Turbulent Combustion Processes Research Articles

Mathematical Modelling of Pool Fire Burning Rates in a Full- Scale Ventilated Tunnel

A computational fluid dynamic model with full coupling between gaseous and liquid phases is developed to predict buring rates of liquid pool fires in ventilated full-scale tunnel. Rates of fuel release are calculated using predictions of flame feedback to the surface of the pool. A pool fire in tunnel is modelled as an unsteady process, from the time of ignition until convergence to a quasi-steady burning rate. This feedback supports sustained flame above the pool surface and controls the burning rate of the fuel. The numerical model solves three dimensional, time-dependent Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. Turbulent combustion process is modelled by an Eddy Dissipation Concept (EDC) by using two chemical reaction steps to CO prediction. The numerical model is shown to possess the ability to predict the effect of ventilation on burning rate and the initial growth period in a fullscale tunnel fire. The current study indicates that CO generation is relatively independent of position in the overfire region, and correlated solely as a function of mixture fraction. While no correlation of soot concentrations in terms of the mixture fraction is found. Abundant CO and soot are formed around the fire base, which is later deflected near the tunnel ceiling, and the backflow brings about the toxic products with a noticeable smoke stratification as the airflow velocity is below a critical value.

Numerical simulations of turbulent jet flames with non-premixed combustion of hydrogen-enriched fuels

Large-eddy simulation is conducted to study the turbulent combustion processes of hydrogen and syngas flames. The subgrid scale momentum transport is performed with a one-equation model using the subgrid scale turbulent kinetic energy, while the linear-eddy model is employed to represent the scalar transport. Reduced chemical kinetics is used to describe the flame chemistry with the extended Zeldovich mechanism to account for the thermal formation of NOx. Results show the effects of the hydrogen content and of the Reynolds number on the characteristics of the flames. Higher hydrogen content contributes to increase the heat released after combustion leading to higher temperature peaks and thicker shear layers. The presence of CO in the fuel stream affects the flame dynamics with the development of a more vortical and wrinkled flow field. The vorticity field and turbulence/chemistry interactions are also discussed. Scalar profiles and comparison against experimental data are presented where a reasonable agreement is observed.

Effect of Swirl Flow on Flame Structure and Stability in Turbulent Combustion

In general, a turbulent flame is observed in most practical combustors. As for the premixed combustion, by controlling the equivalence ratio of fuel and air, it is possible to suppress the emission of harmful substances such as NOx. However, the turbulent combustion process is very complex. Since there is unsteady flame and flow field interaction, it is very difficult to understand the combustion phenomena. To handle such a complex situation, the numerical cost may be too large. Then, by experiments, it is important to investigate turbulent combustion phenomena by laser diagnostics. In this study, we conducted a PIV/OH-PLIF simultaneous measurement of turbulent premixed flames. To focus on the effect of swirl flow on flame structure and stability, we used a cyclone-jet combustor. The extinction limit as well as CO and NOx emissions was experimentally obtained. By changing the flow condition of cyclone nozzles, we discussed the flow field with turbulence and local flame extinction.

A swirler stabilized combustion chamber for a micro-gas turbine fuelled with natural gas

Micro-gas turbines are a good alternative for on-site power generation, since their operation is very reliable. The possibility of operating with various fuels increases versatility and, as a result, the usage of these devices. Focusing on a performance improvement of a tri-fuel low-cost micro-gas turbine, this work presents investigations of the inner flow of its combustion chamber. The aim of this analysis was the characterization of the flame structure by the temperature field of the chamber inner flow. The chamber was fuelled with natural gas. In the current chamber, a swirler and a reversed flow configuration were utilized to provide flame stabilization. The inner flow investigations were done with numerical analysis, which were compared to experimental data. The analysis of the inner flow was done with numerical simulations, which used the RSM turbulence model. A β-PDF equilibrium model was adopted to account for the turbulent combustion process. Different models of heat transfer were compared. Thermal radiation and specially heat conduction in the liner walls played significant roles on results.

