The rising global warming concerns make it inevitable to phase out fossil fuels sooner rather than later, as they contribute to about 60% of the greenhouse gas emissions in the world. One alternative to fossil fuels in heavy industry is green hydrogen. However, hydrogen combustion in air is known to emit the oxides of nitrogen (NOx) which are harmful to health. An improved comprehension of these NOx emissions and their formation pathways is the first step towards their mitigation. This paper intends to improve the methodology to understand the NO formation pathways in a non-premixed, swirl-stabilised, hydrogen/air gas-turbine combustor. To do so, high-fidelity large-eddy simulations (LESs), extended proper orthogonal decomposition (EPOD) technique and zero-dimensional (0D) perfectly stirred reactor (PSR) analyses are combined to utilise the merits of each of these techniques. While the LES ensures that the flow-field and reactions are well captured, EPOD is applied to identify two points featuring high positive and high negative fluctuations of the rate of production of NO, i.e. ROPNO, respectively. The 0D PSR provides a platform for a cost-effective yet detailed chemical pathway analysis at these identified points. Although both NO production and consumption reactions are prevalent at the chosen points, the net effect is that of NO production as was evidenced by the positive value of ROPNO at these points. However, the rate of production of NO differed between the two points. While the major NO formation and consumption reactions were found to be the same at both the points, their contributions to overall ROPNO varied. The major NO production reaction at both the points was NNH+O NH+NO, contributing to 37% and 26% at the points with high positive and high negative ROPNO fluctuations, respectively. Moreover, the species composition was different at the chosen points, which is expected in a non-premixed combustion configuration. For instance, the mass fractions of NNH and O, i.e. the reactants in the major NO production reaction, were respectively 18.28% and 8.77% higher at the point with high positive ROPNO fluctuation compared to the point with high negative ROPNO fluctuation. The above-mentioned differences in the reactions' contributions to ROPNO at the chosen points could be attributed to such variation in local composition, which is highly likely in non- premixed combustion. The methodology proposed in this paper enables a detailed chemical pathway analysis emphasising the points featuring high ROPNO fluctuations. This technique, which conducts a focused analysis of the NO formation pathways in a combustor, can be a useful tool in the effort towards designing cleaner hydrogen combustors.

In accordance with the United Nations Sustainable Development goal #7 – affordable and clean energy, the concept of reversible reactive flow (N2O4/NO2) inside ribbed channel is proposed for low-temperature waste heat recovery. Quasi direct numerical simulations are performed to reveal the relationship between flow, heat/mass transfer, and chemical characteristics with different rib inclined angles (90° and 45°). The analyses indicate that the reaction of N2O4 ⇌ 2NO2 has limited influence on flow patterns inside the ribbed channel, but intensifies the heat transfer considerably. For the 90° reactive case, the enhancement of Nusselt number reaches 112.7 % when Reynolds number is 2000. Although non-equilibrium thermal-chemical phenomenon is observed by instantaneous snapshots, time-averaged results show that the forward endothermic reaction is concentrated close to the heated wall. The flow structures transport fluid pocket consisting of “overheated” gas and triggers local backward exothermic reaction, which decreases the thickness of thermal boundary layer and thereby intensifies the overall heat transfer. For the 45° inclined reactive case, a flow circulation at local equilibrium between heat release and absorption is formed by the rib-induced large-scale vortices. The comprehensive thermal performance is further improved by 24.6 % compared to the 90° reactive case, which attributes to higher Nusselt number and lower friction loss.

Computationally inexpensive reduced order models such as Chemical Reactor Networks (CRN) are encouraging tools to obtain fast numerical solutions. However, the accuracy of such models is usually compromised compared to experiments or high-fidelity numerical simulations. Therefore, continued improvement of such models is necessary to ensure reasonable accuracy and fill the need for computationally inexpensive tools. To that end, inputs from computational fluid dynamics simulations have been used to build CRNs in the last 25 years. This can be further improved by the application of data-driven techniques. The present work uses a novel data-driven analysis based on high-fidelity simulation results where the high-dimensional data is first reduced to a low-dimensional manifold and is then clustered into chemically coherent regions. These results along with the high-fidelity simulation results are used to construct a CRN for wet combustion simulations. Wet combustion is a novel clean combustion technique, where an increase in the level of steam dilution distributes the flame, with a risk for flame extinction. This phenomenon, i.e., humid blowout (HBO), was modelled using the above-mentioned CRN. The HBO predicted by the CRN was also tested for thermal power, equivalence ratio, and fuel conditions different from its design point. The error in HBO prediction was quantified by comparing the steam-dilution level at HBO obtained using CRN and that obtained experimentally. The CRN predicted the HBO with an error magnitude less than 5% when the thermal power was unchanged. The maximum error in all tested conditions was 16.65%. Furthermore, a sensitivity analysis revealed that the inclusion of hot gas recirculation through the central recirculation zone, quantified by the mass flow split between the post-flame region and the CRZ, is important in the prediction of HBO, although its accuracy is inconsequential.

