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.     

Prediction of turbulence radiation interactions of CH4H2/air turbulent flames at atmospheric and elevated pressures

Predicting thermal radiation for turbulent combustion highlights the significance of turbulence radiation interactions (TRI). Thermal radiation behaviors of methane/hydrogen flames under elevated pressures are investigated numerically using the developed TRI module integrated into CFD codes. The updated non-gray weighted sum of gray gases model is used to calculate the radiative properties of participating media. TRI effects have been analyzed with 0%–50% volumetric fraction of hydrogen in the methane/hydrogen blended fuels under 1–5 atm working pressures. Employing the radiation model considering TRI achieves closer predicted consistency to the experimental data. Only thermal radiation makes the flame temperature dropped about 60–140 K, while the predicted radiative source term calculated with TRI is higher than that without TRI, which results in a colder flame (approximately 13–60 K lower). The impact of TRI on the radiation behavior is enhanced in hydrogen-enriched high-pressure flame as the predicted radiation heat flux and radiative source term are increased above 25% than that without TRI. On account of TRI effect, the net radiative heat loss increases almost 50% at elevated pressure. The strong radiation of participating media in methane/hydrogen flames under elevated pressures emphasizes the importance of TRI effect on accurate predictions of thermal radiation and NO emission.     

New correlations for the weighted-sum-of-gray-gases model in oxy-fuel conditions based on HITEMP 2010 database

Oxy-fuel combustion is one of the promising options for carbon dioxide capture in future coal power plants. Radiative properties of combustion gases and heat transfer characteristics inside oxy-fuel furnaces are different from those found in air-fired furnace. Nowadays, few publications provide appropriate radiation property correlations for oxy-fuel conditions. The available correlations are based on previous versions of HITRAN database, which is not accurate for prediction of spectral intensities at high temperature in combustion applications or above 1000 K. This paper considers the determination and evaluation of new correlations for the weighted-sum-of-gray-gases model to predict the radiative transfer in gases under oxy-fuel conditions. The new correlations are fitted from emittance charts calculated from the up-to-date HITEMP 2010 database for molar ratios of water vapor to carbon dioxide between 0.125 and 4, temperature range of 400–2500 K, and pressure path-length varying from 0.001 to 60 bar m. The new correlations are validated by comparing the radiative source term with line-by-line calculations from HITEMP 2010 database for a one-dimensional slab system. The radiative transfer equation is solved with the discrete ordinate method.     

Monte Carlo modeling of radiative transfer in a turbulent sooty flame

The calculation of radiative transfer within a sooty turbulent ethylene-air diffusion jet flame has been carried out by using a Monte Carlo method and an accurate CK model for the gases. The influence of the turbulence-radiation interaction (TRI) has been studied. In the TRI modeling, the radiative properties of the assumed homogeneous turbulent structures are randomly obtained from a multidimensional probability density function (PDF) of the reaction progress variable, of the mixture ratio and of the soot volume fraction. This joint PDF is obtained from an Eulerian-Lagrangian turbulent combustion model and the sizes of the turbulent structures are directly derived from a k-? model. In the considered flame, the TRI effect is an increase of the radiative heat loss by about 30%. The radiative heat loss becomes almost equal to one-third of the chemical heat release. Soot particles play the most important role in the global radiative heat loss but the influence of gaseous species like CO₂ and H₂O can be important in the local energy balance.     

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

Radiative heat transfer is known to play an important role in combustion processes but is often neglected in simulations because of its complexity and the related numerical costs. An original approach is proposed here to perform large eddy simulations of turbulent combustion including radiation: unsteady combustion and radiative heat transfer are computed by two independent codes that exchange data when needed through a specialized language, CORBA, working on an internal computer network or over the Internet. The radiation code gets temperature and mass-fraction fields from the combustion code and returns radiative energy source terms. This coupling technique is easy to implement, portable, flexible, and versatile. Each code keeps its own structure and may be developed and optimized independently, especially when running on massively parallel machines. Preliminary results show that taking radiative heat transfer into account strongly modifies the flame dynamics, probably because the burnt gas temperature decreases, making the flame stabilization weaker and increasing the flame sensitivity to turbulent motions. Comparisons with experimental data are very encouraging.     

