• Nuclear Engineering and Technology
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• v.51 no.7
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• pp.1749-1757
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• 2019

Predicting lower flammability limits (LFL) of hydrogen has become an ever-important task for safety of nuclear industry. While numerous experimental studies have been conducted, LFL results applicable for the harsh environment are still lack of information. Our aim is to develop a calculated non-adiabatic flame temperature (CNAFT) model to better predict LFL of hydrogen mixtures in nuclear power plant. The developed model is unique for incorporating radiative heat loss during flame propagation using the CNAFT coefficient derived through previous studies of flame propagation. Our new model is more consistent with the experimental results for various mixtures compared to the previous model, which relied on calculated adiabatic flame temperature (CAFT) to predict the LFL without any consideration of heat loss. Limitation of the previous model could be explained clearly based on the CNAFT coefficient magnitude. The prediction accuracy for hydrogen mixtures at elevated initial temperatures and high helium content was improved substantially. The model reliability was confirmed for $H_2-air$ mixtures up to $300^{\circ}C$ and $H_2-air-He$ mixtures up to 50 vol % helium concentration. Therefore, the CNAFT model developed based on radiation heat loss is expected as the practical method for predicting LFL in hydrogen risk analysis.

• 한국전산유체공학회:학술대회논문집
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• 1999.05a
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• pp.199-204
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• 1999

Three dimensional numerical analysis for the turbulent reactive flow and radiative heat transfer in the walking beam type of a reheat furnace in POSCO has been carried out by the industrial code FLUENT. Computations an based on the conservation equations of mass, momentum, energy and species with the $k-{\varepsilon}$ turbulence model and mixture fraction/PDF(Probability Density Function) approach for the combustion rate. Radiative heat transfer is computed by the discrete ordinates radiation model in combination with the weighted-sum-of-gray-gas model for the absorption coefficient of gas medium. The predicted temperture distribution in the reheat furnace and energy flow fractions are in reasonable agreement with the measurement data.

• Nuclear Engineering and Technology
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• v.54 no.4
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• pp.1321-1335
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• 2022

This study was conducted to investigate the effect of absorption coefficient models on the P1 radiation model for a premixed hydrogen flame containing the water vapor. A CFD combustion simulation analysis was performed using XiFoam, one of the open-source CFD solvers in OpenFOAM. The solver using the flamelet combustion model has been modified to implement radiative heat transfer. The absorption coefficient models used in this study the grey-mean model and constant model, and for comparison, case without radiation was added. This CFD simulation study consisted of benchmarking the THAI HD-15 and HD-22 experiments. The difference between the two tests is the inclusion of water vapor in the condition before ignition. In the case of the HD-22 experiment containing water vapor in the initial condition, the simulation results show that the grey-mean absorption coefficient model has a strong influence on the temperature decrease of the flame and on the change in pressure inside the vessel.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.265-270
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• 1989

The present study considers the thermal structure variation in a porous medium caused by changing the most important radiative property of porous medium, absorption coefficient, as well as altering the physical dimension of porous medium and the equivalence ratio of premixed gas mixture. The radiation model was introduced as an unsteady differential form using the two-flux gray radiation model. The role of the conductive heat transfer through both gas phase and porous medium was found to be almost insignificant compared with that of the radiative heat transfer. The reaction zone shifted upstream and the flame thickness decreased as either the geometrical length of porous medium increased or the absorption coefficient decreased.

• Journal of the Korean Solar Energy Society
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• v.29 no.6
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• pp.32-38
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• 2009

The effects of surface radiative properties on the radiant cooling of opaque surfaces under clear sky condition are studied. Two types of surfaces, one gray and the other selective, are compared. For the nighttime cooling, black surface gives the lowest plate temperature and on the other hand the ideal selective surface gives the highest temperature. The reverse is true when there is an insolation. Equivalent radiative heat transfer coefficient of radiant cooling without convection is about $1{\sim}7\;W/m^2-K$ for the range of values studied. The surface with black within the $6{\sim}13\;{\mu}m$ band else zero emissivity could be regarded as a black surface for the nighttime radiant cooling purposes. However, lower band limit of $4\;{\mu}m$ is preferred to $6\;{\mu}m$ for small insolation situations.

• Journal of the Korean Society of Combustion
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• v.2 no.1
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• pp.1-8
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• 1997

Premixed combustion within porous ceramic media is numerically studied to understand burning characteristics and to find best configurations for burner implementations. Among many parameters, critical to burner performance, flame location and extinction coefficient are selected as major parameters for this study. The flame structure and burner performance with respect to these two parameters are observed. In the study, it is found that the location of flame is the most important in porous burner operation since it affects the rate of heat transfer and flame structure. Stability of the flame within the porous ceramic burner is discussed with respect to the flame location. It is found that to obtain high radiative output, the flame should be located downstream section of the burner. But the flame is to be unstable at most of downstream section except near the exit plane. To overcome this problem, new porous ceramic burner, using different ceramic properties in one burner instead of single property ceramic, is made and tested. With a combination of ceramics of high extinction coefficient at upstream and another material of low extinction coefficient at downstream of the burner, the flame can be stabilized at wider region of the burner with higher radiative output compared to the original burner configuration.

