Two droplets interaction on substrate

Dynamics of two droplets interactions on a substrate (droplet impact on a sessile droplet) are numerically investigated using OpenFOAM. The impact speed, location of the impacting droplet, viscosity and surface tension were varied in the numerical studies. We found that when the surface tension dominates the flow, the mass canter of two droplets moves to impacting droplet side. When the inertia dominates the flow, the mass centre moves to the opposite direction

A study on reduced chemical mechanisms of ammonia/methane combustion under gas turbine conditions

As an alternative fuel and hydrogen carrier, ammonia is believed to have good potential for future power generation. To explore the feasibility of co-firing ammonia with methane, studies involving robust numerical analyses with detailed chemistry are required to progress towards industrial implementation. Therefore, the objective of this study is to determine a reduced mechanism for simulation studies of ammonia/methane combustion in practical gas turbine combustor conditions. Firstly, five different sized reduced mechanisms of the well-known Konnov’s mechanism were compared. The reduced mechanisms were tested for ignition delay time validation (0D) using ammonia/methane mixtures at high pressure conditions relevant to gas turbine devices. Furthermore, the combustion products of ammonia/methane premixed laminar flames (1D) were validated with the results from the full Konnov’s mechanism. Finally, CFD simulations of a turbulent flame (2D) with all the reduced mechanisms were performed under high temperature and high pressure conditions representative of industrial systems. Results show that several of the reduced mechanisms utilized performed reasonably well in combustion simulation studies under gas turbine conditions. Hence a reaction mechanism with 48 species and 500 elementary reactions is recommended for future studies

Coherent structure impacts on blowoff using various syngases

Swirl stabilized combustion is one of the most successful technologies for flame and nitrogen oxides control in gas turbines. However, complex fluid dynamics and lean conditions pose a problem for stabilization of the flame. The problem is even more acute when alternative fuels are used for flexible operation. Although there is active research on the topic, there are still various gaps in the understanding of how interaction of large coherent structures during the process affect flame stabilization and related phenomena. Thus, this paper approaches the phenomenon of lean premixed swirl combustion of CH4/H2/CO blends to understand the impacts of these fuels on flame blowoff. An atmospheric pressure generic swirl burner was operated at ambient inlet conditions. Different exhaust nozzles were used to alter the Central Recirculation Zone and observe the impacts caused by various fuel blends on the structure and the blowoff phenomenon. Methane content in the fuel was decreased from 50% to 10% (by volume) with the remaining amount split equally between carbon monoxide and hydrogen. Experimental trials were performed using Phase Locked PIV. The Central Recirculation Zone and its velocity profiles were measured and correlated 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 fuel blend, changing stability based on the carbon-hydrogen ratios. Nozzle effects on the shear flow and Re numbers were also observed. Modelling was carried out using the k-ω SST CFD model which provided more information about the impact of the CRZ and the flame nature close to blowoff limit. It was observed that the model under-predicts coherent structure interactions at high methane fuel content, with an over-prediction of pressure decay at low methane content when correlated to the experimental results. Thus, complex interactions between structures need to be included for adequate power prediction when using very fast/slow syngas blends under lean conditions

Two droplets interaction on substrate

Dynamics of two droplets interactions on a substrate (droplet impact on a sessile droplet) are numerically investigated using OpenFOAM. The impact speed, location of the impacting droplet, viscosity and surface tension were varied in the numerical studies. We found that when the surface tension dominates the flow, the mass canter of two droplets moves to impacting droplet side. When the inertia dominates the flow, the mass centre moves to the opposite direction

Characterisation of mist generation through cloud chamber technology

This paper develops understanding and appropriate techniques for characterising mist generation from super-saturated liquid-air systems. Whilst the technology for this technique originates from Wilson (1897), to date mainly a qualitative understanding of the relationship between mist characteristics and initial control conditions exists. Here, an improved design of cloud chamber, which facilitates accurate control, is described, and temporal measurement techniques for thermodynamic control parameters are proposed and appraised. Mist characteristics are quantified using transient laserdiffraction measurements. Expansion is described by a polytropic thermodynamic process with appropriate coefficients, and the influence on mist generation of primary control parameters expansion rate, expansion ratio and initial temperature are quantified and analysed. Applications include fundamental studies of two-phase combustion, quenching explosions by ultra-fine water mists and well as the traditional meteorological interest

Shoot yield drives phosphorus use efficiency in Brassica oleracea and correlates with root architecture traits

