### Recent Submissions

• #### Flame flow field interaction in non-premixed CH4/H2 swirling flames

(International Journal of Hydrogen Energy, Elsevier BV, 2021-07-17) [Article]
Alternative fuels and stocks like biomass or chemical and refinery waste, may potentially be used in gas turbines and industrial applications after gasification. Thus, understanding the role of hydrogen in these fuels is critical to the broader aim of utilising alternative fuels for power generation. In this work, the interaction between the flame and the flow field was studied in a quarl-stabilised swirl non-premixed flame burning CH4 and H2–enriched CH4. Simultaneous high-speed OH-PLIF/PIV imaging at 5 kHz was carried out on these flames to explore the flame-flow interaction. The instantaneous flow fields in the CH4 or CH4+H2 flames showed a small scale vortical structure near the shear layers, which were not apparent in the time-averaged flow fields. Increasing H2% in the fuel jet was observed to dampen the velocity fluctuations. The fuel composition affected the spatial location of the reaction zone; in the CH4 flames, the axial position of the reaction zone is seen to track the relatively large-magnitude axial velocity fluctuations while remaining in locally low-speed regions of the flow. In contrast, in H2-enriched flames, where the flame is more robust, the reaction zone was able to survive longer, in terms of axial distance, in the vicinity of high swirling jet velocity, with less sensitivity to velocity fluctuations. With increasing the H2%, the reaction zone steadily leaves the IRZ towards the swirling jet flow and localised between its outer and inner vortices. This acts as a stabilisation factor where the internal vortices convect hot product towards the fresh mixture. Moreover, the flame curvatures, the vorticity and compressive strain fields interactions with the reaction zone are presented and discussed. This article outlines results that yield more in-depth insight into hydrogen-enriched hydrocarbon non-premixed swirling flames' combustion, which is essential to accelerate the fuel switching from hydrocarbons to hydrogen.
• #### A comprehensive combustion chemistry study of n-propylcyclohexane

(Combustion and Flame, Elsevier BV, 2021-07-11) [Article]
Alkylated cycloalkanes are vital components in gasoline, aviation, and diesel fuels; however, their combustion chemistry has been less investigated compared to other hydrocarbon classes. In this work, the combustion kinetics of n-propylcyclohexane (n-Pch) was studied across a range of experiments including pressurized flow reactor (PFR), jet stirred reactor (JSR), shock tube (ST), and rapid compression machine (RCM). These experiments cover a wide range of conditions spanning low to intermediate to high temperatures, low to high pressures at lean to rich equivalence ratios. Stable intermediate species were measured in PFR over a temperature range of 550–850 K, pressure of 8.0 bar, equivalence ratio (ϕ) of 0.27, and constant residence time of 120 ms. The JSR was utilized to measure the speciation during oxidation of n-Pch at ϕ of 0.5–2.0, at atmospheric pressure, and across temperature range of 550–800 K. Ignition delay times (IDTs) for n-Pch were measured in the RCM and ST at temperatures ranging from 650 to 1200 K, at pressures of 20 and 40 bar, at ϕ = 0.5, 1.0. In addition, a comprehensive detailed chemical kinetic model was developed and validated against the measured experimental data. The new kinetic model, coupled with the breadth of data from various experiments, provides an improved understanding of n-Pch combustion.
• #### Development of a reduced primary reference fuel-PODE3-methanol-ethanol-n-butanol mechanism for dual-fuel engine simulations

