Initial Heat Release Research Articles

Mechanical properties and hydration mechanism of nano-silica modified alkali-activated thermally activated recycled cement

The recycling and reuse of waste concrete are crucial for reducing environmental pollution, minimizing resource waste, and lowering carbon emissions. However, current efforts to improve the performance of recycled cement (RC) using the synergistic effects of alkali activation and thermal activation have been suboptimal. Therefore, this study explores the influence of nano-silica (NS) incorporation on the mechanical properties and hydration mechanism of alkali-activated and thermally activated RC. The research focuses on cement mortar with a thermal activation temperature of 750 °C, an RC replacement rate of 30 %, and an alkali content of 6 %. Various analytical techniques, including thermogravimetric analysis (TG-DTG), X-ray diffraction (XRD), and scanning electron microscopy (SEM), were used to evaluate the impact of different NS dosages (1 %, 2 %, 3 %, and 5 %) on the mechanical properties and hydration characteristics of nano-modified alkali-activated and thermally activated RC mortar. The study reveals the mechanism by which NS enhances the microstructure of alkali-activated and thermally activated RC. The experimental results show that as the NS content increases, the compressive and flexural strengths of RC initially increase and then decrease. The optimal strength was achieved with a 2 % NS content, with a 31.5 % increase in compressive strength compared to RC without NS. At this dosage, RC exhibited a higher hydration rate during the acceleration period of hydration. Additionally, the initial hydration heat release rate of RC increased with rising NS content. While NS incorporation did not alter the phase composition of RC, it activated the pozzolanic effect of RC, causing free active substances such as CaO and gypsum in the components to dissolve and react to form Aft at the start of the hydration reaction. The unsaturated bonds (≡Si-O- and ≡ Si-) on the surface of NS absorbed Ca2⁺ and OH⁻ from the RC, promoting the hydration of C2S and C3S. However, excessive NS content (greater than 3 %) led to a decline in RC strength and the formation of more cracks. This is primarily due to the excess NS particles not contributing to an increase in effective nucleation sites, causing agglomeration, an increase in large pores, and a reduction in transition and gel pores, which negatively affected the microstructure and thus reduced the mechanical properties of the RC. Our findings enhance the understanding of the role of nanomaterials in alkali-activated RC and provide valuable insights for further research into high-performance recycled cement materials.

Study on the hydration characteristics, mechanical properties, and microstructure of thermally activated low-carbon recycled cement

Based on the thermal activation method, utilizing fine particles of waste concrete to prepare low-carbon recycled cement (RC) is currently one of the research hotspots. Based on existing literature research results, this paper identifies four calcination temperature conditions (120, 450, 750, 1000 ℃) and uses replacement rates of 10 %, 30 %, 50 %, 70 %, and 100 % of RC to replace ordinary Portland cement (OPC).The main physical properties, hydration characteristics, mechanical properties, and microstructure of RC are studied. The study elucidated the influence of RC thermal activation temperature, replacement rate, fineness, and age on its hydration characteristics, mechanical properties, and microstructure, establishing a macro-micro correlation for RC and revealing the mechanism of improvement in mechanical properties of thermal-activated RC. The research indicates that in the initial stage of hydration, RC exhibits a relatively fast hydration rate, with higher initial total heat release than OPC. Hydration products include internal hydration products (formation of C-(A)-S-H gel) and external hydration products (formation of plate-shaped AFm). Furthermore, RC exhibits a faster hydration rate at a hot activation temperature of 750°C, with CH and C-S-H gel transforming into the highly hydrated α´L-C2S, forming a denser microstructure at an early stage. Moreover, at a replacement rate of 30 %, the compressive strength of cement mortar reaches its optimum at 28 days, achieving 13.81 MPa, equivalent to 60.7 % of OPC cement mortar, with a 66.67 % reduction in carbon emissions. Microstructure analysis reveals that when the temperature reaches 1000°C, excessive combustion of RC leads to the formation of low-reactivity C2AS and β-C2S, resulting in a decrease in macroscopic mechanical properties. Additionally, increasing the fineness of RC significantly enhances its rehydration capability, especially as the age increases. This study is of great significance for the preparation of green cement-based composite cementitious materials.

