| 1. |
- Yan, Zhenghua, et al.
(författare)
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A two-equation turbulence model and its application to a buoyant diffusion flame
- 1999
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Ingår i: International Journal of Heat and Mass Transfer. - 0017-9310. ; 42:7, s. 1305-1315
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Tidskriftsartikel (refereegranskat)abstract
- A modified k–ε two-equation turbulence model was developed to improve the consideration of the important buoyancy effect on turbulence and turbulent transport, which is a serious deficiency of the standard buoyancy-modified k–ε model. The present model was tested against both plane and axisymmetric thermal plumes and a buoyant diffusion flame. The model was found to be stable, computationally economic, promising and applicable to complex situations. The predicted plume spreading rates and velocity and temperature profiles agreed well with experimental measurements. When compared with the standard buoyancy-modified k–ε turbulence model, this model gives significantly improved numerical results.
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| 2. |
- Yan, Zhenghua, et al.
(författare)
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CFD and experimental studies of room fire growth on wall lining materials
- 1996
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Ingår i: Fire Safety Journal. - 0379-7112. ; 27:3, s. 201-238
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Tidskriftsartikel (refereegranskat)abstract
- CFD simulation and experimental tests have been carried out to study the room corner fire growth on combustible wall-lining materials. In the CFD simulation, the turbulent mass and heat transfer, and combustion were considered. The discrete transfer (DT) method was employed to calculate the radiation with an absorptivity and emissivity model employed to predict the radiation property of combustion products including soot, CO2 and H2O, which are usually the primary radiating species in the combustion of hydrocarbon fuels. The temperature of the solid boundary was determined by numerical solution of the heat conduction equation. A simple and practical pyrolysis model was developed to describe the response of the solid fuel. This pyrolysis model was first tested against the Cone Calorimeter data for both charring and non-charring materials under different irradiance levels and then coupled to CFD calculations. Both full and one-third scale room corner fire growths on particle board were modelled with CFD. The calculation was tested with various numbers of rays and grid sizes, showing that the present choice gives practically grid- and ray number-independent predictions. The heat release rate, wall surface temperature, char depth, gas temperature and radiation flux are compared with experimental measurements. The results are reasonable and the comparison between prediction and experiment is fairly good and promising.
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| 3. |
- Yan, Zhenghua, et al.
(författare)
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Fast, narrow-band computer model for radiation calculations
- 1997
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Ingår i: Numerical Heat Transfer Part B: Fundamentals. - : Informa UK Limited. - 1040-7790 .- 1521-0626. ; 31:1, s. 61-71
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Tidskriftsartikel (refereegranskat)abstract
- A fast, narrow-band computer model, FASTNB, which predicts the radiation intensity in a general nonisothermal and nonhomogeneous combustion environment, has been developed. The spectral absorption coefficients of the combustion products, including carbon dioxide, water vapor, and soot, are calculated based on the narrow-band model. FASTNB provides an accurate calculation at reasonably high speed. Compared with Grosshandler's narrowband model, RADCAL, which has been verified quite extensively against experimental measurements, FASTNB is more than 20 times faster and gives almost exactly the same results.
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| 4. |
- Yan, Zhenghua, et al.
(författare)
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Numerical investigations of rack storage fires
- 1999
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Ingår i: Fire Safety Science. - 0925223255 ; , s. 1075-1086
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A number of numerical simulations of rack storage fires have been carried out, with various fuel types and burner outputs. Both the standard buoyancy-modified k - turbulence model and a recently developed turbulence model which significantly improves the consideration of the buoyancy effect on turbulence and turbulent transport, were used to study the turbulence of the buoyant flow. The flamelet concept, coupled to a prescribed probability density function, was employed to model the non-premixed combustion process. Sooting was modeled by solving the balance equations for mass fraction and number density considering nucleation, surface growth, coagulation and oxidation. The discrete transfer method was used to calculate radiation, with the radiation properties of the main radiating species - carbon dioxide, water vapour and soot, provided by a fast, narrowband model. The results, including heat flux and gas temperature profile, were analyzed and compared with experimental measurements. The comparisons showed considerably improved agreement for the new model. Copyright International Association for Fire Safety Science.
