Over the past decade, additive manufacturing (AM) has gained considerable traction in the gas turbine industry. Manufacturers now look to AM for the production of critical components central to the combustor architecture. However, AM struggles to achieve satisfactory surface finishes, with its increased surface roughness shown to impact axial velocities, heat release, NOx emissions, and stability limits. Correlations between increased roughness and boundary layer flashback (BLF) resistance are of significant interest for lean premixed combustion of high hydrogen content (HHC) fuels. With the development of innovative burner configurations designed to address static stability issues in HHC fuels, and the growing use of AM in their fabrication, studying roughness effects on these designs is essential. This study aims to quantify the impact of surface roughness on an industry-relevant jet burner configuration, building on previous research by the authors using premixed swirl geometries. A premixed jet burner was developed with an interchangeable section to accommodate varying surface textures. The burner was characterized under atmospheric hydrogen-fired conditions at thermal powers ranging from 10 to 25 kW. Both smooth and rough inserts representing traditional machining and selective laser melting manufacturing techniques were utilized. The findings highlight how surface roughness influences flame topology, static stability, and emissions, providing an experimental basis for future numerical studies on roughness sensitivity in advanced burner designs.

Parametric experiments were performed using CH4-H2 blends to fuel a model gas turbine combustor at elevated inlet conditions, unabated and whilst employing exhaust gas recirculation (EGR) as a NOx-abatement technique. The purpose was to quantify the performance of conventional dry, 15% O2 NOx correction (ISO) against mass by heat input (MBHI) normalisation to experimentally verify simulated work from the literature, while expanding on this work to cover NOx-abated combustion. NOx was found to be consistently inflated in H2 combustion when compared to CH4 by the ISO method, whereas the MBHI method was found to closely approximate the mass by work output baseline emissions. Multipliers proposed to correct this inflation were shown to be impractical. A fundamental problem in assessing the effectiveness of EGR using the ISO method was identified as it does not account for the additional volumetric dilution introduced, whereas the MBHI normalisation method avoids this issue.

The additive manufacturing (AM) technique enables the fabrication of advanced burner components to enhance the hydrogen capability of the existing gas turbines (GTs) and reduce the carbon footprints of the power generation sector. This technique produces rough surfaces that may require post-processing to maintain the desired functionality of a burner, particularly for hydrogen fuel with unique thermo-physical properties. This study, therefore, compared the stability of three swirlers of variable surface roughness manufactured using AM and traditional machining methods, with one of the AM swirler post-processed by grit-blasting. The comparison included a conventional benchmark (100% CH4), low carbon (23%volCH4/77%H-vol(2)) and zero carbon (100% H-2) fuels across a range of equivalence ratios. Additionally, the study quantified the flame topology and emissions performance of the fuel blends for each swirler using high-speed OH* chemiluminescence and exhaust gas emissions measurements, respectively. The experimental investigation concluded that the AM-generated surface roughness within the considered range does not detrimentally impact NOX emissions and the stability of the fuel mix. However, the flame location was observed to be influenced by surface roughness and shifted more toward the vertical centerline of the burner with increased roughness. From the practical perspective, the results showed that post-manufacturing surface finishing offers negligible performance advantages, indicating potential cost reductions. It is recommended that further studies should investigate the influence of increased surface roughness on burner performance, as well as numerical modeling techniques which could provide an insight into when AM surfaces are likely to be more influential.

Additive manufacturing (AM) technology can create complex parts that are otherwise impractical to manufacture by traditional methods. However, the process often results in rough and irregular surfaces that can affect performance. In this study, computational fluid dynamics (CFD) is considered as a tool to optimize component design for use in applications such as a gas turbine. However, modeling the interactions between turbulent flows and AM-generated wall roughness affect the predictive capability of numerical models due to difficulty in thoroughly characterizing rough wall texture. To progress toward addressing this issue, this study aims to appraise two common wall roughness approaches within the Reynolds-averaged Navier-Stokes (RANS) framework: the modified "law-of-the-wall" and roughness-resolving approaches. The modified law-of-the-wall is based on the correlation that converts the measured surface roughness parameters to the equivalent sand-grain roughness height. The second approach involves the resolution of the roughness elements within the computational grid. The simulations were compared against the velocity data published for the burner with AM swirl nozzle inserts of different surface finishes. At this stage of development, the realizable k-epsilon turbulence model was selected for all the CFD simulations. The results show that the roughness-resolving approach was better suited than the modified law-of-the-wall correlation, demonstrating good agreement with the experimental velocity data, predicting the velocity shift to the center. The model also revealed the shortened recirculation zone with increasing surface roughness, which is important in predicting flame stability and emissions performance to be studied subsequently.

