Adiabatic Temperature Increase Research Articles

Compaction characteristics and mechanism of the ADC12 alloy powder by laser impact compaction

This paper focuses on the dynamic compaction experimental investigation of ADC12 alloy powder using laser impact, leveraging the high strain rate and controllable precision of pulsed lasers. The effects of varying laser energy (Em) and impactor thickness on the relative density, microstructure, and microhardness of the ADC12 alloy powder billets were examined. Utilizing 2D multi-particle finite element simulation (2D MPFEM), the densification mechanism of these billets was analysed with the help of coordination numbers. The simulation also provided insights into the particle velocity, equivalent plastic strain, and adiabatic temperature increase during compaction. Key findings include that a relative densification of up to 97.27% in the billets is achievable when the laser energy reaches 6J. Notably, a decrease in impactor thickness at constant laser energy leads to a more uniform microstructure and microhardness in the billet. Simulations with the 2D MPFEM demonstrate that thinner impactors allow billets to absorb more energy, thereby increasing particle velocity and plastic strain. This enhances both the relative density and mechanical properties of the billets. The simulation explores the stress distribution during compaction and captures the adiabatic temperature rise on the surface of the pressed billet that occurs in the transient, which is consistent with experimental revelations that heat softens or even locally melts the particles to produce sintering. This study offers new insights into the formation process of these billets from a microstructural standpoint, elucidating the relationship between processing conditions and the resulting material properties.

Dynamic Constitutive Model and Deformation Mechanism of 2219 Aluminum Alloy Based on the Dynamic Interruption Recovery

Dynamic compression and interruption‐recovery experiments are carried out to investigate the dynamic adiabatic and isothermal deformation behavior of the 2219 aluminum alloy at various strain rates (0.001–5000 s−1). The results of the experiments show that adiabatic temperature increase and strain rates have an impact on the compression behavior of 2219 aluminum alloy. The yield strength is strongly associated with strain rates, demonstrating an evident strain rate hardening effect. The increase in adiabatic temperature is the key factor contributing to the decline in adiabatic strain hardening rate. According to microstructural analysis, the adiabatic shear band is first detected with strain rate of 3200 s−1 along the loading direction. At a strain of 30.3%, cracks begin to appear, and shearing fractures occur with a strain rate of 5000 s−1 in the direction of 45°. Finally, the Johnson–Cook constitutive model is modified with the strain rate reinforcement terms corrected and an adiabatic softening term added to reflect the isothermal and adiabatic compression behaviors. The improved model is better at describing the strain rates affect the behavior of isothermal and adiabatic deformation.

Stress analysis and microstructure evolution of Cu/Al composite plate during corrugated rolling

A 3D finite element model was developed. The accuracy of the model was verified through a rolling experiment in which the reduction rate of the plate was 50%. The metal flow law in the rolling process was analyzed from the perspective of time during the whole corrugated rolling process. The deformation experienced by all metal particles is divided into two periods: the filling period and the extension period. The stress change is complex during the filling period, but the stress change trend is approximate during the extension period. The stress state of characteristic positions is two tensile stresses and one compressive stress when the plate starts to deform, which was analyzed by the Lode angle parameter and triaxiality parameter. The microstructure evolution of the Cu plate was analyzed using electron backscatter diffraction (EBSD) technology. Compared with the initial plate, grain refinement occurs at each characteristic position of the Cu plate. There is a phenomenon of grain growth at the trough of the Cu plate after refinement due to the adiabatic temperature increase caused by severe plastic deformation.

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.

