Recent cpl_postprints items

Flame Height Measurement of Laminar Inverse Diffusion Flames

Mon, 2 Oct 2006 00:00:00 +0000

Flame heights of co-flowing cylindrical ethylene-air and methane-air laminar inverse diffusion flames were measured. The luminous flame height was found to be longer than the height of the reaction zone determined by planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH) because of luminous soot above the reaction zone. However, the location of the peak luminous signals along the centerline agreed very well with the OH flame height. Flame height predictions using Roper’s analysis for circular port burners agreed with measured reaction zone heights when using values for the characteristic diffusion coefficient and/or diffusion temperature somewhat different from those recommended by Roper. The fact that Roper’s analysis applies to inverse diffusion flames is evidence that inverse diffusion flames are similar in structure to normal diffusion flames.

Application of Genetic Algorithms and Thermogravimetry to Determine the Kinetics of Polyurethane Foam in Smoldering Combustion

Tue, 25 Apr 2006 00:00:00 +0000

In this work, the kinetic parameters governing the thermal and oxidative degradation of flexible polyurethane foam are determined using thermogravimetric data and a genetic algorithm. These kinetic parameters are needed in the theoretical modeling of the foam’s smoldering behavior. Experimental thermogravimetric mass-loss data are used to explore the kinetics of polyurethane foam and to propose a mechanism consisting of five reactions. A lumped model of solid mass-loss based on Arrhenius-type reaction rates and the five-step mechanism is developed to predict the polyurethane thermal degradation. The predictions are compared to the thermogravimetric measurements, and using a genetic algorithm, the method finds the kinetic and stoichiometric parameters that provide the best agreement between the lumped model and the experiments. To date, no study has attempted to describe both forward and opposed smolder-propagation with the same kinetic mechanism. Thus, in order to verify that...

SMOLDER IGNITION OF POLYURETHANE FOAM: EFFECT OF OXYGEN CONCENTRATION

Thu, 15 Sep 2005 00:00:00 +0000

Experiments have been conducted to study the ignition of both forward and opposed smolder of a high void fraction, flexible, polyurethane foam in a forced oxidizer flow. Tests are conducted in a small scale, vertically oriented, combustion chamber with supporting instrumentation. An electrically heated Nichrome wire heater placed between two porous ceramic disks, one of which is in complete contact with the foam surface, is used to supply the necessary power to ignite and sustain a smolder reaction. The gaseous oxidizer, metered via mass flow controllers, is forced through the foam and heater. A constant power is applied to the igniter for a given period of time and the resulting smolder is monitored to determine if smolder is sustained without the assistance of the heater, in which case smolder ignition is considered achieved. Reaction zone temperature and smolder propagation velocities are obtained from the temperature histories of thermocouples embedded at predetermined...

Space shuttle based microgravity smoldering combustion experiments

Thu, 15 Sep 2005 00:00:00 +0000

Results from four microgravity smoldering combustion experiments conducted aboard the NASA Space Shuttle are presented in this work. The experiments are part of the NASA funded Microgravity Smoldering Combustion (MSC) research program, aimed to study the smolder characteristics of porous combustible materials in a microgravity environment. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal- and microgravity. The microgravity smolder experiments reported here have been conducted to investigate the propagation of smolder through a polyurethane foam sample under both diffusion driven and opposed forced flow driven smoldering. The present experiments, although limited, are unique in that they provide the only available information about smolder combustion in microgravity in sample sizes large enough to allow the self-propagation of the smolder reaction throughout...

An enthalpy-temperature hybrid method for solving phase change problems and its application to polymer pyrolysis and ignition

Thu, 15 Sep 2005 00:00:00 +0000

In this work, an enthalpy-temperature hybrid method is proposed for the numerical solution of generalized phase change problems, and applied to the prediction of polymer pyrolysis and ignition. The basic idea of this method is to treat both enthalpy and temperature as independent variables, and to solve the conservation equations and the constitutive equations (enthalpy-temperature relations) simultaneously. The formula of the enthalpy-temperature relations are not necessary the same for different phases, but can be chosen independently according to the characteristics of physical problems and the convenience of numerical analysis for each respective phase. Therefore this method applies to the problems regardless of the form of the constitutive equations. It overcomes the difficulty or even impossibility encountered in the traditional enthalpy-temperature method, of which either enthalpy or temperature must be consistently and explicitly expressed as a function of the other...

