W. Malalasekera

A Review of Research and an Experimental Study on the Pulsation of Buoyant Diffusion Flames and Pool Fires

This paper reviews the past research, experimental techniques and scaling relationships used in the studies of oscillatory buoyant diffusion flames and reports an experimental investigation conducted to determine the pulsating characteristics of such flames. The experimental data were obtained by using three techniques, namely, pressure fluctuation measurements, thermal imaging and high-speed video photography. Present findings are compared with data sets reported in the literature and correlations for pulsation frequency suggested by previous studies are independently verified. Analysis of the experimental data on frequency of pulsations in different burners shows that for a fixed-diameter flame the pulsation frequency is almost independent of fuel flow rate. The equation f=1.68D−0.5 gives the best approximation for the relationship between pulsating frequency and diameter over a wide range of data. An alternative way of expressing the relationship between the key variables is St=0.52*(1/Fr)0.505. This proves to be a better way of expressing the relationship since it can include the effect of the fuel flow rate. Slight modifications to this expression allows prediction of flame oscillations under elevated/reduced gravity and isothermal buoyant plumes. This relationship and the observations of the present study confirm the hydrodynamic nature of flame puffing: interplay of buoyancy and fluid motion.

Modelling of a Bluff-Body Nonpremixed Flame Using a Coupled Radiation/Flamelet Combustion Model

A coupled radiation/flamelet combustion modelling technique is applied to the simulation of a bluff-body flame. Radiation heat transfer is incorporated into the laminar flamelet model for turbulent combustion through the enthalpy defect. A new method is developed for generating flamelet library with enthalpy defect. The radiation within the flame is modelled using a raytracing approach based on the discrete transfer method. The predicted results are compared with the reported experimental data. Comparison shows that the effects of radiative heat transferr on the temperature and major species are small for the flame considered. However, a significant improvement in the prediction of OH is achieved when radiation heat transfer is included.

COMPARISON OF THE DISCRETE TRANSFER AND MONTE CARLO METHODS FOR RADIATIVE HEAT TRANSFER IN THREE-DIMENSIONAL NONHOMOGENEOUS SCATTERING MEDIA

Modified formulations of the discrete transfer and Monte Carlo methods are presented for the prediction of radiative heat transfer in three-dimensional nonhomogeneous participating media. Numerical solutions found with both algorithms are in good agreement with published benchmark results, which used contemporary methods to determine the radiative transport in a unit cube. New solutions in an arbitrary L-shaped geometry using a nonorthogonal body-fitted mesh are also presented. The average deviation between the two methods is less than 1.2% for both the boundary surface flux and the divergence of radiative flux or gas emissive power within the enclosed, isotropically scattering media.

LES of Recirculation and Vortex Breakdown in Swirling Flames

In this study large eddy simulation (LES) technique has been applied to predict a selected swirling flame from the Sydney swirl burner experiments. The selected flame is known as the SM1 flame operated with fuel CH 4 at a swirl number of 0.5. In the numerical method used, the governing equations for continuity, momentum and mixture fraction are solved on a structured Cartesian grid. The Smagorinsky eddy viscosity model with the localised dynamic procedure of Piomelli and Liu is used as the subgrid scale turbulence model. The conserved scalar mixture fraction-based thermo-chemical variables are described using the steady laminar flamelet model. The GRI 2.11 is used as the chemical mechanism. The Favre-filtered scalars are obtained from the presumed beta probability density function (β-PDF) approach. The results show that with appropriate inflow and outflow boundary conditions LES successfully predicts the upstream recirculation zone generated by the bluff body and the downstream vortex breakdown zone induced by swirl with a high level of accuracy. Detailed comparison of LES results with experimental measurements show that the mean velocity field and their rms fluctuations are predicted very well. The predictions for the mean mixture fraction, subgrid variance and temperature are also reasonably successful at most axial locations. The study demonstrates that LES together with the laminar flamelet model in general provides a good technique for predicting the structure of turbulent swirling flames.

