Zhenghua Yan

CFD and experimental studies of room fire growth on wall lining materials

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.

A two-equation turbulence model and its application to a buoyant diffusion flame

A modified k–e 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–e 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–e turbulence model, this model gives significantly improved numerical results.

CFD Simulation Of Upward Flame Spread Over Fuel Surface

Two-dimensional upward flame spread and subsequent steady burning of a vertical PMMA surface was studied using CFD methodology. Both the turbulent combustion of the gas phase and the pyrolysis of the solid fuel were numerically simulated. The transpired wall function was used to calculate the convection heat transfer with the blowing effect considered. Radiation was considered by using the discrete transfer method. A fast narrowband computer model, FASTNB, which predicts the radiation properties of the combustion products in a general, non-isothermal and non-homogeneous combustion environment, was implemented for the solution of the radiation equation along every ray. An efficient, simple and practical pyrolysis model was adopted to describe the pyrolysis of the solid fuel. The sensitivity of the prediction to grid, time step interval and ray number was analysed. The calculated flame spread velocity, heat fluxes, and the steady burning rate, etc. were analysed and compared with experimental measurements. Good agreement was obtained.

FAST, NARROW-BAND COMPUTER MODEL FOR RADIATION CALCULATIONS

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 Grosshandlers 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.

A numerical study of effect of initial condition on large eddy simulation of thermal plume

Large eddy simulations of thermal plume in two different scenarios have been carried out using a self-developed parallel computational fluid dynamics (CFD) code, SMAFS (Smoke Movement And Flame Spread), with subgrid-scale turbulence modeled using the Smagorinsky model. Two different initial conditions were used in the simulations, and the results were compared to show that the initial condition has a significant effect on the prediction of the plumes evolution behavior. The filtered governing equations were discretized using the finite-volume method, with the variables at the cell faces in the finite-volume discrete equations approximated by a second-order bounded QUICK scheme and the diffusion term computed based on the central difference scheme. All the computations were explicitly time-marched, with the momentum equations solved based on a second-order fractional-step Adams-Bashford scheme and the enthalpy computed using a second-order Runge-Kutta method. The Poisson equation for pressure from the continuity equation was solved using a multigrid solver.

Three-dimensional computation of heat transfer from flames between vertical parallel walls

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. (Less)

Validation of CFD Model for Simulation of Spontaneous Ignition in Bio-mass Fuel Storage

Both numerical simulations and experimental measurements of small scale spontaneous ignition with different biomass fuels have been performed. In the experiments, temperature history was monitored at five different locations inside the fuel bed. The measured temperature history was used for validation of comprehensive threedimensional computer simulations which were carried out using a parallel finite volume CFD code SMAFS (Smoke Movement and Flame Spread) developed by the first author. The computation was based on numerical solution of a set of governing equations including the continuity equation, extended Darcy momentum equations, energy conservation equations for both gas and solid phases, and mass conservation equations for different chemical species. With reliable material properties input data provided by separate measurements, it simulated the temporal state evolution inside the biomass fuel storage. In the simulation, consideration was given to a series of essential physical and chemical processes, including convection and diffusion in porous media, evaporation, condensation and heat generation which is mainly due to chemical oxidation. Numerical results were compared with experimental measurements, showing excellent agreement. (Less)

Numerical investigations of rack storage fires

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.