Influence of Polycarboxylate Superplasticizer on the Properties of Cement-Fly Ash Cementitious Materials and Concrete

Concrete materials often crack due to the temperature field caused by the early heat of hydration, affecting structural safety and normal use. To solve this problem, this paper proposes the method of incorporating polycarboxylate superplasticizer to improve its performance and explore the influence of polycarboxylate superplasticizer on the properties of cement-fly ash composite cementitious materials and concrete. Ordinary silicate was used to prepare cement-fly ash composite cementitious materials. Through isothermal conduction calorimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and other testing methods, the influence of polycarboxylate water-reducing agent on the heat of hydration of cementitious materials was studied. In addition, the hydration products and microscopic morphology of the cementitious materials were analyzed, and the changes in the concrete properties due to the addition of polycarboxylate superplasticizers were discussed. The results showed that the polycarboxylate superplasticizer could delay the onset time of the hydration heat peak of the slurry and reduce the hydration heat peak, inhibit the crystallization of Ca(OH)2 and AFt, improve the density of the slurry, and reduce the amount of chemically bound water. In addition, it could delay the overall hydration process of the cementitious material, where the adiabatic temperature increase rate and the early drying shrinkage rate of the concrete slowed down, and the mechanical properties and impermeability of the concrete improved.

Influence of strain rate on the tensile properties of metastable austenitic stainless CrNi and CrMnNi spring steels

Austenitic stainless steels have a wide area of applications, including spring steels in their hardened condition. A preceding forming process mainly achieves the necessary high yield strength (YS) for spring steel. During strain hardening, strain-induced α′-martensite formation results in high strength characteristics. The driving forces behind this transformation include low stacking fault energy of austenitic base material and high chemical driving force of α′-martensite formation. Both parameters are strongly dependent on temperature. This research represents the temperature dependent mechanical properties in a temperature range between −40 °C and 200 °C of a) nickel reduced manganese and nitrogen alloyed stainless steel X5CrMnNiMoN16-4-4 (4Mn) and b) CrNi steel X10CrNi18-8 (1.4310) at a strain rate of 4.6 × 10−4 s−1. Additionally, the effect of a moderate increase in strain rate to 9.2 × 10−4 s−1 and 6.7 × 10−3 s−1 on quasi-adiabatic heating of the steels was investigated. For both steels, by increasing the strain rate, the adiabatic temperature increases and subsequently decreases the strain-induced α′-martensite formation. The increase in the strain rate results in a strong drop of ultimate tensile strength (UTS) for both steels. Nevertheless, the uniform elongation (UE) remains almost constant at a high level for 4Mn. The UE decrease sharply in case of 1.4310. Independent of strain rate value, a larger strain-induced α′-martensite fraction generated in 4Mn compared with 1.4310. Consequently, under all experimental conditions, higher UTS values were achieved for 4Mn. The strain-induced α′-martensite was quantified by magnetic saturation analysis (MSAT). Subsequently, it was characterised by scanning electron microscopy (SEM) using electron channelling contrast imaging (ECCI) and electron backscatter diffraction technique (EBSD).

Effects of hydrogen addition on combustion characteristics of a methane fueled MILD model combustor

Combined with need of the carbon emissions, the feasibility of Moderate or Intense Low-oxygen Dilution (MILD) combustion fueled with hydrogen/methane blends needs to be investigated. This paper discusses the pollutant emissions, the stable operating range and the flame morphology for a jet-induced MILD model combustor. The hydrogen/methane volume ratios range 0:10 to 5:5. The NOx emissions are less than 5 ppm@15%O2 when the hydrogen content is less than 50% by volume in the atmospheric conditions. The calculation using chemical reactor network (CRN) model demonstrates that the effect of heat loss on NOx emissions increases as the adiabatic combustion temperature increases, which is consistent with the experimental results. The maximum OH∗ signal intensity increased at higher hydrogen content, especially when the hydrogen content exceeds 30% by volume. Due to the increase in turbulent burning velocity and the enhancement in the reaction intensity, the reaction zones shrink with increasing hydrogen content. In addition, with increasing hydrogen content, the stable operation range of the combustor becomes narrower, and the stable combustion is not maintained when the hydrogen content exceeds 50% by volume. The findings of the paper help to further understand the effect of hydrogen content on the formation of MILD combustion in the jet-induced combustor.