As alternative fuels are designated for future energy applications, flexible combustor designs require considerable development to ensure stable operation with reduced NOx emissions. A non-premixed variable swirl burner was used to experimentally appraise changes in NO production pathways, with CH4 NH3, and H2 flames, alongside intermediate fuel blends. Maintaining an equivalent thermal power and flame temperature between fuels, preheated reactants (500 K) were supplied to the burner, with parametric changes made to pressure (1–6 bara) and swirl number (0.8–2.0). NO production was characterized, alongside variations in flame structure and topology, with a correlation demonstrated for exhaust emissions. NO production was shown to be sensitive to combustor pressure, providing an expected increase for CH4 and H2 flames. Emission profiles from both NH3 and H2 flames are shown to be significantly augmented by a change in swirl number. As NH3 fractions were increased in the H2 blend, a decaying trend in NO emissions was observed with an increase in pressure, and as a function of mixture ratio. However, this behaviour was markedly augmented by a change in swirl number and suggests that further reductions may be possible at increased pressure. At the low swirl/high pressure condition the NH3/H2 blend outperformed pure H2, providing lower NO concentrations. Emissions data were normalised using the traditional dry/O2 correction, alongside mass scaled by thermal power, with a comparison provided. The corresponding differences in emission formation pathways were investigated, alongside high-speed OH* chemiluminescence to further elucidate findings.

Renewably generated NH3 has the potential to support future energy demand, however combustor designs and strategies require considerable development to ensure reduced NOx emissions. Expanding on previous work, a turbulent swirl burner was used to appraise potential NOx reduction pathways, both experimentally and numerically, with a premixed NH3/H2/air flame. With a 100-year global warming potential ∼265 times that of CO2, particular emphasis was given to N2O. Maintaining a constant thermal power, reactants were supplied at elevated temperature, with parametric changes made to pressure and humidity. Favourable agreement was demonstrated between exhaust emission measurements and simulations performed using a chemical reactor network model. NO and N2O emissions were shown to be sensitive to operational equivalence ratio, increasing by several orders of magnitude across the experimental range. An increase in combustor pressure was experimentally shown to reduce exhaust N2O concentrations with this globally lean fuel mixture, alongside NO. Steam injection was also explored in detail for the first time and shown to provide contrasting trends, with a reduction in NO, and a rise in N2O, as water loading was increased. Both pressure increase and steam injection were combined to give optimal NOx performance for the evaluated dataset. Changes in chemical kinetic pathways were investigated in detail, and compared to high-speed OH*, NH2*, and NH* chemiluminescence.

This article reviews the critical role of material selection and design in ensuring efficient performance and safe operation of gas turbine engines fuelled by ammonia–hydrogen. As these energy fuels present unique combustion characteristics in turbine combustors, the identification of suitable materials becomes imperative. Detailed material characterisation is indispensable for discerning defects and degradation routes in turbine components, thereby illuminating avenues for improvement. With elevated turbine inlet temperatures, there is an augmented susceptibility to thermal degradation and mechanical shortcomings, especially in the high-pressure turbine blade—a critical life-determining component. This review highlights challenges in turbine design for ammonia–hydrogen fuels, addressing concerns like ammonia corrosion, hydrogen embrittlement, and stress corrosion cracking. To ensure engine safety and efficacy, this article advocates for leveraging advanced analytical techniques in both material development and risk evaluation, emphasising the interplay among technological progress, equipment specifications, operational criteria, and analysis methods.