Effect of High-Amplitude Forcing on Turbulent Combustion Intensity and Vortex Core Precession in a Strongly Swirling Lifted Propane/Air Flame

The present work reports stereoscopic particle image velocimetry (PIV) measurements in a strongly swirling nonreacting jet and partially premixed lifted flame. The spatial distributions of the average velocity and components of turbulent kinetic energy were calculated from the measured ensembles of the instantaneous velocity fields. A pronounced bubble-type vortex breakdown was observed for the studied flows. Based on proper orthogonal decomposition (POD) of the PIV data and on estimates of velocity fluctuation spectra by a laser Doppler velocimetry (LDV) probe, it was concluded that the combustion did not fundamentally affect the type of coherent structures in the strongly swirling flow: a pair of secondary helical vortices was induced by a precessing vortex core in both cases. Because a strongly swirling jet flow is usually insensitive to weak forcing, strong perturbations were superimposed on the flow bulk velocity to force the formation of ring-like vortices in the flow and to investigate the possible outcomes on the turbulent combustion process. The forcing frequency was below that of the precession. Based on the CH* chemiluminescence signal, it was observed that the forcing provided an increase in the turbulent combustion rate near the flame onset, as the entrainment of ambient air to the rich mixture must have increased. Moreover, for a forcing amplitude typically above the magnitude of reverse flow inside the bubble-type recirculation zone, a dramatic suppression of the vortex core precession took place in the reacting case. This effect was accompanied by a quasi-periodical vanishing of the recirculation zone due to interaction of the forced ring-like vortices with the lifted flame.

Investigation of the 3D flow field in an IC engine using tomographic PIV

In-cylinder measurements of the instantaneous 3D flow field are required for further understanding of turbulent mixing and combustion processes in internal combustion (IC) engines as well provide valuable data for the development of large eddy simulation (LES) models for combustion simulation. Tomographic particle image velocimetry (TPIV) is applied within a motored direct-injection spark-ignition (DISI) optical engine to measure the instantaneous volumetric flow field at selected crank-angle degrees (CAD) during the intake and compression stroke. Measurements were conducted using a double-pulse Nd:YAG laser with an average pulse energy of 375mJ for illumination and four CCD cameras arranged circularly around the optically accessible cylinder. A volume of 47×35×4mm3 was imaged located centrally between the intake valves. The uncertainty of the 3D velocity data was addressed using the principle of mass conservation for the inlet flow. A precision within 9% was found for the calculation of the velocity gradient tensor and is comparable to experiments in generic configurations, indicating that TPIV measurements within a motored IC engine are feasible. Based on the volumetric velocity data, the average structure of the flow field was analyzed showing a clear orientation of the average velocity, which changes during the different phases of the cycle. The 3D turbulent kinetic energy (TKE) is computed and compared to the 2D-TKE determined from in-plane velocity components and reveals distinct regions of large out-of-plane velocity fluctuations. Results from volumetric velocimetry furthermore provides insight into the occurrence and orientation of vortical structures that here are identified by using the Q-criterion. Differences between flow structures during intake and compression strokes are discussed on basis of ensemble-averaged and individual cycle velocity maps as well as the full vorticity vector.

Numerical simulation of turbulent flame–wall quenching using a coherent flame model