Numerical study of the combustion process and NOx evolution mechanism of ammonia-hydrogen compound fuel engine under different intake conditions

To explore the distinction between generation atmosphere of fuel NOx and thermal NOx in an ammonia-hydrogen compound fuel engine, a numerical calculation model was established based on a detailed ammonia-hydrogen oxidation mechanism. The combustion process and NOx evolution mechanism of the ammonia-hydrogen compound fuel engine under different intake conditions were studied. The researchresults for different equivalence ratios show that the in-cylinder hydrogen volume is the primary factor influencing the initial heat release. When the equivalence ratio is between 0.89 and 1.14, the mixture exhibits a higher combustion rate, leading to a shorter CD and an earlier CA50. At this point, the heat release process closer to TDC contributes improving engine combustion efficiency and indicated efficiency. Therefore, when the equivalence ratio is between 0.89 and 1.00, combustion efficiency is higher than 98 % and indicated efficiency is higher than 43 %, the ammonia-hydrogen compound fuel engine achieves relatively higher economy. Both over-rich and over-dilute mixtures induce an enhancement of fuel NOx, while the higher in-cylinder temperature caused by stoichiometric combustion leads to a boom of thermal NOx. Consequently, the total NOx emission from the ammonia-hydrogen compound fuel engine is higher at equivalence ratios between 0.80 and 1.00, and the NOx emission reaches its peak at an equivalence ratio of 0.89. Analysis of the evolution process of key N components in the cylinder shows that incomplete combustion plays a significant role in fuel NOx emissions, while thermal NOx emissions are mainly influenced by the distribution of in-cylinder components and combustion temperature. Specifically, thermal NOx emissions primarily depend on the in-cylinder components distribution in the narrow equivalence ratio range around stoichiometric ratio, and are significantly affected by in-cylinder temperature under other conditions. The in-cylinder oxygen concentration significantly affects fuel NOx emissions under most operating conditions. Further optimization of intake temperature shows that combustion efficiency exceeds 90 % when the intake temperature is above 280 K. When the intake temperature reaches 310 K, the combustion efficiency reaches up to 98 %, thus an intake temperature of about 310 K is sufficient to meet the need for improving the combustion efficiency of ammonia-hydrogen compound fuel engine. To achieve high efficiency and lower NOx emissions, appropriate lean combustion combined with an intake temperature slightly above room temperature is recommended.

Thermal decomposition characteristics of BHT and its peroxide (BHTOOH).

2,6-Di-tert-butyl-4-methylphenol (BHT) is an excellent antioxidant that is easily oxidized to 2,6-di-tert-butyl-4-hydroperoxyl-4-methyl-2,5-cyclohexadienone (BHTOOH). For the safety of BHT production and usage, it is meaningful to study the thermal stability and decomposition properties of BHT and BHTOOH. In this paper, the thermal decomposition properties of BHT and BHTOOH were compared by the mini closed pressure vessel test (MCPVT) and differential scanning calorimetry (DSC). Their kinetics of thermal decomposition were studied using thermogravimetric analysis (TGA). The thermal decomposition products of BHT and BHTOOH were analyzed by gas chromatography-mass spectrometry (GC-MS). The results show that there was no significant change in temperature pressure when BHT was warmed up under a nitrogen atmosphere, indicating that BHT was stable within 400K. The thermal decomposition reaction of BHTOOH was rapid with an initial reaction temperature of 375.2K. The initial exothermic temperature (Ti) and heat release (QDSC) of DSC were 384.9K and 865.0Jg-1, respectively. The apparent activation energies (Ea) for the thermal decomposition reactions of BHT and BHTOOH calculated by the Kissinger method were 151.8kJmol-1 and 66.07kJmol-1, respectively. The main decomposition products of BHT were isobutene and 2-tert-butyl-4-methylphenol. The thermal decomposition products of BHTOOH included BHT, 2,6-di-tert-butyl-4-ethylphenol, 3,5-di-tert-butyl-4-hydroxybenzaldehyde, 4,4'-(1,2-ethanediyl) bis [2,6-bis (1,1-dimethylethyl) phenol, etc. Based on the thermal decomposition behavior and products, the reaction pathway has been described. These results indicate that BHT is a potential thermal hazard during production, storage and application. For the safety of the chemical industry, the oxidation of BHT should be avoided.