Non-volatile Particulate Matter (nvPM) from aircraft gas turbine engines are harmful to both human health and the environment, but can be significantly reduced by using low aromatic Sustainable Aviation Fuel (SAF). As part of the Horizon 2020 funded JETSCREEN (JET fuel SCREENing and optimisation platform for alternative fuels) project, nvPM and gaseous emissions were characterised using regulatory compliant sampling and measurement methodologies for a small-scale (<250kW) non-proprietary RQL combustion rig, at pressures ranging from 1.0 to 2.4 bara. The impact of flow conditions, air to fuel ratio and fuel composition was investigated for a selection of conventional aviation Jet-A1 fuels, SAFs, and blended fuels. Measured concentrations were corrected for particle size-dependant system losses using particle size measurements, to be representative of combustor exit concentrations.Across the range of fuels (hydrogen contents 13.51%-15:31%), system-loss-corrected nvPM mass, number, and size were shown to decrease with increasing fuel hydrogen content, in agreement with previous studies. Inverse power law correlations are proposed as the best descriptors of these trends. Average reductions in nvPM mass, number, and sizes of 73%, 54% and 17% respectively, were observed for a near-zero aromatic ATJ fuel compared to a reference Jet A1 fuel, with minimal changes to measured gaseous pollutants. It is noted that without size dependant system loss corrections, nvPM number reductions were overreported (~6% for the ATJ fuel) due to the smaller particle sizes with increasing fuel hydrogen content. It was hypothesised that observed nvPM deviations from the fuel hydrogen content trends were due to fuel physical properties affecting atomisation, however no correlations were found greater than the measurement uncertainty and combustor rig variability.This study provides a unique dataset intended to facilitate combustion model validation, providing full details of combustor geometry, flow conditions and rig conditions, along with the representative combustor exit nvPM and gaseous data.

This experimental work reports, for the first time, observations of an internal flow field involving a DARPA SUBOFF afterbody design aerator body in an inside-out type of effervescent atomizer. The effect of operating parameters like air-to-liquid ratio (ALR), operating pressure, aerator orifice diameter, aeration area, and mixing chamber diameter on internal flow within the effervescent atomizer is studied. The effect of increasing ALR on the internal flow is quantified by identifying different gas injection mechanisms at the aerator orifice (into the mixing chamber) and two-phase mixing chamber flow regimes using high-speed shadowgraphy. In particular, it is observed that as ALR is systematically increased, the gas injection mechanism transits in the following sequence: single bubbling, pulsed bubbling, elongated jetting, atomized jetting, and evacuated chamber. The range of ALRs within which these mechanisms are observed are employed to draw up a flow regime map. Similar analysis on two-phase mixing chamber flow regimes yielded a corresponding regime map for internal two-phase stabilized flow in the mixing chamber. The flow regime transited from bubbly flow, to slug flow, to churn flow, and finally to annular flow as the ALR was increased. The spray characteristics (size and velocity) at the nozzle exit are reported using phase Doppler anemometry measurements. It is observed that dense-bubbly and bubbly-slug flow regimes produce stable sprays with droplet sizes in the range of 50-80 mu m in the range of 0.25%-1.50% ALR.

Strict regulations and acts have been imposed to limit NOx and carbon emissions. The power generation industry has resorted to innovative techniques to overcome such a low level of tolerance. Amongst those in the literature, CO2-argon-steam oxyfuel (CARSOXY) gas turbines have theoretically been proven to offer an economically sustainable solution while retaining high efficiency. Although theoretical studies have characterized CARSOXY, no experimental evidence has been provided in the literature. Therefore, this paper attempts to experimentally assess CARSOXY in comparison to a CH4/air flame. OH* chemiluminescence integrated with OH Planar Laser-Induced Fluorescence (PLIF) imaging has been utilized to study flame stability and flame geometry (i.e., the area of highest heat intensity (AOH¯Max center of highest heat intensity (COH¯Max)) over a range of working fluid Reynolds’ numbers and oxidizing equivalence ratios. In addition, the standard deviation of heat release fluctuations (σOH*/OH¯) has been utilized as the base-criteria to compare the stability performance of CARSOXY to CH4/air combustion. Moreover, turbulence-chemistry interactions have been described using Damköhler numbers and by plotting Borghi regime diagrams. This paper suggests a modified numerical approach to estimate Damköhler numbers and plot regime diagrams for non-premixed combustion by utilizing the Buckingham π theorem based on experimental observations and results. CARSOXY flames showed lower flame intensity than that of the CH4/air flame throughout the entire Re interval by approximately 16%, indicating higher heat release. The Damköhler numbers of the CARSOXY flame were also greater than those of the CH4/air flame in all conditions, indicating more uniform CARSOXY flames. It was found that the tendency of the CARSOXY flame of approaching the concentrated reaction zone is greater than that of the CH4/air flame.

