Effects Of Flame Radiation Research Articles

Effect of flame radiation on aluminium particle combustion time

This work proposes a numerical model to estimate the radiant heat feedback from the flame of a burning aluminium particle. The objective is to evaluate its role on the burning time. It rests on a detailed combustion simulation of a single particle coupled with a simple two-flux approximation for the radiant flux emitted from the oxide smoke. Radiated flux from the flame is found to be around 1 MW/m 2 for atmospheric pressures, going up to more than 5 MW/m 2 for higher pressures (30 bar). Parametric simulations show that aluminium size has a small effect while oxygen-rich atmospheres can double the radiated flux. Overall, the role of flame radiation on aluminium burning rate is calculated to lie roughly between 1 and 25%, depending on conditions. This is the first study that clearly evaluates the role of flame radiation on aluminium combustion ; in most configurations, this effect, however, remains limited.

Effect of Non-Luminous Flame Radiation During Methanol Droplet Combustion

The effect of non-luminous thermal radiation on the combustion of a suspended methanol droplet in a low temperature (300 K) and low pressure (1 atm) environment is discussed in detail. Numerical results are obtained using a predictive, transient, two-phase, axisymmetric numerical model that includes surface tension effects. Radiation is modeled using the optically thin approximation with the product species CO2 and H2O as the radiating species. Results for combustion in a nearly quiescent atmosphere (initial Reynolds number 0.01) and initial droplet diameters in the range of 0.43 mm to 3 mm are presented. In agreement with the reported literature, it is shown that the effect of flame radiation is negligible when the initial droplet diameter is less than approximately 1 mm and becomes increasingly important for larger droplets. As a result, the average evaporation constant decreases with the initial droplet diameter. Radiation and surface tension combined as well as surface tension alone have a significant effect on the predicted extinction diameters of methanol droplets. However, when surface tension is neglected, radiation alone has a negligible effect on the prediction of the extinction diameter. The extinction diameter presents a non-linear variation with the initial droplet diameter for initially large droplets. The agreement with experimental results available in the literature is very good.

The contribution of radiant heat transfer to laboratory-scale fire spread under the influences of wind and slope

Abstract In a previous study, a two-dimensional non-stationary model of fire spread across a fuel bed including slope effects was proposed. Enhancement of the radiant heat transfer process ahead of the fire front under wind and slope conditions is provided in the current paper. Predictions are compared to data recorded during laboratory-scale experimental fires conducted across different pine needles bed under wind and slope conditions. Influence of these environmental factors on rate of spread, temperature–time profiles and fire front shapes is then presented. Predictions reveal that effects of flame radiation alone can account for observations up to limit value of slope angle and wind velocity for which convective mechanisms cannot be neglected. Below this limit observations are quantitatively well reproduced by the model.

Nitric oxide formation in dilute hydrogen jet flames: isolation of the effects of radiation and turbulence-chemistry submodels

Nitric oxide (NO) levels in turbulent flames are sensitive to temperature, O atom concentrations, and residence times at the NO forming conditions. Consequently, accurate prediction of NO formation is a challenge for turbulent combustion models, and NO is a useful scalar to consider when evaluating turbulence-chemistry submodels. Predictions of NO formation in a series of hydrogen flames with varying levels of helium dilution are compared with experimental results. Dilution reduces flame radiation to the point where uncertainties in the radiation calculation have only a small influence on NO emission, and this allows different models for the coupling of turbulence and chemistry to be compared in relative isolation from the effects of radiation. Calculations using the Probability Density Function (PDF) method and the Conditional Moment Closure (CMC) method are carried out using the same turbulence model, radiation model, reduced chemical mechanism, and boundary conditions, so that similarities and differences between these two approaches for the coupling of turbulence and chemistry may be isolated and more fully understood. To accomplish this, flow model constants are adjusted to match measured axial profiles of mean velocity and mixture fraction in the undiluted flame, before detailed scalar comparisons are carried out. The optically thin radiation assumption is shown to be appropriate for these hydrogen flames, and the effects of flame radiation on NO emission are examined. The sensitivities of the calculated results to certain boundary conditions and submodels are also quantified. Both turbulence-chemistry models yield good results for conditional mean temperature and H 2O mole fraction. Results also show that both models yield quantitatively useful predictions for the NO formation and emission. With all other submodels being the same, the PDF method predicts lower NO levels than does the CMC method. This difference is associated with lower O atom concentrations predicted by the PDF method. Both methods underpredict NO formation in the lower portion of the jet flames. This deficiency is attributed to the effects of differential diffusion, which are not included in the models, and to the influence of heat release on turbulence structure near the flame base.

On NO x emissions from turbulent propane diffusion flames

This paper presents an investigation on NO x emissions from turbulent propane diffusion flames. The study includes both experimental measurements and numerical 2D simulations and aims to provide a better understanding of the dominant physical effects associated with the NO x production in turbulent diffusion flames. Measurements of NO x emission indices (EINO x ), performed in unconfined and vertical lifted flames, have been analysed numerically. In this analysis, the effects of flame radiation, flame extinction by stretch and residence time have been studied by means of a mathematical model including a subroutine of the relevant NO chemistry. The numerically calculated values of EINO x , which are in good agreement with the experimental data, are found to be proportional to the residence time of the reaction products in the high-temperature reaction zone. The similar EINO x trends with geometrical or fluid mechanical parameters obtained without taking flame radiation or flame extinction into account reveals that the residence time is the dominant physical parameter associated with NO x scaling for the present flames.

A unified analysis for fire plumes

A unified analysis based on an integral approach is presented for fire plumes involving finite axisymmetric and rectangular sources. The analysis, using Gaussian profiles, obtains the best fits to experimental data found in the literature. Phenomenological constants in the theory are found to give consistent results in that coefficients expected to be numerically similar by theory are found similar among the various data sets. Thorough reviews of the literature data for line and rectangular sources are presented and yield consensus correlations, accordingly. The effect of flame radiation is explicitly included by a radiation fraction that proves to be a significant variable, previously overlooked in experiments. Effective entrainment coefficients for the far-field noncombusting plume are found to be 0.09 to 0.10, and for the flame region about 0.22 for the axisymmetric and rectangle cases as long as D/L >0.1. For D/L <0.1, the infinite line gives nearly double the flame entrainment. Generalized results are presented for entrainment rate and flame height in terms of single algebraic equations that span a wide range of Q * or energy release rate values. For low Q * fires, the effect of diameter and line width are important and expressed by the theory but not enough to address dependence involving Grashof and possibly Froude number effects near the origin. Laminar results are also addressed.

The effects of flame radiation on flame-tube metal temperatures

Details are given of experiments to determine flame radiation, Red Brightness temperatures and metal temperatures in the primary zones of three aircraft combustion chambers; flame temperatures and emissivities are derived. These factors were studied using two pipe chambers (3.75 in. and 5.625 in. diam) over a range of pressures from 4–30 atm and a nominal air-inlet temperature and air/fuel ratio of 800°K and 60/1, respectively. The indications were that flame radiation, temperature, and emissivity increased with pressure, whereas metal temperatures tended to reach a maximum at about 20 atm. Predicted wall temperatures indicated a similar trend, attributed to the convective cooling that increased at a greater rate than flame radiation at the higher pressures. The effects of vitiation and subsequent oxygen replenishment were examined using a tubo-annular combustion chamber operating at a pressure of 4 atm, inlet temperature 777°K, and a nominal air/fuel ratio of 63/1. Primary-zone flame radiation, flame temperature, and metal temperature increased with the oxygen content of the inlet gas. No similar trend was noted with flame emissivity.