Aayush Sharma

Atomistic clustering-ordering and high-strain deformation of an Al0.1CrCoFeNi high-entropy alloy

Computational investigations of structural, chemical, and deformation behavior in high-entropy alloys (HEAs), which possess notable mechanical strength, have been limited due to the absence of applicable force fields. To extend investigations, we propose a set of intermolecular potential parameters for a quinary Al-Cr-Co-Fe-Ni alloy, using the available ternary Embedded Atom Method and Lennard-Jones potential in classical molecular-dynamics simulations. The simulation results are validated by a comparison to first-principles Korringa-Kohn-Rostoker (KKR) - Coherent Potential Approximation (CPA) [KKR-CPA] calculations for the HEA structural properties (lattice constants and bulk moduli), relative stability, pair probabilities, and high-temperature short-range ordering. The simulation (MD)-derived properties are in quantitative agreement with KKR-CPA calculations (first-principles) and experiments. We study AlxCrCoFeNi for Al ranging from 0 ≤ x ≤2 mole fractions, and find that the HEA shows large chemical clustering over a wide temperature range for x < 0.5. At various temperatures high-strain compression promotes atomistic rearrangements in Al0.1CrCoFeNi, resulting in a clustering-to-ordering transition that is absent for tensile loading. Large fluctuations under stress, and at higher temperatures, are attributed to the thermo-plastic instability in Al0.1CrCoFeNi.

Mixed Convection Heat Transfer in a Grooved Channel in the Presence of a Baffle

The paper presents a comprehensive investigation of mixed convection in a differentially heated grooved channel in the presence of an adiabatic baffle supported centrally from the top wall. Both assisting and opposing flow configurations are simulated using in-house CFD code for Richardson number 0.001–1000 and Reynolds number 10–200, considering a baffle height of up to 80% of enclosure height and air as working fluid. The flow and heat transfer characteristics have been analyzed using energy flux vectors, isotherms, averaged Nusselt number, and streamlines. The results show remarkable enhancement of heat transfer in the mixed convection regime in the presence of baffle.

Design of high-strength refractory complex solid-solution alloys

Nickel-based superalloys and near-equiatomic high-entropy alloys containing molybdenum are known for higher temperature strength and corrosion resistance. Yet, complex solid-solution alloys offer a huge design space to tune for optimal properties at slightly reduced entropy. For refractory Mo-W-Ta-Ti-Zr, we showcase KKR electronic structure methods via the coherent-potential approximation to identify alloys over five-dimensional design space with improved mechanical properties and necessary global (formation enthalpy) and local (short-range order) stability. Deformation is modeled with classical molecular dynamic simulations, validated from our first-principle data. We predict complex solid-solution alloys of improved stability with greatly enhanced modulus of elasticity (3× at 300 K) over near-equiatomic cases, as validated experimentally, and with higher moduli above 500 K over commercial alloys (2.3× at 2000 K). We also show that optimal complex solid-solution alloys are not described well by classical potentials due to critical electronic effects.Solid solutions: screening by electronic structureCombining first-principle calculations with electronic alloy design criteria lead to the identification of desirable complex alloys. A team led by Duane Johnson at Iowa State University, USA, applied density functional theory to explore the design space formed by five refractory elements (molybdenum, tungsten, tantalum, titanium, and zirconium) and identified optimal molybdenum-rich alloy compositions for high temperature strength and corrosion resistance. By predicting structural properties such as the Young’s modulus as well as the short-range atomic order, a lattice constant identified the global stability of each composition and allowed for the fast screening of the design space to select the best compositions. Both molecular dynamics and experimental testing confirmed the predicted mechanical properties. This electronic structure approach can help optimize complex solution alloys for enhanced mechanical properties.