Oxidative Coupling of Methane over Pt/Al2O3 at High Temperature: Multiscale Modeling of the Catalytic Monolith

At high temperatures, the oxidative coupling of methane (OCM) is an attractive approach for catalytic conversion of methane into value-added chemicals. Experiments with a Pt/Al2O3-coated catalytic honeycomb monolith were conducted with varying CH4/O2 ratios, N2 dilution at atmospheric pressure, and very short contact times. The reactor was modeled by a multiscale approach using a parabolic two-dimensional flow field description in the monolithic channels coupled with a heat balance of the monolithic structure, and multistep surface reaction mechanisms as well as elementary-step, gas phase reaction mechanisms. The contribution of heterogeneous and homogeneous reactions, both of which are important for the optimization of C2 products, is investigated using a combination of experimental and computational methods. The oxidation of methane, which takes place over the platinum catalyst, causes the adiabatic temperature increase required for the generation of CH3 radicals in the gas phase, which are essential for the formation of C2 species. Lower CH4/O2 ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH4/O2 ratio of 1.4. Thereby, C2 species selectivity of 7% was achieved at CH4/O2 ratio of 1.6, with 35% N2 dilution.

Effects of laser shock peening on microstructure and fatigue behavior of Ti–6Al–4V alloy fabricated via electron beam melting

Laser shock peening (LSP) is a post-treatment process that is widely used to modify the surface microstructure and mechanical properties of parts constructed by additive manufacturing (AM). In this study, the influence of LSP on the microstructure and fatigue behavior of Ti–6Al–4V alloy manufactured via electron beam melting (EBM), a popular method of AM, was investigated. The microstructure of the EBM sample consisted of the β phase (~6 vol%) and α lamellar phase. Grain refinement of the α phase occurred via both dislocation evolution and deformation twinning during LSP. A theoretical description of the microstructural evolution, particularly the distribution of deformation twins, was developed. The fatigue strength and micro-hardness of the EBM samples increased by approximately 17% and 11% after LSP treatment, respectively. The fatigue fracture morphologies at three defined damage stages (crack initiation, crack propagation, and instantaneous rupture) were examined for EBM samples before and after LSP. The dominant mechanism of fatigue strength enhancement by LSP was discussed. The effects of residual compressive stress assistant with adiabatic temperature increase and grain refinement of the α phase produced by LSP reduced the pre-existing crack size, suppressed crack initiation, and increased the required work for fatigue fracture.

Research on the Large Plastic Deformation Damage and Shock Resistance of 7055 Aluminum Alloy

We used the split Hopkinson dynamic impact test and ABAQUS finite element simulation to study the damage mechanism of dynamic impacts on the 7055-T4 aluminum alloy. The results show that the energy absorption phenomenon occurs when the dynamic impact temperature is 220°C. There is a clear positive correlation between the size of the precipitated phase and the dynamic impact temperature. The regression effect of the 7055-T4 aluminum alloy is most obvious when the dynamic impact temperature is 320°C. The adiabatic temperature rise model in the dynamic impact shear zone and the damage model considering the adiabatic temperature increase are established. Moreover, the above model has high accuracy and is verified by projectile impact simulation.

Effect of inorganic salt and organic acid on the thermal runaway of hydrogen peroxide

Hydrogen peroxide with the presence of catalytic metal ions or incompatible organic matters has caused some fire and explosion accidents in recent years. This work further studied the effect of inorganic salt and organic acid on the thermal runaway behaviour of hydrogen peroxide via rapid screening device (RSD), accelerating rate calorimeter (ARC) and a batch reactor. Experimental results revealed that both the metal ions and their corresponding inorganic acid ions have significant influences on the thermal runaway of H2O2: the catalytic effect of metal ion followed Fe2+ > Fe3+ > Cu2+, and the synergistic effect of inorganic acid ion followed Cl− > SO42−. No explosion was detected under RSD and ARC test conditions. In addition, a one-step global reaction kinetic model characterized as adiabatic temperature increase and final exothermic temperature was briefly described to estimate the kinetic parameters of the mixture of H2O2 with/without organic acid. The analysis showed that the runaway reaction of H2O2 with formic acid or acetic acid is a first order reaction at a low temperature range (<100 °C) and the thermal runaway risk of H2O2/HCOOH > H2O2/CH3COOH, along with the decreasing apparent activation energy and pre-exponential factor.

