Doctoral Dissertations

Date of Award

8-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Engineering Science

Major Professor

A.J. Baker

Committee Members

T.G. Carley, K.H. Kim, R.M. Kelso

Abstract

Numerical fire simulations based on computational fluid dynamics (CFD), or field modeling, especially with compressible flows, three dimensions and turbulence have emerged and gained momentum since early 1980's. Nevertheless, none of the germane publications definitively determine the feasibility of buoyant incompressible laminar flows and applicability of thermal radiation. The primary objective of this research is to address these two issues. In addition, all these publications are based on either finite-difference or finite-volume algorithms, thereby rendering the application of finite-element method in this research area genuinely unique and original.

Assuming incompressible Navier-Stokes equations with constant fluid and thermophysical properties, AKCESS.*, a general-purpose finite-element CFD code handling any convective/diffusive fluid simulation, has been adopted. Thus, the major challenge or the crux of this research is to develop and verify thermal radiation submodels therein, which robustly and efficiently handle both non-participating and participating media in conjunction with ad hoc verifications of non-zero Neumann temperature BC's.

To lessen the degrees of freedom of the Navier-Stokes conservation law (or PDE) system, and to circumvent the cumbersome derivation of the spatial discretization for the convection-diffusion-radiation term in the energy equation, the divergence of the radiative flux vector based on Naraghi's methods [60,61] is treated as a legitimate source term constituting the radiation field computed from the temperature field from the previous time station. Due to numerical stability issues and the assumption of laminar flow, certain inconsistencies among fluid, radiation and fire dimensionless groups exist in these equations, resulting in use of a dual dimensionless systems: fluid, and radiation/fire.

An elementary fire scenario in a 3x3x3 m enclosed adiabatic room with a central 10-node column 250 kW fire has been assumed. For an uniform 2-D grid of 16x16 elements, the computational results based on Rayleigh number of 1.E6 reasonably correlate with the overall hot-layer temperature predicted by an experimentally validated 2-zone fire model. A uniform 3-D grid produces similar results. With respect to the temporal-spatial temperature profiles based on 2-D simulations of the first 15 seconds, the radiative source (or sink) terms do not impact the scalar fields due to several orders of magnitude difference between nodal fire and radiative sources. However, further investigations on more complex fire scenarios are needed to generalize this conclusion.

Despite the problematic dual dimensionless systems, current findings of a fundamental fire scenario with easily reproducible results demonstrate that incompressible laminar flows are viable candidates for realistic fire simulations, thereby bridging the dichotomy of over-simplified 2-zone semi-empirical fire models and full-fledged CFD-fire models with complex features that are highly restricted to research community use only.

In order to realistically estimate the combined net (or incident) radiative and conductive (or convective) flux onto an analyst-defined target such as a fire sprinkler, heat detector, fusible link or even human skin located some distance away from the fire plume, or to perform conjugate heat transfer analysis at the fluid-solid interface for structural integrity, one must utilize a similar radiation submodel. This truly original, unique technique is readily transformed into a practical fire protection engineering tool for fire damageability studies, hence advancing the art and science of fire technology.

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