LES Study of Influence of Obstacles on Turbulent Premixed Flames in a Small Scale Vented Chambers

The LES study reported in this paper presents the influence of number and position of the obstacles on turbulent premixed flames. LES simulations have been carried out for a stagnant, stoichiometric propane/air mixture, ignited from rest in a small laboratory scale, vented chamber, capable of rearranging into various configurations based on number and position of baffle plates. The novelty of the present study is two folded. First is the application of novel dynamic flame surface density (DFSD) model to account the sub-grid scale (SGS) chemical reaction rate in LES. Second is the arrangement of these configurations into four families, which facilitate a qualitative comparison with available experimental measurements. The concept of families also offers to understand the flame-flow interactions and the impact of number and position of the baffles with respect to ignition origin

LES Modelling of Propagating Turbulence Premixed Flames using a Dynamic Flame Surface Density Model

A Dynamic flame surface density (DFSD) model, developed recently from experimental images for transient turbulent premixed flames, is implemented and tested using the large eddy simulation (LES) modelling technique. Numerical predictions from DFSD model are compared with those predicted using the flame surface density (FSD) sub-grid scale (SGS) model for reaction rate. In the SGS-DFSD model, dynamic formulation of the reaction rate is coupled with the fractal analysis of the flame front structure. The fractal dimension is evaluated dynamically from an empirical formula based on the sub-grid velocity fluctuations. A laboratory scale combustion chamber with inbuilt solid obstacles is used for model validation and comparisons. The flame is initiated from igniting a stichiometric propane/air mixture from stagnation. The results obtained with the DFSD model are in good comparisons with experimental data and the essential features of turbulent premixed combustion are well captured. It has also been observed that the SGS-DFSD model for reaction rate found to capture the unresolved flame surface density contributions. Further investigations are planned to examine and validate of the SGS-DFSD for different flow geometries

In the blink of an eye: Value and novelty drive saccades

Evidence accumulation is an essential component of value-based decisions. Recent human studies suggest that overt attention correlates with evidence accumulation necessary for optimal decisions. However, the influence of covert attention on decision-making remains relatively unexplored. To investigate this issue, two monkeys were trained to perform a decision-making task where they chose between two stimuli, which were either ‘Overtrained’ or learned that day (‘Novel’). Subjects could freely saccade during choice evaluation and indicated their decision by moving a joystick. Saccades were made within 170 ms of stimulus presentation and were strongly driven by both value and novelty, implying covert stimulus evaluation prior to saccade. This effect was strongest for ‘Overtrained’ choices, but rapidly emerged during learning of ‘Novel’ choices. Though novel stimuli attracted initial saccades, final decisions were guided only by value; implying attentional value comparison processes are at least partially dissociable from value comparison processes that govern final decisions. While subjects made highly optimal decisions, they frequently viewed only one stimulus; final choice was thus best explained by assuming covert evidence accumulation. Our results suggest that the primate brain contains multiple value comparison systems for guiding attention toward highly valuable or novel information while simultaneously optimizing final decision value

Eulerian particle flamelet modelling of a bluff-body CH4/H2 flame

In this paper an axisymmetric RANS simulation of a bluff-body stabilized flame has been attempted using steady and unsteady flamelet models. The unsteady effects are considered in a postprocessing manner through the Eulerian particle flamelet model (EPFM). In this model the transient history of scalar dissipation rate, conditioned by stoichiometric mixture fraction, is required to generate unsteady flamelets and is obtained by tracing Eulerian particles. In this approach unsteady convective–diffusive transport equations are solved to consider the transport of Eulerian particles in the domain. Comparisons of the results of steady and unsteady calculations show that transient effects do not have much influence on major species, including OH, and the structure of the flame therefore can be successfully predicted by steady or unsteady approaches. However, it appears that slow processes such as NO formation can only be captured accurately if unsteady effects are taken into account, while steady simulations tend to overpredict NO. In this work turbulence has been modeled using the Reynolds stress model. Predictions of velocity, velocity rms, mean mixture fraction, and its rms show very good agreement with experiments. Performance of three detailed chemical mechanisms, the GRI Mech 2.11, the San Diego mechanism, and the GRI Mech 3.0, has also been evaluated in this study. All three mechanisms performed well with both steady and unsteady approaches and produced almost identical results for major species and OH. However, the difference between mechanisms and flamelet models becomes clearly apparent in the NO predictions. The unsteady model incorporating the GRI Mech 2.11 provided better predictions of NO than steady calculations and showed close agreement with experiments. The other two mechanisms showed overpredictions of NO with both unsteady and steady models. The level of overprediction is severe with the steady approach. GRI Mech 3.0 appears to overpredict NO by a factor of 2 compared to GRI Mech 2.11. The NO predictions by the San Diego mechanism fall between those of the two GRI mechanisms. The present study demonstrates the success of the EPFM model and when used with the GRI 2.11 mechanism predicts all flame properties and major and minor species very well, and most importantly the correct NO levels

Development of a user-friendly, low-cost home energy monitoring and recording system

