Lean premixed combustion is one of the most successful technologies for flame control in low NOx systems. The characteristics of these flows its good mixing performance, stability and the low emissions. The potential of using new alternative fuels presents a problem in terms due to heating value changes, flame parameters and reactivity. Bio-renewable processes and industrial systems requiring waste gases are just a few examples. The biggest challenge to fuel-flexibility is the large differences between natural gas and the proposed alternative fuels which causes variations in the stability profiles of the combustion process. In this paper, combustion of CH4/H2/CO mixtures was experimentally and numerically studied to understand the impacts of these fuels on the blowoff process. Atmospheric pressure and ambient temperature were used at moderate swirl number. Various nozzles were used to determine the impact of the blends on the Central Recirculation Zones. Methane content in the fuel was decreased from 50% to 0% (by volume) with the remaining amount split equally between carbon monoxide and hydrogen. The Central Recirculation Zone and its turbulence were numerically characterised using the k-ω turbulence model providing details of the structure close to blowoff. The results show how the strength and size of the recirculation zone are highly influenced by the blend, carbon/hydrogen ratio, nozzle geometry and Re numbers.
No abstract is provided for this article.
The use of spherically propagating flames is common for measuring the laminar flame speed in NH3-air, NH3/CH4-air and NH3/H2-air mixtures. However, the radiation-induced uncertainty in such mixtures has not been thoroughly investigated. Due to the low laminar flame speed of ammonia mixtures, it is anticipated that the radiation effect is considerable for such mixtures. This study aims to fill this gap by conducting numerical simulations using different chemical mechanisms and the adiabatic and optical thin radiation models to examine the effects of radiation on spherically propagating NH3-air, NH3/CH4-air and NH3/H2-air flames. The simulations are performed for mixtures at normal temperature and pressure (Tu=298 K and P = 1 atm) and wide range of equivalence ratios. The radiation-induced uncertainty in spherical flames is quantified and compared to planar flames. The importance of the radiation-induced flow and thermal effects in spherical flames is compared between different mixtures and a correlation is developed to determine the radiation-corrected flame speed for spherical NH3-air flames. Considering the radiation effect in NH3-air, it was found that using different mechanisms results in considerable discrepancies in laminar flame speed determination. Some mechanisms showed that the radiation-induced flame speed in spherical flames was underpredicted by more than two times compared to planar flames, and the radiation-induced uncertainty for lean and rich spherically propagating NH3-air flames exceeds 20%. However, the radiation-induced uncertainty at normal temperature and pressure in spherically propagating NH3/CH4-air and NH3/H2-air flames was less significant, not exceeding 11%. Finally, an updated correlation is proposed to determine the radiation-corrected flame speed for NH3-air flames that can be directly used in spherical flame experiments measuring the laminar flame speed.
No abstract is provided for this article.
The first EU Renewable Energy Directive (RED) served as an effective push for world-wide research efforts on biofuels and bioliquids, i.e. liquid fuels for energy purposes other than for transport, including electricity, heating, and cooling, which are produced from biomass. In December 2018 the new RED II was published in the Official Journal of the European Union. Therefore, it is now the right time to provide a comprehensive overview of achievements and practices that were developed within the current perspective. To comply with this objective, the present study focuses on a comprehensive and systematic technical evaluation of all key aspects of the different distributed energy generation pathways using bioliquids in reciprocating engines and micro gas turbines that were overseen by these EU actions. Methodologically, the study originates from the analyses of feedstock and fuel processing technologies, which decisively influence fuel properties. The study systematically and holistically highlights the utilisation of these bioliquids in terms of fuel property specific challenges, required engine adaptations, and equipment durability, culminating in analyses of engine performance and emissions. In addition, innovative proposals and future opportunities for further technical improvements in the whole production-consumption cycle are presented, thus serving as a guideline for upcoming research and development activities in the fast-growing area of bioliquids. Additionally, the paper systematically addresses opportunities for the utilisation of waste streams, emerging from the ever increasing circular use of materials and resources. With this, the present review provides the sorely needed link between past efforts, oriented towards the exploitation of bio-based resources for power generation, and the very recent zero-waste oriented society that will require a realistic exploitation plan for residuals originating from intensive material looping.
The idea of using ammonia as fuel is nothing new as the first well-known Belgium use of buses fleet during World War II. Even if several studies performed during the mid-60’s investigated the possibility to consider ammonia as fuel for internal combustion...
