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Point-scatterer detection plays a key role in medical ultrasound B-mode imaging. Speckle noise and insufficient spatial resolution are important factors affecting point-scatterer detection. To address this issue, normalized spatial autocorrelation in ultrasound B-mode imaging (NSACB) is proposed. First, the acquired data are pre-processed by adding Gaussian white noise (GWN) with a certain signal-to-Gaussian white noise ratio (SGWNR). Next, normalized spatial autocorrelation is applied to the pre-processed data, and the data are divided into several new signals with different spatial lags. Then, the new signals are performed unsigned delay multiply and sum. Finally, the NSACB beamformed data are bandpass filtered by extracting the frequency component around twice the center frequency. Simulated and in vitro experiments were designed for validation. Simulations revealed that the lateral resolution of NSACB measured by the –6-dB mainlobe width can reach as high as 11.11% of delay and sum (DAS), 25.01% of filtered delay multiply and sum (F-DMAS) and 50% of LAG-FDMAS-SCF. The sidelobe level of the NSACB can be reduced at most by 28 dB. Experimental results of simple and complex scatterer phantoms indicate the image resolution of the proposed NSACB can even reach up to 18.76% of DAS, 27.28% of F-DMAS and 14.29% of LAG-FDMAS-SCF. Compared with these methods, the proposed NSACB can reduce the sidelobe level at least by 18 dB. Although the proposed method causes loss of the ability to observe hypo-echoic structures, these results suggest future work to determine the ability to detect breast microcalcifications, kidney stones, biopsy needle tracking and other scenarios requiring scatterer detection. [ABSTRACT FROM AUTHOR]

• Characteristics of DOM in coastal and inland aquifers are contrasting. • Types of DOM and N-containing DOM utilized differ in coastal and inland aquifers. • Less-degraded DOM leads to higher production efficiency and level of NH 4 + in coastal aquifer. • Heteroatom-containing DOM is predominantly of microbial-derived and accumulates gradually. Elevated concentration levels of geogenic ammonium in groundwater arise from the mineralization of nitrogen-containing natural organic matter in various geological settings worldwide, especially in alluvial-lacustrine and coastal environments. However, the difference in enrichment mechanisms of geogenic ammonium between these two types of aquifers remains poorly understood. To address this knowledge gap, we investigated two representative aquifer systems in central Yangtze (Dongting Lake Plain, DTP) and southern China (Pearl River Delta, PRD) with contrasting geogenic ammonium contents. The use of optical and molecular characterization of DOM combined with hydrochemistry and stable carbon isotopes has revealed differences in DOM between the two types of aquifer systems and revealed contrasting controls of DOM on ammonium enrichment. The results indicated higher humification and degradation of DOM in DTP groundwater, characterized by abundant highly unsaturated compounds. The degradation of DOM and nitrogen-containing DOM was dominated by highly unsaturated compounds and CHO+N molecular formulas in highly unsaturated compounds, respectively. In contrast, the DOM in PRD groundwater was more biogenic, less degraded, and contained more aliphatic compounds in addition to highly unsaturated compounds. The degradation of DOM and nitrogen-containing DOM was dominated by aliphatic compounds and polyphenols and CHO+N molecular formulas in highly unsaturated compounds and polyphenols, respectively. As DOM degraded, the ammonium production efficiency of DOM decreased, contributing to lower ammonium concentrations in DTP groundwater. In addition, the CHO+N(SP) molecular formulas were mainly of microbial-derived and gradually accumulated with DOM degradation. In this study, we conducted the first comprehensive investigation into the patterns of groundwater ammonium enrichment based on DOM differences in various geological settings. [Display omitted] [ABSTRACT FROM AUTHOR]

This study investigated the effect of the H 2 fraction on reaction dynamics and NO x formation in a premixed NH 3 /H 2 /air jet flame within a hot coflow (JHC) through numerical simulations. We found that even a small mole fraction of H 2 (e.g., X H2, F = 5%) significantly enhances the ignition of the premixed NH 3 /air flame. However, the presence of H 2 markedly intensifies the main combustion reactions only when X H2, F ≥ 30%, resulting in a sharp increase in temperature and heat release rate. Moreover, as X H2, F increases, the NO x emission first increases until X H2, F = 75% and then decreases. Without any H 2 addition, a high emission of unburned NH 3 occurs in the pure NH 3 JHC flame. On the other hand, when X H2, F ≥ 50%, the emissions of unburned H 2 and OH increase rapidly due to extremely high temperatures. Notably, maintaining X H2, F between 5% and 20% minimizes the emissions of NO x , NH 3 , H 2 , and OH. Furthermore, the preferential diffusion of H 2 plays a key role in enhancing the production of active radicals (OH, O, and H), thereby significantly boosting the initiation and combustion reactions of NH 3 /H 2 blended fuel. [Display omitted] • Effect of H 2 addition on the premixed NH 3 /H 2 /air jet flames is investigated. • A small amount of H 2 (5%) in the fuel notably enhances the ignition of NH 3 flame. • H 2 significantly boosts major combustion reactions only when its proportion ≥30%. • Optimal H 2 proportion is 5%–20% to reduce emissions of NO x , unburned NH 3 and H 2. • Preferential diffusion of H 2 greatly enhances the ignition and combustion reactions. [ABSTRACT FROM AUTHOR]