Clinkering and hydration of alite-belite-ye'elimite cement with increasing ye'elimite percentage

Alite-belite-ye'elimite (ABY) cement is considered to be eco-cements with adequate mechanical properties and fewer CO2 emissions. The phase assemblage and ye’elimite content of ABY cement play a very important role in cement properties. In this study, ABY clinkers with increasing ye'elimite percentage (0.3 wt%-36.4 wt%) were prepared at 1300 °C and the phase composition was characterized by Rietveld Quantitative Phase Analysis combined with energy dispersive X-ray spectrometry. The ye'elimite formed with a calculated composition of Ca3.92Mg0.02Al2.81Fe0.05Si0.04S1.16, incorporating additional sulfur compared to the theoretical value. In addition, the hydration properties of heat evolution, hydration products and compressive strength of ABY cement are studied. The result shows that the increase of ye'elimite content leads to a more intense initial heat release and a higher cumulative heat release. The presence of ye'elimite inhibits the hydration of alite at early ages. ABY mortars with increasing ye'elimite (15.8 wt%) exhibited a higher strength (29.6 MPa) at 3 days by the contribution of ye'elimite hydration, and an increase (47.4 MPa) at 28 days. An adequate proportion of ye'elimite (<15 wt%) is favorable to the performance of ABY cement, and releases 12% less CO2.

DL‐malic acid for inhibiting the re‐ignition of long‐flame coal and its micro‐reaction mechanism

AbstractTo study the metal ion chelating agent (DL‐malic acid) inhibiting oxidation characteristics of residual coal in goaf, the microscopic thermal transformation characteristics of the treated coal samples were obtained using apparent morphology, infrared spectroscopy, free radical testing, and thermal analysis. The results demonstrate that after the addition of malic acid, the sphericity and roundness of the coal samples decrease, and the small fractured particles accumulate in the pores of the oxidized coal samples and, thereby, reduce the number of coal–oxygen contact sites. The addition of 10% malic acid promotes the conversion of functional groups (FG) and free radicals in pre‐oxidized coal, inhibits the catalytic effect of metal ions, generates more active sites, and increases the total heat release during the combustion process. There are strong endothermic points in the malic acid‐treated coal samples, and the free water evaporation blocked by small particles in the holes delays the initial heat release temperature of the coal samples. Pre‐oxidation can promote changes in the valence state of metal ions in coal, increase the adhesion between broken pre‐oxidized coal particles via malic acid chelation, and lose the catalytic effect of metal ions; this inhibits the process of oxidative re‐ignition of coal samples.

Effects of NaIO4 treatment on the chemical and physical properties of polyacrylonitrile fibers

Polyacrylonitrile (PAN) fiber modification can promote the stabilization efficiency and mechanical properties of the subsequent carbon fibers; however, it could cause damage to the fibers to some extent. In this article, the effects of NaIO4 treatment on the chemical and physical properties of PAN fibers were investigated. After NaIO4 modification, the cyclization reaction has partially occurred, owing to the FTIR revealed that the C≡N peak was weakened, but the C=N stretching vibration peak, the –N = C=O stretching vibration peak, the C=O stretching vibration absorption peak, and the short sequence conjugate C=N characteristic peak were enhanced simultaneously, indicating a successful cyclization and dehydrogenation reaction. Moreover, owing to more oxygen-containing functional groups successfully attached to the PAN fibers and the cyclization reaction taking place, the initial temperature, peak temperature, and heat release of cyclization decreased, which means a more efficient and safer stabilization process. X-ray diffraction (XRD) showed that the (100) crystal plane microcrystal size was the smallest at 40 °C of the modification temperature, whereas the (002) crystal plane microcrystal size was the largest at 60 °C. The modified stabilized PAN fibers presented lower crystallinity and a more aromatic structure compared with untreated fibers. Raman spectra revealed a significant increase in the intensity of the G peak after NaIO4 modification; the R-value decreased from 1.65 to 1.43, demonstrating a transformation of the sp3 hybrid carbon structure into a more graphite-like sp2 hybrid C=C structure. Meanwhile, scanning electron microscopy (SEM) images disclosed additional grooves and cracks on the fiber surface after modification.

