Radiative Heat Flux Distribution Research Articles

Modelling flame-to-fuel heat transfer by deep learning and fire images

In numerical fire simulations, the calculation of thermal feedback from the flame to the solid and liquid fuel surface plays a critical role as it connects the fundamental gas-phase flame burning and condensed-phase fuel gasification. However, it is a computationally intensive task in CFD fire modelling methods because of the requirement of a high-resolution grid for calculating the interface heat transfer. This paper proposed a real-time prediction of the flame-to-fuel heat transfer by using simulated flame images and a computer-vision deep learning method. Different methanol pool fires were selected to produce the image database for training the model. As the pool diameters increase from 20 to 40 cm, the dominant flame-to-fuel heat transfer shifts from convection to radiation. Results show that the proposed AI algorithm trained by flame images can predict both the convective and radiative heat flux distributions on the condensed fuel surface with a relative error below 20%, based on the input of real-time flame morphology that can be captured by a larger grid size. Regardless of growing or decaying fires or puffing flames induced by buoyancy, this method can further predict the non-uniform distribution of heat transfer coefficient on the interface rather than using empirical correlations. This work demonstrates the use of AI and computer vision in accelerating numerical fire simulation, which helps simulate complex fire behaviours with simpler models and smaller computational costs. Highlights A total framework between AI model and fire simulation software is designed to further enhance the reliability of AI-based fire simulations. A standard pool fire simulation database is built using numerical model recommend by the IAFSS Computation Group. A deep learning model is developed to predict both the convective and radiative heat flux distributions on the condensed fuel surface using numerical images database. The demonstration showcases the application of AI and computer vision to accelerate numerical fire simulation.

Segmented Line Heat Source Model for Thermal Radiation Calculation of Jet Fires in Chemical Plants

Abstract The jet fire caused by the leakage of combustible materials is one of the biggest threats to the safety of chemical plants. Thermal radiation of the jet fire brings severe damage to nearby facilities and people's health. To evaluate the damage of jet fires, a precise model for the calculation of thermal radiation is indispensable. Classical thermal radiation models of jet fires either have a lower prediction accuracy or a higher computation complexity. To overcome such deficiencies, this paper proposes a novel segmented line heat source (SLHS) model for jet fires. Because the length of the jet fire is often much larger than the width, the jet fire is viewed as a line heat source, with all the heat radiated from the centerline of the jet fire. The jet fire is divided into three segments along the flame length according to the temperature distribution and thermal radiation characteristics of the flame. Based on the SLHS model, three types of thermal radiation models called cone-cylinder-cone, ellipsoid-cylinder-ellipsoid, and ellipsoid-cylinder-cone models are built for computing the radiant heat flux distribution around the jet fire. The effectiveness and advantages of the proposed models are illustrated with the experimental data and a numerical simulation.

Consequence analysis of heptane multiple pool fire in a dike

Multiple Pool Fire (MPF) reasonably enhances the flame height, the rate of fuel combustion and irradiation due to cascading effect and flame merging of multiple pools. The distribution of temperature and radiative heat flux from the source is the key parameter in predicting safety distance. The current study’s major objective is to develop a Computational Fluid Dynamics (CFD) model to calculate the safety distance of MPF and validate the predicted results (flame temperature, radiative heat flux, CO2 and O2 mass fraction) with the experimental data. The flame structure and its thermal properties have been evaluated using the Reynolds Averaged Navier Stokes (RANS) and the Large Eddy Simulation (LES) model. The CFD results are found to be in close agreement with the experimental findings. The LES turbulence model predicts more accurately the flame structures and the fluctuations with an error of less than 3%, while the Re-Normalization Group k-ε model predicts the average characteristics. This authenticates the accuracy of computational methodology and robustness of the LES turbulence model to predict MPF flame characteristics. Furthermore, this computational methodology can be utilised by the industries for quantitative risk assessment and the worst-case scenario of various pool fires to save the human and material of the workplace beforehand.