Surface gradient microstructural characteristics and evolution mechanism of 2195 aluminum lithium alloy induced by laser shock peening

The peak-aged 2195 aluminum lithium alloy was treated by laser shock peening (LSP). The surface gradient microstructural characteristics of this alloy induced by ultrahigh strain rate deformation during LSP were systemically examined with transmission electron microscope (TEM), the observed results suggested that the grains refined and precipitates partially dissolved in the surface after LSP. The original coarse grains with average size of about 16 μm in length and 5 μm in width were refined instantly to equiaxed grains with size of about 91 nm at the top surface after LSP. The quantitative calculation of recrystallization kinetics proved that the grain refinement was the result of rotation dynamic recrystallization (RDR). The adiabatic temperature increase, the generation of high-density dislocations around the precipitates, and the increase of grain boundary area caused by grain refinement provided the thermodynamics and kinetics conditions for partial dissolution of precipitates. The microhardness tested results showed gradient distribution characteristics of microhardness values after LSP, and the maximum of microhardness was at the top surface of this alloy. The refined grains and deformed substructures played important roles on the enhancement of surface microhardness.

Characterization and magnetic properties of nanocrystalline Mg1-xCdxFe2O4 (x = 0.0–0.8) ferrites synthesized by glycine-nitrate autocombustion method

Sol-gel autocombustion method using glycine as fuel and complexing agent was utilized to synthesize nanocrystalline Mg1-xCdxFe2O4 (x = 0–0.8) ferrites. An increase in adiabatic flame temperature with x during the combustion reaction was demonstrated, based on the thermodynamic calculations. The X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) results confirmed that a single-phase spinel structure in all samples is formed at a remarkably lower temperature and with a larger production rate compared to the standard ceramic and other wet-chemical methods. Crystallites growth from 98 nm to 320 nm, enhanced X-ray density and a linear increase in both theoretical and experimental lattice constants, with comparable values were observed with introduction of cadmium. Analysis of the XRD patterns, using the Bertaut method along with a detailed Rietveld analysis reflected a different cations distribution and evolution of some structural parameters with cadmium, compared to the earlier studies. The Field-emission scanning electron microscopy (FESEM) results showed that the average particle size of the synthesized ferrites increases from 161 to 520 nm for x = 0 to x = 0.8, respectively. The magnetic measurements revealed that the saturation magnetization (Ms) initially increases and reaches the maximum value of 46 A m2 kg−1 at x = 0.2, and subsequently decreases, while the coercivity (Hc) and remanent magnetization (Mr) continuously decline with x values. The present magnetic results for x = 0.0 − 0.4, show a noticeable improvement in magnetic softness compared to previous researches on the Mg-Cd ferrites obtained by other chemical methods.

Experimental and kinetic investigations of the effect of H2/CH4/C2H4 addition on the burning properties of practical jet fuel

Abstract Laminar flame speeds of premixed jet fuel/air with the addition of hydrogen, methane and ethylene are measured in a constant-volume bomb at an initial temperature of 420 K, initial pressure of 3 atm, equivalence ratios of 0.6–1.5 and gas mass fractions of 0–50%. The experimental results show that the addition of hydrogen and ethylene can significantly improve the laminar flame speed of the liquid jet fuel, while the addition of methane shows a weak inhibitory effect, and these effects are relatively remarkable on the fuel-rich conditions. The laminar flame speed of the dual fuels/air is linearly dependent on the additional gas mass fraction. A kinetic analysis indicates that the gas addition causes both thermodynamic and chemical kinetic effects on the laminar flame speed of the dual fuels/air. The adiabatic temperature increases and decreases with the addition of hydrogen/ethylene and methane, respectively. A sensitivity analysis shows that the reactions concerning to the H, CH3 and C2H3 radicals become significant with the addition of hydrogen, methane and ethylene, respectively, and that the different values of the rate of product (ROP) of these species via the critical reactions lead to a different promotional or inhibitory effect on the fuel-rich and fuel-lean conditions.