Flame quenching by fine mesh is one of the oldest known methods for mitigating flame propagation. Sir Humphry Davy pioneered the study of flame–wall quenching while developing the mining safety lamp. Installing a quenching mesh around the equipment is an effective preventive technique against hazardous flame propagation. Quenching of flame at the wall is due to the coupled thermo physical process involving heat transfer, flame stretch and preferential diffusion. While laminar flame–wall quenching has been extensively studied both theoretically and numerically, the reported studies of turbulent flame–wall interactions are limited to some Direct Numerical Simulations (DNS), which have subsequently led to improvement of models for predicting flame characteristics in the vicinity of the wall.Large Eddy Simulation (LES) of turbulent flows is considered as a powerful tool to predict the occurrence of instabilities due to heat release, hydrodynamic flow fields and acoustic waves. It provides a better description of turbulent–combustion interaction than the classical Reynolds Averaged Navier Stokes techniques (RANS). In the present study, the single mesh quenching of turbulent flame deflagration is investigated in stoichiometric methane–air mixture, which is equi-diffusive with unit Lewis number. It is well known that flame stretch and preferential diffusion has negligible influence in flame quenching for equi-diffusive flames. Therefore a unity Lewis number flamelet formulation can be used for simulating the turbulent combustion process within the premixed flamelet regime.In the present study, LES predictions are performed using the OpenFOAM CFD toolbox solver. The Coherent Flame Model (CFM) in the LES context as proposed by Richard et al. (2007) is implemented for modeling flame deflagration. During the flame/wall interactions, enthalpy loss through the wall affects the flamelet speed, flamelet annihilation and flame propagation; and the decrease in turbulence scales near the wall affects turbulent diffusion and flame strain. These flame–wall interactions are accounted for through extending the closures proposed by Bruneaux, Poinsot, and Ferziger (1997) for the CFM–RANS model to the LES context. Preliminary testing has demonstrated good potential of the modified CFM to capture the quenching effect of the wall.

Simulation of spray development and turbulent combustion processes in low and high speed diesel engines by the CMC-ISR model

Simulation is performed to analyse the characteristics of turbulent spray combustion in conventional low and high speed diesel engine conditions. Turbulence–chemistry interaction is resolved by the Conditional Moment Closure (CMC) model in the spatially integrated form of an Incompletely Stirred Reactor (ISR). After validation against measured pressure traces, characteristic length and time scales and dimensionless numbers are estimated at the locations of sequentially injected fuel groups. Conditional flame structures are calculated for sequentially evaporated fuel groups to consider different available periods for ignition chemistry. Injection overlaps the combustion period in the high rpm engine, while most combustion occurs after injection and evaporation are complete in the low rpm engine. Ignition occurs in rich premixture with the initial peak temperature at the equivalence ratio around 2–4 as observed in Dec [2]. It corresponds to the most reactive mixture fraction of the minimum ignition delay for the given mixture states. Combustion proceeds to lean and rich sides in the mixture fraction space as a diffusion process by turbulence. The mean scalar dissipation rates (SDRs) are lower than the extinction limit to show stability of diffusion flames throughout the combustion period.

Analysis of the interaction between turbulent combustion and thermal radiation using unsteady coupled LES/DOM simulations

Radiation exchanges must be taken into account to improve the prediction of heat fluxes in turbulent combustion. The strong interaction with turbulence and its role on the formation of polluting species require the study of unsteady coupled calculations using Large Eddy Simulations (LESs) of the turbulent combustion process. Radiation is solved using the Discrete Ordinate Method (DOM) and a global spectral model. A detailed study of the coupling between radiative heat transfer and LES simulation involving a real laboratory flame configuration is presented. First the impact of radiation on the flame structure is discussed: when radiation is taken into account, temperature levels increase in the fresh gas and decrease in the burnt gas, with variations ranging from 100K to 150K thus impacting the density of the gas. Coupling DOM and LES allows to analyze radiation effects on flame stability: temperature fluctuations are increased, and a wavelet frequency analysis shows changes in the flow characteristic frequencies. The second part of the study focuses on the Turbulence Radiation Interaction (TRI) using the instantaneous radiative fields on the whole computational domain. TRI correlations are calculated and are discussed along four levels of approximation. The LES study shows that all the TRI correlations are significant and must be taken into account. These correlations are also useful to calculate the TRI correlations in the Reynolds Averaged Navier–Stokes (RANS) approach.

