Supersonic combustion modelling using the conditional moment closure approach

This work presents a novel algorithm for supersonic combustion modelling. The method involved coupling the Conditional Moment Closure (CMC) model to a fully compressible, shock capturing, high-order flow solver, with the intent of modelling a reacting hydrogen-air, supersonic jet. Firstly, a frozen chemistry case was analysed to validate the implementation of the algorithm and the ability for CMC to operate at its frozen limit. Accurate capturing of mixing is crucial as the mixing and combustion time scales for supersonic flows are on the order of milliseconds. The results of this simulation were promising even with an unexplainable excess velocity decay of the jet core. Hydrogen mass fractions however, showed fair agreement to the experiment. The method was then applied to the supersonic reacting case of ONERA. The results showed the method was able to successfully capture chemical non-equilibrium effects, as the lift-off height and autoignition time were reasonably captured. Distributions of reactive scalars were difficult to asses as experimental data was deemed to be very inaccurate. As a consequence, published numerical results for the same test case were utilised to aid in analysing the results of the presented simulations. Due to the primary focus of the study being to assess non-equilibrium effects, the clustering of the computational grid lent itself to smeared and lower magnitude wall pressure distributions. Nevertheless, the wall pressure distributions showed good qualitative agreement to experiment. The primary conclusions from the study were that the CMC method is feasible to model supersonic combustion. However, a more detailed analysis of sub-models and closure assumptions must be conducted to assess the feasibility on a more fundamental level. Also, from the results of both the frozen chemistry and the reacting case, the effects of assuming constant species Lewis number was visible

Direct numerical simulation of a high-pressure hydrogen micromix combustor: flame structure and stabilisation mechanism

A high-pressure hydrogen micromix combustor has been investigated using direct numerical simulation with detailed chemistry to examine the flame structure and stabilisation mechanism. The configuration of the combustor was based on the design by Schefer [1], using numerical periodicity to mimic a large square array. A precursor simulation of an opposed jet-in-crossflow was first conducted to generate appropriate partially-premixed inflow boundary conditions for the subsequent reacting simulation. The resulting flame can be described as a predominantly-lean inhomogeneously-premixed lifted jet flame. Five main zones were identified: a jet mixing region, a core flame, a peripheral flame, a recirculation zone, and combustion products. The core flame, situated over the jet mixing region, was found to burn as a thin reaction front, responsible for over 85% of the total fuel consumption. The peripheral flame shrouded the core flame, had low mean flow with high turbulence, and burned at very lean conditions (in the distributed burning regime). It was shown that turbulent premixed flame propagation was an order-of-magnitude too slow to stabilise the flame at these conditions. Stabilisation was identified to be due to ignition events resulting from turbulent mixing of fuel from the jet into mean recirculation of very lean hot products. Ignition events were found to correlate with shear-driven Kelvin-Helmholtz vortices, and increased in likelihood with streamwise distance. At the flame base, isolated events were observed, which developed into rapidly burning flame kernels that were blown downstream. Further downstream, near-simultaneous spatially-distributed ignition events were observed, which appeared more like ignition sheets. The paper concludes with a broader discussion that considers generalising from the conditions considered here

2023 Roadmap on ammonia as a carbon-free fuel

The 15 short chapters that form this 2023 ammonia-for-energy roadmap provide a comprehensive assessment of the current worldwide ammonia landscape and the future opportunities and associated challenges facing the use of ammonia, not only in the part that it can play in terms of the future displacement of fossil-fuel reserves towards massive, long-term, carbon-free energy storage and heat and power provision, but also in its broader holistic impacts that touch all three components of the future global food-water-energy nexus

Investigation of numerical resolution requirements of the Eulerian Stochastic Fields and the Thickened Stochastic Field approach

The stochastic fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the stochastic fields equations that require grid spacing much finer than the filter scale used for the Large Eddy Simulation. An investigation into numerical resolution requirements is conducted through simulation of a series of one-dimensional stochastic fields simulations of freely-propagating turbulent premixed flames. The investigation involved various stochastic field simulations at different combustion regimes and numerical resolutions. It was concluded that the conventional approach of using a numerical grid spacing equal to the filter scale can yield substantial numerical error; specifically towards the flamelet regime. However, using a numerical grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Thickened Stochastic Fields approach is developed in this thesis in order to provide physically and numerically-accurate solutions of the stochastic fields equations with reduced compute time compared to a fully resolved simulations. The Thickened Stochastic Fields formulation bridges between the conventional stochastic fields and conventional Thickened-Flame approaches depending on the sub-filter combustion regime and numerical grid spacing utilised. One-dimensional stochastic fields simulations of freely-propagating turbulent premixed flames are used in order to obtain a criteria for the thickening factor required as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the stochastic fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory premixed Bunsen flame. The results demonstrate that the Thickened Stochastic Fields method produces accurate predictions even when using a grid spacing equal to the filter scale

