed well at low Re numbers either failed to converge or had a computationally prohibitive convergence rate at high Re numbers. The additive correction method of Settari and Aziz and an iterative incomplete lower and upper (ILU) solver were used in a multigrid approach that performed well in the whole range of Re numbers considered (from 1000 to 10,000) and for uniform as well as nonuniform grids. At high Re numbers, point or...

COMPUTATIONAL FLUID DYNAMICS FOR THE ANALYSIS OF LIGHT WATER REACTOR FLOWS

Abstract Benchmark experiments simulating flows in a pressurized water reactor rod bundle were analyzed to evaluate the performance of a state-of-the-art computational fluid dynamics (CFD) code. For the simulation of turbulence a number of standard k-ε models were used. Away from components that cause significant flow deflections, the difference between mean velocity predictions and measurements is within the experimental error. Near such components there is significant discrepancy between velocity predictions and measurements. Even in rod bundles without flow deflectors, the turbulence predictions of standard k-ε models show significant discrepancy with measurements. These discrepancies are greater near components that cause flow deflections. Turbulence generated by vanes on spacer grids significantly enhances thermal mixing. To improve the fidelity of CFD simulations of flows in reactor rod bundles, the development of Reynolds averaging of the Navier-Stokes equations turbulence models based on such flows is needed.

Jet breakup and spray formation in a diesel engine

The breakup of injected fuel into spray is of key interest to the design of a fuel efficient, nonpolluting diesel engine. We report preliminary progress on the numerical simulation of diesel fuel injection spray with the front tracking code FronTier. Our simulation design is set to match experiments at ANL, and our present agreement is semi-quantitative. Future efforts will include mesh refinement studies, which will better model the turbulent flow.

An optimization study for the reactor vessel auxiliary cooling system of a pool liquid-metal reactor

This paper reports on the effects of design parameters on the performance of the reactor vessel auxiliary cooling system (RVACS) of a pool liquid-metal reactor (LMR). These parameters include stack height, size of the airflow gap, system pressure loss, fins on the guard vessel or the baffle wall, and repeated ribs on the airflow channel walls. As a measure of performance , the peak sodium pool temperature during transient following a reactor scram from full power was used. Horizontal ribs with a 0.003-m height and a 0.015-m pitch gave the best performance, i.e., the lowest peak sodium pool temperature during the scram transient. For a 3500-MW(thermal) LMR, they gave peak hot pool and peak cladding temperatures that were 52{degrees}C lower than those obtained with a reference RVACS having smooth airflow channel walls.

Liquid-metal fast breeder reactor intermediate heat exchanger transient modeling for faster than real-time analysis

A method was developed for faster than real-time liquid-metal fast breeder reactor intermediate heat exchanger (IHX) analysis for purposes of continuous on-line data validation, plant state verification, and fault identification. The basic feature of this method is the utilization of spatial nodes whose sizes vary with time. The use of time-variant node sizes leads to adequately accurate solutions with a few nodes and at short computation times. Applications of this methodology to reference IHX problems with the IBM 3033 machine showed that the computation time for steady-state analysis was --6 ms.

Improved Simulations of Heat Transfer in Liquid-Metal Flows

Abstract In liquid-metal flows, the predictions of the Nusselt number (heat transfer) by Reynolds-averaged Navier–Stokes models of turbulence that use the assumption of a constant turbulent Prandtl number can be significantly off. Heat transfer analyses were performed with a number of turbulence models for flows in a triangular rod bundle and in a pipe, and model predictions were compared with experimental data. Emphasis was placed on the low Reynolds (low-Re) number k-ε model that resolves the boundary layer and does not use “logarithmic wall functions.” The high Reynolds (high-Re) number k-ε model underpredicts the Nusselt number up to 30%, while the low-Re number model overpredicts it up to 34%. For high Peclet number values, the low-Re number model provides better predictions than the high-Re number model. For Peclet numbers higher than 1500, the predictions of the Reynolds stress model (RSM) are in very good agreement with experimental measurements, but for lower Peclet number values its predictions are significantly off. A relationship was developed that expresses the turbulent Prandtl number as a function of the ratio of the turbulent viscosity to the molecular viscosity. With this modified turbulent Prandtl number, for the flow in the rod bundle the predictions of the low-Re number model are well within the spread of the experimental measurements. For pipe flow, the model predictions are not as sensitive to the correction of the turbulent Prandtl number as they are in the case of the flow in a bundle. The modified low-Re number model underpredicts the limited experimental data by 4%.

Numerical predictions of turbulent flow around a structure via the k-ε model for reactor vessel auxiliary cooling system performance analysis

Turbulent airflows around structures are important in many engineering applications. Such flows can have a significant impact on the thermal performance of the reactor vessel auxilliary cooling system (RVACs) of advanced liquid-metal reactor designs. The adequacy of the high-Reynolds-number form of the k-e model in analyzing turbulent airflow around structures like the RVACS stacks is evaluated. An experiment of simulated atmospheric turbulent flow around a cube is analyzed with the computer code COMMIX, and numerical predictions for pressure and velocity distributions are compared with experimental measurements. Considering the complexity of the problem and the approximation involved in the k-e model, the overall agreement between numerical predictions and measurements of pressure coefficients and velocities is good. The largest discrepancies between predictions and measurements are in the pressure coefficient at the sections of the top and side cube surfaces very close to the upwind edges and in the spanwise velocity distribution downstream from the cube

Central difference-like approximation for the solution of the convection-diffusion equation

A method was developed for the numerical solution of the convection-diffusion equation. It is based on a central difference-like approximation and the requirement that cell face values of the convected variable must be bounded by their adjacent cell center values. To evaluate its performance, a two-dimensional problem that has bean used in the literature to test a large number of methods was used as a benchmark. The predictions of the proposed method are in very good agreement with the benchmark for all Peclet numbers. They are free of oscillations and far superior to those of the upwind scheme. At high Peclet numbers, spatial oscillations are the weakness of central difference schemes and Galerkin finite-element methods. Also, at high Peclet values, due to false diffusion, the accuracy of first-order upwind and hybrid schemes in practical meshes is poor.

A Semianalytic Method for the Solution of the Steady-State Steam Generator Equations

A model was developed for steady-state steam generator analysis for purposes of continuous on-line data validation, plant state verification, and fault identification. The basic features of this model are as follows: 1. The water (steam) temperature in the subcooled and superheated regions is written as a quadratic function of enthalpy and the system equations are solved analytically. 2. The same equations are also solved analytically in the nucleate and film boiling regions. 3. The departure from nucleate boiling quality is determined from the iterative solution of a transcendental equation. These features allow the use of large nodes in each region and the direct determination of the region boundaries. Applications to typical steam generator problems showed computation times of 0.025 s. The analysis of four Energy Technology Engineering Center experiments showed very good agreement between model predictions and measurements.