Effect of moisture release on radiation behavior of burning coal bed between enclosed confined surfaces

In this study, a numerical analysis was performed to predict the radiation characteristics of burning coal layers heated on confined surfaces in an enclosed chamber. The one-dimensional (1D) unsteady energy balance equation and radiative transfer equation (RTE) were solved simultaneously using the semi-implicit Runge–Kutta method (SIRK) and discrete ordinate method (DOM). In addition to the 1D analysis, a commercial software was used to analyze the two-dimensional (2D) radiation behavior of the burning coal layers using the DOM.The heat generation rate (relevant to the combustion rate) and particle absorption coefficient were independently determined using combustion and thermal emission experiments. The gas absorption coefficient was calculated from gray gas models to account for the change of H2O vapor along coal and gas mixture with high moisture content. The dimensionless radiative heat flux and temperature distribution were determined for the three calculation cases. Two different predictions based on 1D and 2D geometries were validated against measured heat flux and temperature data. The effect of the moisture release inside the particle layer was particularly examined. The result indicates that the condensation of H2O is probably more responsible than the absorption of H2O vapor for a reduced heat flux observed in the combustion condition of high moisture coal burning at high pressure.

Flame evolution and radiation hazards of rectangular fires between two parallel walls

The manuscript investigated experimentally the influence of two parallel walls on the flame geometric parameters and radiative heat flux distributions along the wall from fires on rectangular burners. Four sets of burners with equal area and different aspect ratios were used. The burner aspect ratios, separation distance between two parallel walls and the fire heat release rates were systematically varied in the tests. Measurements were conducted for the flame height evolution and radiation hazards. The results were analysed to build the change trends of the vertical flame height and radiant heat flux with the change of the separation distance between two parallel walls. A normalized flame height equation incorporating the separation distance was proposed. The results also revealed that the radiant heat fluxes along a vertical target do not change monotonously. Comparison between the measurements and the radiant heat fluxes calculated by some published empirical models revealed relatively large discrepancies. A view factor based formula was hence proposed by assuming the flame shape as a triangular prism based on the probability flame contours for relatively larger burner aspect ratios (n ≥ 3) and found to correlate well with the measurements.

Effects of longitudinal wind and sidewall restriction on downstream radiative heat flux and temperature rise distribution in tunnel fires

This paper presents an experimental study on downstream radiative heat flux and temperature rise distribution of tunnel fires under the combined effects of longitudinal wind and sidewall restriction. Two fire locations, i.e., centerline fire and sidewall fire, were considered, and the wind speeds were varied between 0 and 3 m/s. The results show that under smaller ventilation velocity, the downstream radiative heat flux of sidewall fire is larger than that of centerline fire. As the ventilation velocity increases, the radiative heat flux of two fire locations both firstly increase and then decrease with increasing distance. The position of the peak radiative heat flux depends on the evolution of the flame drag and moves away from the fire center with increasing wind speed and heat release rate (HRR). Based on the solid flame assumption, thermal radiation models are proposed and validated by experimental results. A similar spacial distribution trend was observed for temperature rise with varying fire locations and wind speeds. The distance from peak temperature rise to fire center is related to the flame drag front and increases with HRR and ventilation velocity. Finally, correlations for peak temperature rise, its locations and the temperature rise attenuation beyond the peak value were established.

