Two-region Approach Research Articles

Effect of offset distance on the performance of two-region porous inert medium burners at low thermal power operation

A modified technique to improve the performance of the conventional two-region approach used in stabilizing the combustion wave within porous inert media (PIM) burners operating at low power was proposed. This was achieved by detaching the two regions by an adjustable offset distance aiming to control the effects of the various interacted heat transfer mechanisms between the two remoted regions. The validation of this technique was experimentally examined for three offset distances of 13, 26 and 52 mm by comparing its performance with the conventional approach at 1 and 2 kW. The axial temperature distributions were measured versus time for wide ranges of stable and unstable operating conditions. Almost-constant temperature zone was created in the gap between the two regions. The stability limits range was improved by an excess air ratio of 0.2 at 1 kW and 52 mm. At 2 kW, the entire stability diagram was shifted down by an excess air ratio of 0.1 into hotter regime. A tremendous ultra-low CO-emission of ≤1 ppm, beside the ultra-low NOx-emissions of ≤2 ppm, was obtained. The CO-emission was significantly reduced with the offset distance. This may solve the high CO-emission experienced with the conventional PIM burners operating at low power.

Low-Reynolds-number diffusion-driven flow around a horizontal cylinder

Diffusion-driven flow occurs when any insulated sloping surface is in contact with a quiescent stratified and viscous fluid. This startling fluid motion is generally very slow, and is caused by a hydrostatic pressure imbalance due to bending of isotherms near the surface. In contrast to previous studies of the phenomenon, the low-Reynolds-number case is considered here, for which the induced steady motion is influenced by viscous and density diffusion over a much larger length scale than the size of the insulated object. The relevant linear equations of steady two-dimensional motion in a linearly stratified fluid are solved for a circular cylinder using a matched two-region approach that yields analytical solutions for the streamfunction and the density variations both close to and far from the object. The exact analytical expressions for the solutions in the ‘outer-flow region’ are new, and after matching enable accurate solutions to be evaluated easily at any point. Similar qualitative behaviour is expected under similar conditions near isolated objects of other shapes, including for a sphere. Implications for multiple objects are also discussed.

Wind flow simulation over forested areas using a 3D RANS solver with a tree – scale approach

Wind flow over forested areas is simulated using a 3-D Reynolds Averaged Navier–Stokes solver, employing the Wilcox k–ω model modified for atmospheric flows. Each tree is simulated individually with a two-region approach considering the leaf area and the trunk area. Each area affects the surrounding flow acting as a momentum sink term in the RANS equations. Leaf area sink terms are computed via Leaf Area Density which is described by an empirical relation. Predictions are compared against measurements and simulation results of other methods. A good agreement with measurements is observed as a result of a more detailed simulation of the canopy in comparison to simpler methods. A parametric analysis investigates the effect of the forest density as well as of the type and shape of the trees on the surrounding flow. A reduction of wind speed deficit up to 25% is predicted when the tree shape changes from cylinder to cone.

Integrated simulation approach for laser-driven fast ignition.

An integrated simulation approach fully based on the particle-in-cell (PIC) model is proposed, which involves both fast-particle generation via laser solid-density plasma interaction and transport and energy deposition of the particles in extremely high-density plasma. It is realized by introducing two independent systems in a simulation, where the fast-particle generation is simulated by a full PIC system and the transport and energy deposition computed by a second PIC system with a reduced field solver. Data of the fast particles generated in the full PIC system are copied to the reduced PIC system in real time as the fast-particle source. Unlike a two-region approach, which takes a single PIC system and two field solvers in two plasma density regions, respectively, the present one need not match the field solvers since the reduced field solver and the full solver adopted respectively in the two systems are independent. A simulation case is presented, which demonstrates that this approach can be applied to integrated simulation of fast ignition with real target densities, e.g., 300 g/cm(3).