Numerical Simulation of 3-D Chemical Non-Equilibrium Flow in the Afterburning Chamber of Solid Ducked Rocket

In order to reveal the complex turbulent combustion processes in the afterburning chamber of Solid Ducked Rocket (SDR), Based on the fundamental equations of hydrokinetics and elementary principles of radical reaction kinetics, using multi-component chemical reaction equation of containing Mg and Al components, the numerical simulation of chemical non-equilibrium flow in the afterburning chamber of SDR is processed and effects of the air inlet angle on the afterburning chamber is studied by using Fluent software. The results show that pressure distribution is more balanced, temperature distribution is very uneven and flow is extremely complicated in the afterburning chamber. Combustion efficiency can be increased by improving the air inlet angle.

Numerical study of gaseous reactive flow over a ram accelerator projectile in subdetonative velocity regime

Computational fluid dynamics solutions of the Reynolds Averaged Navier-Stokes equations have been used to numerically predict the thrust of a thermally choked ram accelerator in subdetonative velocity regime. Studies were focused on a projectile operating in a 38-mm-diameter ram accelerator tube loaded with premixed propellant gas; methane/oxygen/nitrogen at 5.15 MPa fill pressure. Simulations were carried out for a series of incoming velocities. The shear stress transport turbulence model (SST) and the eddy dissipation combustion model (EDM) with five-step reaction mechanism were used to simulate the fully turbulent reactive flow field around the projectile. The predicted projectile thrust-velocity agreed well with the experimental measurements, in addition, the CFD predicted pressure variation and magnitude along projectile axial direction also agreed well with the test data. The present investigation reveals some key features of the shock-wave system around the projectile, which are important in determining the characteristics of the thermally choked propulsive mode. These findings are useful in understanding the characteristics of high speed turbulent combustion process in the ram accelerator.

Evaluation of different numerical approaches for a ventilated tunnel fire

The objective of this study is to examine the feasibility of two combustion models combined with a large-eddy simulation for a full-scale ventilated tunnel fire. The numerical model solves 3D, time-dependent Navier—Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. Turbulent combustion process is modeled by an eddy dissipation concept (EDC) and mixture fraction (MF) approach by using two chemical reaction steps to CO prediction. The predicted temperature in the smoke and flame shape was compared through a Blind method to the experimental data of Apte et al. [Apte VB, Green AR and Kent JH. Pool fire plume flow in a large-scale wind tunnel. Proceedings of the Third International Symposium on Fire Safety Science, July 8-12, Edinburgh, UK: Elsevier; 1991, pp. 425—434] and Fletcher et al. [Fletcher DF, Kent JH, Apte VB and Green AR. Numerical simulations of smoke movement from a pool fire in a ventilated tunnel. Fire Safety J 1994; 23(4): 305—325]. The predicted results of Fletcher et al. [Fletcher DF, Kent JH, Apte VB and Green AR. Numerical simulations of smoke movement from a pool fire in a ventilated tunnel. Fire Safety J 1994; 23(4): 305—325] with k-ε model and of Gao et al. [Gao PZ, Liu SL, Chow WK and Fong NK. Large eddy simulations for studying tunnel smoke ventilation. Tunn Undergr Sp Tech 2004; 19(6): 577—586] with large-eddy simulation, both based on a single chemical reaction, are also included. Two boundary conditions, such as adiabatic wall and heat loss inside wall, were applied on the tunnel structure. Both the EDC and MF approaches are shown to possess the ability to predict the general behavior of the experimentally determined temperature field as the heat loss inside the tunnel walls is taken into account. While, it is felt that EDC is more convenient for simulation of a large-scale under-ventilated tunnel fire where the CO and soot yields required by using MF approach, vary significantly with the air to fuel ratio by mass.

Numerical modeling of fires on gas pipelines

When natural gas is released through a hole on a high-pressure pipeline, it disperses in the atmosphere as a jet. A jet fire will occur when the leaked gas meets an ignition source. To estimate the dangerous area, the shape and size of the fire must be known. The evolution of the jet fire in air is predicted by using a finite-volume procedure to solve the flow equations. The model is three-dimensional, elliptic and calculated by using a compressibility corrected version of the k - ξ turbulence model, and also includes a probability density function/laminar flamelet model of turbulent non-premixed combustion process. Radiation heat transfer is described using an adaptive version of the discrete transfer method. The model is compared with the experiments about a horizontal jet fire in a wind tunnel in the literature with success. The influence of wind and jet velocity on the fire shape has been investigated. And a correlation based on numerical results for predicting the stoichiometric flame length is proposed.