Hydrogen fuel has significant potential for various applications under MILD combustion conditions. This study investigates the impact of H 2 mass fraction (ranging from 10 % to 100 %) on the structure of the combustion zone and the emission rules of NO x in the JHC burner utilizing the detailed mechanism GRI-Mech 2.11. A contraction of the reaction zone and a decrease in the ignition delay is observed when the H 2 ratio increases. Increasing H 2 concentration results in the gradual loss of MILD combustion characteristics in the reaction area, and the downstream region gradually transforms to High Temperature Combustion. The MILD reaction volume at Y O2 = 6 % is only 69 % compared to that at Y O2 = 3 %. Higher Y O2 has a more significant impact on the relative increment of NO emissions. The NNH route consistently predominates, and increasing H 2 ratio amplifies the production of thermal NO x and weakens the NO-reburning mechanism. • Transition of reaction area from MILD to HTC is observed in pure hydrogen cases. • The boundary of pure hydrogen fuel under MILD combustion conditions is depicted. • Contributions of different NO formation routes under MILD conditions are conducted. • Trend of NO generation pathways with changing hydrogen ratio is revealed. [ABSTRACT FROM AUTHOR]

• LES-SWF well predicts oxy-flame characteristics across a wide Re range. • Higher Re reduces radial flame size, enhancing effects of vortex on reaction zone. • Increasing Re weakens the beneficial effect of preferential diffusion of H 2. • Adverse effect of CO 2 on combustion increases with increasing Re. Due to differences in the physical and chemical properties of CO 2 and N 2 , along with a reduction in the momentum ratio between oxidant and fuel streams, the local extinction of oxy-fuel flames is significantly more pronounced compared to conventional air–fuel flames, which poses challenges in the design and operation of oxy-fuel burners. This study further validates the species-weighted flamelet/progress variable (FPV) model proposed in previous work [Jiang et al., 2023], particularly in oxy-fuel flames characterized by highly local extinction. Large eddy simulations were conducted on the Sandia oxy-fuel jet diffusion flame at various Reynolds numbers. The predictions are systematically compared with experimental data, and the influence of Reynolds number on local extinction is thoroughly analyzed. The results demonstrate that the numerical simulation effectively predicts mean temperature, species mass fractions, differential diffusion parameters (Z HC), and local extinction in oxy-fuel jet flames across a wide range of Reynolds numbers. The errors in predicted mean temperature and mass fractions exhibit a slight increase with rising Reynolds numbers, yet remain below 15 %. As the Reynolds number increases from 12,000 to 18,000, the predicted peak Z HC decreases by 30 %, and the beneficial effect of preferential diffusion of H 2 weakens, while the adverse effect of CO 2 on combustion becomes stronger. [ABSTRACT FROM AUTHOR]

• The external laser based on femtosecond-written birefringent fiber Bragg grating (FBG) achieves excellent linewidth characteristics. • The hybrid laser based on ridge waveguide emission gain chip and femtosecond-written birefringent FBG attains high integration and low cost. • Linear polarization output is realized by the orthogonal polarization eigenmode and effective external cavity mode selection of the birefringent FBG. We demonstrate an external-cavity semiconductor laser (ECSL) with linear polarization and a narrow linewidth. A birefringent Bragg grating prepared by a femtosecond laser point-by-point technique in a polarization-maintaining fiber (PMF) is used to provide external-cavity feedback, and its polarization-dependent characteristics enable selection of the main polarization mode and linewidth narrowing of the ridge waveguide emission gain chip (GC). This compact and robust ECSL achieves an output power of > 60 mW and a polarization extinction ratio (PER) of > 30 dB. We simulate and calculate the linewidth and injection current, while a Lorentz linewidth of 2.58 kHz is obtained based on delayed self-heterodyne beat frequency measurement. This flexible and cost-effective solution allows realization of a compact ECSL with linear polarization and a narrow linewidth. [ABSTRACT FROM AUTHOR]