The environmental and financial costs of using inorganic phosphate fertilizers to maintain crop yield and quality are high. Breeding crops that acquire and use phosphorus (P) more efficiently could reduce these costs. The variation in shoot P concentration (shoot-P) and various measures of P use efficiency (PUE) were quantified among 355 Brassica oleracea L. accessions, 74 current commercial cultivars, and 90 doubled haploid (DH) mapping lines from a reference genetic mapping population. Accessions were grown at two or more external P concentrations in glasshouse experiments; commercial and DH accessions were also grown in replicated field experiments. Within the substantial species-wide diversity observed for shoot-P and various measures of PUE in B. oleracea, current commercial cultivars have greater PUE than would be expected by chance. This may be a consequence of breeding for increased yield, which is a significant component of most measures of PUE, or early establishment. Root development and architecture correlate with PUE; in particular, lateral root number, length, and growth rate. Significant quantitative trait loci associated with shoot-P and PUE occur on chromosomes C3 and C7. These data provide information to initiate breeding programmes to improve PUE in B. oleracea

Laminar flame speed and markstein length characterisation of steelworks gas blends

An outwardly propagating spherical flame configuration has been used to characterise the combustion of different blended steelworks gas compositions, under atmospheric ambient conditions. A nonlinear extrapolative technique was used to obtain values of laminar burning speed and Markstein length for combustion with air and change in equivalence ratio. Peak burning speed was shown to reach almost 1 m s−1 for the combustion of coke oven gas under marginally rich conditions, and the influence of flame stretch on burning speed also shown to increase with equivalence ratio. The molar fraction of coke oven gas (COG) was then blended in the range 0–15% with four blast furnace gas mixtures (BFG) containing 1–7% H2 fractions, representative of the inherent compositional fluctuation experienced in production. Profiles for change in burning speed resulting from this addition of COG are presented, and the dampening extent of fluctuation resulting from the H2 variation has been quantified. Results are also presented for the relative change in gross calorific value and corresponding Wobbe index of the variable blended gases across the tested limits. Modelled results were generated using the PREMIX coded CHEMKIN-PRO, and the performance of specified chemical reaction mechanisms evaluated relative to the experimental data

On flammability hazards from pressurised high-flashpoint liquid releases

Hazardous area classification is well established for dust and vapours, however this is not the case for high flashpoint liquid fuels. This study highlights the limitations of current guidance in relation to flammable mists, through demonstration of flammability of a representative high flashpoint fuel for releases in the range of representative industrial operating pressure, complemented by a phenomenological analysis and semi-quantification of the results observed. Flammability results are presented from low-pressure practical releases ( 61 °C), through a plain orifice, at temperatures well below its flashpoint. Based on a proposed two-phase flow-regime diagram, a semi-quantitate analysis of the results observed is offered via a simple 1-D phenomenological model, accommodating jet breakup length, spray quality, air entrainment and droplet dynamics. The complex scenario of liquid releases impinging onto an unheated flat surface is also considered. An impingement model is utilised to show the relative increase in volume of fine secondary spray induced post-impingement relative to the unobstructed case, resulting in a significant volume of flammable mist. This is demonstrated experimentally by showing flammability of a 5 barg release post impingement whereas the unobstructed 10 barg case would not ignite

The effect of hydrogen containing fuel blends upon flashback in swirl burners

Lean premixed swirl combustion is widely used in gas turbines and many other combustion Processes due to the benefits of good flame stability and blow off limits coupled with low NOx emissions. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends, especially those derived from gasification of coal and/or biomass/industrial processes such as steel making, cause concerns in this area. Thus, this paper describes a practical experimental approach to study and reduce the effect of flashback in a compact design of generic swirl burner representative of many systems. A range of different fuel blends are investigated for flashback and blow off limits; these fuel mixes include methane, methane/hydrogen blends, pure hydrogen and coke oven gas. Swirl number effects are investigated by varying the number of inlets or the configuration of the inlets. The well known Lewis and von Elbe critical boundary velocity gradient expression is used to characterise flashback and enable comparison to be made with other available data. Two flashback phenomena are encountered here. The first one at lower swirl numbers involves flashback through the outer wall boundary layer where the crucial parameter is the critical boundary velocity gradient, Gf. Values of Gf are of similar magnitude to those reported by Lewis and von Elbe for laminar flow conditions, and it is recognised that under the turbulent flow conditions pertaining here actual gradients in the thin swirl flow boundary layer are much higher than occur under laminar flow conditions. At higher swirl numbers the central recirculation zone (CRZ) becomes enlarged and extends backwards over the fuel injector to the burner baseplate and causes flashback to occur earlier at higher velocities. This extension of the CRZ is complex, being governed by swirl number, equivalence ratio and Reynolds Number. Under these conditions flashback occurs when the cylindrical flame front surrounding the CRZ rapidly accelerates outwards to the tangential inlets and beyond, especially with hydrogen containing fuel mixes. Conversely at lower swirl numbers with a modified exhaust geometry, hence restricted CRZ, flashback occurs through the outer thin boundary layer at much lower flow rates when the hydrogen content of the fuel mix does not exceed 30%. The work demonstrates that it is possible to run premixed swirl burners with a wide range of hydrogen fuel blends so as to substantially minimise flashback behaviour, thus permitting wider used of the technology to reduce NOx emissions