Simulation — a bridge between theory and reality: the case of electric circuits

Abstract This study aimed to examine the role of a simulation as a potential aid that may help students bridge the gap between theory and reality, in the case of electric circuits. Sixty-three pairs of students aged about 15 years were presented with two tasks involving real circuits. An open simulation environment was available as an optional aid for the experimental group. Significant differences were found between the achievements of the groups. The use of the simulation contributed to students' confidence and enhanced their motivation to stay on-task. The detailed analysis revealed the role of the simulation as a source of constructive feedback, helping students identify and correct their miscon-ceptions and cope with the common difficulties of relating formal representations to real circuits and vice versa.     

3‐D Simulations of an Internal Airlift Loop Reactor using a Steady Two‐Fluid Model

Three‐dimensional (3‐D) simulations of an internal airlift loop reactor in a cylindrical reference frame are presented, which are based on a two‐fluid model with a revised k‐? turbulence model for two‐phase bubbly flow. A steady state formulation is used with the purpose of time saving for cases with superficial gas velocity values as high as 0.12 m/s. Special 3‐D treatment of the boundary conditions at the axis is undertaken to allow asymmetric gas‐liquid flow. The simulation results are compared to the experimental data on average gas holdup, average liquid velocity in the riser and the downcomer, and good agreement is observed. The turbulent dispersion in the present two‐fluid model has a strong effect on the gas holdup distribution and wall‐peaking behavior is predicted. The CFD code developed has the potential to be applied as a tool for scaling up loop reactors.     

Loading Dependence of Self‐Diffusivities of Gases in Zeolites

Experimental data on the self‐diffusivities, D_i,self, of a variety of gases (CH₄, N₂, Kr, C₂H₆, and C₃H₈) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q_i. In LTA, D_i,self appears to increase with q_i for all molecules except N₂. In FAU and in MFI the D_i,self shows a sharp decrease with increasing q_i. In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self‐diffusivities of seven gases (CH₄, N₂, Kr, C₂H₆, C₃H₈, Ar, and Ne) in six different all‐silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of D_i,self with q_i is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one‐dimensional channels (AFI) and intersecting channel structures (MFI), the D_i,self invariably decreases with increasing q_i. For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the D_i,self decreases with q_i for all molecules. If the windows are narrow (CHA, DDR and LTA), the D_i,self often exhibits a sharp increase with q_i, reaches a maximum and reduces to near‐zero values at saturation. The sharpness with which D_i,self increases with q_i, is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D_i,self with q_i.     

HERSCHEL—PACS Bolometer Arrays for Submillimeter Ground-Based Telescopes

The Herschel Space Observatory will carry onboard a new kind of bolometric architecture for the PACS (Photodetector Array Camera and Spectrometer) submillimeter photometer. These new generation CCD-like multiplexed bolometer arrays are buttable and enable the conception of large fully sampled focal planes either for space or for ground-based telescopes. We present here some development for ground-based applications in the context of the ARTEMIS (ARchitecture de bolomètres pour des TElescopes sub-MIllimétriques au Sol) project. We have developed an electro-thermal numerical model that simulates the performances of these semiconducting bolometers under specific ground-based conditions (different wavelengths and background powers for example). This model permits to determine the optimal parameters for each condition and shows that the bolometers can be background limited in each atmospheric transmission window between 200 and 450 microns. We also describe the optical system that provides a high optical efficiency in each submillimeter atmospheric window. Astronomical observations made with a prototype on the APEX telescope are presented.     

Simulations of flow behavior of gas and particles in spouted bed with a porous draft tube

Flow behaviors of particles in a two-dimensional spouted bed with a porous draft plates are studied using a fluid dynamic computation with kinetic-frictional stresses models. Gas flow through the porous draft tube is simulated by Brinkman-Darcy-Forchheimer formulation. The distributions of concentration and velocities of particles are predicted. Simulated results predict the solid circulation rate and gas flux rate measured in the spouted bed with a porous draft tube (Ishikura et al., 2003). The solid circulation rate in the spouted bed with a porous draft tube is larger than that with a non-porous draft tube. The predicted bed pressure drop with the porous draft tube is high in comparison to the spouted bed with non-porous draft tube. The effect of the porosity of the porous draft tube on distributions of gas flux rate through the annulus and solid circulation rate are discussed.     