(Energy, Elsevier BV, 2021-07-07) [Article]
Polyoxymethylene dimethyl ethers (PODEn) have attracted worldwide attention, which could be adopted as an alternative or additive of diesel in compression ignition engines. This work focuses on the implementation of PODE for dual-fuel engine combustion. The effects of four dual-fuel combinations on the engine combustion performance and emissions were studied, including PODE-isooctane, PODE-methanol, PODE-ethanol, and PODE-n-butanol. First, a new reduced mechanism of primary reference fuel-PODE3-methanol-ethanol-n-butanol was developed and validated. This mechanism was then employed for the three-dimensional numerical investigation. The results demonstrated that at an exhaust gas recirculation (EGR) rate of 45%, PODE-n-butanol and PODE-isooctane generated higher thermal efficiencies than PODE-methanol and PODE-ethanol, primarily due to the higher reactivity of n-butanol and isooctane compared to methanol and ethanol. The highest thermal efficiency of 47.0% was obtained using PODE-NC4H9OH with a start of injection timing of −7.5°. With an EGR rate of 52%, NOx emissions of all cases met the regulation limit, which, however, were achieved at a sacrifice of engine efficiency; additionally, the highest thermal efficiency of 43.3% was achieved using PODE-CH3OH with an injection timing of −17.5°.
• #### A reinforcement learning-based economic model predictive control framework for autonomous operation of chemical reactors

(Chemical Engineering Journal, Elsevier BV, 2021-07-03) [Article]
Economic model predictive control (EMPC) is a promising methodology for optimal operation of dynamical processes that has been shown to improve process economics considerably. However, EMPC performance relies heavily on the accuracy of the process model used. As an alternative to model-based control strategies, reinforcement learning (RL) has been investigated as a model-free control methodology, but issues regarding its safety and stability remain an open research challenge. This work presents a novel framework for integrating EMPC and RL for online model parameter estimation of a class of nonlinear systems. In this framework, EMPC optimally operates the closed loop system while maintaining closed loop stability and recursive feasibility. At the same time, to optimize the process, the RL agent continuously compares the measured state of the process with the model's predictions (nominal states), and modifies model parameters accordingly. The major advantage of this framework is its simplicity; state-of-the-art RL algorithms and EMPC schemes can be employed with minimal modifications. The performance of the proposed framework is illustrated on a network of reactions with challenging dynamics and practical significance. This framework allows control, optimization, and model correction to be performed online and continuously, making autonomous reactor operation more attainable.
• #### Polyether-Based Block Co(ter)polymers as Multifunctional Lubricant Additives

(ACS Applied Polymer Materials, American Chemical Society (ACS), 2021-06-30) [Article]
A series of polyether-based diblock copolymers were synthesized by sequential organocatalytic ring-opening polymerization (ROP) of either hexene oxide (HO) or octene oxide (OO) as the first monomer with propylene oxide (PO) as the second monomer. In addition, one triblock terpolymer of OO, PO, and styrene oxide (SO) was synthesized following the same method. The ROP was catalyzed by triethyl borane (TEB)/(phosphazene base t-BuP2) with eicosanol as the initiator. The resulting co(ter)polymers have a low polydispersity index and good solubility in hydrocarbon-based oils and are metal-free. By blending polyoctene oxide (POO) and polyhexene oxide (PHO) homopolymers with a Group II base oil (AramcoPrima 230) (2.5 to 5.0 wt %), the viscosity index (VI) increased from 100 to 160, and the thermal stability enhanced up to 50 °C. By using diblock copolymers, POO-block-poly(propylene oxide) (POO-b-PPO) and PHO-block-PPO (PHO-b-PPO), instead of the homopolymers, the VI and the thermal stability are almost the same, but the oil exhibits superior lubrication performance, with friction and wear decreasing up to 46 and 86%, respectively. The addition of the PSO block to the POO-b-PPO chain (POO-b-PPO-b-PSO) further improves the thermal stability but worsens the rheological and tribological properties (i.e., VI, friction, and wear) of lubricating oils.
• #### Polyether-Based Block Co(ter)polymers as Multifunctional Lubricant Additives