A Hardware-Accelerated Quantum Monte Carlo framework (HAQMC) for N-body systems

Interest in the study of structural and energetic properties of highly quantum clusters, such as inert gas clusters has motivated the development of a hardware-accelerated framework for Quantum Monte Carlo simulations. In the Quantum Monte Carlo method, the properties of a system of atoms, such as the ground-state energies, are averaged over a number of iterations. Our framework is aimed at accelerating the computations in each iteration of the QMC application by offloading the calculation of properties, namely energy and trial wave function, onto reconfigurable hardware. This gives a user the capability to run simulations for a large number of iterations, thereby reducing the statistical uncertainty in the properties, and for larger clusters. This framework is designed to run on the Cray XD1 high performance reconfigurable computing platform, which exploits the coarse-grained parallelism of the processor along with the fine-grained parallelism of the reconfigurable computing devices available in the form of field-programmable gate arrays. In this paper, we illustrate the functioning of the framework, which can be used to calculate the energies for a model cluster of helium atoms. In addition, we present the capabilities of the framework that allow the user to vary the chemical identities of the simulated atoms. Program summary Program title: Hardware Accelerated Quantum Monte Carlo (HAQMC) Catalogue identifier: AEEP_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEEP_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 691 537 No. of bytes in distributed program, including test data, etc.: 5 031 226 Distribution format: tar.gz Programming language: C/C++ for the QMC application, VHDL and Xilinx 8.1 ISE/EDK tools for FPGA design and development Computer: Cray XD1 consisting of a dual-core, dualprocessor AMD Opteron 2.2 GHz with a Xilinx Virtex-4 (V4LX160) or Xilinx Virtex-II Pro (XC2VP50) FPGA per node. We use the compute node with the Xilinx Virtex-4 FPGA Operating system: Red Hat Enterprise Linux OS Has the code been vectorised or parallelized?: Yes Classification: 6.1 Nature of problem: Quantum Monte Carlo is a practical method to solve the Schrödinger equation for large many-body systems and obtain the ground-state properties of such systems. This method involves the sampling of a number of configurations of atoms and averaging the properties of the configurations over a number of iterations. We are interested in applying the QMC method to obtain the energy and other properties of highly quantum clusters, such as inert gas clusters. Solution method: The proposed framework provides a combined hardware–software approach, in which the QMC simulation is performed on the host processor, with the computationally intensive functions such as energy and trial wave function computations mapped onto the field-programmable gate array (FPGA) logic device attached as a co-processor to the host processor. We perform the QMC simulation for a number of iterations as in the case of our original software QMC approach, to reduce the statistical uncertainty of the results. However, our proposed HAQMC framework accelerates each iteration of the simulation, by significantly reducing the time taken to calculate the ground-state properties of the configurations of atoms, thereby accelerating the overall QMC simulation. We provide a generic interpolation framework that can be extended to study a variety of pure and doped atomic clusters, irrespective of the chemical identities of the atoms. For the FPGA implementation of the properties, we use a two-region approach for accurately computing the properties over the entire domain, employ deep pipelines and fixed-point for all our calculations guaranteeing the accuracy required for our simulation.

Simulator performance upgrade and evaluation of loss of RHR during midloop operation for Maanshan PWR plant

Because a greater risk than expected is introduced during midloop operation for typical PWRs, the performance of training simulator of Taiwan’s Maanshan 3-loop PWR plant was verified and upgraded for midloop operation (MLO) simulation. Besides, plant specific midloop abnormal operation procedure (AOP) also was quantitatively evaluated. Instead of modifying existing RCS module, a thermal-hydraulic code, namely ROSE (Reactor Outage Simulation and Evaluation) has been developed and transplanted into the training simulator of Maanshan PWR plant. A two-region approach with a modified two-fluid model was adopted as the theoretical basis of the ROSE code. The success of the simulator performance upgrade for the MLO was demonstrated by comparisons to W-GOTHIC MLO calculation as well as the original simulator performance. Moreover, regarding the evaluation of the associated AOP for MLO after loss of RHR, the most risky plant configuration as well as associated crucial timing before core uncovery was also identified by the upgraded training simulator.

Simulation of ordering in large defect clusters in gadolinium-doped ceria

Defect clusters in Gd-doped ceria containing up to four oxygen vacancies and eight dopant cations have been simulated by the Mott–Littleton two-region approach. With increasing cluster size, the magnitude of the binding energy of the clusters increases so that the clusters become more and more stable. Moreover, the oxygen vacancies tend to form curved chains in the clusters, and the adjacent oxygen vacancies are preferentially separated by <110>/2 in the clusters containing more than three oxygen vacancies. It is suggested that the structure of the nano-sized domains that have been reported in heavily doped ceria can be described as an aggregation and combination of these clusters.