Numerical Study of the Swirl Effect on a Coaxial Jet Combustor Flame Including Radiative Heat Transfer

A numerical study of the swirl effect on a coaxial jet combustor flame including radiative heat transfer is presented. In this work, the standard k-ε model is applied to investigate the turbulence effect, and the eddy dissipation model (EDM) is used to model combustion. The radiative heat transfer and the properties of gases and soot are considered using a coupled of the finite-volume method (FVM), and the narrow-band based weighted-sum-of-gray gases (WSGG-SNB) model. The results of this work are validated by experiment data. The results clearly show that radiation must be taken into account to obtain good accuracy for turbulent diffusion flame in combustor chamber. Flame is very influenced by the radiation of gases, soot, and combustor wall. However, swirl is an important controlling variable on the combustion characteristics and pollutant formation.     

The turbulent burning velocity of iso-octane/air mixtures

Turbulent burning velocities of iso-octane air mixtures have been measured for expanding flame kernels within a turbulent combustion bomb. High speed schlieren images were used to derive turbulent burning velocity. Turbulent velocity measurements were made at u′ = 0.5, 1.0, 2.0, 4.0, 6.0 m/s, equivalence ratios of 0.8, 1.0, 1.2, 1.4 and pressures of P = 0.1, 0.5, 1.0 MPa. The turbulent burning velocity was found to increase with time and radius from ignition, this was attributed to turbulent flame development. The turbulent burning velocity increased with increasing rms turbulent velocity, and with pressure; although differences were found in the magnitude of this increase for different turbulent velocities. Generally, raising the equivalence ratio resulted in enhanced turbulent burning velocity, excepting measurements made at the lowest turbulent velocity. The results obtained in this study have been compared with those evaluated for a number turbulent burning velocity correlations and the differences are discussed.     

Experimental study on flame height and temperature profile of buoyant window spill plume from an under-ventilated compartment fire

Fire experiments were carried out in a scale model, consisting of an 0.8 m cubic fire compartment with six window like geometries and an attached 3 m (wide) × 5 m (high) façade wall. A propane porous gas burner with controlled fuel supply rate was the fire source. Gas temperature profiles were measured inside the compartment and near the façade wall. The outside spill flame heights were recorded by a CCD Digital camera. Temperature and flame heights are correlated with heat release rate and the window geometry using physically non-dimensional analysis. The steady gas temperatures inside the compartment are determined by an overall energy balance between the heat release rate inside the compartment and the wall conduction and opening radiation heat losses using an effective overall heat loss coefficient. Flame heights on the façade are non-dimensionally correlated by the excess fuel heat release rate outside the enclosure and a characteristic length scale for the window. These results agree with previous results in the literature. Vertical gas temperatures near the façade wall outside the enclosure are non-dimensionally correlated with the total convective heat flow rate above the flames and the same characteristic window length scale as the flame height, with the additional necessary determination of a virtual origin of the convective flow above the flame. These results and correlations are new and a significant improvement over previous results in the literature.     

Detailed numerical simulation of thermal radiation influence in Sandia flame D

In order to investigate the influence of thermal radiation in turbulent combustion processes, Sandia flame D is numerically simulated, with multiple-time scale (MTS) k–ε turbulence model for turbulence, the combination of probability density function (PDF) transportation method, Lagrangian flamelet model (LFM) and the detailed chemical reaction mechanism GRI 3.0 (consisting of 53 species and 325 elemental reactions) for combustion and finite volume/correlated-k (FV/CK) method for radiation heat transfer. To account for turbulence’s influence on radiation, the effects of turbulence–radiation interactions (TRI) are investigated in radiation calculations and it is recommended that for detailed numerical simulation TRI should be considered. Numerical results with and without radiation influence being taken into accounted are compared with experimental data. Different from reports by other researchers, our simulation results show that although the magnitude of thermal radiation is relatively small, its influence on combustion process is significant. It is suggested that turbulence and chemical reactions may magnify the influence of thermal radiation.     