Blending of partially cracked ammonia with biosyngas is an attractive strategy for improving NH3 combustion. In practice, products of biomass gasification and those of thermo-catalytic cracking of NH3 are subject to some compositional uncertainties. Despite their practical importance, so far, the effects of such uncertainties on combustion systems remained largely unexplored. Hence, this paper quantifies the effects of small compositional uncertainties of reactants upon combustion of partially cracked NH3/syngas/air mixtures. An uncertainty quantification method, based on polynomial chaos expansion and a data-driven model, is utilised to investigate the effects of uncertainty in fuel composition on the laminar flame speed (SL) and adiabatic flame temperature (Tad) at different inlet pressures (Pi). The analysis is then extended to the power output of a combined-cycle gas turbine fuelled by the reactants. It is found that 1.5% fuel compositional uncertainty can cause 12–21% of SL uncertainty depending on the inlet pressure. Furthermore, the effect of compositional uncertainty on Tad increases at higher ratios of H2 to NH3. Sensitivity analysis reveals that the uncertainty of CO contribution to SL uncertainty is higher than that of NH3, while the trend is reversed for the Tad uncertainty. In addition, the power output from the combined-cycle gas turbine system varies between 4 and 6% with 1.5% of fuel compositional uncertainty. This become more noticeable at elevated Pi [5–10 atm], particularly when the fuel mixture contains high H2 which is the main contributor to Tad variability.

In line with the Paris Agreement and the United Nations Sustainable Development Goals, the use of fossil fuels is to be phased out in the coming decades. Fossil-free hydrogen is a promising candidate for decarbonising the energy sector. However, hydrogen combustion is known to produce undesirable emissions such as nitrogen oxides (NOx) and calls for mitigation measures such as steam-diluted hydrogen combustion, which is known to reduce the NOx emissions. This study focuses on improving the understanding of NOx formation in a swirl-stabilised, steam-diluted, non-premixed hydrogen/air combustion using high-fidelity large eddy simulation (LES) and zero-dimensional (0D) perfectly stirred reactors (PSR). The LES results are used to identify four salient points in the domain where the NOx chemistry is analysed in detail. These points are chosen in the main flame, flame tail, post-flame and central recirculation regions. Note, the high heat release rate (HRR) region is referred to as the main flame while the flame tail denotes the low-HRR and V-shaped flame region adjacent to the main flame. Based on the LES data at these chosen points, the contribution of the major NO production pathways such as the Zeldovich mechanism, extended Zeldovich mechanism, N2O route and the NNH route are studied and reveal different rates of production of NO (i.e. ROPNO). Also, the role of individual reactions in NO production and consumption is investigated. Furthermore, reaction pathway diagrams illustrating the chemical pathways leading to NO are visualised. The sensitivity of NO to each reaction is also reported. These analyses provide insights into the NO chemical pathways observed at the salient points across the domain. The dominance of the Zeldovich mechanism remains but is much less compared to that in dry hydrogen combustion as its contribution to ROPNO is less than 20% at all the points considered. However, the extended Zeldovich mechanism is significant at all the points. The NNH route is observed to be important in the flame regions. The major NNH reaction contributing to ROPNO in the flame regions is NNH+O⇌NH+NO. Furthermore, the complex NO chemistry at all the identified points is presented in great detail using the above-mentioned analyses and illustrations. This work highlights the possibility of combining a high-fidelity simulation and a simple 0D ideal reactor to decipher the nuances of NO chemical pathways in a practical swirl-stabilised gas turbine combustor and provide new understanding for assisting engineers in designing clean burners.

While fundamental mechanisms of non-reactive jet impingement have been extensively researched, the reactive jet impingement is yet unexplored although it offers the potential to increase further the heat transfer. The present work fills this knowledge gap via high-fidelity numerical simulation of reactive jet impingement invoking the finite rate one-step chemistry. Endo-/exothermic reactions are found to have very limited influence on time-averaged fluid flow, while they induce substantial modifications in heat transfer characteristics. This influence is manifested as discernible changes in the momentary or instantaneous fluid dynamics. Specifically, the primary vortices are responsible for downwashing cold fluid (mostly N2O4) towards the hot plate, posing low local convective heat transfer. The secondary vortices are responsible for extracting heat from the hot plate via the dissociation reaction N2O4→2NO2, followed by transporting NO2 away from the hot plate and towards the upwashing side of the primary vortex. At the upside of the primary vortex, hot NO2 is cooled by the environment, therefore reforming N2O4 to give out energy. This intense and long-lasting heat absorption and heat release process explains the outstanding performance of reactive jet impingement.