Atomistic simulation of the mechanisms of noble gas incorporation in minerals

Atomistic simulations have been carried out to investigate the mechanisms of noble gas incorporation in minerals using both the traditional two-region approach and the “supercell” method. The traditional two-region approach has been used to calculate defect energies for Ne, Ar, Kr and Xe incorporation in MgO, CaO, diopside and forsterite in the static limit and at one atmosphere pressure. The possibilities of noble gas incorporation via both substitution and interstitial mechanisms are studied. The favored mechanism varies from mineral to mineral and from noble gas to noble gas. In all minerals studied, the variation of the solution energies of noble gas substitution with atomic radius appears approximately parabolic, analogous to those for 1+, 2+, 3+ and 4+ trace element incorporation on crystal lattice sites. Noble gas solution energies thus also fall on a curve, similar to those previously observed for cations with different charges, but with much lower curvature.The “supercell” method has been used to investigate the pressure dependence of noble gas incorporation in the same systems. Results indicate a large variation of the solubility of the larger noble gases, Kr and Xe with pressure. In addition, explicit simulation of incorporation at the (001) surface of MgO shows that the solubility of the heavier noble gases may be considerably enhanced by the presence of interfaces.

Temperature effects on phase equilibrium and diffusion in mesopores

The equilibrium and dynamic properties of fluids confined to mesoporous material have been studied using nuclear magnetic resonance (NMR) methods. Molecular diffusion of n -pentane in Vycor porous glass within closed sample tubes has been measured by means of the pulsed field gradient (PFG) NMR method for temperatures notably exceeding the boiling point of the neat liquid. It is found that the temperature dependence of the diffusivity dramatically depends on the state of the fluid surrounding the mesoporous monoliths. In an oversaturated sample, i.e., in a sample containing some amount of the liquid also outside of the porous material, the diffusivity in the mesopores followed the Arrhenius dependence. In samples with only the mesopores saturated by the liquid, i.e., without any excess fluid, with increasing temperature the diffusivity notably deviated from the Arrhenius dependence towards higher diffusivities. The analysis of the intensities of the respective NMR signals from the fluid within the porous material and in the surrounding phase has revealed that this anomaly is accompanied by the formation of a space free of liquid within the pore system. With the measured pore filling factors, the resulting overall diffusivity is estimated by a two-region approach with diffusion occurring in either the liquid phase or the free space within the pore volume. It is shown that this procedure, free of any fitting parameters, yields excellent agreement with the experimental data.

Trace element incorporation into pyrope-grossular solid solutions: an atomistic simulation study

We have performed atomistic computer simulations on trace element incorporation into the divalent dodecahedral X-sites of pyrope (Py — Mg3Al2Si3O12) – grossular (Gr — Ca3Al2Si3O12) solid solutions. An ionic model and the Mott–Littleton two-region approach to defect energies were used to calculate the energetics of substitution by a range of divalent trace-elements and of charge-balanced substitution by trivalent ions in the static limit. Results are compared with experimental high-temperature, high-pressure garnet-melt trace element partitioning data obtained for the same garnet solid solution to refine our understanding of the factors controlling element partitioning into solid solutions. Defect energies (Udef,f), relaxation (lattice strain) energies (Urel), and solution energies (Usol) were derived using two different approaches. One approach assumes the presence of one type of hybrid X-site with properties intermediate between pure Mg and Ca sites, and the other assumes discrete Mg and Ca X-sites, and thus two distinct cation sublattices. The hybrid model is shown to be inadequate, since it averages out local distortions in the garnet structure. The discrete model results suggest trace elements are more soluble in Py50Gy50 than in either end-member compound. Physically this is due to small changes in size of the X-sites and the removal of unfavourable interactions between third nearest neighbours of the same size. Surprisingly, depending on the local order, large trace element cations may substitute for Mg2+ and small trace elements for Ca2+ in Py50Gr50. These solubilities provide an explanation for the anomalous trace-element partitioning behaviour along the pyrope–grossular join observed experimentally.

Assessment and Application of the ROSE Code for Reactor Outage Thermal-Hydraulic and Safety Analysis

The currently available tools, such as RELAP5, RETRAN, and others, cannot easily and correctly perform the task of analyzing the system behavior during plant outages. Therefore, a medium-sized program aiming at reactor outage simulation and evaluation, such as midloop operation (MLO) with loss of residual heat removal (RHR), has been developed. Important thermal-hydraulic processes involved during MLO with loss of RHR can be properly simulated by the newly developed reactor outage simulation and evaluation (ROSE) code. The two-region approach with a modified two-fluid model has been adopted to be the theoretical basis of the ROSE code.To verify the analytical model in the first step, posttest calculations against the integral midloop experiments with loss of RHR have been performed. The excellent simulation capacity of the ROSE code against the Institute of Nuclear Energy Research Integral System Test Facility test data is demonstrated. To further mature the ROSE code in simulating a full-sized pressurized water reactor, assessment against the WGOTHIC code and the Maanshan momentary-loss-of-RHR event has been undertaken. The successfully assessed ROSE code is then applied to evaluate the abnormal operation procedure (AOP) with loss of RHR during MLO (AOP 537.4) for the Maanshan plant. The ROSE code also has been successfully transplanted into the Maanshan training simulator to support operator training. How the simulator was upgraded by the ROSE code for MLO will be presented in the future.