(ACS Applied Polymer Materials, American Chemical Society (ACS), 2021-06-30) [Article]
A series of polyether-based diblock copolymers were synthesized by sequential organocatalytic ring-opening polymerization (ROP) of either hexene oxide (HO) or octene oxide (OO) as the first monomer with propylene oxide (PO) as the second monomer. In addition, one triblock terpolymer of OO, PO, and styrene oxide (SO) was synthesized following the same method. The ROP was catalyzed by triethyl borane (TEB)/(phosphazene base t-BuP2) with eicosanol as the initiator. The resulting co(ter)polymers have a low polydispersity index and good solubility in hydrocarbon-based oils and are metal-free. By blending polyoctene oxide (POO) and polyhexene oxide (PHO) homopolymers with a Group II base oil (AramcoPrima 230) (2.5 to 5.0 wt %), the viscosity index (VI) increased from 100 to 160, and the thermal stability enhanced up to 50 °C. By using diblock copolymers, POO-block-poly(propylene oxide) (POO-b-PPO) and PHO-block-PPO (PHO-b-PPO), instead of the homopolymers, the VI and the thermal stability are almost the same, but the oil exhibits superior lubrication performance, with friction and wear decreasing up to 46 and 86%, respectively. The addition of the PSO block to the POO-b-PPO chain (POO-b-PPO-b-PSO) further improves the thermal stability but worsens the rheological and tribological properties (i.e., VI, friction, and wear) of lubricating oils.
• #### The effect of oxygen content on the turbulent flame speed of ammonia/oxygen/nitrogen expanding flames under elevated pressures

(Combustion and Flame, Elsevier BV, 2021-06-25) [Article]
Ammonia is a promising, but weakly reactive, carbon-free fuel and a promising hydrogen carrier for renewable energy. In this study, the turbulent flame speed (ST) of stoichiometric ammonia/oxygen/nitrogen mixtures under oxygen enrichment conditions was investigated at elevated pressures using a fan-stirred constant volume combustion chamber. Turbulent flame speed is found to increase with oxygen content at all pressures and turbulent intensities (u′) studied. In contrast, the turbulent-to-laminar flame speed ratio (ST/SL) was found to decrease with the oxygen content, mainly due to SL increasing faster than ST. The self-similar propagation characteristics of ammonia flame as flame radius develops are similar to other fuels and scale as a one-half power law, but it bends down as oxygen content increases. Several correlations of turbulent flame speed are validated against the present data, and it is found that a fitting relationship including pressures and turbulent intensities is independent of fuel type and performs best among literature proposed correlations. Moreover, the correlations of ammonia get similar results with other fuels when turbulent length scale adopted same definition of laminar flame thickness, in contrast, the ST/SL is larger for ammonia compared to methane at the same Reynolds number. Two new correlations of turbulent flame speed based on Karlovitz number (Ka) and Damköhler number (Da) are presented and narrowed the data scatter. Ammonia/oxygen/nitrogen mixture with ηO2 = 0.4 get similar turbulent flame speed as methane/air. A flame surface wrinkling analysis and stretch sensitivity analysis showed that the sensitivity of the higher stretch rate combined with the wrinkling acceleration was responsible for the observed increase in ST/SL ratio.
• #### Chemical effects of anisole and toluene addition to n‑heptane on PAH characteristics in laminar premixed flames by LIF measurement and kinetic model

(Fuel, Elsevier BV, 2021-06-18) [Article]
Anisole is a candidate renewable fuel that displays satisfying combustion characteristics, but its sooting characteristics are not well known. The goal of this study is to investigate the chemical effects of anisole and toluene on PAH formation in n-heptane laminar premixed flames, using LIF measurement and chemical kinetic simulation. To focus on chemical effects, the equivalence ratios, dilution ratios, and flame temperatures were kept nearly unchanged when anisole and toluene were blended separately into n-heptane flame. LIF experimental results indicated that PAH formation was promoted with the addition of anisole and toluene, and the effect of toluene was stronger than the promotional effect of anisole. The chemical kinetic model predicted the observed PAH tendencies well in the LIF experiments. Based on this model, reaction pathway and sensitivity analyses were performed to interpret the chemical effects. Results revealed that, due to their different molecular structures, the difference in chemical effects between anisole and toluene was notable in PAH growth processes. Anisole decomposed first via the O-CH3 bond dissociation reaction, and then proceeded to a CO elimination reaction to yield an important PAH precursor, C5H5, which contributed to PAH formation via styrene and indene reaction networks. In the toluene added flame, PAH formation was enhanced because of ring expansion reaction on the dehydrogenated branch of the toluene.
• #### Local flame displacement speeds of hydrogen-air premixed flames in moderate to intense turbulence