Performance and mechanism of amino acids (AAs) on the gypsum setting-time control

Protein retarders can not only prolong the setting time of gypsum in application, but also lower the reduction in strength. The chemical activity of proteins is determined by the basic unit of amino acids (AAs). To enhance the understanding of protein retarding action mechanism, this paper focuses on the AAs-adsorbed gypsum structure, and investigated the retarding effects, crystal morphologies, and nanoscale interactions. Results indicate that the AAs had varying retarding effects on gypsum, and the effects from greatest to smallest were L-arginine, L-glutamic, and L-leucine acid. The rate of initial hydration heat release was reduced, and the peak of hydration release was delayed. Retardation of AAs on gypsum involves chemical adsorption on the surface of dihydrate gypsum crystals, which mainly occurs on the (1 2 0) face and affect gypsum morphology. The mechanism of AAs adsorption involves electrostatic interactions between charged functional groups of AAs and gypsum particles, as well as hydrogen bonding at additional carboxyl and amino groups. The adsorption strength depends on longer chained structure and number of functional groups of the AAs. And intrinsic strength of AAs-adsorbed gypsum nanocrystals contributes to the strength reduction on the macroscale. This paper provides basic data for designing and tailoring enhanced protein retarders for gypsum.

Modeling the impact of the fuel injection strategy on the combustion and performance characteristics of a heavy-duty GCI engine

Gasoline compression ignition (GCI) is a promising strategy to achieve high thermal efficiency and low emissions with limited modifications to the conventional diesel engine hardware. It is a partially premixed concept, which derives its superiority from higher volatility and longer ignition delay of gasoline-like fuels combined with higher compression ratio typical of diesel engines. The present study investigates the combustion process in a GCI engine operating with different injection strategies using computational fluid dynamics (CFD). Simulations are carried out on a single cylinder of a multi cylinder heavy-duty compression ignition engine, which operates at a compression ratio of 17:1 and an engine speed of 1038 rev/min. Two different injection strategies viz., late injection (LI), and early pilot injection (EP) are investigated to understand their impact on combustion and performance of the engine. Renormalized group (RNG) k-ε model is used to describe in-cylinder turbulence and KH RT model is used to simulate the fuel spray breakup. The developed CFD methodology is validated against relevant experimental data under a wide range of operating conditions for each injection strategy. The developed CFD methodology was found to capture the engine combustion behavior quite well. Based on the validated CFD model, the differences in the progress of combustion event for the two injection strategies is highlighted. It was found that a larger pilot fuel mass fraction results in a steeper rise in the initial heat release rate which in turn influences the transition to mixing controlled combustion. In line with the experimental data, the study showed that the late pilot injection strategy with three injection pulses, results in higher performance compared to the other conditions.