Effect of dispersed retained γ-Fe on brittle crack arrest toughness in 9% Ni steel in cryogenic temperatures

In structural steels, high crack arrest toughness is required to stop brittle cracks from propagating to prevent eventual fracture of the structure. It is known that Ni can reduce the risk of cleavage fracture of α-Fe, as it exists in a solid solution as a substitutional alloying element. Additionally, Ni can enhance the stability of γ-Fe between the martensite-laths, leading to improved toughness. Although there are several interpretations of the effect of retained γ on the fracture toughness, especially regarding dynamic crack propagation, there is little knowledge on the role of retained γ. In this study, the relationship between the amount of transformation and crack arrest toughness is evaluated by an experimental approach. As a result, it is revealed that the γ-α transformation at the propagating crack tip enhances the crack arrest properties. Furthermore, compared to static loading conditions, γ-Fe seems reluctant to transform to martensite as a result of an adiabatic temperature increase due to plastic work. Stress relaxation effects due to deformation-induced transformation of γ-Fe are the proposed mechanism of enhancement of the crack arrest properties.

Experimental study on the thermal runaway of hydrogen peroxide with in-/organic impurities by a batch reactor

Hydrogen peroxide, as a very versatile reagent for many industrial processes, has caused many accidents in recent years. Therefore, understanding the thermal hazards of hydrogen peroxide with or without impurities is essential for the prevention of fire and explosion accidents. In this work, batch reactor tests were conducted to study the thermal runaway behavior of hydrogen peroxide under the contaminations of Fe3+ ion, formic/acetic acid and ethanol/acetone. Experimental results revealed that the thermal runaway risk of H2O2 increased significantly with increasing initial temperature and Fe3+ ion concentration. Comparatively, the potential risk of organic impurities on the thermal stability of hydrogen peroxide was found to be much larger, because several violent explosions were measured resulting from a series of rapid and complicated decomposition reactions. The explosion severity of H2O2 with organic matters followed acetone > ethanol > formic acid > acetic acid. In addition, a modified runaway scenario characterized as adiabatic temperature increase and time to maximum rate was briefly described to theoretically evaluate the thermal risk of H2O2 decomposition. This scenario was found to be valid to estimate thermal runaway hazards. These results improve our understanding of thermal runaway behavior and explosion risk of H2O2 with incompatible impurities.

Application of Ground Granulate Blast Furnace Slag-Steel Slag Composite Binder in a Massive Concrete Structure under Severe Sulphate Attack

A composite mineral binder was prepared by mixing ground granulate blast furnace slag (GGBS) and steel slag (GGBS/steel slag ratios are 1 : 1 or 3 : 2 by mass). The application of a composite binder in a massive concrete structure under severe sulphate attack is discussed by determining the hydration heat, adiabatic temperature increase, compressive strength, elastic modulus, chloride ion permeability, and sulphate attack resistance. The results show that the hydration heat of the composite binder decreases greatly when the cement replacement ratio increases to 50% at 45°C. The adiabatic temperature rise of the concrete containing the composite mineral admixture decreases significantly. Concrete containing the composite mineral admixture has a lower early elastic modulus and satisfactory late-age compressive strength. The composite mineral admixture can improve the resistance to chloride ion permeability and sulphate attack resistance of concrete, especially during temperature match curing.