(arXiv, 2021-06-15) [Preprint]
Comprehensive knowledge of local flame displacement speed, $S_d$, in turbulent premixed flames is crucial towards the design and development of hydrogen fuelled next-generation engines. Premixed hydrogen-air flames are characterized by significantly higher laminar flame speed compared to other conventional fuels. Furthermore, in the presence of turbulence, $S_d$ is enhanced much beyond its corresponding unstretched, planar laminar value $S_L$. In this study, the effect of high Karlovitz number ($Ka$) turbulence on density-weighted flame displacement speed, $\widetilde{S_d}$, in a H$_2$-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of $\widetilde{S_d}$ from $S_L$, for moderately turbulent flames. An interaction model for the same has also been proposed. In this work, we seek to test the interaction model's applicability to intensely turbulent flames characterized by large $Ka$. To that end, we investigate the local flame structures: thermal, chemical structure, the effect of curvature, along the direction that is normal to the chosen isothermal surfaces. Furthermore, relative contributions of the transport and chemistry terms to $\widetilde{S_d}$ are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced $\widetilde{S_d}$ is driven by interactions among complete flame structures, $\widetilde{S_d}$ enhancement in high $Re_t$ and high $Ka$ flame is predominantly governed by local interactions of the isotherms. It is found that enhancement in $\widetilde{S_d}$ in regions of large negative curvature occurs as a result of these interactions, evincing that the interaction model is useful for high $Ka$ turbulent premixed flames as well.
• #### Experimental and kinetic modeling study of α-methyl-naphthalene pyrolysis: Part II. PAH formation

(Combustion and Flame, Elsevier BV, 2021-06-13) [Article]
α-Methyl-naphthalene plays an important role as a functional material in petrochemical industries and as a precursor of soot particles. The formation chemistry of polycyclic aromatic hydrocarbons (PAHs) from α-methyl-naphthalene, therefore, warrants detailed investigations. In this work, we studied PAH formation from its pyrolysis using experiments and kinetic models. Flow reactor pyrolytic experiments at low and atmospheric pressures (30 and 760 Torr) were performed using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). A kinetic model was then developed to predict PAH formation from α-methyl-naphthalene. According to the kinetic analysis of the proposed model, naphth-1-yl-methyl, benzo-fulvenallene, and benzo-fulvenallenyl are three critical intermediates in the formation of large PAHs. Other than the traditional H-abstraction acetylene-/vinylacetylene-addition mechanisms, three prototypical PAH formation pathways are identified in α-methyl-naphthalene pyrolysis: 1) addition and cyclization reactions of naphth-1-yl-methyl and naphth-1-yl radicals; 2) recombination of resonance stabilized radicals (indenyl, benzo-fulvenallenyl, phenalenyl, etc.) and the subsequent ring expansion reactions; 3) sequential propargyl addition reactions.
• #### Furan formation pathways exploration in low temperature oxidation of 1,3-butadiene, trans-2-butene, and cis-2-butene