Analysis of radiative heat transfer in two-dimensional irregular geometries by developed immersed boundary–lattice Boltzmann method

The Immersed Boundary–lattice Boltzmann method (IB-LBM) is developed to analyze radiative heat transfer in two-dimensional (2D) irregular geometries. Lattice Boltzmann method (LBM) has been adopted to discretize the radiative heat transfer equation in an absorbing and emitting media, and the immersed boundary method (IBM) is used to model the boundaries of irregular geometries. Due to the mismatch between boundary and computational nodes in the Immersed Boundary method, a radiative density term based on the difference of radiation intensity is added to the lattice Boltzmann equation to satisfy the boundary condition. Radiative heat flux distribution is obtained for several irregular geometries which are maintained at an isothermal media or radiative equilibrium condition in the enclosure. The results are successfully validated by comparing the results obtained from other methods of irregular geometries analysis, such as the blocked-off domain, embedded boundary, and body-fitted grid method. According to the obtained results, their accuracy, and numerical cost, the combined IB-LBM benefits from the unique and worthy advantages of both IBM and LBM in solving the complicated analysis of 2D radiative heat transfer problems. Compared to the traditional methods used for the radiative analysis of irregular geometries, the immersed boundary method can solve the complexities of modeling curved boundaries effortlessly. It should be pointed that for optically thin media, the proposed model shows poor performance.

Jet fires involving releases of gas and solid particle

The high-pressure leakage of underground pipeline could entrain the nearby sands and soils, and thus cause the jet fire of gas-solid mixtures. In addition, the thermal runaway of Lithium ion batteries could also cause a typical gas-solid jet diffusion flame. In comparison with the gaseous jet diffusion flame and the gas-solid-air jet premixed flame, the gas-solid jet diffusion flame lacks of exploration. Thus, a new facility consisting of a jet fire apparatus and a sand feeder, was designed to explore the gas-solid jet fire. The test repeatability was justified for the proposed facility. The dynamical differences of gas-solid and gaseous jet fires are clarified. It is found that the gas-solid jet fire holds a larger lift-off height, a less visible flame height and a less radiative fraction than the purely gaseous jet fire. In particular, for gas-solid jet fires at high exit velocities, the lift-off height increases, but the visible flame height and radiative fraction seem to decrease, as the particle size of sand decreases to increase the jet height of sand. (1) Inside the flame volume, the sand plays a role of heat sink to decrease the flame temperature and thus the flame burning velocity. Moreover, in the lift-off region, the sand dilutes the gas fuel and isolates the surrounding oxygen to inhibit the combustion. Both effects of sand cause a large lift-off height, especially for the sand of little particle size. (2) A large lift-off height means the increase of the air mass entrained from flame base or the portion of the premixed flame to the whole flame. In addition, the sand particle-laden flow would significantly increase the ambient air entrainment rate of fire plume. Both effects result in a less visible flame height. (3) The portion of the premixed flame increases in the bottom, which reduces the total soot amount inside the whole flame volume. The decrease of the flame temperature and the total soot amount reduces the radiative fraction. (4) The sand significantly affects the flame geometry and radiative fraction, which alters the radiant heat flux distribution and challenges the development of thermal radiation model for gas-solid jet fires. The gas-solid jet fire holds a less peak radiant heat flux but a higher positon of peak radiant heat flux than the gaseous jet fire. In addition, the line source radiation model available in literature is revised to well predict the radiant heat flux distribution of the gas-solid jet fires.

Study on the merged flame height and radiative heat flux distribution from square propane fire arrays

SummaryThe remote ignition and accelerated flame spread in multiple fires accidents are mainly due to the thermal radiation. The merged flame height and external radiative heat flux distribution of the square fire arrays were investigated numerically. Different burner spacing, total number of burners and heat release rate (HRR) of an individual burner were considered. Results show that the fire plume centerline temperature can be predicted accurately by achieving grid independence on a fine enough grid. A correlation for predicting the merged flame height of the multiple fires is developed involving the number of burners, HRR of each burner and burner spacing. The comparison of calculated and simulated data in this work validated the accuracy of the proposed correlation. The radiative heat flux distributions surrounding the fire array can be divided into two regions, that is, the near‐ and far‐field regions. The total heat release rate of the fire array is shown to be the dominant effect on the radiation distribution throughout the whole region, whilst the burner spacing is shown to only have an effect in the near‐field region. To distinguish the two regions, the critical distance of 0.7 ( is the characteristic fire array length) was obtained.