Hypergolic ignition response to oxidizer droplet properties

The hypergolic ignition response to oxidizer droplet properties is an important consideration for oxidizer injector design in hybrid rocket motors. Hence, the present work focused on hypergolic ignition characterization as a function of droplet properties and impact dynamics for a promising propellant. The propellant for the present work consisted of rocket grade hydrogen peroxide (RGHP) as oxidizer, high-density polyethylene (HDPE) as fuel and sodium borohydride (NaBH4) as the hypergolic additive embedded in the HDPE matrix. This propellant was chosen for this detailed study since it showed promising results in prior studies, exhibiting low (<10 ms) ignition delays. For this work, in a drop-test setup, RGHP droplet diameters between 1.90±0.11 mm and 4.35±0.26 mm were dropped on the HDPE + NaBH4 fuel samples at velocities between 0.3 m/s to 1.93 m/s. The ensuing ignition event was studied using both visible and infrared high-speed imaging. Post-impact droplet parameters were determined to enable a deeper analysis of observed phenomena. While ignition delay was the focus of this work, flame spread areas and heat release were also studied. Higher impact velocities generally led to lower ignition delays while splashing oxidizer drops increased probability of larger flame areas and higher heat release. Additionally, the initial ignition/heat release event was found to originate in a small region at the thinnest area of the post-impact droplet. Surface temperature measurements were also performed using infrared imaging. The data suggested that the initial heat release was found to be likely due to RGHP-NaBH4 reaction with no contribution from RGHP decomposition. Regression and random forest machine learning analysis found ignition delay depended strongly on the minimum post-impact drop layer thickness. Based on the data analysis, a simple mathematical model was developed whose results matched experimentally observed trends. The study is expected to further understanding of the processes that control hypergolic ignition in addition to aiding rocket engineers design better oxidizer feed systems. Novelty and SignificanceThis work constitutes a novel in-depth study on the influence of drop impact properties on hypergolic ignition. Using innovative techniques, this work is the first to show the overwhelming importance of residual droplet layer thickness on ignition delay. Additionally, a simple mathematical model based on that finding performed well against experimental data which is of substantial significance, given the complex and multi-parametric nature of hypergolic ignition. Finally, the work establishes that operating in the splashing regime enhances heat release and flame spread. The candidate propellant for this work was hydrogen peroxide and high-density polyethylene with embedded NaBH4. However, the findings of this study are likely generalizable over a range of solid-liquid hypergolic propellants involving solid fuels with embedded energetic materials, of which there are many. The findings of this work can enhance understanding of hypergolic ignition in addition to aiding in better rocket injection system design.

Experimental and numerical investigation of an innovative complex piston with enhanced free spray length for the heavy-duty engine applications

The combustion and emission formation process in heavy-duty compression ignition engines are strongly influenced by the design of the piston bowl, the cylinder charge motion and the performance of the injection system. This paper proposes an innovative complex piston bowl design approach called as ’flower piston’ to improve the combustion process. It can be manufactured using the conventional machining approaches without use of still expensive additive manufacturing techniques. The flower piston features a scooped cavity with an intention of increasing the free fuel spray penetration. The potential of this innovative flower piston is evaluated using a validated 3D CFD model and compared with the results of single cylinder engine experiments at operating points with minimum fuel consumption. The results of the experimental investigations with the flower piston show an improvement in thermal efficiency in comparison to the conventional piston by up to 0.5%. This is due to a faster initial heat release and increased high pressure cycle work. However, the smoke emissions increased threefold due to fuel entrapment into the flower cavities, leading to formation of fuel-rich pockets. Also, the wall heat losses increased twofold with an increase in NOx raw emissions. With increased spray targeting angle, there was a drastic reduction in smoke emissions and an additional improvement in the thermal efficiency.

Influence of Activation Parameters on the Mechanical and Microstructure Properties of an Alkali-Activated BOF Steel Slag

Steel slag (SS) is a secondary material from steelmaking production with little commercial value. Its volumetric expansion and low reactivity limit the use of SS in Portland cement (PC)-based materials. This study investigated the potential use of basic oxygen furnace (BOF) slag as a single precursor in alkali-activated matrices (AAMs). Six AAM pastes were assessed by changing the silica modulus (0.75, 1.50 and 2.22) and the sodium concentration (4% or 6% Na2O—wt. SS). The early hydration was assessed using isothermal calorimetry (IC), followed by the assessment of the mechanical performance (compressive strength), apparent porosity, and structure and microstructure characterization (X-ray diffraction, thermogravimetric analysis and scanning electron microscopy). The results indicated that although the BOF slag may be considered a low-reactivity material, the alkaline environment effectively dissolved important crystalline phases to produce hydrates (reaction products). An optimized combination of activator sources was achieved with 4% Na2O and a silica modulus of 1.50–2.22, with a compressive strength up to 20 MPa, a significant amount of reaction products (C-S-H/C-A-S-H gels), and low initial and cumulative heat release. Those properties will help to promote SS recycling use in future engineering projects that do not require high-strength materials.