Numerical simulation of the interaction between turbulence and radiation in reactive flows

The interaction between turbulence and radiation (TRI) in reactive flows has been demonstrated experimentally, theoretically and numerically, and results from the highly non-linear coupling between fluctuations of radiation intensity and fluctuations of temperature and chemical composition of the medium. The instantaneous and the time-averaged form of the radiative transfer equation (RTE) are presented, and the TRI effects resulting from time-averaging are discussed. Methods to account for TRI in practical calculations are surveyed, and works where such methods have been employed are reviewed. These include both decoupled and coupled fluid flow/radiative transfer calculations. It is shown that the solution of the RTE using instantaneous scalar data is the most accurate way to deal with TRI, but it is computationally prohibitive for coupled problems. Hence, this approach has been mainly used to calculate the radiation intensity along lines of sight. The generation of time series of instantaneous scalar data may be accomplished using stochastic or deterministic models, which are also surveyed. Coupled fluid flow/radiative transfer problems are generally solved using the time-averaged form of the RTE or the Monte Carlo method, and rely on the optically thin fluctuation approximation, which neglects the correlation between fluctuations of the absorption coefficient and fluctuations of the radiation intensity. Experimental data and numerical calculations demonstrate that turbulent fluctuations may significantly increase the mean spectral radiation intensity in both non-luminous and luminous flames. Turbulent fluctuations contribute to decrease the flame temperature below the level observed without fluctuations, particularly for optically thick flames. The net radiative power and the fraction of radiative heat loss increase due to TRI, particularly in the case of optically thin flames. Recent direct numerical simulations provide additional insight on the role of different correlations responsible for TRI, and on how they are influenced by the optical thickness of the medium.     

Flame-flow interactions and flow reversal

The interaction of a premixed methane/air flame with flow unsteadiness is studied computationally using a stagnation-point flow configuration. The problem is of fundamental interest and also relevant for turbulent combustion in the laminar flamelet regime. In the present study, of particular interest is the flame residing in a weakly strained flow field such that the flame is stabilized away from the viscous boundary layer adjacent to the stagnation plane and is free to move in response to flow perturbations. An unsteady sinusoidal strain rate field is imposed on the flame, and an extensive parametric study is conducted by varying the frequency and amplitude of strain rate fluctuation. It is found that for high frequencies and large amplitudes, flow direction reverses upstream of the flame, thereby establishing a new stagnation plane in the preheat zone ahead of the flame. This observation indicates that the flame strongly affects the upstream flow field and could also possibly explain the reported occurence of flow reversal in experimental studies of turbulent jet flames. Effects of other key parameters such as the Lewis number, mean flame speed, and gas expansion ratio on flame-flow interaction and flow reversal are studied by investigating highly lean (Le < 1) and rich (Le > 1) hydrogen/air flames. The physical mechanism responsible for flow reversal phenomenon is explained.     

Parallel Computation for Radiative Heat Transfer Using the DOM in Combustion Applications: Direction,Frequency, Subdomain Decompositions,and Hybrid Methods

In the framework of coupled large-eddy/discrete ordinates method (LES/DOM) computations of turbulent combustion problems, various decompositions for parallel calculations of the radiative heat transfer based on the DOM are investigated. The methods analyzed are: (A) a task decomposition on the discrete directions and frequencies with two numeric strategies: Message Passing Interface (MPI) with distributed memory and OpenMP with shared memory for the direction decomposition; (B) a new algorithm for a DOM subdomain decomposition, which is proposed and tested using MPI; and (C) hybrid methods combining an OpenMP strategy for direction and MPI for tasks and subdomain decomposition. It is shown for the case of coupled simulations that the convergence and the parallel efficiency of the domain decomposition (B) are optimal. This method is limited in this work to 25 sub-domains, at which point the efficiency stagnates. Combining the directions with frequency and/or domain decompositions in a hybrid method (C) results in very good efficiency up to 1,200 processors. This hybrid strategy is also very efficient in terms of memory usage. This work shows that the best way to perform massively parallel computation for radiative heat transfer with the DOM is to combine different decomposition levels. The analysis performed in this work shows the best parallel strategy to be used in coupled simulations between radiation and LES on massively parallel architectures.     