This study of effervescent atomization, a two-phase gas-liquid spray generation technique that offers many advantages over conventional atomizers, shows the advantage of streamlined aerator design over flat-end aerator type with respect to formation of gas-void in the aerator wake in the interior of an inside-out type of effervescent atomizer. The experiments use high-speed shadowgraphy visualizations. It is observed that, in the conventional flat-end type of aerator design, the formation of gas-void is undesirable and leads to spray characterized by instabilities, causing fluctuating spray properties. The existence of gas-void also prevents the formation of bubbly flow inside the effervescent atomizer, which is actually preferred in these types of atomizers to enable stable spray generation and fine atomization. The formation and existence of gas-void is a result of aerator bluff body recirculation and gas phase buoyancy effects. Four different streamlined aerator designs with tips in the shape of circular arc, circular arc/conical hybrid, conical, and DARPA SUBOFF afterbody design (which is common in the conventional ship designs) are evaluated to determine the best among them with respect to mitigating the unwanted gas-void in the interior of an effervescent atomizer. These are evaluated by their ability to produce bubbly flow over comparatively large operating range and the ability to impart minimum wake (of aerator body) effect. It is concluded, upon careful experimental observations, that the DARPA SUBOFF afterbody design is the best among the streamlined aerator designs.

Steelmaking is energy intensive, with manufacturing facilities representing some of the biggest point-source carbon dioxide (CO2) emitters in the UK. Efficiency improvements are essential with rising energy costs, driving significant investment from the UK iron and steel sector. However, the industry still finds it difficult to justify waste heat recovery (WHR) projects, as individual schemes incorporating waste heat capture and an end-use for the waste heat often incur high capital costs, resulting in long payback times. This paper describes the conceptual and numerical development of a strategy for the deployment of WHR using a large integrated steel works as a case study. An existing asset was utilised to link individual waste heat schemes together with a single end-user; thereby reducing the capital requirement for each subsequent project. The proposed strategy and its development is discussed, followed by the resultant carbon dioxide and energy savings (estimated to be 2.3 Mt and equivalent to £45 million), over the 6-year period since its implementation.

With developing interest in NH3 as a prospective energy carrier, combustor designs and fuelling concepts require optimisation to reduce NOx emissions. Through the introduction of staged combustor concepts, pathways have previously been identified that limit NOx production whilst improving combustor efficiency and reducing unburned NH3. However, the efficacy of secondary air staging is sensitive to the primary flame behaviour, and whilst low NOx emissions can be achieved at rich conditions, high unburned NH3 leads to greater global NOx concentrations from downstream production. Here, time-resolved OH*, NH2* and NH* chemiluminescence were employed together for the first time for NH3-air and NH3H2-air flames to investigate a primary flame configuration that produced the lowest combined emissions concentration. A generic, fuel-flexible burner was developed to enable partial and full premixing, together with operation of a swirl-stabilised non-premixed flame. Initially, NH3H2-air flames were employed in a range of configurations and produced markedly different chemiluminescence and emissions results as functions of global equivalence ratio. The performance of a pure NH3-air flame was subsequently investigated and compared to the blended fuel results. Optical trends complemented changes in sampled exhaust emissions, enabling analysis of intermediate chemistry. Burner inlet temperature and pressure were then increased proportionally to maintain equivalent bulk nozzle exit velocities. Contrasting trends were identified as functions of fuel composition and equivalence ratio, with a comprehensive database of optical and analytical results generated. Results obtained for NH3H2-air suggest the most favourable configuration resulted from a partially premixed flame employing H2 as a pilot, operating under rich conditions (Φ=1.2). However, at higher temperatures and pressures, the trends observed for non-premixed NH3-air flames will lead to superior performance, particularly with a small increase in equivalence ratio.

