Ronald G. Rehm

Large eddy simulations of smoke movement

An approach to field modeling of fire phenomena in enclosures is presented. The conservation equations of mass, momentum and energy are calculated with sufficient temporal and spatial resolution to yield a truly three-dimensional, dynamic picture of the fire plume and its surroundings. The large-scale eddies are simulated directly and sub-grid scale motion is represented by a constant eddy viscosity. Efficient flow solving techniques make it possible to simulate fire scenarios using computational grids in excess of a million cells on modern workstations. Several examples of the methodology are presented.

The wildland-urban interface fire problem - current approaches and research needs

Wildfires that spread into wildland–urban interface (WUI) communities present significant challenges on several fronts. In the United States, the WUI accounts for a significant portion of wildland fire suppression and wildland fuel treatment costs. Methods to reduce structure losses are focussed on fuel treatments in either wildland fuels or residential fuels. There is a need for a well-characterised, systematic testing of these approaches across a range of community and structure types and fire conditions. Laboratory experiments, field measurements and fire behaviour models can be used to better determine the exposure conditions faced by communities and structures. The outcome of such an effort would be proven fuel treatment techniques for wildland and residential fuels, risk assessment strategies, economic cost analysis models, and test methods with representative exposure conditions for fire-resistant building designs and materials.

Numerical simulation of smoke plumes from large oil fires

Abstract A large eddy simulation (LES) model of smoke plumes generated by large outdoor pool fires is presented. The plume is described in terms of steady-state convective transport by a uniform ambient wind of heated gases and particulate matter introduced into a stably stratified atmosphere by a continuously burning fire. The Navier-Stokes equations in the Boussinesq approximation are solved numerically with a constant eddy viscosity representing dissipation on length scales below the resolution limits of the calculation. The effective Reynolds number is high enough to permit direct simulation of the large-scale mixing over two to three orders of magnitude in length scale. Particulate matter, or any non-reacting combustion ;product, is represented by Lagrangian particles which are advected by the fire-induced flow field. Background atmospheric motion is described in terms of the angular fluctuation of the prevailing wind, and represented by random perturbations to the mean particle paths. Results of the model are compared with two sets of field experiments.

Simulating fire whirls

A numerical investigation of swirling fire plumes is pursued to understand how swirl alters the plume dynamics and combustion. One example is the ‘fire whirl’ which is known to arise naturally during forest fires. This buoyancy-driven fire plume entrains ambient fluid as heated gases rise. Vorticity associated with a mechanism such as wind shear can be concentrated by the fire, creating a vortex core along the axis of the plume. The result is a whirling fire. The current approach considers the relationship between buoyancy and swirl using a configuration based on fixing the heat release rate of the fire and imposing circulation. Large-eddy methodologies are used in the numerical analyses. Results indicate that the structure of the fire plume is significantly altered when angular momentum is imparted to the ambient fluid. The vertical acceleration induced by buoyancy generates strain fields which stretch out the flames as they wrap around the nominal plume centreline. The whirling fire constricts radially and stretches the plume vertically.(Some figures in this article are in colour only in the electronic version; see www.iop.org)

Calculations of Three Dimensional Buoyant Plumes in Enclosures

Abstract A computational model of the three-dimensional buoyant convection and aerosol dynamics induced by a weak volumetric source of heat and mass is presented. The hydrodynamics is directly based on the time-dependent inviscid Boussinesq equations. No turbulence model or other empirical parameters are introduced. The use of Lagrangian particle tracking together with an exact solution of the Smoluchowski equation allows prediction of smoke aerosol transport and coagulation. The combined calculations represent predictions involving five independent variables. Flow features from three different configurations are illustrated with both Eulerian and Lagrangian displays of information. Sample aerosol coagulation results are compared with data reported from a wood fire. The computer resources required are discussed, and an assessment of the current feasibility of large-eddy simulations in fire research is made.

Simulation of smoke plumes from large pool fires

A large eddy simulation model of smoke plumes generated by large outdoor pool fires is presented. Theplume is described in terms of steady-state convective transport by a uniform ambient wind of heated gases and particulate matter introduced into a stably stratified atmosphere by a continuously burning fire. The Navier-Stokes equations in the Boussinesq approximation are solved numerically with a constant eddy viscosity representing dissipation on length scales below the resolution limits of the calculation. The effective Reynolds number is high enough to permit direct simulation of the large scale mixing over two to three orders of magnitude in length scale. Particulate matter, or any nonreacting combustion product, is represented by Lagrangian particles that are advected by the fire-induced flow field. Background atmospheric motion is described in terms of the angular fluctuation of the prevailing wind and represented by random perturbations to the mean particle paths. Sample computations are presented and compared with plumes generated by large crude oil pool fires. Also presented is an assessment of the potential environmental hazard posed by burning marine oil spills.

Large eddy simulation of buoyant turbulent pool fires

Three-dimensional large eddy simulations (LES) of two buoyant flows were performed in conjunction with a Smagorisky turbulence model. The flows included a non-reacting helium plume previously simulated with two-dimensional simulations and a methane/air turbulent diffusion flame. In comparison with the previous two-dimensional simulation results, the three-dimensional LES results for the helium plume show much better agreement with the available experimental data. A relatively simple combustion model involving Lagrangian thermal elements with a single empirical constant involving the time to burn out of notional fuel parcels was adequate for obtaining reasonable predictions of the mean velocity and vorticity fields. The model also captured the temperature distribution patterns in the methane/air diffusion flames reasonably well. The agreement between the measurements and predictions for these two flows establishes the basic capabilities of LES for buoyant fire applications.