Code Development and Assessment for Reactor Outage Thermal-Hydraulic and Safety Analysis – Midloop Operation with Loss of Residual Heat Removal

Although only a few percent of residual power remains during plant outages, the associated risk of core uncovery and corresponding fuel overheating has been identified to be relatively high, particularly under midloop operation (MLO) in pressurized water reactors. However, to analyze the system behavior during outages, the tools currently available, such as RELAP5, RETRAN, etc., cannot easily perform the task. Therefore, a medium-sized program aiming at reactor outage simulation and evaluation, such as MLO with the loss of residual heat removal (RHR), was developed. All important thermal-hydraulic processes involved during MLO with the loss of RHR will be properly simulated by the newly developed reactor outage simulation and evaluation (ROSE) code. Important processes during MLO with loss of RHR involve a pressurizer insurge caused by the hot-leg flooding, reflux condensation, liquid holdup inside the steam generator, loop-seal clearance, core-level depression, etc. Since the accuracy of the pressure distribution from the classical nodal momentum approach will be degraded when the system is stratified and under atmospheric pressure, the two-region approach with a modified two-fluid model will be the theoretical basis of the new program to analyze the nuclear steam supply system during plant outages. To verify the analytical model in the first step, posttest calculations against the closed integral midloop experiments with loss of RHR were performed. The excellent simulation capacity of the ROSE code against the Institute of Nuclear Energy Research Integral System Test Facility (IIST) test data is demonstrated.

Modeling the kinetics of neutral particles in low-pressure chemically active planar microwave plasmas

Low-pressure planar microwave plasmas exhibit a pronounced spatial structure. If the discharge vessels are wide enough, they have a thin, plane homogeneous excitation region adjacent to a microwave coupling window, and they strongly decay in the direction normal to the window. The decay length of plasma parameters and activated neutrals differ by about one order of magnitude. This suggests a two-region approach for modeling of neutral particle kinetics assuming spatial homogeneity inside these two regions. The gaseous reactor volume is subdivided by a virtual interface on which mass exchange by directed gas flow, local microconvective mixing or diffusion, and exchange of enthalpy between the two different regions can be balanced. The main differences between the two regions are that electron-induced generation of activated neutrals is restricted to the excitation region (1) and that basic loss processes for activated neutrals due to the substrate surface and volume reactions are considered to occur in the second, chemical reaction region (2). Processes of loss of active neutral atoms by recombination at inert surfaces of the vessel are considered in both regions. Effectively, the model represents a system of two-coupled continuous flow stirred tank reactors given by a set of differential equations including unsteady-state Bernoulli equations. For the case of an example process concerning an oxygen plasma surface treatment, numerical solutions for initial values at the startup of the reactor are computed. The results for the dynamical behavior and the steady states of the concentrations, the temperatures, and the mass loss are in qualitative agreement with the experimental behavior of a real reactor arrangement.

On the outer-edge problem of a hypersonic boundary layer

Existing analytical treatments of the hypersonic strong interaction problem adopt the two-region structure of the classical boundary-layer theory; however, the uneven heating and external vorticity created by the highly curved, leading-edge shock wave leads to the question of the uniform validity of the boundary-layer approximation near the boundary-layer edge. Recently Bush (1966), using a non-linear viscosity-temperature law (μ ∞ Tω, ω &lt; 1) instead of the linear one (μ ∞ T) used by previous investigators, demonstrated the need to analyze separately a transitional layer intermediate between the inviscid region and the boundary layer. In this paper, an asymptotic analysis of the Navier-Stokes equations in von Mises's variables, allowing a three-region structure, is carried out for the case of μ ∞ T. Results, with the second-order effects associated with heating and vorticity accounted for, show that a separate analysis of the transitional region is not strictly necessary in this case, and hence the equivalence of the two-region approach is confirmed. On the other hand, it is shown that the second-order boundary-layer correction owing to the heating and vorticity effects, not considered by Bush, is necessary in order to determine a uniformly valid temperature distribution in the physical variables. Numerical results for an insulated and a cold flat plate, considerably different from those of others, are obtained.