Histogram Reweighting Method for Dynamic Properties

The histogram reweighting technique, widely used to analyze Monte Carlo data, is shown to be applicable to dynamic properties obtained from Molecular Dynamics simulations. The theory presented here is based on the fact that the correlation functions in systems in thermodynamic equilibrium are averages over initial conditions of functions of the trajectory of the system in phase-space, the latter depending on the volume, the total number of particles and the classical Hamiltonian. Thus, the well-known histogram reweighting method can almost straightforwardly be applied to reconstruct the probability distribution of initial states at different thermodynamic conditions, without extra computational effort. Correlation functions and transport coefficients are obtained with this method from few simulation data sets.Comment: 4 pages, 3 figure

Nitrogen activation in a Mars-van Krevelen mechanism for ammonia synthesis on Co3Mo3N

Co3Mo3N is one of the most active catalysts for ammonia synthesis; however, little is known about the atomistic details of N2 adsorption and activation. Here we examine whether N2 can adsorb and activate at nitrogen surface vacancies. We have identified the most favorable sites for surface nitrogen vacancy formation and have calculated vacancy formation free energies (and concentrations) taking into account vacancy configurational entropy and the entropy of N2 at temperature and pressure conditions relevant to ammonia synthesis (380–550 °C, 100 atm) via a semiempirical approach. We show that 3-fold hollow bound nitrogen-containing (111)-surfaces have surprisingly high concentrations (1.6 × 1016 to 3.7 × 1016 cm–2) of nitrogen vacancies in the temperature range for ammonia synthesis. It is shown that these vacancy sites can adsorb and activate N2 demonstrating the potential of a Mars–van Krevelen type mechanism on Co3Mo3N. The catalytically active surface is one where 3f-hollow-nitrogens are bound to the molybdenum framework with a hexagonal array of embedded Co8 cobalt nanoclusters. We find that the vacancy-formation energy (VFE) combined with the adsorption energy can be used as a descriptor in the screening of materials that activate doubly and triply bonded molecules that are bound end-on at surface vacancies

Coupled Heat Transfer Analysis in Regeneratively Cooled Thrust Chambers

A computational procedure able to describe the coupled hot-gas/wall/coolant environment that occurs in most liquid rocket engines and to provide a quick and reliable prediction of thrust-chamber wall temperature and heat flux as well as coolant-flow characteristics, like pressure drop and temperature gain in the regenerative circuit is presented and demonstrated. The coupled analysis is performed by means of an accurate CFD solver of the Reynolds-Averaged Navier-Stokes equations for the hot-gas flow and a simplified quasi-2D approach, which widely relies on semi-empirical relations, to study the problem of coolant flow and wall structure heat transfer in the cooling channels. Coupled computations of the Space Shuttle Main Engine Main Combustion Chamber are performed and compared with available literature data. Results show a reasonable agreement in terms of coolant pressure drop and temperature gain with nominal data, whereas the computed wall temperature peak is quite closer to hot-firing data than to the nominal value. © 2012 by B. Betti, M. Pizzarelli, F. Nasuti

Soil

Soil properties and Ethylene Dibromide (EDB) concentrations were measured at two locations with histories of EDB applications. The objective was to explain the presence of EDB residues in a well near one location and the lack of residues in a well near the other location. The soil profile was sampled at each location from the surface to ground water, and ground water samples were collected at the profile base. A combination of statistical analyses (stepwise linear regression and discriminant analyses) was applied to soil data to interpret differences within each location. EDB concentrations occurred at location 1 between 0 and 3.2 feet and ranged from 0.3 to 12.5 ppb, correlating with organic carbon. Location 1, composed of a silty clay with a mean soil moisture content of 23 % contained a deeper band of EDB lying between 7.7 and 9.8 feet below the soil surface. Within the band, EDB concentrations ranged from 0.2 to 0.6 ppb. The presence of this deeper band was not correlated with any measured variables, including organic carbon, and ma

Application of a Real-Gas-Library Multi-Fluid-Mixing Model to Supercritical Single Injector Flow

In this paper we report on supercritical single injector computations using a new type of real gas CFD model. This Euler-Euler model is an extension to the DLR TAU CFD code. By storing fluid data in a library, we were able to decouple equation of state (EOS) accuracy from runtime performance. The library covers all fluid states effciently and robust, including gaseous, liquid, supercritical, and multiphase states. In our new multifluid mixing model, an EOS is solved for each species. Computations were carried out using a modifed Benedict-Webb-Rubin high fidelity equation of state for cryogenic oxygen, with negligible penalty in performance compared to a pure ideal gas computation. Additional species (H, H2, O, OH, H2O, H2O2) were treated as perfect gases. The immediate goal is to create a flow solver for industrial application, i.e. to support design by enabling a fast turnaround. Thus, we focus on 2D RANS modeling in this first step. The baseline model is applied to the canonical Mascotte A60 test case. The chamber pressure is well met, the flame dimensions are within the spread found among other CFD results. In accordance with experimental results, the reaction zone is very thin. Maximum OH* occurrences are correctly predicted in the shear layer, reducing in magnitude towards shoulder and flame tip. The fluid library allows to pinpoint the extent of the liquid oxygen core, the length is determined to 20 LOX injector diameters. It is found to be embedded in a gaseous oxygen shell. Within this RANS context, H2 and O2 do not coexist in a premixed form. Finally, we show that numerical OH* concentration differs significantly from OH mass fraction distributions, the latter are thus no appropriate data to compare to experiments