Comparison on early hydration of Portland cement and sulphoaluminate cement in the presence of nano ettringite

This paper proposed adopting ettringite-based nucleation seeds to induce ettringite to precipitate in the pore solution, and thus facilitated early hydration reaction. In present work, sulphoaluminate cement (SAC) and Portland cement (PC) paste were considered, and 0 ∼ 5 % nano-ettringite prepared by hydration-wet grinding method was extra incorporated. The hydration kinetic of the blends was studied by hydration heat, chemical shrinkage and setting time. The effects of nano-ettringite on the dissolution behavior were also examined by dynamic conductivity of cement slurry, ion concentration and conductivity of pore solution. Meanwhile, the hydrate assemblage was determined by XRD, TGA, FTIR and 27Al NMR, and the compressive strength was also tested. On this basis, the hydration mechanism was discussed. Results indicated that nano-ettringite significantly hastened ions dissolution and ettringite precipitation, whether it was SAC or PC paste. The initial hydration heat release and chemical shrinkage were obviously promoted, and even cracks were generated in SAC paste; besides, the setting time was also shortened dramatically. In the presence of 1 ∼ 5 % nano-ettringite, the compressive strength of PC pastes at 8 h was increased by 54 ∼ 180 %. However, the strength of SAC paste at all ages was degraded by adding nano-ettringite, due to the appearance of cracks and reduced amount of AH3.

Thermodynamic variation law and influence mechanism of low-temperature oxidation of lignite samples with different moisture contents

Coal spontaneous combustion (CSC) is one of the main disasters that threaten the safe production in coal mines. Moisture, which is the main factor affecting coal oxidation, has a considerable impact on CSC. In this paper, the variation characteristics of TG-DTG and DSC-DDSC curves of two kinds of lignite during low-temperature oxidation were investigated. The following results were obtained: Oxidation-induced heat production (OHP) starts to play a leading role in the temperature range of 50–75 °C; pyrolysis-induced heat absorption (PHA) grows faster than OHP in the temperature range of 70–130 °C; and OHP is dominant in the temperature range of 130–260 °C. Furthermore, for coal samples from Shengli (SL) Coal Mine and Yuanbaoshan (YB) Coal Mine, the initial heat release temperature and the end temperature both conform to the Lorentz function T=T0+2A(B4(M−Mc)2+B2). Thermodynamic parameters reach the maximum under the moisture content of 12%–17% where the CSC is the most likely to occur. In the oxygen adsorption and weight gain stage, the activation energy decreases and gradually stabilizes with the increase in moisture content. In the accelerated oxidation stage, moisture in coal plays a chemical catalytic role in the processes of oxygen chemical adsorption and intermediate complex formation and decomposition. However, excessive moisture cannot accelerate the coal-oxygen composite reaction.

Process hazard evaluation and thermal runaway prediction for styrene polymerization initiated by peroxide-azo composites

