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.

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.

Low-grade heat recovery is an indispensable solution towards high energy efficiency of power electronics. The fast pace of sustainable digitalisation calls for developing alternative solutions to create a sustainable loop for decreasing the energy footprint. However, heat transfer under low-temperature differences challenges effective heat recovery processes. Therefore, in this paper, reactive fluid performance in a practical heat exchanger is investigated using high-fidelity finite rate chemistry method, which is a key step to deploy the attractive Ericsson cycle for low-temperature heat-to-electricity conversion. Under fixed thermal efficiency, it is found that replacing non-reactive fluid by N2O4 reactive fluid can immediately boost electrical efficiency of an Ericsson cycle by at least 260%. The needed primary heat exchanger component in an integrated cooling and power electronic system can be 54.8% smaller in volume whilst enabling a 26% higher thermal performance, provided that the hot source temperature is low (<403 K). For thermal processes involving high temperature hot source, substantial limitation of chemical reaction rate on the effectiveness of an Ericsson cycle is identified. Remarkably, low temperature difference is not a limitation for reactive heat transfer that continuous endo-/exothermic reaction happening throughout a heat exchanger improves Nusselt number Nu = 7.5 by a factor of similar to 1.3 than the corresponding value (Nu = 5.9) for non-reactive fluid. Turbulence is found beneficial for reactive heat transfer, suggesting the use of corrugated-type heat exchangers for better thermal exchange rates.

E-fuels are promising alternatives to fossil fuels in the transition towards zero-emission energy system. In this study, a novel chemical-recuperated and humidified gas turbine concept aiming at the application of e-fuel ammonia is proposed to overcome the technical hitches and exploit the waste heat to enhance the cycle performance. The thermodynamic analysis shows that the highest net electrical efficiency (56.7%) under the design conditions exceeds that of the ammonia-fueled Brayton cycle by 20.6%-points. The chemical recuperation of fuel contributes to the efficiency improvement by 7%-points under dry condition, while steam injection provides 8%-points to 12%-points efficiency increase corresponding to the ammonia decomposition ratio of 5%–96%. A non-monotonic relation between the net electrical efficiency and steam-to-air ratio is found to be the result of the competition between the enthalpy change from to the varied steam and air mass flow rates. Analyzing the flame characteristics in the combustor under the design conditions, we found that conditions with high decomposition ratio (>88%) and high steam-to-air ratio (>0.3) exhibit similar flame speed with that of methane fueled combustor and thus existing designs can be reused. The prediction shows the NOx emission can be restricted when the steam-to-air ratio exceeds 0.4.

Waste heat recovery is an indispensable solution towards high energy efficiency in various industrial processes. While many methods are available to recuperate waste heat of medium-to-high temperature range, limited solutions are applicable at low-temperature (<373 K). The present work presents a poten-tially reasonable cost while less understood method, namely the reactive heat transfer using reversible exo-/endothermic reactions for harvesting low-grade heat. Invoking high-fidelity direct numerical simu-lation, the interplay amongst turbulence, heat transfer, and chemical reactions is investigated in a heated channel flow. We consider a temperature difference between hot source and reacting working fluid of 100 K and show a remarkable improvement of heat transfer coefficient by 600% compared to non-reacting working fluid. This is associated with similar to 17% higher total energy absorption across the geometry of inter-est. These improvements are proven to be related to the existence of mild exothermic reactions near the channel core, the molar expansion, and the mild endothermic reaction close to the hot source which contributes to a thin thermal boundary layer, etc.

The interest in climate action and building sustainable energy system, has highlighted electricity derived fuels (eFuels) and has promoted studies considering ammonia as a eFuel for sustainable transportation and as a mean of energy storage. Ammonia combustion, however, poses several challenges due to the low reactivity and potential high NOx emissions. A promising solution is to add hydrogen and steam into the fuel/air mixture. Steam contributes to the enhancement of specific power output and efficiency in humidified gas turbine cycle, and hydrogen promotes reactivity of the eFuel and can be obtained by ammonia decomposition on-site. In the present study, wet combustion of ammonia/hydrogen blends is systematic studied using one-dimensional flame analysis and Large eddy simulation (LES) to understand the flame stabilisation and NOx formation mechanisms. When hydrogen content is low (H-2%<30%vol) and T-ad = 1750 K, flame speed is nearly independent while flame thickness is significantly dependent on the equivalence ratio Phi due to the steam dilution. At high hydrogen content, the reverse applies. The impact of the H-2% on the emission is non-monotonic, peaking between 60%similar to 70% at various Phi. Rich and stoichiometric combustion benefits NO reduction at a balance of H-2% and steam dilution. High fidelity LES demonstrates that the steam diluted hydrogen/ammonia blends can be stably burnt in a swirl burner in a MILD regime. The wet combustion distributes the flame at low H-2% and reduces local NO emission. In rich conditions, low NO emission attributes to the activation of NO reburning progress. Compared to dry condition, low steam addition (Omega = 0.1) in the reactant lifts the flame and enlarges the opening angle of the swirling flow. When flame speed is low, POD analysis shows two PVC related structures - a single and a double helix, which dominate the flow and flame dynamics. While when flame speed is high, quick combustion process damps the precessing motion.