Large eddy simulation of isothermal turbulent swirling jets

Abstract This article investigates the application of the large eddy simulation (LES) technique to turbulent isothermal swirling flows. The aim was to improve our understanding of the flow physics and turbulence structure of unconfined swirling flows and examine the capability of LES to predict the formation of the vortex breakdown (VB) and recirculation zones. In this study, the filtered Navier-Stokes equations are closed using the Smagorinsky eddy viscosity model with the localized dynamic procedure of Piomelli and Liu (1995). The Sydney University swirl burner experiments are simulated as test cases. Three different test cases have been investigated covering a range of swirl numbers and stream wise annular velocities. The cases considered have swirl numbers ranging from 0 to 1.59 and Reynolds numbers from 32400 to 59000. The LES calculations confirm that the combination of lower swirl number and higher axial velocity of the primary annulus leads to the establishment of the downstream vortex breakdown region. For the cases considered, the LES calculations were successful in predicting observed recirculation zones, vortex breakdown and showed good agreement with experimentally measured mean velocities, their rms fluctuations and Reynolds shear stresses.

Radiative heat transfer calculations in three-dimensional complex geometries

This article is closed access. It was published in the Journal of Heat Transfer [© ASME]. To obtain this article please visit the ASME Digital Library at: http://www.asmedl.org/

Modelling of a bluff body stabilized CH4/H2 flame based on a laminar flamelet model with emphasis on NO prediction

Abstract A laminar flamelet model prediction of a bluff body stabilized CH4/H2 flame is reported. The predicted results of temperature and major and minor species as well as NO are compared against the Raman/Rayleigh/LIF data available from the University of Sydney. The effect of differential diffusion is also studied. The calculation shows that temperature and concentration of major and minor species are better predicted by the unity Lewis number flamelet. It is also shown that the current steady state flamelet approach has difficulties in predicting experimental NO levels.

Fire computation: The ‘flashover’ phenomenon

This paper describes the application of a predictive code of the ‘field model’ type to the prediction of compartment fires. The flashover problem, in so far as it is influenced by heat transfer, is given special consideration. This emphasis is facilitated by the refined nature of the method employed to calculate thermal radiation transfer. Data from the test cell of the Lawrence Livermore Laboratories and from that of the Swedish National Testing Institute are deployed in the validation exercises. The latter possess the advantage of having been obtained for a broad spectrum of tests in which the time to flashover was specifically recorded. On the whole, agreement between the predictions and the data is good. However the prediction of the time to flashover is severely hindered by the lack of a suitable model technique for the prediction of flame spread. For those Swedish tests where flashover was not preceded by significant flame spread, the time to flashover is predicted by reference to the critical heat flux. We conclude that: 1) the total heat transfer to solid surfaces, a critical factor influencing flashover, is seemingly well predicted by the present code, 2) a flame spread model suitable for incorporation in field model type codes needs to be devised and validated, and 3) that correspondingly specialist flame spread experiments require to be performed to provide the necessary validation data.

A mathematical model for airflow and heat transfer through fibrous webs

Abstract A mathematical model based on computational fluid dynamics has been developed to investigate the airflow and heat transfer through fibrous webs. The model is based on the porous media concept and involves solving equations for continuity, momentum, and energy. A thermal energy equation is developed, which incorporates the heat of fusion of fibres in the fibrous web. Local flow information such as air velocity, temperature, and melt fraction of fibres is obtained from the simulations. An important outcome of the simulation is the prediction of time required to melt fibres in the web under different working conditions. This information can be used potentially in the design of through-air bonding process for nonwovens manufacturer.

Effects of Swirl on Intermittency Characteristics in Non-Premixed Flames

Swirl effects on velocity, mixture fraction, and temperature intermittency have been analyzed for turbulent methane flames using large eddy simulation (LES). The LES solves the filtered governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modeling based on the localized dynamic Smagorinsky and the steady laminar flamelet models, respectively. Probability density function (PDF) distributions demonstrate a Gaussian shape closer to the centerline region of the flame and a delta function at the far radial position. However, non-Gaussian PDFs are observed for velocity and mixture fraction on the centerline in a region where center jet precession occurs. Non-Gaussian behavior is also observed for the temperature PDFs close to the centerline region of the flame. Due to the occurrence of recirculation zones, the variation from turbulent to nonturbulent flow is more rapid for the velocity than the mixture fraction and therefore indicates how rapidly turbulence affects the molecular transport in these regions of the flame.