A study has been undertaken to experimentally and numerically evaluate the use of carbon dioxide or steam as premixed fuel additive in hydrogen-air flames to aid in the development of lean premixed (LPM) swirl burner technology for low NOx operation. Chemical kinetics modelling indicates that the use of CO2 or steam in the premixed reactants reduces H2-air laminar flame speed and adiabatic flame temperature within the well-characterized range of preheated LPM methane-air flames, albeit in markedly different proportions; for example, nearly 65 %vol CO2 as a proportion of the fuel is required for a reduction in laminar flame speed to equivalent CH4-air values, while approximately 30 %vol CO2 in the fuel is required for an equivalent reduction in adiabatic flame temperature, significantly impacted by the increased heat capacity of CO2. The 2nd generation high-pressure generic swirl burner, designed for use with LPM CH4-air, was therefore utilized to experimentally investigate the influence of CO2 and steam dilution on pressurized (up to 250 kW/MPa), preheated (up to 573 K), LPM H2-air flame stability using high-speed OH* chemiluminescence. In addition, exhaust gas emissions, such as NOx and CO, have been measured in comparison with equivalent thermal power conditions for CH4-air flames, showing that low NOx operation can be achieved. Furthermore, pure LPM H2-air flames are characterized for the first time in this burner, stabilized at low equivalence ratio (approximately 0.24) and increased Reynolds number at atmospheric pressure compared to the stable CH4-air flame (equivalence ratio of 0.55). The influence of extinction strain rate is suggested to characterize, both experimentally and numerically, the observed lean flame behavior, in particular as extinction strain rate has been shown to be non-monotonic with pressure for highly-reactive and diffuse fuels such as hydrogen.

Aircraft engines are a source of harmful non-volatile Particulate Matter (nvPM) emissions, negatively affecting human health and the global environment. To mitigate this, new sources of fuel are being assessed for the commercial aviation sector. Sustainable Aviation Fuels (SAF) show significant promise as replacements to conventional aviation fuels, with the potential to reduce lifecycle CO2 and nvPM emissions because of lower aromatic contents and higher hydrogen content. Towards better understanding of the nvPM emissions from aircraft combustors operating with SAF, this work outlines results from the RAPTOR experimental test campaigns performed at Cardiff University’s Gas Turbine Research Centre (GTRC). Several aviation fuels of varying physiochemical properties were burned in a non-proprietary Rich-Quench-Lean (RQL) combustor rig. The nvPM emissions were measured using the European nvPM reference system, with data corrected for particle loss in the sampling and measurement system using additional particle size measurement. nvPM emission reductions were achieved for fuels of higher hydrogen content, and system loss correction was required to accurately quantify those reductions. Additionally, independent control of the air supply to the combustor rig allowed the impact of fuel spray quality to be decoupled from AFR, demonstrating that small improvements in spray droplet atomisation predicted from benchmarking fuel spray experiments (~5% reduction in SMD) consistently yielded significant reductions in nvPM emissions, ranging from 5-72% for nvPM EImass, 11-89% for nvPM EInumber, and 1-7% for GMD.

This paper aims to conduct a parametric study for five gas turbine cycles (namely, simple, heat exchanged, free turbine and simple cycle, evaporative, and humidified) using a CO2-argon-steam-oxyfuel (CARSOXY) mixture as a working fluid to identify their optimal working conditions with respect to cycle efficiency and specific work output. The performance of the five cycles using CARSOXY is estimated for wet and dry compression, and a cycle is suggested for each range of working conditions. The results of this paper are based on MATLAB codes, which have been developed to conduct the cycle analysis for CARSOXY gas turbines, assuming a stoichiometric condition with an equivalence ratio of 1.0. Analyses are based on the higher heating value (HHV) of methane as fuel. This paper also identifies domains of operating conditions for each cycle, where the efficiency of CARSOXY cycles can be increased by up to 12% compared to air-driven cycles. The CARSOXY heat exchanged cycle has the highest efficiency among the other CARSOXY cycles in the compressor pressure ratio domain of 2–3 and 6–10, whereas, at 3–6, the humidified cycle has the highest efficiency. The evaporative cycle has intermediate efficiency values, while the simple cycle and the free turbine-simple cycle have the lowest efficiencies amongst the five cycles. Additionally, a 10% increase in the cycle efficiency can be theoretically achieved by using the newly suggested CARSOXY blend that has the molar fractions of 47% argon, 10% carbon dioxide, 10% H2O, and 33% oxyfuel at low compressor inlet temperatures, thus theoretically enabling the use of carbon capture technologies.