CFD Analysis of Turbulent Combustion Processes by a Stochastic Approach

CFD Analysis of Turbulent Combustion Processes by a Stochastic Approach

Numerical and Experimental Characterization of a Natural Gas Engine With Partially Stratified Charge Spark Ignition

This paper outlines the development of a comprehensive numerical framework for the partially stratified charge (PSC) lean-burn natural gas engine. A 3D model of the engine was implemented to represent fluid motion and combustion. The spark ignition model was based on the works of Herweg and Maly (1992, “A Fundamental Model for Flame Kernel Formation in SI Engines,” SAE Technical Publication, Paper No. 922243) and Tan and Reitz (2006, “An Ignition and Combustion Model Based on the Level-Set Method for Spark Ignition Engine Multidimensional Modeling,” Combust. Flame, 145, pp. 1–15). The EDC model (Ertesvåg and Magnussen, 2000, “The Eddy Dissipation Turbulence Energy Cascade Model,” Combust. Sci. Technol., 159, pp. 213–235) with a two-step mechanism was used to model natural gas turbulent combustion process. An open geometry simulation strategy was adopted to account for intake-exhaust gas and valve movements. Each simulation was executed for multiple cycles to produce a representative residual gas fraction. The numerical results were compared with the experimental data obtained on the Ricardo Hydra single cylinder research engine for both homogeneous and PSC cases and they were found to be in excellent agreement in pressure trace and heat release rate. The detailed investigation of the numerical data showed the development of an ignitable mixture under PSC cases, allowing stable kernel growth well beyond the lean misfire limit of the bulk mixture. Furthermore, limits on successful ignition can be identified using the ignition model, which exhibited self-similar behavior in terms of flame speed and turbulent fluctuation. It can also be shown that, at ultralean air-fuel ratios, the PSC plume helps replicate the ignition conditions that can be found under stoichiometric operation.

New Perspectives on Turbulent Combustion: Multi-Parameter High-Speed Planar Laser Diagnostics

Over the past three decades laser combustion diagnostics have guided an improved understanding of turbulent combustion processes. Until recently, this was based on statistically independent sampling using sampling rates much slower than typical integral time-scales of turbulent flames. Recent developments in laser and camera technology enabled an increase in sampling rates by more than three orders of magnitudes. Using these new instruments for particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) at high sampling rates (high-speed diagnostics) allowed the resolution of integral time-scales of turbulent flames. This statistically dependent sampling is increasingly used to temporally track transients in turbulent combustion, such as flame extinction, ignition, flashback and cycle-to-cycle variations in IC engines. The simultaneous application of flow and scalar field measurements makes insights into these transients possible that were not when using statistically independent sampling with low data acquisition rates. Conditioning on distinct flame features with high-speed diagnostics enables the inclusion of time as an additional dimension. This paper reviews the emerging field of multi-parameter, high-speed, planar laser diagnostics in combustion applications. The benefit of high data acquisition rates in turbulent combustion applications is discussed in detail as well as requirements and constraints imposed by the time-scales of the investigated phenomenon are addressed. Recent developments in laser and detector hardware are highlighted, as these are the limiting factors of the sampling rate. Finally, multi-parameter high-speed measurements in combustion are summarized, with a few examples discussed in more detail.

A DRD finite element formulation for computing turbulent reacting flows in gas turbine combustors

An effective multiscale treatment of turbulent reacting flows is presented with the use of a stabilized finite element formulation. The method proposed is developed based on the streamline-upwind/Petrov–Galerkin (SUPG) formulation, and includes discontinuity capturing in the form of a new generation “DRD” method, namely the “DRDJ” technique. The stabilized formulation is applied to finite-rate chemistry modelling based on mixture-fraction approaches with the so-called presumed-PDF technique. The turbulent combustion process is simulated for an aero-engine combustor configuration of RQL concept in non-premixed flame regime. The comparative analysis of the temperature and velocity fields demonstrate that the proposed SUPG+DRDJ formulation outperforms the stand-alone SUPG method. The improved accuracy is demonstrated in terms of the combustor overall performance, and the mechanisms involved in the distribution of the numerical diffusivity are also discussed.