[Display omitted] • A benchmark model to evaluate the performance of pressurized oxy-coal systems is developed. • The performance and process characteristics of two pressurized oxy-coal systems are compared. • Detailed analysis of the factors affecting the pressurized oxy-coal efficiency is conducted. • The efficiency increases of two pressurized oxy-coal systems are obtained and analyzed. Process simulation and analysis are demonstrated to provide significant benefits for pressurized oxy-fuel combustion systems. This study compares the performance of pressurized oxy-fuel combustion systems and a first-generation oxy-combustion system under the same standard conditions. On this basis, the increase in efficiency of pressurized oxy-fuel combustion systems is evaluated. Process simulation and analysis are carried out for the first-generation oxy-combustion system in a 600 MW supercritical primary intermediate reheat power plant without heat integration (Base Case) and with heat integration (Case A). The validity of the simulation results is then verified. Considering Case A as a benchmark model, the process simulations of a pressurized oxy-coal combustion system and a staged pressurized oxy-combustion system are performed. The simulation results are compared with results in other literature studies. Compared with the benchmark model, the results indicate that the gross generation efficiency of the pressurized oxy-coal combustion system is improved by 1.12 % (LHV), and the net generation efficiency is improved by 0.47 % (LHV). In addition, the gross generation efficiency of the staged pressurized oxy-combustion system is enhanced by 1.78 % (LHV), and the net generation efficiency is improved by 2.4 % (LHV) compared with the benchmark model. [ABSTRACT FROM AUTHOR]

• NO x formation in staged oxy-combustion has been numerically studied. • The fuel staged mode reduces NO concentration in furnace outlet by 5.6%. • Oxygen-fuel two-way staged mode reduces NO concentration in furnace outlet by 17%. • The NO formation pathway of HCN → NCO → NO is suppressed, and NO reduction pathway of NO → HCN is enhanced in staged oxy-combustion. In this study, high-fidelity simulations are conducted on a 35 MW pulverized coal oxy-fuel boiler to investigate NO x formation characteristics under staged oxy-fuel combustion, including both fuel-staged and oxygen-fuel two-way staged modes. Detailed volatile components are considered by using the chemical percolation devolatilization model. A skeletal reaction mechanisms consisting of 35 species developed for fuel-NO x formation under oxy-fuel conditions are employed for the gas phase combustion and NO x modeling. The radiative property models are also optimized for oxy-fuel combustion. The results show that in the fuel-staged mode, a reduction atmosphere is established in the reburning zone to reduce the NO x concentration in the flue gas. A reduction of 5.6% in the average NO concentration at the furnace outlet is observed relative to the baseline oxy-fuel case. Moreover, the application of oxygen-fuel two-way staged combustion leads to a further reduction in NO x formation. The average NO concentration at the furnace outlet is decreased by 17% compared to the baseline oxy-fuel case. Pathway analysis indicates a reduction in the NO formation pathway involving HCN → NCO → NO, while the NO reduction pathway of NO → HCN is enhanced. [ABSTRACT FROM AUTHOR]

• Ignition process is controlled by both mixing and chemistry for all jet flames. • Flow mixing greatly affects both ignition and stabilization of low-oxygen flames. • Rich-side reactions are much slower than lean-side and are far from equilibrium. • Premixing of fuel jet and coflow before ignition is crucial for low-oxygen flames. • Low-oxygen flames develop by autoignition versus high-oxygen ones by propagation. This study numerically investigates the ignition and evolution of CH 4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, Y O2,C = 6 % and 9 %) and high temperature combustion (HTC, Y O2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base. [ABSTRACT FROM AUTHOR]

• NH 3 flame has much lower temperature rise and larger reaction zone than CH 4. • NO x emission of NH 3 flame is two orders higher than CH 4 at equivalence ratio < 1. • NO x emissions of both flames increase rapidly with coflow oxygen concentration. • NO x emission mainly comes from NO while N 2 O is important at low temperature. • MILD combustion can reduce the NO x emission of NH 3 flame by 1–2 orders. Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NO x emissions (E NO x) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH 3 /air jet flame in hot coflow (JHC) under different jet equivalence ratios (Φ J), coflow temperatures (T C), and oxygen levels (X O2,C). For comparison, similar CH 4 /air MILD-JHC flame characteristics are calculated. The results show that the NH 3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH 4. This suggests that the NH 3 flame can develop into a MILD regime more easily than CH 4. E NO x of the NH 3 flame is two orders of magnitude higher than that of CH 4 flame at all T C and X O2,C values for Φ J < 1. At Φ J > 1, E NO x of the NH 3 flame rapidly decreases to nearly zero, but unburned NH 3 emissions (E NH3) and H 2 emissions (E H2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, E NO x increases rapidly (gradually) with X O2,C (T C), whereas E NH3 and E H2 are considerably low at T C ≥ 1300 K and X O2,C ≥ 1% for the NH 3 flame. E NO x primarily originates from NO; however, N 2 O is also important at low Φ J and T C values. Therefore, considering E NO x , E NH3 , E H2 and the burning efficiency, the optimal conditions for NH 3 MILD combustion are Φ J = 1, T C ≥ 1500 K, and X O2,C = 1%–3%. In addition, compared with conventional flames, NH 3 MILD combustion can reduce E NO x by one to two orders of magnitude. [ABSTRACT FROM AUTHOR]