The current work investigates the swirling flow of a gas fuel injector utilized in the Lean Direct Injection (LDI) combustion system. Planer particle image velocimetry (PIV) measurements and large eddy simulation (LES) numerical analysis are conducted to have a profound understanding of the swirling flow characteristics. Specifically, the impacts of the level of confinement with a rectangular cross-section and different Reynolds number are examined. Increasing the Reynolds number increases the strength of swirling jets and reverse flow region. More significant changes occurred on the mean flowfield due to the confinement effect such as increasing the width of the reverse flow region and increasing/decreasing the size of the recirculation zones which in turn effects the inlet jet penetration. The inlet jet spreads at a larger angle as the size of the outer recirculation zone (ORZ) increases with the confinement ratio. The shape of the inner recirculation zone (IRZ) vortex structure on the unconfined flow is characterized to be a thin and short vortex and located on top of the nozzle exit, and it becomes thicker and longer vortex located further downstream from the nozzle exit upon confinement. The increased size of the IRZ vortex structure in confined cases is an indication of the increased thickness of the inner shear layer (ISL) that increases linearly as the confinement ratio increases. LES results reveled there is a connection channel between the reverse flow region and the ORZ of the swirling flow emanating from the multiple-jet LDI nozzle. Higher level of turbulence is associated with the location of the IRZ vortex structure. Proper orthogonal decomposition (POD) analysis is preformed to extract coherent fluctuating flow features. The swirling flow of the LDI nozzle exhibits the single-helical and double-helical precessing vortex core (PVC) modes, with the first one being the most energetic mode. The general flow structure of the coherent single-helix PVC mode on the unconfined flow consists of four vortices: two corner vortices rotating in opposite of each other, and a tiny vortex on top of the nozzle exit followed by a huge central vortex rotating in a different direction. Upon confinement the outer vortices attached to the wall of the combustor and the central vortex becomes about twice bigger. The preexistence of the outer vortices on the unconfined flow suggests that the formation of the ORZ is not caused by the confinement, but rather it is a part of the natural behavior of a highly turbulent swirling flow which magnified in the case of confined environment. The single-helix PVC mode gains higher energy value and becomes less-sensitive to the increase of the Reynolds number as the confinement ratio decreases. This is linked to the asymmetry mode shapes, and energy content linearity between the axial and radial components associated with the single-helix PVC mode.

A numerical study has been performed on a linear array of five nozzles in a lean direct injection (LDI) combustor for three mass flow rates and four different inter-nozzle spacings, using two equation realizable k − ɛ turbulence model. It is observed that the mass flow rates do not affect the flow patterns in this five nozzle configuration. The two smaller inter-nozzle spacings, s = 1.25d and 1.5d, developed asymmetric flow patterns. Especially at 1.5d, the asymmetry is quite dominant in the core flows of nozzles N2 and N4, due to the non-merging of jets in the shear layers. But, at higher nozzle spacings, s = 1.75d and 2d, the jets merge in the shear layers and move downstream as a single jet. Due to the slower expansion of the flow in the radial direction, strong and compact central toroidal recirculation zones (CTRZ) are formed at smaller inter-nozzle spacings, 1.25d and 1.5d. These compact CTRZs contribute to higher turbulence kinetic energy (TKE) in regions between the nozzles and closer to the dome-plate. These regions correspond to higher velocity and higher shear stress in the flow. As the inter-nozzle spacing is increased, the intensity of TKE decreases between the nozzles.

Using steam as a heat carrier and working media has merits to increase electric efficiency up to 60% and decrease NOx emission to single-digit compared to dry gas turbine cycles. These attribute primarily to the physical properties of steam as having high heat capacity to reduce local flame temperature, and hence reduce emissions by inhibiting the thermal NOx forward reaction rate. In this work, ultrahigh steam content with a steam-to-air mass ratio of up to 40% is premixed with methane-air mixture before entering into a swirl-stabilized high pressure (HP)-burner for combustion. A significant change of flame from V-shape (attached) to M shape (detached) is observed through a transparent combustion chamber whilst changing steam content. The measurement of chemiluminescence OH* is conducted with an intensified CCD-camera bandpass filtered at 320nm. Following these measurements, large eddy simulation (LES) is used to capture reacting flow features. Reasonably well agreements between experimental data and numerical results are obtained for both attached and detached flames in terms of the OH* distribution. Slight inconsistency of OH* intensity is mainly due to uncollected wall temperature, which leads to either over- or underprediction of chemical reaction rate depending on the experimental flame positions. Distributed flame front is clearly identified with LES for wet methane combustion associated with 35% steam-to-air ratio corresponding to a high Karlovitz number flame. Slightly unstable combustion is observed when the steam-to-air ratio exceeds 40% featuring an onset of flame blow-off. In addition, interaction between precessing vortex core (PVC) and the flame is presented for different level of steam dilution, and conclusions are drawn regarding the flame stabilization. The in-depth understanding of the ultrawet combustion is an important step toward the use of sustainable, steam-diluted biosyngas for electricity production.

Towards developing humidified gas turbines (HGT) capable of running at high electrical efficiencies and low emissions, wet/steam-diluted combustion in a premixed swirl burner is investigated using large eddy simulation and a partially stirred reactor method. Chemical explosive mode and extended combustion mode analyses are performed to promote the understanding of wet flame structures. The former identifies the key features of the wet methane oxidation processes, and the latter extends the flame regime classification method to describing the combustion status of fluid parcels using local properties. Three combustion regimes are extensively discussed: the swirl stabilized (SS), colorless distributed (CDC) and non-combustible. Using the combined analyses of the two approaches, it is found that compared to dry flames, wet flames present more fluid parcels defined in the practical CDC regime where local heat release is low and Damköhler number is smaller than unity. The wet fluid parcels are capable of self-igniting via radical explosion, while dry fluid parcels self-ignite via thermal runaway. The species CH2O and temperature are the first and second highest contributors towards the explosivity of dry flames, while temperature is insignificant to that of wet flames. The species C2H6 is found an important source to the self-ignitability of wet fluid parcels in the practical CDC regime due to the activation of the three-body ethane formation reaction R148: 2CH3 + M = C2H6 + M in the low O2% wet combustion environment. Proper use of proposed methods to quantify wet flame behavior guides stable and low emission operation of practical HGT.