Computation of fire induced flow and smoke coagulation

Mathematical models for the calculation of the dynamics of smoke and hot gases induced by enclosure fires are presented. The models predict the evolution of the size distribution of smoke aerosol under the influence of coagulation, as well as the large scale fluid motion and temperature fields. The calculations contair three main ingredients: a finite difference solution of a hydrodynamics problem, the computer evaluation of an exact solution to the aerosol coagulation equation, and a Lagrangian particle tracking scheme to imbed the coagulation dynamics in the hydrodynamics. The hydrodynamics model is a time dependent variable density, two dimensional, infinite Grashof number flow driven by a prescribed heat source. No turbulence model is employed; the large scale eddy motion is calculated directly from the equations of motion. The mathematical particles each represent a large ensemble of aerosol particles, distributed initially in size according to the experimentally observed Junge distribution. They are introduced into the spatial grid in a random fashion near the heat source. The subsequent evolution of the size distribution in space and time is calculated deterministically from the solution to the Smoluchowski equation for the size distribution and the Lagrangian equations of motion for the spatial coordinates. Sample results of the hydrodynamic and aerosol properties are presented. Comparisons between calculations and relevant experiments are shown.

Fire-front propagation using the level set method

Propagation of an outdoor fire front in wildland or in a combination of wildland and structural fuels (the so-called wildland-urban interface or WUI fire), can be modelled as an initial-value problem using either a Lagrangian or an Eulerian description. The equations associated with each description are presented, and the methods used to solve the equations are discussed. Some comparisons between the two methods are also made. The emphasis in this report is on the Eulerian equations and on the level-set numerical method. Earlier studies had presented the Lagrangian formulation, and a method-of-lines solution. Advantages of the Eulerian/level-set method are discussed, and several examples that illustrate these advantages are presented.

Community-scale fire spread

This paper addresses community-scale fires, which have also been called urban/wildland interface or intermix fires. These fires arise when wildland fires invade the built environment and attack structures as well as wildland fuels. The prediction of the spread of wildland fires, such as those occurring out West during the summer of 2000, has been accomplished through ”operational” mathematical models. These models are based on empirical correlations for wildland fuels and have generally performed well. They fail, however, when the fire spreads to the built environment where the empirical correlations no longer apply and where there is greatly increased potential for property damage, injury and death. The Oakland and Berkeley Hills fire of October 21, 1991, and the Los Alamos fires of May 2000 are examples of community-scale fires. The potential fuel loadings for various land uses demonstrates that structures generally provide much higher loadings than wildlands do. While this comparison is useful, it could also be misleading since generally, not all of the potential fuel in either the wildland or the built environment will burn. Furthermore, often the time scales for ignition and the heat release rates for the wildland fuel and the fuel in the structures will be widely disparate, and these differences will influence both the spread rate of the fire and its persistence. Although the NIST computational model known as the Fire Dynamic Simulator (FDS) was developed to study building fires, it is now being extended to study community-scale fires. These extensions require much higher resolution data on local topography, buildings, vegetation, and meteorological conditions. They also require additional research on the mechanisms by which fires spread in the built environment between discrete elements, such as structures or structures and trees. This paper appeared as pp 126-139 in: Blonski, K.S., M.E. Morales and T.J. Morales, 2002. Proceedings of the California’s 2001 Wildfire Conference: Ten Years After the 1991 East Bay Hills Fire, 10-12 October 2001, Oakland California Technical Report 35.01.462. Richmond CA; University of California Forest Products Laboratory. Published by: University of California Agriculture & Natural Resources, Forest Products Laboratory, 1301 South 46th Street, Richmond CA 94804, www.ucfpl.ucop.edu. Proceedings of the California’s 2001 Wildfire Conference:10 Years After the 1991 East Bay Hills Fire1 COMMUNITY-SCALE FIRE SPREAD R.G. Rehm, A. Hamins, H.R. Baum, K.B. Mcgrattan and D.D. Evans, Building and Fire Research Laboratory, National Institute of Standards & Technology, Gaithersburg, MD 28099 Email: Ronald.Rehm@nist.gov ABSTRACT This paper addresses community-scale fires, which have also been called urban/wildland interface or intermix fires. These fires arise when wildland fires invade the built environment and attack structures as well as wildland fuels. The prediction of the spread of wildland fires, such as those occurring out West during the summer of 2000, has been accomplished through ”operational” mathematical models. These models are based on empirical correlations for wildland fuels and have generally performed well. They fail, however, when the fire spreads to the built environment where the empirical correlations no longer apply and where there is greatly increased potential for property damage, injury and death. The Oakland and Berkeley Hills fire of October 21, 1991, and the Los Alamos fires of May 2000 are examples of community-scale fires. The potential fuel loadings for various land uses demonstrates that structures generally provide much higher loadings than wildlands do. While this comparison is useful, it could also be misleading since generally, not all of the potential fuel in either the wildland or the built environment will burn. Furthermore, often the time scales for ignition and the heat release rates for the wildland fuel and the fuel in the structures will be widely disparate, and these differences will influence both the spread rate of the fire and its persistence. Although the NIST computational model known as the Fire Dynamic Simulator (FDS) was developed to study building fires, it is now being extended to study community-scale fires. These extensions require much higher resolution data on local topography, buildings, vegetation, and meteorological conditions. They also require additional research on the mechanisms by which fires spread in the built environment between discrete elements, such as structures or structures and trees.