Radiative properties of alumina/aluminum particles and influence on radiative heat transfer in solid rocket motor

The thermal radiation of micron-sized condensed phase particles plays a dominant role during the heat transfer process in aluminized Solid Rocket Motors (SRMs). Open research mainly focuses on the radiative properties of alumina particles while the study considering the presence of aluminum is lacking. In addition, the thermal radiation inside the SRM with consideration of the participating particles is seldom studied. In this work, the multiscale method of predicting the thermal environment inside SRMs is established from the particle radiation at microscale to the two-phase flow and heat transfer at macroscale. The effective gray radiative properties of individual particles (alumina, aluminum, and hybrid alumina/aluminum) and particles cloud are investigated with the Mie theory and approximate method. Then a numerical method for predicting the thermal environment inside SRMs with considering particle radiation is established and applied in a subscale motor. The convective and radiative heat flux distributions along inner wall of motor are obtained, and it is found that the heat transfer in the combustion chamber is dominated by thermal radiation and the radiative heat flux is essentially a constant of 5.6–6.8 MW/m2. The convective heat transfer plays a dominant role in the nozzle and the heat flux reaches the maximum value of 11.2 MW/m2 near the throat. As the combustion efficiency of aluminum drops, the radiative heat flux remains unchanged in most regions and increases slightly along the diverging section wall of the nozzle.

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.

Experimental study and analysis of radiation heat fluxes received by a floor beneath an inclined ceiling

SummaryThis study experimentally investigates the radiation heat flux distribution received on the floor due to fire plume impinging upon an inclined ceiling, which has not been quantified previously. The radiation heat fluxes were measured on the floor for 160 experimental conditions, involving various fire source heat release rates, source‐ceiling heights, angles of ceiling inclination and dimensions (aspect ratios) of the rectangular sources. The main findings include that the declining rate of the radiation heat flux along with distance received by the downstream floor decreases, while that received by the upstream floor increases, with the increasing of ceiling inclination angle. The radiation heat flux received by the floor is higher as the ceiling inclination angle is smaller for the downstream side, while it is lower as the ceiling inclination angle is smaller for the upstream side. Both of these variations can be explained by change of the flow distribution as well as flame length due to combustion and heat released in the two directions beneath the inclined ceiling. Further, a model with various fire source heat release rates, source‐ceiling height, and ceiling inclination angles is proposed, to globally describe the radiation heat flux received by both the upstream and downstream floors.

Conductive-radiative heat transfer in a 2D semi-transparent medium with a square centered obstacle

The present paper deals with an exact semi-analytical formulation of a combined conductive-radiative heat transfer, applied to a two-dimensional semi-transparent medium carrying a square centered obstacle. The gray participating medium with black boundaries absorbs, emits but does not scatter radiation. One intends to evaluate the temperature and radiative heat flux distributions within the semi-transparent medium. The radiative transfer equation has been solved using an exact analytical expansion of Bickley-Naylor and Altaç angular integrated Bickley-Naylor functions, then solved numerically with Gauss quadrature. Energy equation has been directly discretized and approximated numerically using the centered finite differences method and consequently the dimensionless temperature has been obtained after an iterative scheme. The results of radiative quantities obtained have been verified with benchmark with an excellent agreement, both for simple and complex geometries. Simulations have been performed to obtain results for different sizes of the centered obstacle and the optical thickness. The effects of the conduction-radiation parameters, discrete directions and the size of the obstacle have also been investigated.