(Combustion and Flame, Elsevier BV, 2021-06-11) [Article]
Furan is one of the smallest organic compounds with heterocycle ring. With this particular molecular structure, furan is considered as a highly toxic and carcinogenic combustion pollutant, and furan may contribute to the formation of oxygenated soot. In this work, furan formation pathways from 1,3-butadiene, trans-2-butene and cis-2-butene were comprehensively explored. The potential energy surfaces, reaction rate coefficients, and thermodynamics were calculated by quantum chemistry using high level of theories including the CCSD (T) and G3 methods. The proposed reaction pathways were then implemented into the AramcoMech 3.0 model uniformly or independently to examine the model performance with the experimental data. The oxidation experiments of 1,3-butadiene, trans-2-butene and cis-2-butene were performed in a jet stirred reactor (JSR) in the low temperature regime (500–830 K). The JSR is coupled with time-of-flight molecular beam mass spectrometry (ToF-MBMS) using synchrotron radiation as photon ionization source for species identification and quantification. Compared with experiments, both updated models (the independent and uniform model) showed better prediction of furan than the base AramcoMech 3.0 model, which highlighted the contribution of the proposed pathways. Reaction pathway analyses reveal that in the proposed reaction pathway, both reactions C4H6 + OH ⇌ S1–4 (H2C[dbnd]CH-ĊH–CH2OH, but‑1-en-3-yl-4-ol) and C4H6 + HO2 ⇌ C4H61–3OOH4 (H2C[dbnd]CH-ĊH–CH2OOH, but‑1-en-3-yl-4-peroxide) not only contribute to furan formation, but also to fuel consumption. Furthermore, the kinetic uncertainty from activation energy calculated by the CCSD series methods, CBS-ANPO, and G4 methods was evaluated for reaction C4H6 + HO2 ⇌ C4H61–3OOH4. Instead of developing a new kinetic model, this work aims at proposing and validating new reaction pathways to advance the understanding of furan formation chemistry in low temperature oxidation, and provide guidance for future model development.
• #### Chemiluminescence signature of premixed ammonia-methane-air flames

(Combustion and Flame, Elsevier BV, 2021-06-07) [Article]
This paper reports on the chemiluminescence footprint of premixed ammonia-methane-air flames. The chemiluminescence spectrum of laminar twin-flames stabilized with a counterflow burner were measured between 200 and 457 nm for large ranges of equivalence ratio (0.60 ≤ ϕ ≤ 1.30), ammonia fraction (pure methane to pure ammonia), and strain rate (55/s ≤ a ≤ 300/s). Relevant excited radicals were identified, namely, NO*, OH*, NH*, CN*, CH*, and CO2* and evolutions of their chemiluminescence intensity were analyzed as a function of equivalence ratio, ammonia fraction, and strain rate. These measurements produced an unprecedented database on the chemiluminescence of ammonia-methane-air flames, which could be used in the future for model validation. A total of 15 ratios of chemiluminescence intensity were also considered and 5 ratios showing promise for the development of chemiluminescence-based flame sensors were identified. The CN*/OH* ratio is a potential surrogate for equivalence ratio even if the ammonia fuel fraction in the fuel blend is not known accurately – as long as it exceeds 0.3 by volume. The CN*/NO* ratio is another possible surrogate for equivalence ratio if the ammonia fraction in the fuel blend is below 0.5. The OH*/CH* ratio, often used to sense equivalence ratio in hydrocarbons flames, is not recommended for ammonia-methane flames. The NH*/CN* ratio is a potential surrogate for the ammonia fraction in the fuel blend if equivalence ratio is larger than ϕ = 0.7 and if the ammonia fraction in the fuel blend is below 0.4. Other ratios may be combined to provide a simultaneous measure of equivalence ratio and ammonia fraction in the fuel blend with an extended range of validity, for example NH*/OH* and CN*/NH*.
• #### On the origins of lubricity and surface cleanliness in ethanol-diesel fuel blends

(Fuel, Elsevier BV, 2021-06-04) [Article]
Ethanol is the most used bio-derived fuel additive. However, adding ethanol in diesel fuel may negatively impact lubricity or surface cleanliness, which is critical for high-pressure fuel injection systems employed in compression ignition engines. This work investigates surfaces lubricated by ethanol–diesel blends. Adding 5 wt% ethanol in diesel showed negligible changes in fuel lubricity, while blending 10, 20, and 40 wt% ethanol increased wear rates by 46, 81, and 239% respectively. These increases in wear rates (with increases in ethanol by wt%) correlate with the evolution of electrical contact resistance (ECR) values over time. As more ethanol was added, the ECR values signaled thinner fuel films, more metal-to-metal contacts, and a delayed onset of frictional product growth. Raman spectra showed that forming frictional species produced by tribochemical reactions enhanced fuel lubricity. The absence of some frictional species in ethanol lubricated surfaces points to simultaneously improved surface cleanliness and reduced lubricity.
• #### A Eulerian Multi-Fluid Model for High-Speed Evaporating Sprays