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| 5. |
- Yan, Zhenghua
(författare)
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Numerical Modeling of Turbulent Combustion and Flame Spread
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Theoretical models have been developed to address several important aspects of numerical modeling of turbulent combustion and flame spread. The developed models include a pyrolysis model for charring and non-charring solid materials, a fast narrow band radiation property evaluation model (FASTNB) and a turbulence model for buoyant flow and flame. In the pyrolysis model, a completely new algorithm has been proposed, where a moving dual mesh concept was developed and implemented. With this new concept, it provides proper spatial resolution for both temperature and density and automatically considers the regression of the surface of the non-charring solid material during its pyrolysis. It is simple, very efficient and applicable to both charring and non-charring materials. FASTNB speeds up significantly the evaluation of narrow band spectral radiation properties and thus provides a potential of applying narrow band model in numerical simulations of practical turbulent combustion. The turbulence model was developed to improve the consideration of buoyancy effect on turbulence and turbulent transport. It was found to be simple, promising and numerically stable. It has been tested against both plane and axisymmetric thermal plumes and an axisymmetric buoyant diffusion flame. When compared with the widely used standard buoyancy-modified model, it gives significant improvement on numerical results. These developed models have been fully incorporated into CFD (Computational Fluid Dynamics) code and coupled with other CFD sub-models, including the DT (Discrete Transfer) radiation model, EDC (Eddy Dissipation Concept) combustion model, flamelet combustion model, various soot models and transpired wall function. Comprehensive numerical simulations have been carried out to study soot formation and oxidation in turbulent buoyant diffusion flames, flame heat transfer and flame spread in fires. The gas temperature and velocity, soot volume fraction, wall surface temperature, char depth, radiation and convection heat fluxes, and heat release rate were calculated and compared with experimental measurements. In addition, to provide comprehensive data for comparison, experiments on room corner fire growth were undertaken, where the gas temperature, solid fuel surface temperature, radiative heat flux, char depth and heat release rate were all measured.
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| 6. |
- Yan, Zhenghua, et al.
(författare)
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Numerical Prediciton of Heat Flux from Flame in Room Fire
- 1997
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A number of CFD (Computational Fluid Dynamics) calculations were carried out to simulate the large scale room corner fire, which is an important scenario for the evaluation of the fire performance of the surface lining material. Considered are turbulent gas flows, turbulent combustion, radiation and heat conduction inside solid boundary. Heat transfer from flame and hot gas is calculated, with the important radiation component presented by discrete transfer (DT) method and the convection heat transfer considered by the wall function. An absorptivity and emissivity model was employed to predict the radiation property of combustion products including soot, CO2 and H2O, which are usually the primary radiating species in the combustion of hydrocarbon fuels. Configurations are a square burner flame in the corner of the standard full scale fire room, with three different standoff distances: 0 cm, 5 cm and 10 cm, and two different burner outputs: 40 kW and 150 ItW. Totally, six cases were studied. The results, including the temperature and heat fluxes, are discussed and compared with experimental measurements.
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| 7. |
- Yan, Zhenghua, et al.
(författare)
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Three-dimensional computation of heat transfer from flames between vertical parallel walls
- 1999
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Ingår i: Combustion and Flame. - 0010-2180. ; 117:3, s. 574-588
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Tidskriftsartikel (refereegranskat)abstract
- The heat transfer from turbulent diffusion flames between vertical walls has been computed for different wall and burner configurations. The buoyancy-modified k- model was used to study the turbulent characteristics of the flow. The flamelet concept, coupled to a prescribed probability density function, was employed to model the nonpremixed combustion process. With the nucleation, surface growth, coagulation, and oxidation considered, sooting was modeled by solving the balance equations for mass fraction and number density. The radiation from the main radiating species - carbon dioxide, water vapor and soot - was calculated using the discrete transfer method. A recently developed fast, narrow-band model was adopted to provide the radiation properties of the radiating species. Computations were performed for different cases by varying the wall separation and burner output. The results were analyzed and compared with experimental measurements, with which they showed good agreement. The effects of wall separation and burner output on heat transfer were faithfully reproduced.
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