Styrene is an important monomer for synthetic resins, ion exchange resins and synthetic rubber. Styrene polymerization requires the use of initiators to increase the reaction rate, with composite initiators showing promise for increased reaction rates. However, increased reaction rates in polymerization, if uncontrolled, can lead to thermal runaway with disastrous consequences. Numerous runaway incidents have been documented, indicating inadequate awareness of the thermal hazards of polymerization reactions. This study focuses on determining via calorimetric techniques the thermal hazards of styrene polymerization using azodiisobutyronitrile (AIBN) and tert-Butyl peroxybenzoate (TBPB) composite initiators. Differential scanning calorimetry (DSC) is employed to investigate the thermal decomposition properties of composite initiators with varying composition. Non-isothermal experiments and adiabatic experiments are used to determine the thermal hazard parameters including initial exothermic temperature and heat release of styrene polymerization. The risk of secondary reactions is evaluated by reaction calorimetry (RC1e) and product thermogravimetric analysis (TGA). Key safety parameters of the exothermic reaction, such as the onset temperature, heat release, time to maximum rate under adiabatic condition as well as activation energy, are presented. The results show that the thermal hazard of the polymerization reaction is lowest when the ratio of AIBN to TPPB in the composite initiator is 1:1. In this scenario, the temperature reached by the uncontrolled reaction does not provoke the decomposition of the products, yet the runaway consequences are still unacceptable. This work provides extensive data as a reference for the process optimization of styrene polymerization from the perspective of safety.

The thermal stability of fluorine modified and nickel modified aluminum-based active fuel

The addition of F and Ni modification of Al can significantly enhance the ignition and combustion properties of the propellants. DSC/TGA was used to obtain the reaction thermodynamic process of Al, Al95-PTFE5 (containing Al-polytetrafluoroethylene composites), and Al95-Ni5 (containing Al and Ni composites). The addition of F and Ni greatly increased the mass percentage and exotherm of Al in the first stage (about 580–680 °C), which caused a decrease in temperature that might trigger ignition. Considering the initial reaction temperature and heat release that might trigger ignition, the thermal stability was ranked as Al > Al95-Ni5 > Al95-PTFE5. The activation energy values of the first stage were ranked as Al95-PTFE5 > Al > Al95-Ni5. In addition, the saturated moisture state significantly affected the thermal stability of the nickel-modified aluminum-based active fuel and reduced the thermal stability of Al95-Ni5 at a constant temperature of 30 °C and 50 °C. The conclusions obtained from this study can substantially improve the safety of aluminum and aluminum-based active fuels in use.

Study on the effect of mixed particle size on coal heat release characteristics

ABSTRACT To study the oxidation characteristics of coal under different conditions of mixed particle sizes, a C80 microcalorimeter and ESR spectrometer were used to study the heat release characteristics and laws of free radical parameter changes for coal samples with different mixed particle sizes. Research indicates that a smaller average particle size in the mixed coal sample results in stronger low-temperature oxidation and greater heat release from coal and a smaller apparent activation energy and initial heat release temperature. The concentration of free radicals in coals increases with decreasing average particle size in the mixed coal samples. When the average particle size in the mixed coal sample increased from 0.09 mm to 0.246 mm, the cumulative heat release of the GJW coal sample and the YHP coal sample decreased by 36.67% and 20.13%, and the free radical concentration decreased by 4.34 × 1017/g and 3.66 × 1017/g. In addition, In the mixed coal samples, the mean particle size mutation value was 0.126, which makes the heat release characteristics of coal at this particle size opposite to the overall trend. Further analysis established the relationship model between the heat of different mixed particle sizes and the free radical concentration of different mixed particle sizes as follows: y = ax3+ bx2+ cx+d.

Research on application of asymmetrical Pre-chamber in Air-Assisted direct injection kerosene engine