(Processes, MDPI AG, 2021-05-26) [Article]
Advancements in internal combustion technology, such as efficiency improvements and the usage of new complex fuels, are often coupled with developments of suitable numerical tools for predicting the complex dynamic behavior of sprays. Therefore, this work presents a Eulerian multi-fluid model specialized for the dynamic behavior of dense evaporating liquid fuel sprays. The introduced model was implemented within the open-source OpenFOAM library, which is constantly gaining popularity in both industrial and academic settings. Therefore, it represents an ideal framework for such development. The presented model employs the classes method and advanced interfacial momentum transfer models. The droplet breakup is considered using the enhanced WAVE breakup model, where the mass taken from the parent droplets is distributed among child classes using a triangular distribution. Furthermore, the complex thermal behavior within the moving droplets is considered using a parabolic temperature profile and an effective thermal conductivity approach. This work includes an uncertainty estimation analysis (for both spatial and temporal resolutions) for the developed solver. Furthermore, the solver was validated against two ECN Spray A conditions (evaporating and non-evaporating). Overall, the presented results show the capability of the implemented model to successfully predict the complex dynamic behavior of dense liquid sprays for the selected operating conditions.
• #### Line-strengths, collisional coefficients and narrowing parameters in the ν3 band of methane: H2, He, N2, O2, Ar and CO2 collider effects

(Journal of Quantitative Spectroscopy and Radiative Transfer, Elsevier BV, 2021-05-25) [Article]
The ν3 fundamental absorption band of methane (12CH4) dominates the 3.3–3.4 μm infrared spectral window and has been extensively used for atmospheric, planetary and astrophysical investigations. Spectroscopic data, such as line positions, line-strengths and foreign-collisional coefficients, are needed to analyze the infrared spectra of planetary atmospheres for the understanding of relevant physical chemistry. In this study, high-resolution infrared spectra of 12CH4 belonging to the ν3 fundamental vibrational band are recorded over 2884–2969 cm−1 with a difference–frequency–generation (DFG) laser having a linewidth of Δν ≈ 0.0002 cm−1. Line-strengths as well as H2-, He-, N2-, O2-, Ar- and CO2- broadening coefficients are determined for 49 methane transitions at room temperature. Data are retrieved using Voigt and Galatry profiles to compute the measured line shape of each individual transition at various pressures. Narrowing coefficients due to Dicke effect are also determined. The new data extend and supplement previous measurements over the target spectral region (2884–2969 cm−1) where very fewer studies are reported. In particular, the present work is the first known complete determination of broadening data and narrowing parameters at high rotational quantum numbers of the P-branch of the ν3-CH4 band.
• #### Effects of fuel trapping in piston crevice on unburned hydrocarbon emissions in early-injection compression ignition engines

(Combustion and Flame, Elsevier BV, 2021-05-24) [Article]
Early injection strategy is employed in many advanced combustion concepts of modern compression-ignition engines to promote the fuel-air premixing, but it could result in fuel spray impingement on the cylinder wall and potential fuel trapping in the crevice regions. The impact of the fuel trapping on the formation and distribution of unburned hydrocarbon (UHC) in the advanced compression-ignition concepts is not well understood. In this study, the planar laser-induced fluorescence (PLIF) technique was applied in a single-cylinder optical engine to visualize the fuel distribution before combustion and the formaldehyde formation during combustion. A three-dimensional computational model was established to explore the detailed mechanism of UHC formation in the piston crevice. Three cases with injection timings of 20°, 40° (SOI-40), and 100° (SOI-100) were compared. The PLIF and simulation results indicate that, under early injection conditions, the squish region and piston crevice trap a considerable amount of fuel, resulting in increased UHC emission and reduced engine work, and the main combustion zone resides in the squish region. The simulation shows that the amount of UHC formation from the trapped fuel is highly dependent on the local equivalence ratio distribution formed by different injection timings. The local equivalence ratio within the piston crevice region for the SOI-40 case exceeds 2 before combustion; the charge sequentially undergoes both low- and high-temperature heat release (LTHR and HTHR) processes and produces less UHC in the crevice region. However, the SOI-100 case produces an overall leaner mixture with a local equivalence ratio lower than 1 before combustion; the charge undergoes LTHR without noticeable HTHR and becomes UHC near the cylinder wall. The injector dribbling results in more UHC formation in the central part of the cylinder under very early injection timing, and thus an accurate injector dribbling model is required to better reproduce the UHC emissions.
• #### Experimental study of flame evolution, frequency and oscillation characteristics of steam diluted micro-mixing hydrogen flame

