Effects of reformed exhaust gas recirculation on the HC and CO emissions of a spark-ignition engine fueled with LNG

Reformed exhaust gas recirculation (REGR), which can generate onboard hydrogen-rich gas (i.e., the reformate including H₂, CO, unreformed hydrocarbon, etc.) via catalytic reforming of fuel and engine exhaust gas, is an attractive way to improve the performance and emissions characteristics of the engine fueled with liquefied natural gas (i.e., NG engine). However, the leakage during the valve overlap period and incomplete burning of the added reformate may lead to extra HC and CO emissions from the engine with REGR. In the present study, a multi-dimensional computation fluid dynamics model coupled with a detailed chemical kinetic mechanism was developed to investigate the effects of the ratio of reformate addition (R_ref) and exhaust valve closed (EVC) timing on the total emissions characteristics as well as the sources of HC and CO emissions from the engine. The emissions from the combustion and the leaking were included to calculate the total emissions. Moreover, the unburned CO from the added reformate was distinguished from the total CO emissions by adding marked-species. Results show that the unburned CH₄ in the cylinder is the main component of the total CH₄ emissions. Due to the increase of the concentrations of OH, O and H radicals during the combustion process, the oxidization of CH₄ is promoted with the increase of R_ref at high load, and therefore the total CH₄ emissions decrease. However, the total CO emissions increase with the increase of R_ref, and it is demonstrated that the unburned CO from the added reformate increases and turns to be the main sources of the total CO emissions. At R_ref of 10%, the total CH₄ and CO emissions firstly remain nearly constant and then increase dramatically with the delay of EVC timing. Therefore, low concentration of CO in the reformate and short valve duration are recommended to achieve low HC and CO emissions for the NG engine with REGR.     

In-plane shear behaviour of fibre composite sandwich beams using asymmetrical beam shear test

The in-plane shear behaviour of a new generation composite sandwich beam made up of glass fibre skins and modified phenolic core material was investigated to determine its application as shear loading component in a structural beam. Iosipescu shear test was conducted to characterise the shear properties of the fibre composite skins and the phenolic core material. The fibre composite sandwich beams were then tested under asymmetrical beam shear to determine its behaviour under in-plane shear loading. The results show that the in-plane shear behaviour of the composite sandwich beam is similar to that of the skins. A theoretical prediction of the in-plane shear strength of the composite sandwich beam was proposed and showed a good agreement with the experimental results. Based on the results of the study, the asymmetrical shear test is recommended as a test method for determining the shear properties of sandwich structures with high strength core materials.     

Numerical simulation of transcritical strained laminar flames

We investigate reactive and non-reactive strained flows associated with high pressure cryogenic rocket engines. A detailed high pressure fluid model based on thermodynamics of irreversible processes, statistical mechanics as well as kinetic theory of dense gases is used. This model insures the positivity of chemical entropy production and of molecular transport related entropy production. We first investigate a mixing layer between cold hydrogen and oxygen and the dramatic influence of nonideal transport near thermodynamic instabilities. Diffusion and partially premixed H₂–O₂ transcritical flame structures are then studied as well as strain extinction limits and dilution extinction limits.     

Effect of overload on crack closure in thick and thin specimens via digital image correlation

The displacement field of compact tension (CT) specimens have been mapped by digital image correlation (DIC) local to growing fatigue cracks to study overload effects for plane stress and plane strain. We have extracted crack opening displacement (ΔCOD) and stress intensity (K) determined by a Muskhelishvili fit to the crack tip displacement field to infer the closure load. In both cases a classical knee was observed upon unloading consistent with closure which disappeared during the accelerated growth following OL, before increasing during retardation. In both cases following OL the crack growth rate is perturbed for a distance similar to the plastic zone.     

The Modified Manson–Coffin Curve Method to estimate fatigue lifetime under complex constant and variable amplitude multiaxial fatigue loading

This paper investigates the accuracy of the so-called Modified Manson–Coffin Curve Method (MMCCM) in estimating fatigue lifetime of metallic materials subjected to complex constant and variable amplitude multiaxial load histories. The MMCCM postulates that fatigue damage is maximised on that material plane experiencing the maximum shear strain amplitude. In the present investigation, the orientation of the critical plane was determined through that direction along which the variance of the resolved shear strain reaches it maximum value. Under variable amplitude complex load histories, this direction was also used to count the resolved shear strain cycles via the classic Rain-Flow method. Further, the degree of multiaxiality and non-proportionality of the time-variable stress states at the assumed critical locations was directly quantified through a suitable stress ratio which accounts for (i) the mean value and the variance of the stress perpendicular to the critical plane as well as for (ii) the variance of the shear stress resolved along the direction experiencing the maximum variance of the resolved shear strain. The accuracy and reliability of the proposed approach was checked against approximately 650 experimental data taken from the literature and generated by testing un-notched metallic materials under complex constant and variable amplitude multiaxial load histories. The sound agreement between estimates and experimental results which was obtained strongly supports the idea that the proposed design technique is a powerful engineering tool allowing metallic materials to be designed against constant and variable amplitude multiaxial fatigue by always reaching a remarkable level of accuracy. This approach offers a complete solution to the strain based multiaxial fatigue problem.     

Application of the weakest link analysis to the area of fatigue design of steel welded joints

A new approach in the area of fatigue life assessment of steel welded joints is being proposed with the following features: (i) methodology of fatigue life calculation is independent from geometry of welded element; (ii) fatigue life assessment is based on fatigue characteristic of introduced efficient material – suitable for different steel welded joints; (iii) the fatigue life assessment is carried on the desired level of failure probability.In the proposed method a material volume surrounding the weld is divided into volume elements and regarded as a serial system having its definition in the reliability theory (the weakest link concept). Failure probability distribution of the welded structure is characterized by the proposed S–N curve for efficient material and the shape parameter introduced to describe the volume effect.     

Dynamic response of clamped sandwich beam with aluminium alloy foam core subjected to impact loading

The dynamic response of clamped sandwich beam with aluminium alloy open-cell foam core subjected to impact loading is investigated in the paper. The face sheet and the core of the sandwich beam have the different thickness. And the sandwich beam is impacted by a steel projectile in the mid-span. The impact force is recorded by using accelerometer. The results show that tensile crack and core shear are the dominant failure modes. And the impact velocity and the thickness of the face sheet and the foam core have a significant influence on the failure modes and the impact forces. Combining with the inertia effect and experimental results, the failure mechanisms of the sandwich beams are discussed. The thickness of the foam core plays an important role in the failure mechanism of the sandwich beam. In present paper, the failure of the sandwich beam with a thin core is dominated by the bending moment, while the sandwich beam with a thick core fails by bending deformation in the front face sheet and the bottom face sheet in opposite direction due to the plastic hinges in the front face sheet.     

Deformation and tearing of clamped circular work-hardening plates under impulsive loading

An approximate theory is presented in this paper to predict the deformation and tearing of clamped circular work-hardening plates subjected to uniformly distributed impulsive loads. Based on a power law stress-strain relationship throughout, various equations are obtained and an effective strain failure criterion is suggested for the rupture of the plates. It is shown that the theoretical predictions are in good agreement with the experimental observations in terms of the maximum permanent transverse displacements and the critical input impulses which caused the tensile tearing failure of the plates under impulsive loading when material strain rate sensitivity is taken into account. It is also shown that, to a first approximation, the theory developed for circular plates is applicable to rectangular plates.