Thermal instability-driven multiple solutions in a grooved channel

ABSTRACT We present rich non-linear characteristics like multiple-stable states, bifurcation, and transition to chaos as a result of thermal instability-driven multiple solutions in a bottom-heated grooved channel undergoing natural convection with Rayleigh number varying from 1 × 105 to 9 × 105. From our analysis, we identify two different routes in which the solution of the system evolves along with the critical Rayleigh numbers associated with such transitions. In one route, the solution transforms from steady symmetric to chaotic solutions, while for the second route it first undergoes pitchfork bifurcation, where steady symmetric solution transforms to steady asymmetric, followed by Hopf bifurcation to periodic solutions. The two distinct solution routes clearly bring out the dependence of system solutions on initial conditions. The route to chaos in the grooved channel system has been successfully identified, which is supported by the results of time-series, power-spectra density and three-dimensional phase plots, apart from the positive values of largest Lyapunov exponent.

Analysis of Different Radiation Models in a Swirl Stabilized Combustor

A computational study on spray combustion, using kerosene (C12H23) as fuel, in a model gas turbine combustor has been carried out. The numerical modelling of radiation heat transfer is carried out in a three-dimensional swirl stabilized, liquid-fuelled combustor. The Favre-averaged governing equations are solved using Ansys Fluent 14.5 as the CFD package. The turbulence parameters are computed using realizable k-e with standard wall functions model. Eulerian-Lagrangian approach is used to track stochastically the motion of the evaporation species in the continuous gas phase. The effect of different radiation models — Discrete Ordinate (DO), P1 and Discrete Transfer Radiation Model (DTRM) along with Soot are analysed in the present study. To validate the results of radiation modelling carried out in the present work, the computational results have been compared with previous experimental data for the same combustor geometry. The numerical data considering effect of soot along with radiation is shown to closely approximate the experimental data. An attempt has also been made to introduce a liner in the combustor and evaluate its effect and the heat transfer across the liner for the present numerical model.Copyright

Author Correction: Design of high-strength refractory complex solid-solution alloys

The caption of Fig. 6 and the main text contained an error in the chemical formula of the “(MozW1−z)0.85Ta0.10(TiZr)0.05” alloy; it has now been corrected to “(Mo1−zWz)0.85Ta0.10(TiZr)0.05”. Figure 5 also contained an erroneous text box, which has now been removed. This has now been corrected in both the PDF and HTML version of this article.

Correlation of Equivalence Ratio with Spectrometric Analysis for Premixed Combustion

The present study deals with the application of spectrometry in correlating flame colour with relevant equivalence ratios in the entire flammability limit, extending from lean blowout to rich blowout. A swirl-stabilized, LPG-fueled dump combustor was used for this purpose. The predominance of a specific colour at a particular equivalence ratio is noted viz. the predominance of red wavelength in richer regimes of operation, and that of blue wavelength in leaner regimes. The practical application of blow out prediction is significant in the gas-turbine industry. The salient contribution from the present study is the development of prediction parameters for both lean and rich blow out at low flow rates. Apart from spectrometric analysis, an attempt has also been made to predict RGB-intensity values using image processing tools.

Effect of Flow Pulsations in Premixed, Swirl Stabilized Combustor

In the present work, a numerical model has been developed using ANSYS Fluent 14.5 to simulate the combustion phenomenon in a partially premixed, swirl-stabilized, LPG-fueled gas turbine combustor. In a practical gas turbine combustor, pulsations in the flow at the air side cannot be avoided which can lead to thermoacoustic instabilities. The primary objective of the study is to numerically analyze the effect of such pulsations on the fluid flow and combustion process inside the combustor. Different parameters like static temperature, progress variable and product formation rate are compared at the outlet plane of the combustor. The effect of change in the parameters like amplitude and frequency of the sinusoidal air flow input has also been investigated in the present study. It is observed that the solution changes from periodic to quasi-periodic at a higher amplitude condition. The numerical model was qualitatively validated against experiments performed on a laboratory-scale premixed, swirl-stabilized, gas turbine combustor.© 2014 ASME