The consumption of fossil fuels and their greenhouse emissions have increased research to develop new mechanisms for the generation of energy and a variety of industrial processes. However, many of the mechanisms under scrutiny have only been measured by indirect simulations. Swirling flows represent this category. Even thought they have been extensively analyzed, there are many uncertainties concerned to their behavior, especially those related to coherent structures and their relationship to the Central Recirculation Zone (CRZ), which is responsible for enhanced mixing and combustion stability. Although extensive programs on the coherent structures generated have been developed, structures such as the Precessing Vortex Core (PVC) and indeed the CRZ remain barely understood. Recent work using numerical simulations (DNS, LES, etc.) have predicted relationships between the PVC and CRZ. The problem lies in the lack of detailed experimental data to validate the results of the interaction, since the measurement has only been done using indirect techniques. Therefore, this paper adopts the approach of producing direct fundamental data on these structures in order to visualize the phenomenon under different conditions. Here the effects of combustion are ignored. Phase Locked Particle Image Velocimetry (PIV) provides results about the interaction of the PVC and CRZ, giving details of shape and dependence on non-dimensional parameters. Various cases were analyzed to find operational regions where strong perturbations occurre d, which lead to the selection of cases that were inspected in detail. 3D holograms were produced in MatLab showing the real spatial interaction of these large structures. Shapes and interactions between different structures are compared and discussed in order to detail the relationship shared between different cases. The most stable and recognizable configuration was analyzed in detail under unconfined and confined conditions. The use of different triggering levels confirmed the accuracy of the technique, with implications of using different signals filters for future projects. No bifurcations or major perturbations were observed during the process, but the appearance of new structures made evident the high correlation between the geometry and type of flow in the burner. The results refuted the normally assumed helical shape of the PVC with a complex spiraling mechanism being revealed. The CRZ was shown in fact to consist of two normally separate, but intertwined CRZs both of which interacted with the PVC.
Lean premixed combustion using swirl flame stabilisation is widespread amongst gas turbine manufacturers. The use of swirl mixing and flame stabilisation is also prevalent in many other non-premixed systems. Problems that emerge include loss of stabilisation as a function of combustor geometry and thermo-acoustic instabilities. Coherent structures and their relationship with combustion processes have been a concern for decades due to their complex nature. This paper thus adopts an experimental approach to characterise large coherent structures in swirl burners under isothermal conditions so as to reveal the effects of swirl in a number of geometries and cold flow patterns that are relevant in combustion. Aided by techniques such as Hot Wire Anemometry, High Speed Photography and Particle Image Velocimetry, the recognition of several structures was achieved in a 100kW swirl burner model. Several varied, interacting, structures developed in the field as a consequence of the configurations used. New structures never observed before were identified, the results not only showing the existence of very well defined large structures, but also their dependency on geometrical and flow parameters. The PVC is confirmed to be a semi-helical structure, contrary to previous simulations performed on the system. The appearance of secondary recirculation zones and suppression of the vortical core as a consequence of geometrical constrictions are presented as a mechanism of flow control. The asymmetry of the Central Recirculation Zone in cold flows is observed in all the experiments, with its elongation dependent on Re and swirl number used.
There is growing interest in the application of renewably-generated NH3 to support future energy requirements, however combustor designs and strategies require considerable development to reduce NOx emissions in particular. A turbulent swirl burner was used to experimentally and numerically appraise potential pathways for operational NOx reduction with a premixed NH3/H2/air flame. Reactants were supplied at elevated temperature with parametric changes made to pressure and humidity. Favourable agreement was demonstrated between exhaust gas measurements and chemical kinetic simulations with a reactor network model, showing NOx emissions to be sensitive to operational equivalence ratio, increasing by several orders of magnitude across the experimental range. The lowest NOx concentrations were achieved at the richest conditions, accompanied by high unburned fuel fractions in the product stream. An increase in combustor pressure remarkably reduced exhaust NOx concentrations primarily due to enhanced NH2 formation, and subsequent NO consumption in the post-flame zone. Reactant humidification was explored in detail for the first time with this fuel, and shown to reduce NOx production limiting thermal pathways with the extended Zel'dovich mechanism. NO consumption in the post-flame zone was also enhanced through an increase in OH-produced NH2, and together with pressure, resulted in elevated exhaust NH3 concentrations. Whilst this effect was comparatively small, it meant that leaner humidified operation could be employed to reduce unburned fuel fractions without a NOx penalty. Emissions performance was further improved by the application of staged combustion, with secondary airflow used to improve fuel burnout. Humidity and pressure were optimised in the staged configuration to achieve operation with sampled respective NOx and NH3 exhaust fractions of 32 and 50 ppmvd (15%O2), at a globally lean equivalence ratio. There is considerable scope for further system optimisation through improved mixing of secondary air and increased ambient pressure.