(Fuel, Elsevier BV, 2021-05-21) [Article]
Effects of steam dilution on flame evolution, frequency and oscillation characteristics were investigated using an optically accessible micro-mixing combustor under the lean operating condition (equivalence ratio 0.4 to 0.9). The flame profile was visualized by 5 kHz OH planar laser-induced fluorescence (PLIF), meanwhile the structural zoning analysis, frequency spectrum and dynamic mode decomposition (DMD) methods were used to investigate flame instability. The effects on frequency-shift and temporal-spatial heat-release distribution characteristics were demonstrated. Results indicated that heat-release frequency got blue-shift with the increase of hydrogen equivalence ratio while flame length gradually extended, though the second harmonic frequency only occurred in the flame arm zone. The steam content influenced the heat-release obviously, and the obvious periodic oscillation existed in the dilution ratio of 25%. Furthermore, the increasing or decreasing steam content will make the OH radical concentration and distribution change significantly, meanwhile the intensity and location of oscillation zone were also be influenced. The mode decomposition analysis revealed the growth of oscillation zone was closely related to the flame arm zone (FAZ) and experienced the transition to the flame tail zone (FTZ) with the increase of equivalence ratio.
• #### Numerical investigation on the combustion and emission characteristics of a heavy-duty natural gas-diesel dual-fuel engine

(Fuel, Elsevier BV, 2021-05-13) [Article]
Natural gas (NG)-diesel dual-fuel combustion is an effective approach to reduce soot and greenhouse gas emissions and mitigate the liquid fossil fuel crisis. In this work, a comprehensive numerical study on the combustion and emission characteristics of a NG-diesel dual-fuel engine operating at the high load condition was performed. Six significant parameters such as the start of injection (SOI) timing, exhaust gas recirculation (EGR), injection and intake pressures, nozzle number, and NG substitution ratio were investigated. A pathway to achieve highly efficient and clean combustion was proposed. It was demonstrated that at least an EGR rate of 40% should be employed to meet the NOx Euro VI regulation limit. Although a higher injection pressure enhanced the diesel-air mixing process and promoted engine efficiency, the highest achievable engine efficiencies were similar using different injection pressures. An elevated intake pressure with an earlier SOI timing promoted the oxidation process. Therefore, it resulted in a lower combustion loss and thus higher engine efficiency. Furthermore, the increase of nozzle number effectively promoted the air utilization rate and expedited the combustion heat release, which resulted in a higher engine efficiency but lower soot emission. But the growing trend of engine efficiency was limited and a nozzle number of 11 was found to be the optimal option. Finally, a peak engine efficiency of about 47.6% was achieved with a NG substitution ratio of 95% and an optimized SOI timing, and meanwhile, both the NOx and soot emissions were below the Euro VI emission regulation limits.
• #### Energy Distribution Analysis of Multiple Injectors for the Double Compression Expansion Engine Concept