• Solar Energy
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• v.15 no.3
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• pp.91-103
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• 1995

Non-gray radiation with convection in the entrance region of a smooth tube is numerically investigated. The fluid is a mixture of carbon dioxide, water vapor, and nitrogen to simulate combustion products of propane. The flow is assumed to be laminar and hydrodynamically and thermally developing. The P-1 approximation is used to simplify the radiative transfer equation and the exponential wide band model is adapted to model the spectral absorption coefficients of non-gray gas mixture. The bulk mean temperature and Nusselt number variation along the tube axis are shown for several inlet and wall temperature pairs to show the effect of temperature on the heat transfer characteristics. Nusselt numbers for simultaneously developing flow are compared to those for thermally developing flow. In addition, the effect of the mole fraction of the non-gray gases on convective and radiative Nusselt numbers is investigated.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.2
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• pp.235-251
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• 1997

A numerical simulation on the combined natural convection and radiation is carried out in a partially open rectangular enclosure with a heater by using the finite volume and the S-8 discrete ordinate methods. The fluid inside the enclosure is considered as an absorbing, emitting and anisotropic scattering media. The heater causes a natural circulation of the fluid (10$^{5}$ $^{9}$ ) which results in significant in-flow of the ambient cold fluid through the partially open wall. Comparing the results of pure convection with those of the combined convection- radiation, the combined heat transfer results with small Planck numbers (P$_{l}$ <1.0) show much stronger circulation than those of the pure convection, and the fluid circulation is more evident for larger Rayleigh numbers. When one of three radiative properties - the medium absorption coefficient, the wall reflectivity, and the scattering albedo - increases, the fluid circulation and the heat transfer in the enclosure are reduced. The location of the heater and the open ratio of the right wall are also shown to affect the fluid circulation and heat transfer significantly. However, the anisotropy of the scattering phase function is shown to be unimportant for the fluid circulation and heat transfer within the enclosure considered in this study.

• Journal of the Korean Society of Industry Convergence
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• v.6 no.1
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• pp.65-71
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• 2003

Infrared heating has been traditionally used in industrial applications for processes such as dehydration of food industrial. This heating method involves the application of radiation in the wavelength range of 5~50 micrometers. In this work, simultaneous heat balance equations were developed to simulate the infrared radiation heating of red peppers. The equations assume that moisture diffuses to the outer boundaries of the material in liquid form and evaporation occurs at the surface of the red peppers. Energy for moisture evaporation is supplied by the infrared radiant energy. The equations were validated with experimental data on surface temperature and average moisture content of red peppers. Average deviations of predicted surface red peppers temperature and average red peppers moisture from experimental data were 323~353K and 50~80%, respectively. The spectral extinction coefficients in the wavelength range $1.5<{\lambda}<27$ micrometer at 293K for Red Peppers were determined from results of reflection measurements and the four flux radiative heat transfer calculation. The radiation extinction coefficients were obtained from effective drying factor the temperature 373K.

• Journal of the Korean Society of Industry Convergence
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• v.8 no.1
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• pp.47-55
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• 2005

Infrared heating has been traditionally used in industrial applications for processes such as dehydration of food industrial. This heating method involves the application of radiation in the wavelength range of 2 to 50 micrometers. In this work, simultaneous heat balance equations were developed to simulate the infrared radiation heating of agricultural products. The equations assume that moisture diffuses to the outer boundaries of the material in liquid form and evaporation occurs at the surface of the agricultural products. Energy for moisture evaporation is supplied by the infrared radiant energy. The optimum temperature and drying time for the best drying conditions of changing the red peppers with the moisture content of 18% and the restore rate of 80~85% are $80^{\circ}C$ and 44 hours. The performance of radiation tubes coating with the radiation paint developed in this research has the energy of $2.27{\times}103W/m^2{\mu}m$, $150^{\circ}C$ within the scope of radiation wave length of $2{\sim}30{\mu}m$ and has the radiation 0.92~0.93, which is superior to the general radiation tubes. The extinction coefficient according to the band pass filter using the 4 flux theory ha higher dependability on wave length, accounting for $2{\sim}17{\mu}m$ and $5{\times}105{\sim}106m-1$. A comparison between the theoretical energy transfer whose figures are interpreted according to 4 flux theory and the experimental energy transfer of far infrared dryer leads to the findings of the agreement less than 5%.