• Effects of asymmetric jet pre-chamber on combustion process of kerosene AADI engines were studied. • The simulation of jet pre-chamber schemes was conducted for combustion optimization. • Engine test on combustion comparison of different ignition schemes was performed. • The asymmetric jet pre-chamber was found to be advantageous to improve combustion performance. Aiming at the power improvement of the aviation kerosene engine caused by the mixture preparation, and slow combustion rate, this paper introduced the design and application of the asymmetric jet pre-chamber on a spark ignition piston-type aviation engine equipped with the kerosene low-pressure air-assisted direct injection (AADI) system. Simulation calculation was conducted on combustion differences between the asymmetric pre-chamber and the traditional symmetrical pre-chamber. Meanwhile, the impacts on the flame combustion duration and heat release process in the main combustion chamber, which was divided into zone A and zone B, by the different jet nozzle orifice diameters and spatial distribution angles in the asymmetric jet pre-chamber were analyzed in the paper; and engine bench tests were performed on the influence of the combustion characteristics and power performance by the asymmetric jet pre-chamber ignition scheme under different control strategies. The simulation results showed that the asymmetric pre-chamber could further improve the combustion reaction rate by 7% and the cumulative heat release rate by 17%, thereby effectively promoting the engine combustion efficiency. And an optimal orifice diameter by 2.00 mm of the two jet nozzle in the zone B existed. Compared with the spatial angle α of these two jet nozzle in zone B by 80°, the spatial angle α of 50° can further be helpful to strengthen the heat transfer between the jet flame in the pre-chamber and the mixture in the main combustion chamber. The test results showed that the asymmetric pre-chamber was beneficial to improve the engine combustion performance by 11.4% under low load and by 16.7% under heavy load conditions. The optimal combustion performance can be obtained by the asymmetric pre-chamber at the condition of λ = 0.7, which resulted in higher jet flame ignition energy and faster combustion progression at the condition of richer mixture. As a conclusion, it is benefit to increase the initial heat release and enhance heat work transfer efficiency in the main combustion chamber of the AADI aviation kerosene engine.

Influence of water on thermo-oxidative behavior and kinetic triplets of shale oil during combustion

The oxidation reactions between oxygen and shale oil directly affect the feasibility and performance of air injection. In this work, the influence of water on thermo-oxidative behavior of shale oil was studied using high-pressure differential scanning calorimetry (HP-DSC). Subsequently, the kinetic triplets for shale oil oxidation in the absence and presence of water were determined using Friedman and Ozawa-Flynn-Wall (OFW) models as well as one advanced integral master plots method. The HP-DSC results showed that at 5 MPa, the shale oil was vulnerable to obviously higher heat release at the low-temperature oxidation (LTO) region compared to the high-temperature oxidation (HTO) region, resulting in shortening the oxidative induction time and achieving in-situ upgrading of shale oil. Water delayed the initial heat release due to the fact that heat release yielded by LTO compensated the heat adsorption yielded by evaporation of water and light hydrocarbons. Additionally, the heat release was reduced with the elevated water saturation as a result of the reduction in exothermic rates and intensified evaporation of water and light hydrocarbons. The higher energy was needed to trigger off the LTO reactions while increasing water saturation from 10% to 20%. Therefore, it was believed that the presence of water extended the oxidative induction time, thereby hindering the smooth and rapid transition from the LTO to HTO stages. Water changed the reactivity of each oxidation stage for shale oil, resulting in higher energy required for the initial LTO reactions and lower energy required for the subsequent oxidation reactions. The water saturation range of 10% to 15% could accelerate the occurrence of high-temperature combustion most effectively, demonstrated by values of activation energy for the conversion greater than 0.6. The appropriate oxidation reaction models for shale oil alone and water saturations of 10%, 15%, and 20% were determined to be D2, F1, F3, and R3, respectively. The determination of kinetic triplets assisted in building the reaction schemes for modelling the air injection process of shale oil reservoirs.

Hydration evolution mechanisms of magnesium ammonium phosphate cement within three days of curing

This study systematically investigated the hydration evolution mechanisms of magnesium ammonium phosphate cement (MAPC) within 3 d of curing. The effects of magnesia-to-phosphate (M/P) ratios were mainly focused. The results showed that the samples with a high M/P ratio generated more struvite within 1 h of curing, which led to a faster initial heat release rate supported by microscopic analyses of hydration process and products. The lower M/P ratio group exhibited better performance after curing 3 h, and the amorphous intermediate phases continuously transferred and crystallized into struvite. The pore structure and electrical impedance evolution characterized by low-filed nuclear magnetic resonance and alternating current impedance spectroscopy further indicated that the attenuation amplitude of signal intensity and the increase rate of resistance were both inversely proportional to M/P ratio of MAPC within 3d. Considering both microstructure and performance, M/P ratios in the range of 3/1–3.5/1 were suggested for filed construction.