Multimode heat transfer in cyclic flow reversal combustion in a porous medium

Experimental and numerical studies of combustion and multimode heat transfer in a porous medium, with and without a cyclic flow reversal of a mixture through a porous medium, were performed. Parametric studies were done in order to understand combustion characteristics such as maximum flame temperature and radiative heat flux using a one‐ dimensional conduction, convection, radiation and premixed flame model. The porous medium was assumed to emit and absorb radiant energy, while scattering is ignored. Non‐local thermodynamic equilibrium between the solid an d gas is taken into account by introducing separate energy equations for the gas and the solid phase. As a prelimina ry study, the combustion regime was described by a one‐step global mechanism with an internal heat source uniformly dist ributed along the reaction zone. The effects of the flame position, cyclic flow reversal, period of the cyclic flow rever sal, the optical thickness and the flow velocity on the burner performance were clarified by a rigorous radiation analysis. Th e model was validated by comparing the theoretical results with the experiments. It was shown that, for maximizing the fl ame temperature and the net radiative heat flux feedback, the flame should be stabilized near the centre of the po rous medium with a cyclic flow reversal, the period of which should be as small as possible. A high optical thickness prod uced a high flame temperature and a high net radiative feedback. Also, a high flow velocity at low period of the cyclic f low reversal of mixture yielded a high value of both the flame temperature and the net radiative feedback. Thermal structure predictions in terms of the gas‐phase and the solid‐phase temperature distributions along the axis of the combustor show good agreement with the experimental ones. Copyright © 1999 John Wiley & Sons, Ltd.     

Numerical study of radiative heat transfer effects on a complex configuration of rack storage fire

This work describes the application and the performance of a new radiation model in CFD calculations for the simulation of thermal radiation transfer effects on a fire scenario. A 3D Cartesian coordinates radiative heat transfer procedure based on coupling of the FTn finite volume method (FTnFVM) with the bounded high-order resolution CLAM scheme is developed. The narrow-band based weighted-sum-of-gray-gases (NB-WSGG) model is applied to take account of nongray effects by CO₂, H₂O and soot. To treat irregular boundaries, the present model used the blocked-off-region procedure. This radiation code is implemented in the Fire Dynamics Simulator (FDS), a Computational-Fluid-Dynamics-based fire model, where a the combustion is represented by means of the mixture fraction with a single step chemical reaction model and the Large Eddy Simulation (LES) is used to model the dissipative processes. Computational results with and without radiation effects are compared against available experimental data and quasi-steady state law correlations of in-rack storage fire, which consists a complex configuration of double tri-wall corrugated paper cartons placed onto a wood pallet. Sensibility analyses of spatial and angular grids demonstrate the improvements due to the FTnFVM and to the CLAM scheme in the configuration studied. Results show that the simulations of the flame height, the gas temperature and the gas velocity are strongly influenced by thermal radiation. Overall, simulations predicted closer profiles to the experimental results only when the nongray-sooting radiation model was incorporated and an over-prediction of the gas temperature and the flame height is found when radiation is neglected. A sensibility analysis has shown that the flame characteristics are strongly affected by the soot yield.     

A computational study of spherical diffusion flames in microgravity with gas radiation. Part II: Parametric studies of the diluent effects on flame extinction

Spherical diffusion flame in microgravity was investigated computationally, considering gas radiation with statistical narrow band model (SNB) and discrete ordinate method (DOM). The parametric studies explored the relative effectiveness of fuel- versus oxidizer-side dilution on the flame radius and temperature behavior, and it was discovered that the oxidizer-side dilution has a stronger effect on flame transient behavior than the fuel-side dilution, thereby suggesting a more effective means to induce flame extinction by dilution. Study on different oxidizer-side dilution cases shows that CO₂ has a larger suppression effect than helium and nitrogen with the same dilution level. CO₂ dilution has multiple effects on flame behavior including radiation, thermodynamic, diffusion, and chemical effects. Quantitative analysis shows that the radiation effect is the primary factor accounting for flame temperature drop by approximately 60%, as compared to the thermal/diffusion (30%), and chemical effect (10%). Computational results over a wide range indicated a critical flame temperature of 1130 K at extinction, which appears to be a valid unified extinction criterion for the flame under study. Therefore, it is concluded that extinction of spherical diffusion flame is primarily dictated by the local condition in the flame zone rather than by the volumetric radiative heat transfer in the surrounding gases. Investigation on steady flame solution within different domain sizes shows that, a steady state spherical diffusion flame does not exist in microgravity because the flame keeps growing with the non-zero gradient on the flame outer edge, additionally flame temperature constantly decreases with gas radiation which eventually extinguishes the flame when the flame temperature drops down to a critical value.     

