Engineering topics
Lu, Zhen
Publications and source records attributed to Lu, Zhen.
Mechanical Properties and Microstructure Evolution During Bending–Unbending Deformation of Pure Titanium Sheet
In this study, the pure titanium sheets with thickness of 1 mm are deformed 1-5 times in the form of bending–unbending cyclic deformation. The bending angles before and after unbending are 120° and 180°, respectively. Mechanical properties of samples during cyclic deformation, including M/(BH){sup 2}-surface strain curve and spring back in each bending cycle, have been investigated. Moreover, the twinning/detwinning behavior and texture evolution in three typical bending layers (i.e., the tensile layer, neutral layer and compressive layer) in cyclic deformation have been tested by EBSD and TEM, and analyzed systematically. The results indicate that the most intensive twins, consist of similar proportion of tensile and compressive ones, distribute in the inner layer after 1st bending. While they disappear almost entirely after 1st unbending. The twins in outer layer could also be detwinned, and the new twins could be activated during 1st unbending. Besides, the twin density increases obviously and the residual twins could not be detwinned clearly after multiple passes of cyclic deformation. Moreover, the Schmid factor along the σ{sub 2} direction functions in the activation of twin variants regardless of the three-direction stress states.
Analysis of operator splitting errors for near-limit flame simulations
High-fidelity simulations of ignition, extinction and oscillatory combustion processes are of practical interest in a broad range of combustion applications. Splitting schemes, widely employed in reactive flow simulations, could fail for stiff reaction–diffusion systems exhibiting near-limit flame phenomena. The present work first employs a model perfectly stirred reactor (PSR) problem with an Arrhenius reaction term and a linear mixing term to study the effects of splitting errors on the near-limit combustion phenomena. Analysis shows that the errors induced by decoupling of the fractional steps may result in unphysical extinction or ignition. The analysis is then extended to the prediction of ignition, extinction and oscillatory combustion in unsteady PSRs of various fuel/air mixtures with a 9-species detailed mechanism for hydrogen oxidation and an 88-species skeletal mechanism for n-heptane oxidation, together with a Jacobian-based analysis for the time scales. The tested schemes include the Strang splitting, the balanced splitting, and a newly developed semi-implicit midpoint method. Results show that the semi-implicit midpoint method can accurately reproduce the dynamics of the near-limit flame phenomena and it is second-order accurate over a wide range of time step size. For the extinction and ignition processes, both the balanced splitting and midpoint method can yield accurate predictions, whereas the Strang splitting can lead to significant shifts on the ignition/extinction processes or even unphysical results. With an enriched H radical source in the inflow stream, a delay of the ignition process and the deviation on the equilibrium temperature are observed for the Strang splitting. On the contrary, the midpoint method that solves reaction and diffusion together matches the fully implicit accurate solution. The balanced splitting predicts the temperature rise correctly but with an over-predicted peak. For the sustainable and decaying oscillatory combustion from cool flames, both the Strang splitting and the midpoint method can successfully capture the dynamic behavior, whereas the balanced splitting scheme results in significant errors.
A kinetics-based method for constraint selection in rate-controlled constrained equilibrium
Not provided.