Experimental study of natural gas flame enriched by hydrogen and oxygen in a coaxial burner

In air combustion, nitrogen brings about a low yield of combustion and a high energetic consumption because the nitrogen contained in the air acts as energy ballast. The substitution of air by oxygen or the oxygen addition to the air permits the improvement of the heat yield, the rise of the adiabatic flame temperature and the extension of the flammability limits. The hydrogen addition to the natural gas combustion shows an increase in adiabatic flame temperature, a reduction in flame thickness and further enhances the auto-ignition characteristics and the global rate of heat release. This work presents the effects of enrichment with oxygen and hydrogen on a non-premixed flame behavior. More specifically, the study focuses on the flame stability, the pollutants emissions such as NOx, CO2, and CO and the jet flow dynamic. Chemiluminescence of OH∗ radical and Particle Image Velocimetry (PIV) are conducted to describe the structure, the stability and the dynamic of the flame. The measurements are conducted at the stoichiometry for oxygen rate varying from 20 to 50% by volume and hydrogen varying from 0 to 15%. The results show that the addition of oxygen and hydrogen improves the flame stability, increases the CO2emission, decreases the CO formation but favors the NOx formation.

A Lagrangian investigation of hot and cold temperature extremes in Europe

Since temperature extremes have a strong impact on environment and society, it is crucial to understand their underlying mechanisms. While their relationship to the large‐scale atmospheric circulation has been comprehensively investigated, the role of physical processes in the formation of air masses with extreme temperatures is less well understood. This study presents a Lagrangian analysis of the 0.1% most extreme six‐hourly hot and cold events in three European regions (UK, Central Europe, Balkans) for the time period 1989–2009. The results provide insight into typical transport patterns and physical processes (adiabatic compression, radiation, surface heat fluxes) occurring along the trajectories of extreme temperature air masses. Cold events in Europe are most frequently induced by advection of cold air masses from the Arctic and Russia. The transport to the target region is characterized by a temperature increase caused by adiabatic compression and, in the maritime setting of the UK, also by diabatic heating due to surface sensible heat fluxes. Despite the warming along the trajectories, the extremeness of the associated 2 m temperature increases, due to the dislocation of the air mass to regions with a milder climate. Hot events are generally associated with weaker horizontal transport, but strong adiabatic warming and local temperature increase caused by enhanced radiation and surface heat fluxes. This in situ warming is particularly strong in Central Europe. Evaluating the temperature evolution along the trajectories reveals that hot and cold extremes develop on a similar time‐scale of 2–3 days. This time‐scale is mostly set by physical processes for hot extremes and controlled by advective transport for cold extremes. The diagnostics applied in this study lead to an improved process understanding that can provide a basis for more accurate predictions of temperature extremes.

Molecular dynamics investigation of thermite reaction behavior of nanostructured Al/SiO2 system

In this study we have investigated the thermite reaction of Al/SiO2 layered structure by classical molecular dynamics simulation in combination with the reactive force field function. Under the adiabatic conditions, we simulate the structural changes and energetic properties of the system at six different initial temperatures (600, 700, 800, 900, 1000 and 1100 K). These results show that the thermite reaction of Al/SiO2 is the self-heating reduction-oxidation (redox) reaction. When the initial temperatures are 900 and 1000 K, the Al layers change into liquid-like structure under melting points. The thermite reaction happens with a much faster rate. At other initial temperatures such as 600, 700, 800 and 1100 K, the thin Al-O layer at the interface is quite weak for the higher initial temperature. The adiabatic reaction temperature increases and the effective reaction time decreases with the increasing of the initial temperature. the reaction self-heating rates are 3.4, 3.5, 4.7 and 5.4 K/ps for the initial temperatures of 600, 700, 800 and 1100 K, respectively. The results reveal that the thermite reaction self-heating rates depend on the thickness of interfacial diffusion barrier in the nanoparticle. In addition, the thermite reaction of the Al/SiO2 system leaves the Si, which accord well with the experimental result.