(SAE International Journal of Engines, SAE International, 2021-05-12) [Article]
The present study shows the impact of multiple injectors with distinctive spray and injection angles on engine efficiency, heat transfer (HT), and exhaust losses. It is well known from previous studies that multiple injectors, particularly two injectors, improve the air-fuel mixing while simultaneously reducing the HT losses by keeping hot reaction zones away from the combustion cylinder walls. However, it is unclear how the spray-to-spray, umbrella, and spray-orientation angles would impact the overall energy distribution in both the combustion cylinder and double compression expansion engine (DCEE) concept. In this study, three-dimensional (3D) Reynolds-averaged Navier-Stokes (RANS) simulations were conducted in a heavy-duty single-cylinder engine, comparing three-injectors and two different side-injectors cases with a baseline case of the centrally mounted single injector. To obtain the baseline case and validate the computational fluid dynamics (CFD) data, the engine was operated at 1200 rpm, and the fuel mean effective pressure (FuelMEP) of 40 bar using conventional diesel fuel was fixed. One-dimensional (1D) simulations then utilized the CFD-based in-cylinder pressure and rate of heat release (RoHR) traces to analyze the energy distribution of the DCEE concept. The CFD results show that the three-injectors and the best side-injectors case lead to 1.2%-points and 0.2%-points improvement of gross indicated efficiency (GIE) and heat losses reduction of 1.3%-points and 2%-points, respectively, compared to the baseline single-injector case. This is due to the longer injector-wall distance from the side injections, thereby reducing the HT losses, compared to the large coverage area of the high-temperature gases for the baseline case. The 1D results show that a high brake thermal efficiency (BTE) of 53% and 52.7% can be achieved with the three-injectors and the best side-injectors case, respectively, compared to 51.9% of the baseline case. This was due to the advanced combustion phasing, lower HT losses, and higher conversion efficiency of exhaust losses from the combustion cylinder into useful work in the expansion cylinder of the DCEE concept, resulting in an overall higher engine efficiency for multiple injectors.
• #### Pyrolysis of Waste Tires in a Twin-Auger Reactor Using CaO: Assessing the Physicochemical Properties of the Derived Products

(Energy & Fuels, American Chemical Society (ACS), 2021-05-11) [Article]
This work assesses the effect of adding CaO during the pyrolysis of waste tires (WT) using a twin-auger reactor on the properties of the pyrolysis derived products. Pyrolysis was conducted in a lab-scale facility at a reactor temperature of 475 °C, solid residence time of 3.5 min, WT mass flow rate of 1.16 kg/h, and N2 flow rate of 300 mL/min. CaO was continuously fed at ratios of 10, 15, and 20 wt %, according to the WT mass flow rate, using two particle size ranges: fine (105-149 μm) and coarse (149-841 μm). The resulting tire pyrolysis oil (TPO) was initially characterized in terms of sulfur content, and the sample with the lowest sulfur content, named TPO[CaO], was further studied by different analytical techniques, including GC-MS and 1H NMR. The tire pyrolysis gas (TPG) and the tire pyrolysis solid (TPS) related to TPO[CaO], so-called TPG[CaO] and TPS[CaO], respectively, were also characterized by gas chromatography, and elemental, proximate, and XRF analyses, respectively. Lastly, an acid demineralization process was carried out to remove some of the inorganic elements in the TPS[CaO]. The addition of 15 wt % of coarse CaO during the pyrolysis of WT resulted in a sulfur reduction in TPO of 26.10%, while viscosity and water content were significantly reduced. The GC-MS analysis revealed a significant presence of benzene, toluene, xylene, and limonene in both TPO and TPO[CaO]. Likewise, 1H NMR suggested an increase of hydrogen atoms in aromatic, naphthenic, and olefin structures in the TPO[CaO], and a decrease of these atoms in paraffinic structures. Similarly, H2 and some CxHy compounds increased, while CO2, CO, and H2S decreased in TPG[CaO], which supports the hypothesis of the participation of CaO in several reactions during the pyrolysis of WT. Although the ash content in TPS[CaO] was significantly high after pyrolysis (57.5 wt %), the acid demineralization process was effective at removing 80% of its inorganic content, improving its surface area and porosity. The information presented in this work aims at providing some insights toward the advancement of in situ upgrading strategies for the resulting products derived from pyrolysis of WT.