This article presents a joint numerical study on the Multi Regime Burner configuration. The burner design consists of three concentric inlet streams, which can be operated independently with different equivalence ratios, allowing the operation of stratified flames characterized by different combustion regimes, including premixed, non-premixed, and multi-regime flame zones. Simulations were performed on three LES solvers based on different numerical methods. Combustion kinetics were simplified by using tabulated or reduced chemistry methods. Finally, different turbulent combustion modeling strategies were employed, covering geometrical, statistical, and reactor based approaches. Due to this significant scattering of simulation parameters, a conclusion on specific combustion model performance is impossible. However, with ten numerical groups involved in the numerical simulations, a rough statistical analysis is conducted: the average and the standard deviation of the numerical simulation are computed and compared against experiments. This joint numerical study is therefore a partial illustration of the community's ability to model turbulent combustion. This exercise gives the average performance of current simulations and identifies physical phenomena not well captured today by most modeling strategies. Detailed comparisons between experimental and numerical data along radial profiles taken at different axial positions showed that the temperature field is fairly well captured up to 60 mm from the burner exit. The comparison reveals, however, significant discrepancies regarding CO mass fraction prediction. Three causes may explain this phenomenon. The first reason is the higher sensitivity of carbon monoxide to the simplification of detailed chemistry, especially when multiple combustion regimes are encountered. The second is the bias introduced by artificial thickening, which overestimates the species' mass production rate. This behavior has been illustrated by manufacturing mean thickened turbulent flame brush from a random displacement of 1-D laminar flame solutions. The last one is the influence of the subgrid-scale flame wrinkling on the filtered chemical flame structure, which may be challenging to model.

In line with the United Nation Sustainable Development goal #7 (clean and affordable energy), new carbon -neutral fuels need to be investigated. Methanol is a promising alternative e-fuel to fossil fuels for the application in gas turbines. The paper presents a numerical study of the efficient use of green methanol using in a wet Brayton cycle with chemical recuperation. The 1D flame analysis shows the steam addition affects the oxidation pathway in terms of the H-atom abstraction reactions. The high fidelity LES results show that steam addition leads to distributed flames denoted by increased area of heat release and decrease of temperature gradient. The latter solely occurs in the inner shear layer. The conservative representation of Chemical explosive mode analysis (CCEMA) shows that the more flame is distributed, the more autoignition mechanism dominates the ignition process. It is found that autoignition mode becomes more dominant globally while the area featuring local extinction mode is lightly increased since the flame area is increased. The increasingly predominant role of autoignition is accompanied by the emergence of high-temperature reactions that generates HO2 and OH radicals contributing the booming of radical pool.

The quest for decarbonisation of industry calls for phasing out fossil fuels and in some cases replacing them with green hydrogen in line with the Global Sustainable Goals. To that end novel hydrogen burners will be developed. In this work, the non-premixed combustion of steam-diluted hydrogen combustion is studied numerically in a swirl-stabilised combustor. The flame shape and stabilisation is discussed along with the effects of flow-field, stratification and temperature distribution on the flame. The characteristics of the swirl-stabilised flame is illustrated using several time-averaged and instantaneous contours of parameters including heat-release rate, local equivalence ratio and temperature. The effect of mixing and temperature distribution in the non-premixed configuration is studied quantitatively, subsequently identifying several flame regions. The combustion modes in these regions are also reported. Importantly, neither local hotspots nor flame flashback is observed in this steam-diluted hydrogen combustion. While a typical short hydrogen flame is observed near the centreline of the combustor, a weak V-shaped flame tail also co-exists. The heat-release rate in the flame tail is about two orders of magnitude lower than the main flame. Examining the flame structure using a normalised flame index showed that the combustion in the main flame is premixed-like while that in the main flame is non-premixed. The effect of the flow-field on the transport of fuel and air seems to influence this observation in the flame tail. The high heat-release rate regions of the flame comprised lean (ϕ<1), moderately hot (T = 800 to 1400 K) mixture. The flame tail region resulted from lean, hot (T >1500 K) mixtures. A flame region with rich mixture (ϕ = 1 to 2), featuring moderate heat-release rate, was found to be trapped between an axial fuel jet and the central recirculation region. All these flame regions had flameless combustion as the dominant mode of combustion. In particular, the flame tail region burned completely in the flameless mode. The lean and dominantly flameless combustion in the main flame regions are likely to lead to lower NOx.

In line with the decarbonisation of power sector, carbon-free fuels are currently being investigated. In particular, green ammonia or e-ammonia is a candidate fuel which will be playing a key role in many energy-intensive industries. It calls for an in-depth under-standing of eFuels combustion characteristics in the fuel flexible combustors. Therefore, the present work for the first time numerically investigates the combustion regimes of steam-diluted, decomposed eNH₃ in a novel multi-nozzle direct injection (MDI) burner. Although the MDI burner is not equipped with a conventional swirler, strong flow-flame interaction is observed. The two-layer, angled channels create swirling flows featuring swirl numbers larger than 0.9 in general. The centre recirculation region can help stabilise highly steam-diluted decomposed ammonia with a maximum steam-to-air ratio of 74%. This highest H2% containing, wettest ammonia flame case is found to emit the lowest total emission (NH₃+NO + NO₂+N₂O) of -400ppmvd@15%O2 at stoichiometric conditions. The wall heat loss is confirmed responsible for the formation of N₂O in distributed flame, suggesting the need of reducing pollution through good chamber wall insulation. However, for flames sitting in the conventional regimes, the impact of wall heat loss is found insignificant. Further, extensive data and flame regime analyses show that NNH can al-ways accurately mark the high heat release region of all types of flames, while OH is only an effective marker for thin flames.