Mechanical Engineering

Permanent URI for this communityhttp://hdl.handle.net/1903/2263

Browse

Search Results

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Prediction of Upward Flame Spread over Polymers
    (2016) Leventon, Isaac Tibor; Stoliarov, Stanislav I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this work, the existing understanding of flame spread dynamics is enhanced through an extensive study of the heat transfer from flames spreading vertically upwards across 5 cm wide, 20 cm tall samples of extruded Poly (Methyl Methacrylate) (PMMA). These experiments have provided highly spatially resolved measurements of flame to surface heat flux and material burning rate at the critical length scale of interest, with a level of accuracy and detail unmatched by previous empirical or computational studies. Using these measurements, a wall flame model was developed that describes a flame’s heat feedback profile (both in the continuous flame region and the thermal plume above) solely as a function of material burning rate. Additional experiments were conducted to measure flame heat flux and sample mass loss rate as flames spread vertically upwards over the surface of seven other commonly used polymers, two of which are glass reinforced composite materials. Using these measurements, our wall flame model has been generalized such that it can predict heat feedback from flames supported by a wide range of materials. For the seven materials tested here – which present a varied range of burning behaviors including dripping, polymer melt flow, sample burnout, and heavy soot formation – model-predicted flame heat flux has been shown to match experimental measurements (taken across the full length of the flame) with an average accuracy of 3.9 kW m-2 (approximately 10 – 15 % of peak measured flame heat flux). This flame model has since been coupled with a powerful solid phase pyrolysis solver, ThermaKin2D, which computes the transient rate of gaseous fuel production of constituents of a pyrolyzing solid in response to an external heat flux, based on fundamental physical and chemical properties. Together, this unified model captures the two fundamental controlling mechanisms of upward flame spread – gas phase flame heat transfer and solid phase material degradation. This has enabled simulations of flame spread dynamics with a reasonable computational cost and accuracy beyond that of current models. This unified model of material degradation provides the framework to quantitatively study material burning behavior in response to a wide range of common fire scenarios.
  • Thumbnail Image
    Item
    Large Eddy Simulation of Boundary Layer Combustion
    (2013) Bravo, Luis Giovanni; Trouve, Arnaud C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Numerical simulations of turbulent non-premixed flames occurring in the presence of solid surfaces is a prevalent topic of interest due to the complexity of the near wall physics and the technical modeling challenges it presents. Near wall combustion phenomena is relevant in a variety of combusting environments including but not limited to the occurrence of fire spread, as a result of a heating load to a flammable wall leading to fire growth in enclosure settings; and in engine combustion configurations where the interaction with a cooled surface combined with occurrences of short flame wall distances can lead to extinction events adversely affecting combustion performance. The interaction between the flame and surface can result in a reduction of flame strength near the cold wall region while gas phase heat fluxes can take peak values at flame contact. To address the aforementioned modeling challenges, an advanced computational fluid dynamics (CFD) solver has been developed by adapting a preexisting numerical simulation solver from a boundary layer code to a code with variable mass density and combustion capabilities to produce high-fidelity simulations of turbulent non-premixed wall-flames. A series of verification studies have been developed using several benchmark laminar flow problems for the following canonical configurations: a binary diffusion controlled mixing problem, Poiseuille flow with heat transfer, and classical Blasius boundary layer flow. The turbulence LES modeling capability is validated by performing wall-resolved heated/non-heated turbulent channel flow and transpired boundary layer simulations to capture the effects of heat and mass transfer on the turbulent eddy structure and statistics. Lastly, an application of a simplified non-premixed wall flame configuration is presented in which the fuel corresponds to pyrolysis products supplied by a thermally-degrading flat sample of polymethyl methacrylate (PMMA) and the oxidizer corresponds to a cross-flow of ambient air with controlled mean velocity and turbulence intensities. Comparisons between numerical results and experimental data are made in terms of flame length, wall surface heat flux and flame structure and the ability of the solver in modeling non-premixed turbulent wall-flames is successfully demonstrated. The solver extends the present state of the art in fire modeling (limited to laminar flows) by providing a high quality numerical tool to study the heat transfer aspects of turbulent wall flame phenomena