This paper reports research undertaken to develop a user-friendly home energy monitoring system which is capable of collecting, processing and displaying detailed usage data. The system allows users to monitor power usage and switch their electronic appliances remotely, using any web enabled device, including computers, phones and tablets. The system aims to raise awareness of consumer energy use by gathering data about usage habits, and displaying this information to support consumers when selecting energy tariffs or new appliances. To achieve these aims, bespoke electrical hardware, or ‘nodes’, have been designed and built to monitor power usage, switch devices on and off, and communicate via a Wi-Fi connection, with bespoke software, the ‘server’. The server hosts a webpage which allows users to see a real-time overview of how power is being used in the home as well as allowing scheduled tasks and triggered tasks (which respond to events) to be programmed. The system takes advantage of well standardised networking specifications, such as Wi-Fi and TCP, allowing access from within the home, or remotely through the internet. The server runs under Debian Linux on a Raspberry Pi computer and is written in Python, HTML and JavaScript. The server includes advanced functionality, such as device recognition which allows users to individually monitor several devices that share a single node. The openPicus Flyport is used to provide Wi-Fi connectivity and programmable logic control to nodes. The Flyport is programmed with code compiled from C

Simulation of premixed combustion and near wall flame quenching in spark ignition engines with an improved formulation of the Bray-Moss-Libby model

Theoretical and experimental based modifications have been investigated, such that the BML model can be applied to wall-bounded combustion modelling eliminating the wall flame acceleration problem. Estimation of integral length scale of turbulence has been made dynamic so that allowance for spatial inhomogeneity of turbulence is made. A new dynamic formulation has been proposed based on the Kolmogorov- Petrovski-Piskunov analysis and fractal geometry to evaluate the mean flame wrinkling scale. In addition, a novel empirical correlation to quantify the quenching rates in the influenced zone of the quenching region near solid boundaries has been derived based on experimentally estimated flame image data. The proposed model was then applied to simulate the premixed combustion in spark ignition engines. Full cycle combustion in a Ricardo E6 engine for different operating conditions was simulated. Results show that the present improvements have been successful in eliminating the wall flame acceleration problem, while accurately predicting the in-cylinder pressure rise

An improved formulation of the Bray-Moss-Libby (BML) model for SI engine combustion modelling

In this paper an improved version of the BML model has been developed so that it could be applied to wall-bounded combustion modelling, eliminating the wall flame acceleration problem. Based on the Kolmogorov-Petrovski-Piskunov (KPP) analysis and fractal theory, a new dynamic formulation has been proposed to evaluate the mean flame wrinkling scale making necessary allowance for spatial inhomogeneity of turbulence. A novel empirical correlation has been derived based on experimentally estimated flame image data to quantify the quenching rates near solid boundaries. The proposed modifications were then applied to simulate premixed combustion in two spark ignition engines with different operating conditions. Results show that the present improvements have been successful in eliminating the wall flame acceleration problem found with the original BML model, while accurately predicting the in-cylinder pressure rise, mass burn rates and heat release rates

Effects of radiation on predicted flame temperature and combustion products of a burning liquid fuel spray

The effects of radiative heat transfer calculations on the predicted temperature rise in a burning liquid-fuel spray, are studied. The adiabatic temperature rise resulting from a comprehensive spray combustion model is adjusted for heat transfer to the chamber cooling water by incorporating a radiation model using the Discrete Transfer Technique. The spray combustion model used is of the ‘mixed-is-burnt’ type where combustion is treated as a post process event. The data needed for the combustion post-processor are obtained from an effective property Locally Homogeneous CFD flow model, incorporating a droplet evaporation model to account for the liquid phase. The combustion model itself is based on the minimisation of Gibbs free energy and incorporates kinetic sub-modules for soot formation and oxidation. The results from the combustion model are fed into a radiation sub model for calculating cell emmisivities. These are used to calculate corrective terms for incorporation within the energy balance employed by the combustion model resulting in corresponding temperature (and, subsequently, composition) corrections. The convergence of this iterative process yields results of product concentrations and of temperature throughout the combustion chamber. The predicted results are compared with existing experimental result in a case study. The results are also compared with those obtained from the combustion model with no radiation correction and also with ones obtained with empirical corrections

Numerical study of bluff-body non-premixed flame structures using laminar flamelet model

A laminar flamelet model is applied for bluff-body stabilized flames to study the flow field, mixing pattern, and the flame structure at two different velocities. The k 1 turbulence model is applied for accounting the turbulence fluctuations. It is found that the recirculation zone dominates the near field, while the far field structure is similar to the jet flow. The intermediate neck zone is the intense mixing region. The computation shows that the fuel jet velocity has significant effect on the structure of the flow field, which in turn has significant effect on the combustion characteristics. The laminar flamelet model is found to be adequate for simulating the temperature and the flame composition inside the recirculation zone. The flamelet model has, however, failed to account for the local extinction in the neck zone. Possible limitation of the laminar flamelet model to predict the local extinction is discussed

Simulation of engine combustion with ethanol as a renewable fuel

Ethanol as a fuel is an important bio-fuel for future energy needs. In this work the combustion process of gasoline-ethanol blends in spark ignition engines was investigated using computational fluid dynamics and turbulent combustion modeling. A modified flame surface density approach developed for gasoline engine combustion was adapted to calculate fuel-burning rate of the blend. The rise in in-cylinder peak pressure and temperature for blends up to E20 was found relatively small compared to E00. A significant reduction of CO and an increment of NOx were observed for optimized combustion with adjusted ignition timing