A thickened stochastic fields approach for turbulent combustion simulation

The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddys Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Partially-Thickened Stochastic Fields approach is developed in this study in order to provide physically accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Partially-Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised, and converges towards Direct Numerical Simulation in the limit of full-resolution. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory Bunsen flame, demonstrating that the method leads to numerically-converged simulations that agree with results of conventional Stochastic Fields simulations using orders of magnitude more grid points. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems

Resolution requirements in stochastic field simulation of turbulent premixed flames

The spatial resolution requirements of the Stochastic Fields probability density function approach are investigated in the context of turbulent premixed combustion simulation. The Stochastic Fields approach is an attractive way to implement transported Probability Density Function modelling into Large Eddy Simulations of turbulent combustion. In premixed combustion LES, the numerical grid should resolve flame-like structures that arise from solution of the Stochastic Fields equation. Through analysis of Stochastic Fields simulations of a freely-propagating planar turbulent premixed flame, it is shown that the flame-like structures in the Stochastic Fields simulations can be orders of magnitude narrower than the LES filter length scale, implying that the usual practice of setting the LES filter length scale equal to grid spacing leads to severe under-resolution, to numerical thickening of the flame, and to substantial error in the turbulent flame speed. The under-resolution is worst for low Karlovitz number combustion, where the thickness of the Stochastic Fields flame structures is similar to the laminar flame thickness. The effect of resolution on LES predictions is then assessed by performing LES of a laboratory Bunsen flame and comparing the effect of refining the grid spacing and filter length scale independently. The Bunsen flame LES results confirm that setting the LES filter length scale equal to the grid spacing gives substantial numerical error, and that this error affects the Stochastic Fields solution to a greater extent than it affects the flow field solution. The present results have important implications for application of the Stochastic Fields approach to simulation of high-pressure industrial premixed combustion systems where the grid spacing is necessarily much larger than the laminar flame thickness and suggests that some amount of artificial flame thickening might be needed in order to make such simulations numerically accurate

A thickened stochastic fields approach for turbulent combustion simulation

The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddys Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Partially-Thickened Stochastic Fields approach is developed in this study in order to provide physically accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Partially-Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised, and converges towards Direct Numerical Simulation in the limit of full-resolution. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory Bunsen flame, demonstrating that the method leads to numerically-converged simulations that agree with results of conventional Stochastic Fields simulations using orders of magnitude more grid points. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems

Data for "Resolution Requirements in Stochastic Field Simulation of Turbulent Premixed Flames"

Raw data used in production of the published article (also presented at Mediterranean Combustion Symposium, Naples, Italy): Picciani, M. A., Richardson, E. S., &amp; Navarro-martinez, S. (2018). Resolution requirements in stochastic field simulation of turbulent premixed flames. Flow Turbulence and Combustion, 1-16. </span

Data for &quot;A Thickened Stochastic Fields Approach For Turbulent Combustion Simulation&quot;

Raw data used for preparation of the published manuscript (also presented at Tenth Mediterranean Combustion Symposium, Napoli, Italy.): Picciani, M. A., Richardson, E. S., &amp; Navarro-martinez, S. (2018). A thickened stochastic fields approach for turbulent combustion simulation. Flow Turbulence and Combustion, 1-18.</span

Experimental Characterisation of the Dynamics of Partially Premixed Hydrogen Flames in a Lean Direct Injection (LDI) Combustor

Hydrogen continues to show significant promise as a zero-carbon energy carrier in the pursuit of global decarbonisation targets. Hydrogen has wide flammability limits which means it can operate at considerably leaner conditions for reduced NOx emissions. However, fuel-lean operation makes these systems more susceptible to thermoacoustic instabilities and flame blow-off. Combustor configurations such as jet-in-crossflow are gaining popularity in industry for 100% hydrogen as they can help mitigate risk of flashback, but detailed characterisation of flame dynamics is still necessary. In this study, the combustion dynamics of partially premixed hydrogen flames in a lean direct injection (LDI) multi-cluster combustor were investigated at atmospheric conditions. The combustor inlet consisted of nine circular air channels, with hydrogen injected inwards through two diametrically opposite holes into each air channel. Dynamic pressure and OH* chemiluminescence measurements were employed to study the effect of varying key parameters, such as Reynolds number and global equivalence ratio, on combustor dynamics. High-speed OH-PLIF imaging was conducted to understand flame dynamics. The results showed that self-excited oscillations were observed at all tested conditions and the dynamical behaviour of the combustor was complex with strong dependency on global equivalence ratio and bulk velocity conditions. The magnitude of self-excited thermoacoustic oscillations initially increased with a decrease in global equivalence ratio, but subsequently decreased at leaner conditions (below global equivalence ratio 0.3). Similar observations were noted for all bulk velocities. High speed OH-PLIF imaging indicated that the heat release oscillations were influenced by vortex-flame roll up and possible global lean extinction events. The results from this work have the potential to inform design efforts towards development of new architectures for stable, low-emission 100% hydrogen combustors