Effects of preferential transport in turbulent bluff-body-stabilized lean premixed CH4/air flames

Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, bluff-body stabilized, lean premixed methane–air flames (Le = 0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent bluff-body burner are cause by preferential transport of H₂ and H₂O through the preheat zone ahead of CO₂ and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the bluff body and is apparently amplified by the presence of a strong recirculation zone where excess CO₂ is accumulated.     

From Large-Eddy Simulation to Direct Numerical Simulation of a lean premixed swirl flame: Filtered laminar flame-PDF modeling

Large-Eddy Simulations (LES) and Direct Numerical Simulation (DNS) are applied to the analysis of a swirl burner operated with a lean methane–air mixture and experimentally studied by Meier et al. [19]. LES is performed for various mesh refinements, to study unsteady and coherent large-scale behavior and to validate the simulation tool from measurements, while DNS enables to gain insight into the flame structure and dynamics. The DNS features a 2.6 billion cells unstructured-mesh and a resolution of less than 100 microns, which is sufficient to capture all the turbulent scales and the major species of the flame brush; the unresolved species are taken into account thanks to a tabulated chemistry approach. In a second part of the paper, the DNS is filtered at several filter widths to estimate the prediction capabilities of modeling based on premixed flamelet and presumed probability density functions. The similarities and differences between spatially-filtered laminar and turbulent flames are discussed and a new sub-grid scale closure for premixed turbulent combustion is proposed, which preserves spectral properties of sub-filter flame length scales. All these simulations are performed with a solver specifically tailored for large-scale computations on massively parallel machines.     

Characterisation of an oxy-coal flame through digital imaging

This paper presents investigations into the impact of oxy-fuel combustion on flame characteristics through the application of digital imaging and image processing techniques. The characteristic parameters of the flame are derived from flame images that are captured using a vision-based flame monitoring system. Experiments were carried out on a 0.5 MW_th coal combustion test facility. Different flue gas recycle ratios and furnace oxygen levels were created for two different coals. The characteristics of the flame and the correlation between the measured flame parameters and corresponding combustion conditions are described and discussed. The results show that the flame temperature decreases with the recycle ratio for both test coals, suggesting that the flame temperature is effectively controlled by the flue gas recycle ratio. The presence of high levels of CO₂ at high flue gas recycle ratios may result in delayed combustion and thus has a detrimental effect on the flame stability.     

Evaluation of the performance of biodiesel from waste vegetable oil in a flame tube furnace

This work presents a theoretical and experimental study of the biodiesel (ethyl ester from a waste vegetable oil) performance in a flame tube furnace. The heat transfer rate was analysed in several sections along the furnace and the performance of the biodiesel was compared to that of diesel oil. The flow of heat from the burn of each fuel in the direction of the walls of the combustion chamber was evaluated under the same fuel injection pressure. The peak of the heat transfer occurred around 0.45 m far from the fuel injection nozzle in a 0.305 m inner diameter combustion chamber. The diesel oil showed a higher heat transfer rate in most parts exposed to the flame. In the region where the body of the flame is not present, the heat transfer of biodiesel becomes higher.     

Experimental investigation of single-phase turbulent flow of R-134a in a multiport microchannel heat sink

This experimental study aims to investigate the heat transfer characteristics of single-phase turbulent flow of R-134a refrigerant in a rectangular multi-micro channel heat sink having 27 channels where each channel has a hydraulic diameter of 421 μm. Experimental results were obtained for inlet temperatures ranging from 24 to 33 °C, mass fluxes from 1485 to 2784 kg m^− 2 s^− 1 and wall heat fluxes from 3 to 24 kW m^− 2. The results indicate that the heat transfer coefficients are found to be higher at lower inlet temperatures than those at higher ones. In addition, when equal amount of heat supplied to the heat sink, the heat transfer coefficients increase with increasing the mass flux of refrigerant. They were also compared with 12 well-known correlations and it was seen that 4 of 12 were in good agreement with each other with the average deviation < 10%. The findings demonstrate that well-known correlations in fundamental sources can be used to predict the heat transfer coefficient of R-134a during its single phase flow in a multiport microchannel heat sink under turbulent regime.