Numerical simulation of thermal response behavior of floating-roof tanks exposed to pool fire

In industrial plants, the fire spread of separative fuel tanks could be related to the tank rupture caused by the thermal radiation from nearby fires. The present paper develops a theoretical framework for analyzing the thermal response behavior and failure time of floating-roof atmospheric storage tanks when exposed to a pool fire. Multi-layer cylindrical flame radiation model is used to predict the radiant heat flux field on the surface of tanks. This radiant heat, considered as the thermal load for a finite element analysis (FEA), influences the tank transient response under thermo-mechanical coupling effects. The paper investigates the respective and combined effects of the tank thermo-physical properties, the non-uniform radiant heat flux distribution, the generated stress as well as the failure time of target tank. For comparative purposes, an empty tank is used as a reference. The results show that the empty and full-filled storage tanks differ in their thermal response modes and their thermos-mechanical behaviors. Within the values in the temperature range of interest adopted for the tank thermo-physical properties, their variations with the temperature have little influence on the tanks’ behavior. The non-uniform radiant heat along the vertical and around circumferential directions of tank has a significant effect on the tanks’ behavior. The stored liquid mediums considerably affect the tanks’ behavior. The failure time of empty tank is over three times as long as that of full-filled storage tank.

Tomography-based pore level analysis of combined conductive-radiative heat transfer in an open-cell metallic foam

Open-cell metallic foams are increasingly used in various thermal systems. The temperature distributions are significant for the comprehensive understanding of the foam-based engineering applications. This study aims to numerically analyze combined conductive-radiative transfer in a tomography-based open-cell metallic foam. The method used to carry out the simulations consists of a Monte Carlo ray-tracing method based on mesh-agglomeration approach, which avoids the error caused by block-off treatment of porous media surfaces, while greatly minimizing the computation cost for solving the Radiative Transfer Equation. A validation test case has been considered for the distribution of wall heat flux against other benchmark available in the literature. The detailed absorbed radiative flux at local strut surface is analyzed for the real nickel foam, and great difference up to 100% between the front and the back of the strut is found. In addition, the effects of average temperature (i.e. the arithmetic mean temperature of the cold and hot boundary) on the distribution of temperature, the absorbed radiative flux and radiative heat flux are investigated. The results show great fluctuation of absorbed radiative flux and radiative heat flux distribution due to the irregular porous structure. On this basis, the effective thermal conductivity and radiative conductivity of nickel foam at different temperatures are also evaluated. Results show when the temperature increases from 800 K to 1100 K, the radiative conductivity increases from 0.18 W/m•K to 0.46 W/m•K, and the effective thermal conductivity decreases from 4.81 W/m•K to 4.76 W/m•K first and then increases to 4.92 W/m•K, which indicates though the value of radiative conductivity is relatively small, it still influences the variation trend of the effective thermal conductivity with the temperature. The results also show that radiative thermal conductivity and the effective thermal conductivity are strongly dependent upon porosity.

Experimental study of radiative heat transfer from laminar non-premixed methane flames diluted with CO2 and N2

This paper presents an experimental study about the effect of fuel dilution with inert gases on the radiative heat transfer characteristics of laminar non-premixed coflow methane flames. The visible flame lengths were determined and flickering is characterized when necessary. Radiative heat flux distributions were measured and used to determine the radiative fraction of each flame. In total, 55 flames were studied with powers up to 0.554 kW and with CO2 and N2 dilutions up to 60%. The flames were grouped in series of constant power and constant flow rate. It is shown that the addition of CO2 may decrease or increase the radiative fraction depending on the dilution level and on the characteristic residence time of the flame. Dilution with N2 always leads to a decrease of the radiative fraction. The results are relevant for practical applications as in biogas combustion and also proved to be useful for validation of radiative heat transfer models.

Application of decentralized fuzzy inference method for the inverse geometry design of radiative enclosures

The decentralized fuzzy inference method (DFIM) is applied to solve the inverse geometry design of a 2D radiative enclosure filled with gray participating medium. The heat transfer process in the irregular enclosure is solved using the discrete ordinate method in body-fitted coordinate. The simulated surface heat flux is employed as measurement signals. The design task is formulated as an inverse problem which is solved by the DFIM. Retrieval results illustrate that the design purpose of producing a uniform distribution of radiative heat flux on the design surface can be successfully satisfied using the present method. The DFIM is proved to be more efficient than other inverse techniques. Moreover, the effects of the number of measurement points and radiative properties of participating medium on the inverse design results are investigated. The DFIM is effective in designing the geometry of radiative enclosures for different kinds of participating media.