散热器和低温水辐射采暖系统不同连接方式及控制策略动态仿真

散热器和低温水辐射采暖系统最佳连接方式一直是业界探讨的重要课题之一。本文以动态模型为手段,模拟和分析直接、混水泵及间接连接方式系统动态响应和能耗。仿真结果表明,地板热情性的提高延长了室内温度达到稳态时间;地暖系统直接连接方式是不适宜的;混水泵和间接连接系统可节省燃料消耗约17％。从控制角度看,间接连接系统具有更好的稳定性和控制精度。     

A Three‐Dimensional Non‐isothermal Model of High Temperature Proton Exchange Membrane Fuel Cells with Phosphoric Acid Doped Polybenzimidazole Membranes

High temperature proton exchange membrane fuel cells (HT‐PEMFCs) with phosphoric acid doped polybenzimidazole (PBI) membranes have gained tremendous attentions due to its attractive advantages over conventional PEMFCs such as faster electrochemical kinetics, simpler water management, higher carbon monoxide (CO) tolerance and easier cell cooling and waste heat recovery. In this study, a three‐dimensional non‐isothermal model is developed for HT‐PEMFCs with phosphoric acid doped PBI membranes. A good agreement is obtained by comparing the numerical results with the published experimental data. Numerical simulations have been carried out to investigate the effects of operating temperature, phosphoric acid doping level of the PBI membrane, inlet relative humidity (RH), stoichiometry ratios of the feed gases, operating pressure and air/oxygen on the cell performance. Numerical results indicate that increasing both the operating temperature and phosphoric acid doping level are favourable for improving the cell performance. Humidifying the feed gases at room temperature has negligible improvement on the cell performance, and further humidification is needed for a meaningful performance enhancement. Pressurising the cell and using oxygen instead of air all have significant improvements on the cell performance, and increasing the stoichiometry ratios only helps prevent the concentration loss at high current densities.     

Influence of scalar dissipation on flame success in turbulent sprays with spark ignition

A Direct Numerical Simulation (DNS) study is performed to determine a quantitative indicator of imminent global extinction in spray flames ignited by a spark. The cases under consideration have Group Combustion numbers sufficiently small that each droplet has an individual flame form around it, which subsequently merge. The structure of the flames is examined, including identification of non-premixed behaviour in the core of the flame and premixed flame fronts except in the presence of droplets, which cause strong non-premixed behaviour. The reaction progress variable c is studied and its dissipation rate is identified as being a key indicator of whether a flame will globally extinguish after being ignited by the spark. Specifically, immediately after the spark is deactivated, the volume containing the end of the flame front and hot products is studied in detail with respect to c. For successful flames, it is observed that regions of zero dissipation of c   were predominantly restricted to the highest reaction progress variable (c>0.98)$(c>0.98)$, with zero probability within the range 0.95<c<0.98$0.95<c<0.98$ and low probability within 0.9<c<0.95$0.9<c<0.95$. In contrast, cases which subsequently extinguished had substantial probability of zero dissipation for 0.95<c<0.98$0.95<c<0.98$. This region was a secondary structure separate from the main flame kernel that was unable to evaporate sufficient liquid to create a self-sustaining flame and therefore contributed to the subsequent quenching of the flame. In the successfully-burning case under consideration, this region was part of the main flame structure. The low reaction rate contributed to a thickened flame structure near the hot core, which reduced the heat transfer to the flame front and prevented effective evaporation and preheating of the fluid ahead of the flame front. Calculation of the conditional probability of c   for its dissipation rate being zero could provide a quantitative measure to determine whether a flame is likely to extinguish within a relatively short timeframe. This is equivalent to detecting that, for every value of 0.9<c<1$0.9<c<1$, there are volumes of significant size where the value of c   is uniform. Note that a successful flame must have a volume of substantial size with c=1$c=1$. From a practical perspective, if each individual flame kernel is monitored, then extinction is imminent if secondary structures of incomplete reactions are present when the spark ceases adding energy.     

Modeling the kinetics of atomic ordering in high temperature intermetallics

Canonical ensemble Monte Carlo study was performed for the analysis of ordering characteristics of the high temperature X 3 Al (X=Fe and Ni)-based ordered intermetallics. Calculated partial ordering energies by means of the electronic theory of alloys in pseudopotential approximation were utilized as input data to determine the Hamiltonian of the system. Bragg-Williams long-range order (LRO) and Cowley-Warren short-range order (SRO) parameters were calculated from the equilibrium configurations attained at the end of Monte Carlo simulation for each predefined temperature and Al concentration levels of X 3 Al intermetallics. It was shown that the current predictions agree well with the experimental values of LRO and SRO parameters, indicating that the systems have transformed into perfect ordered superlattices by forming a single domain in the lattice structures. It seems that this present approach on the combination of Monte Carlo simulation methods with the electronic theory of alloys in pseudopotential approximation can be successfully applied to elucidate the ordering characteristics of high temperature Fe 3 Al and Ni 3 Al intermetallics.