Investigation of HCCI Combustion of Diethyl Ether and Ethanol Mixtures Using Carbon 14 Tracing and Numerical Simulations

Wed, 24 Aug 2005 00:00:00 +0000

Despite the rapid combustion typically experienced in Homogeneous Charge Compression Ignition (HCCI), components in fuel mixtures do not ignite in unison or burn equally. In our experiments and modeling of blends of diethyl ether (DEE) and ethanol (EtOH), the DEE led combustion and proceeded further toward completion, as indicated by 14C isotope tracing. A numerical model of HCCI combustion of DEE and EtOH mixtures supports the isotopic findings. Although both approaches lacked information on incompletely combusted intermediates plentiful in HCCI emissions, the numerical model and 14C tracing data agreed within the limitations of the single zone model. Despite the fact that DEE is more reactive than EtOH in HCCI engines, they are sufficiently similar that we did not observe a large elongation of energy release or significant reduction in inlet temperature required for light-off, both desired effects for the combustion event. This finding suggests that, in general, HCCI combustion...

A Comparison of Three Fire Models in the Simulation of Accidental Fires

Mon, 22 Aug 2005 00:00:00 +0000

The assumptions and the results of applying three fire modeling approaches to study three accidental fires that occurred in single-family dwellings, are presented in this work. The modeling approaches used are: a simplified analytical model of fire growth, a zone model (CFAST) and a field model (FDS). The fires predicted are: a house fire of suspected initial location but of unknown ignition source, a small-apartment fire initiated by the ignition of a sofa which extinguished due to oxygen depletion, and a one-story house fire started by a malfunctioning gas heater. The input to each model has been kept as independent as possible from the other models while consistent with the forensic evidences. The predictions from the models of the fires’ characteristics are analyzed in the context of the forensic evidences for each accidental fire to compare the models’ predictive capabilities. It is found that in spite of the differences in the sophistication of these three modeling approaches,...

Transition from Forward Smoldering to Flaming in Small Polyurethane Foam Samples

Sat, 23 Apr 2005 00:00:00 +0000

Experimental observations are presented of the effect of flow velocity, oxygen concentration, and a thermal radiant flux, on the transition from smoldering to flaming in forward smoldering of small samples of polyurethane foam with a gas/solid interface. The experiments are part of a project studying the transition from smoldering to flaming under conditions encountered in spacecraft facilities, i.e., microgravity, low velocity variable oxygen concentration flows. Because the microgravity experiments are planned for the International Space Station, the foam samples had to be limited in size for safety and launch mass reasons. The feasible sample size is too small for smolder to self propagate because of heat losses to the surroundings. Thus, the smolder propagation and the transition to flaming had to be assisted by reducing heat losses to the surroundings and increasing the oxygen concentration. The experiments are conducted with small parallelepiped samples vertically placed...

The effect of buoyancy on opposed smoldering

Thu, 10 Mar 2005 00:00:00 +0000

An experimental investigation on the effects of buoyancy on opposed-flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction of the propagation of the Smolder front. These data were compared with opposed-forced-flow tests conducted aboard STS-69, STS-77, and STS-105 and their ground-based simulations. Thermal measurements were made of the smolder reaction to obtain peak reaction temperatures and smolder velocities as a function of the ambient pressure in the MSC chamber. The smolder reaction was also observed using high-frequency ultrasound pulses as part of the ultrasound imaging system (UIS). The UIS measurements were used Lis a second means of providing smolder propagation velocities Lis well...

Forced forward smoldering experiments in microgravity

Thu, 10 Mar 2005 00:00:00 +0000

Results from two forward forced-flow smolder tests on polyurethane foam using air as oxidizer conducted aboard the NASA Space Shuttle (STS-105 and STS-108 missions) are presented in this work. The two tests provide the only presently available forward smolder data in microgravity. A complimentary series of ground-based tests were also conducted to determine, by comparison with the microgravity data, the effect of gravity on the forward smolder propagation. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal- and microgravity. The data consists of temperature histories from thermocouples placed at various axial locations along the fuel sample centerline, and of permeability histories obtained from ultrasonic transducer pairs also located at various axial positions in the fuel sample. A comparison of the tests conducted in normal- and microgravity indicates that smolder...

Modeling of One-Dimensional Smoldering of Polyurethane in Microgravity Conditions

Thu, 10 Mar 2005 00:00:00 +0000

Results are presented from a model of forward smoldering combustion of polyurethane foam in microgravity. The transient one-dimensional numerical-model is based on that developed at the University of Texas at Austin. The conservation equations of energy, species and mass in the porous solid and in the gas phases are numerically solved. The solid and the gas phase are not assumed to be in thermal or in chemical equilibrium. The chemical reactions modeled consist of foam oxidation and pyrolysis reactions, as well as char oxidation. The model has been modified to account for new polyurethane kinetics parameters and radial heat losses to the surrounding environment. The kinetics parameters are extracted from thermogravimetric analyses published in the literature and using Genetic Algorithms as the optimization technique. The model results are compared with previous tests of forward smoldering combustion in microgravity conducted aboard the NASA Space Shuttle. The model calculates...