Experimental study of jet fire radiation and a new approach for optimizing the weighted multi-point source model by inverse methods

The present study brings new experimental data on radiation from subsonic jet fires and proposes a new method for employing the inverse analysis technique to optimize the weighting coefficients of the multi-point model. First, detailed measurements of radiative heat flux distributions are obtained for different flame powers, injector diameters and sensor distances. All results fall onto a single curve with an adequate non-dimensionalization. A new correlation for the radiative heat transfer rate from these flames was obtained with a better performance compared to other options from literature. Then, an inverse method is employed to obtain the coefficients of the weighted multi-point model for predicting the radiative heat transfer from flames. By using the non-dimensional heat flux distributions, the inverse method became independent of the flame radiated fraction. A new optimization parameter that emerges from the non-dimensional model showed the consistency of the inverse method and leads to a correlation for the maximum heat flux as a function of the radial distance from the flame. Finally, the new weighting coefficients and the radiative heat transfer rate correlation were used to compute the dimensional radiative heat flux distributions. The comparison with the experimental data showed expressive improvements of the model prediction.

Numerical study of inlet air swirl intensity effect of a Methane-Air Diffusion Flame on its combustion characteristics

In this paper, the effect of inlet air swirl number of a Methane-Air Diffusion Flame on dynamic flow behavior, temperature, and radiation heat flux distribution was investigated using ANSYS-Fluent CFD code. Based on the swirling effect on dynamic flow behavior, a specific equation in terms of axial and tangential velocity components was used to reach the swirl number. The modeling of the chemical reaction was carried out by applying the Eddy Dissipation Model (EDM). Furthermore, radiation heat flux and turbulent flow characteristics were performed by using P-1 and standard k-ϵ models.The results showed that the elevating swirl number of the inlet air from 0.0 to 0.6 develops the furnace internal recirculation zone which leads to producing the combustion products in the internal recirculation zone. Consequently, fuel and air are mixed more efficiently, which results in the enhancement of combustion efficiency by removing the high-temperature zones as the leading cause of producing nitrogen oxides (NOx). Moreover, as the swirl number increases, the radial flow distribution improves, and the flame heat exchange area enhances regardless of the maximum flame temperature reduction, which will increase the flux radiation efficiency by 36.5% and reduces the pollutant NOx by 58.6%.

Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection

The aim of the present work is to investigate the effect of low-flow-rate injection of silver-water nanofluid on the enhancement of thermal and radiative characteristics and reduction of NOx emission in natural gas flame. In our method, 0.009 Lit/min of 100-ppm silver-water nanofluid is directly injected into the natural gas flame at the equivalence ratio of 0.65. An SBG01 water cooled heat flux sensor is used to measure the total radiation of flame and IR photography method and luminosity measurement are used to determine the spectral emissivity of flame in infrared and visible wavelengths. Furthermore, a KIGAS 310 gas analyzer measures the concentrations of pollutant emissions. The results indicate that nanofluid injection enhances the IR radiation of natural gas flame and creates a high-value and uniform radiative heat flux distribution for flame. Solid nanoparticles, as foreign surfaces, enhance the nucleation rate and surface growth of soot particles, which are the best radiative species in flame. The other parameters which affect the flame radiation are increase in H2O concentration, as another radiative species, and increased number of tiny high temperature dark nanoparticles, as highly emissive bodies. Furthermore, although the nanofluid injection does not significantly change the concentration of carbonaceous pollutant emission, it decreases the NOx emission as much as 22.6%. The reduction of NOx emission is caused by decrease in the maximum flame temperature, due to absorption of heat by nanofluid droplets, and increase in the concentration of OH radicals, due to dissociation of H2O at the high temperatures of flame reaction zone.