In this study, two Inconel 625 swirl nozzle inserts with identical bulk geometry were constructed via additive layer manufacturing (ALM) for use in a generic gas turbine swirl burner. Further postprocessing by grit blasting of one swirl nozzle insert results in a quantifiable change to the surface roughness characteristics when compared with the unprocessed ALM swirl nozzle insert or a third nozzle insert which has been manufactured using traditional machining methods. An evaluation of the influence of variable surface roughness effects from these swirl nozzle inserts is therefore performed under preheated isothermal and combustion conditions for premixed methane-air flames at thermal power of 25 kW. High-speed velocimetry at the swirler exit under isothermal conditions gives evidence of the change in near-wall boundary layer thickness and turbulent fluctuations resulting from the change in nozzle surface roughness. Under atmospheric combustion conditions, this influence is further quantified using a combination of dynamic pressure, high-speed OH* chemiluminescence, and exhaust gas emissions measurements to evaluate the flame stabilization mechanisms at the lean blowoff and rich stability limits. Notable differences in flame stabilization are evident as the surface roughness is varied, and changes in rich stability limit were investigated in relation to changes in the near-wall turbulence intensity. Results show that precise control of in-process or postprocess surface roughness of wetted surfaces can positively influence burner stability limits and NOx emissions and must, therefore, be carefully considered in the ALM burner design process as well as computational fluid dynamics (CFD) models.

Due to growing concerns about carbon emissions, Carbon Capture and Storage (CCS) techniques have become an interesting alternative to overcome this problem. CO2-Argon-Steam-Oxy (CARSOXY)-fuel gas turbines are an innovative example that integrates CCS with gas turbine powergen improvement. Replacing air-fuel combustion by CARSOXY combustion has been theoretically proven to increase gas turbine efficiency. Therefore, this paper provides a novel approach to continuously supply a gas turbine with a CARSOXY blend within required molar fractions. The approach involves H2 and N2 production, therefore having the potential of also producing ammonia. Thus, the concept allows CARSOXY cycles to be used to support production of ammonia whilst increasing power efficiency. An ASPEN PLUS model has been developed to demonstrate the approach. The model involves the integrations of an air separation unit (ASU), a steam methane reformer (SMR), water gas shift (WGS) reactors, pressure swing adsorption (PSA) units and heat exchanged gas turbines (HXGT) with a CCS unit. Sensitivity analyses were conducted on the ASU-SMR-WGS-PSA-CCS-HXGT model. The results provide a baseline to calibrate the model in order to produce the required CARSOXY molar fraction. A MATLAB code has also been developed to study CO2 compression effects on the CARSOXY gas turbine compressor. Thus, this paper provides a detailed flowsheet of the WGS-PSA-CCS-HXGT model. The paper provides the conditions in which the sensitivity analyses have been conducted to determine the best operable regime for CARSOXY production with other high valuable gases (i.e., hydrogen). Under these specifications, the sensitivity analyses on the (SMR) sub-model spots the H2O mass flow rates, which provides the maximum hydrogen level, the threshold which produces significant CO2 levels. Moreover, splitting the main CH4 supply to sub-supply a SMR reactor and a furnace reactor correlates to best practices for CARSOXY. The sensitivity analysis has also been performed on the (ASU) sub-model to characterise its response with respect to the variation of air flow rate, distillation/boiling rates, product/feed stage locations and the number of stages of the distillation columns. The sensitivity analyses have featured the response of the ASU-SMR-WGS-PSA-CCS-HXGT model. In return, the model has been qualified to be calibrated to produce CARSOXY within two operability modes, with hydrogen and nitrogen or with ammonia as by-products.