Radiative heat transfer during turbulent combustion process

We investigate the radiative heat transfer in a co-flowing turbulent nonpremixed propane-air flame inside a three-dimensional cylindrical combustion chamber. The radiation from the luminous flame, which is due to the appearance of soot particles in the flame, is studied here, through the balance equation of radiative transfer which is solved by the Discrete Ordinates Method (DOM) coupling with a Large Eddy Simulation (LES) of the flow, temperature, combustion species and soot formation. The effect of scattering is ignored as it is found that the absorption dominates the radiating medium. Assessments of the various orders of DOM are also made and we find that the results of the incident radiation predicted by the higher order approximations of the DOM are in good agreement.

Acoustic radiation from turbulent premixed flames

Turbulent combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized their total sound power, their spectral characteristics are not well understood. This work investigates these acoustic spectral features, including the flame's low- and high-frequency characteristics and the scaling of the frequency of peak acoustic emissions. The spatiotemporal characteristics of the flame's chemiluminescence emissions, used as a marker of heat release fluctuations, were measured and used to determine the heat release spectrum, spatial distribution and spatial coherence characteristics. These heat release characteristics were then used as inputs to an integral solution of the wave equation and compared to measured acoustic spectra obtained over a range of conditions and burners and at several spatial locations. The spectral characteristics of the flame's acoustic emissions are controlled by two processes: the underlying spectrum of heat release fluctuations that are ultimately the combustion noise source, and the transfer function relating these heat release and acoustic fluctuations. An important result from this work is the clarification of the relative roles of these two processes in controlling the shape of the acoustic spectrum. This transfer function is primarily controlled by the spatiotemporal coherence characteristics of the heat release fluctuations which are, in turn, strongly influenced by burner configuration/geometry and operating conditions. Low-frequency acoustic emissions are controlled by the heat release spectrum essentially independent of flame geometry. Both the heat release spectrum and heat release-acoustic transfer function are important at intermediate and high frequencies. An important feature of the investigated geometry that controls the heat release phase dynamics is the high-velocity flow relative to the flame speed and anchored character of the flame. This leads to convection of flame sheet disturbances (i.e. heat release fluctuations) along the front that dominates the high frequency and peak frequency scaling of the flame's acoustic emissions.

Prediction of soot and carbon monoxide production in a ventilated tunnel fire by using a computer simulation

Numerical study is performed by using a large eddy simulation to give a quantitative description of the toxic products (CO, soot), temperature stratification and heat fluxes from a circular pool fire in a longitudinally ventilated tunnel. The turbulent combustion process is modelled by an eddy break-up concept by using two sequential, semi-global steps to CO prediction. The numerical model solves three-dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The smoke movement is modelled as an unsteady process, from the time of ignition until convergence to a quasi-steady state. The computed, time-averaged temperatures are compared with experimental data from a full-scale ventilated tunnel, and a relatively good agreement is attained. Below a critical ventilation velocity, the back layer flow carries the CO and soot production towards the entry, and an inverse dependence of the backflow length to ventilation velocity is predicted. For the tunnel hydraulic height of 3.3 m considered here, the heat-release rate (HRR) approaches a maximum of approximately 35 MW, beyond which the critical ventilation velocity is unchanged even though the theoretical HRR continues to increase. In the case where HRR reaches to a maximum, the high-temperature level ( T>500 °C) in the smoke layer and the strong heat flux (>20 kW/m 2) pose significant threat to people or combustible objects exposed downstream the fire source, potentially inducing flashover within tunnel. This study highlights the utility of field modelling for the analysis of fires and smoke (CO and soot) movement in tunnels.