Six-flux Model Research Articles

The Development of a CFD Based Simulator for Water Mist Suppression Systems: The Development of the Fire Submodel

The FIRE Detection and Suppression Simulation (FIREDASS) project was concerned with the development of water misting systems as a possible replacement for halon based fire suppression systems currently used in aircraft cargo holds and ship engine rooms. As part of this program of work, a computational model was developed to assist engineers optimize the design of water mist suppression systems. The model is based on Computational Fluid Dynamics (CFD) and comprised of the following components: fire model; mist model; two-phase radiation model; suppression model; detector/activation model. In this paper the FIREDASS software package is described and the theory behind the fire and radiation sub-models is detailed. The fire model uses prescribed release rates for heat and gaseous combustion products to represent the fire load. Typical release rates have been determined through experimentation. The radiation model is a six-flux model coupled to the gas (and mist) phase. As part of the FIREDASS project, a detailed series of fire experiments were conducted in order to validate the fire model. Model predictions are compared with data from these experiments and good agreement is found.

Radiative heat transfer in natural gas-fired furnaces

The performance of the discrete transfer and of the six-flux radiation models is assessed in a swirling natural gas diffusion flame confined in an axisymmetric furnace. The predictions are evaluated as part of a complete prediction procedure involving the modeling of the simultaneously occurring flow, combustion, convection and radiation phenomena. Computational results with and without radiation effects are compared with experimental data and the two radiation models are evaluated in terms of computational efficiency, ease of application and predictive accuracy. The results have demonstrated that the effect of thermal radiation is important even in light flames, and that the six-flux model can be applied in industrial gas furnaces with relative ease, yielding accurate predictions.

Numerical Modeling of Radiative Heat Transfer in Integrated CFD Fire Modeling

Thermal radiation, although considered an important mode of heat transfer in high temperature conditions, is often neglected in fire modeling, mainly due to the complex physics involved. The aim of this study is to provide modelers with guidance on the engineering treatment of radiation transfer; two widely used radiation models are reviewed and their performance is assessed in a benchmark experimental enclosure fire case. The discrete transfer and the six-flux radiation models are compared in terms of computational efficiency, ease of application, and predictive accuracy. The predictions are evaluated as part of a complete prediction procedure involving the modeling of the simultaneously occurring flow, convection, and radiation phenomena. The results have demonstrated that the effect of thermal radiation is important, even in small fires. Comparison between the two models indicates that the simple six-flux model suffices for small compartment fires, up to 100 kW. Language: en

Performance of numerical spray combustion simulation

The aim of this study is to examine the performance of numerical spray combustion simulation. A numerical simulation for the prediction of local properties of heavy oil spray flames stabilized by a baffle plate is described. Time-averaged governing conservation equations are solved to estimate the combustion gas flow, gas composition and temperature fields in the experimental combustor. The κ-ε turbulence model is used to describe the turbulent flow field. The behavior of fuel droplets in the turbulent combustion gas flow is calculated by the Lagrangian method. The combustion rate of fuel vapor is estimated by the eddy dissipation model. The effects of radiation are accounted for by the six-flux model of radiation. The performance of the simulation is examined by comparison with measured data. In the isothermal (cold) case, the calculated flow pattern is compared with the data measured by LDA, and it is clear that the calculated results show quantitative agreement with the measured data. In the combustion case, however, the simulation cannot predict well the measured profiles of temperature, O 2 and CO 2 concentrations near the baffle plate. It is inferred that this simulation cannot estimate accurately the interaction between the recirculation flow induced by the baffle plate and fuel droplets.

Numerical prediction of flame spread over solid combustibles in a microgravity environment

Abstract A computational model of three-dimensional, time-dependent flame spread in a microgravity environment is presented. The solid is assumed to be a thermally thin, pyrolysing cellulosic sheet. The gas-phase model includes the full Navier-Stokes equations with density and pressure variations and a six-flux model of radiation heat transfer. The solid phase model consists of continuity and energy equations whose solution provides boundary conditions for the gas phase equations. In the numerical procedure, the gas- and solid-phase equations are solved separately and iteratively at each time step. Predictions have been made of flame spread in slow forced flow under gravitational acceleration normal to the fuel surface and flame spread in a quiescent environment in an enclosed chamber under gravitational acceleration parallel to the fuel surface. Numerical simulations show that, under microgravity and slow-flow conditions, the flame spread process is highly unsteady with the upstream flame spreading faster than the downstream flame after a period of ignition. It has also been shown that the level of microgravity has a significant effect on the flame spread process.

Evaluation of flux models for radiative transfer in rectangular furnaces

Three flux-type models for three-dimensional radiative heat transfer were applied to the prediction of the radiative flux density and the source term of a box-shaped enclosure problem based on data reported previously on a large-scale experimental furnace with steep temperature gradients. The models, which are a six-term discrete ordinate model and two Schuster-Schwarzschild type six-flux models, were evaluated from the viewpoints of both predictive accuracy and computational economy by comparing their predictions with exact solutions produced previously. The comparison showed that the six-flux model based on angular subdivisions related to the enclosure geometry produces more accurate results and is computationally less expensive than the other two models.

An assessment of techniques for predicting radiation transfer in aqueous media

Five methods of determining radiative transfer in aqueous suspensions have been compared on the basis of their ability to predict radiative flux and absorption profiles within the suspension, as well as overall absorption and reflection by the suspension. They include the forward scattering technique, the delta-Eddington method, a three-flux technique, the six-flux model, and the more rigorous method of discrete ordinates. Calculations have been performed for a planar aqueous suspension over a wide range of independent parameters which include the scattering albedo, bottom reflectance, overall optical depth, and directional distribution of the incident radiation. Reasonable agreement is obtained between the three-flux, six-flux and discrete ordinate results for the range of conditions. In contrast suspension absorption effects are consistently underpredicted and overpredicted, respectively, by the forward scattering and delta-Eddington methods.

Prediction of Radiation Absorption and Scattering in Turbid Water Bodies

In this study four methods of solving the radiative transfer equation applied to shallow ponds have been compared. The methods include a simple version of Beer’s law, a representative two-flux model, a six-flux model and the method of discrete ordinates. Calculations have been performed for a wide range of representative conditions typical to natural water. It was found that scattering was unimportant when the scattering albedo is small (ω ≲ 0.5) and the phase function is highly forward peaked. Consequently, even the simple Beer’s law expression yielded accurate predictions. However, in general only the six-flux method provided a suitable compromise between accuracy and operational convenience for all the conditions considered. The two-flux method was found to be the least accurate primarily due to its inability to account for refraction.

Comparison of predictions with measurements for radiative transfer in an algal suspension

The radiation field within a scattering—absorbing suspension of unicellular algae has been determined experimentally and theoretically. A submerged fiber optic probe was used to measure the angular variation of the radiation intensity at different depths in a diffusely irradiated culture of Chlorella pyrenoidosa. The measurements were also used to determine the variation of the radiative flux with depth in the culture. The measured directional distributions agree well with predictions of the radiation field based on the method of discrete ordinates. Good agreement with the data for the radiative flux is also obtained from predictions based on a six-flux model as well as on the method of discrete ordinates. However, predictions based on the simpler two-flux and Beer's law models deviate significantly from the measured results.