Ronald S. Sheinson

The physical and chemical action of fire suppressants

This work measures the suppression action of inert gas phase agents in extinguishing an air/liquid organic fuel pool fire. It then gives a predictive model for determining the physical contribution involved in non-inert agent fire suppression. Chemical suppression effectiveness can then be calculated. Further, studying the CF3Y and SF5Y (Y = F, Cl, Br, I) series allows quantitation of chemical suppression action of the individual radical moieties. CF3 is shown to be a strong chemical suppressant while SF5 is a flame promoter. CF3Br (Halon 1301) suppression action is 20% physical, 25% chemical due to CF3, and 55% chemical due to Br. Such quantitation provides guidance in selecting alternative fire suppressants to replace ozone layer depleting halons.

Chemiluminescence spectra from cool and blue flames: Electronically excited formaldehyde

Resolved emission spectra of electronically excited formaldehyde (1A2 → 1A1) have been obtained from two-stage ignition in a Vertical Tube Reactor combustion flow system, using acetaldehyde and n-butane as fuels. The very low intensity UV-visible emission from both the cool flame reaction zone (200–400°C) and blue flame reaction zone (400–800°C) is due to formaldehyde chemiluminescence. The “continuum” observed by previous researchers is due to formaldehyde band overlap, with typical hot flame emission from radicals, CO flame bands and continuum, and black body radiation being absent except for minor contributions in the acetaldehyde blue flame (800°C). Differences in the spectra band structure and underlying ‘continuum’ are due to the temperatures of the various flames. The cool flame formaldehyde emission yield per reacting fuel molecule is ∼10−8 for the fuels studied.

Suppression of nonpremixed flames by fluorinated ethanes and propanes

Abstract Suppression of methane/air and propane/air nonpremixed counterflow flames, and n-heptane and methanol cup burner flames by fluorinated hydrocarbons was investigated. Four fluorinated ethanes, 10 fluorinated propanes, four bromine- or iodine-containing halons, and the inert agents CF 4 , SF 6 , and N 2 were tested in some or all of the flames. Laser Doppler velocimetry (LDV) determinations of peak velocity gradients in the oxidizer flow of the counterflow flames were found to be linearly correlated with the expression for global strain rate derived for plug flow boundary conditions. This correlation was used to estimate strain rate values at extinction. The bromine- or iodine-containing agents are more effective on a molar basis than the fluorinated propanes, followed by the fluorinated ethanes, and finally SF 6 , CF 4 , and N 2 . Agent effectiveness increases with the number of CF 3 groups present in the agent molecular structure. Numerical investigations of the flame speed reduction of methane/air mixtures doped with either CHF 2 CHF 2 or CF 3 CH 2 F predict that the latter is the better agent, in accord with experimental observations. Chemical contributions to suppression account for less than 35% of the total suppression offered by fluorinated hydrocarbons not containing bromine or iodine. At strain rates below 100 s −1 , suppression effectiveness rankings in methane and propane counterflow flames are similar to those obtained in n-heptane and methanol cup burner flames. Methanol flames are more difficult to extinguish than the alkane flames investigated, particularly with the chemical agent CF 3 Br.

Dynamics and suppression effectiveness of monodisperse water droplets in non-premixed counterflow flames

In-situ measurements of velocity and size distributions of initially monodisperse water mists of initial diameters ranging from 14 μm to 44 μm seeded into the air stream of nonpremixed propane/air counterflow flames are reported. Droplets were generated piezoelectrically, and the size and velocity distributions and the number density were determined by phase-Doppler particle anemometry. Droplets having initial diameters of 18 μm underwent complete vaporization in a counterflow flame at a strain rate of approximately 170 s -1 , while droplets of 30 μm penetrated slightly beyond the visible flame zone. Measurements of the effect of water droplets on the extinction strain rates of propane/air counterflow flames were performed. Droplets of 14 μm and 30 μm were found to have similar suppression effectiveness, while droplets of 44 μm were substantially less effective at reducing the extinction strain rate. Both the 14 μm and 30 μm water droplets were found to be more effective, on a mass basis, than CF3Br. The present experimental results are in excellent agreement with the predictions of recent modeling studies exploring the behavior of various sized water droplets in counterflow

Decomposition of methane in an AC discharge

This paper presents qualitative and quantitative product analysis results from an atmospheric-pressure AC discharge of nitrogen containing trace levels of methane and oxygen. In the absence of oxygen the primary products were unreacted methane, hydrogen, and hydrogen cyanide. Methane destruction efficiency was unaffected by trace oxygen addition; however, hydrogen and hydrogen cyanide levels decreased and the concentrations of carbon monoxide, carbon dioxide, and water increased as the level of added oxygen increased. The only cyanide compound that persisted with air as the bulk gas was cyanogen. A chemical mechanism is presented which qualitatively explains the observed product distributions.

In situ determination of molecular oxygen concentrations in full-scale fire-suppression tests using tunable diode laser absorption spectroscopy

The fast and accurate determination of oxygen in air is important for fire research. Available O2 sensors (paramagnetic, electrochemical, ZrO2) are only of limited use because of significant errors caused by the specific sampling or measurement process, so that a purely optical, in situ detection is of great interest. Optical methods can account for the dilution of O2 by water (both vapor and drops) and are thus valuable tools for studying the effectiveness of water for replacing halogenated fire suppressants. To fulfill this need, a tunable diode laser based absorption spectrometer (TDLAS) has been developed for the in situ detection of molecular oxygen at 760 nm (A band, b1Σ+g←X3Σ−g). The device was successfully tested during fullscale fire-suppression tests carried out at the Naval Research Laboratory Chesapeake Bay Detachment facility in a 28-m3 rest compartment. A specially protected open-path Herriott multipass setup with an absorption length of 1.8 m was developed to restrict the probe volume to a base length of 30 cm. Various scenarios including water mist only, unsumppressed fires, and water-suppressed fires (methanol and n-heptane, both pool and cascading fires up to 400 kW) were investigated. O2 concentrations were measured at a 2.5 Hz repetition rate with a resolution of 0.01 to 1 vol % O2 depending on the transmission conditions. This demonstrated for the first time the capability for in situ oxygen measurements under fire-suppression conditions with large and rapid obscuration changes (transmission of as little as 0.8% of the emitted laser power). It also showed that the TDLAS results account for dilution of O2 by water vapor without any interference of other species.

FORMATION AND CHEMILUMINESCENT DECOMPOSITION OF DIOXETANES IN THE GAS PHASE

Abstract— High resolution chemiluminescence spectra have been obtained of the singlet electronically excited products of O2(1Δ) plus alkene, dioxetane forming, reactions. The experiments were conducted in a flow apparatus at pressures of 1–5 torr. The spectra are a measure of the unrelaxed initial distribution of energy in the excited product. Results are reported for ethylene, 1, 1‐difluoroethylene. methyl vinyl ether, ethyl vinyl ether, n‐butyl vinyl ether, ketene, ketene‐d2, allene, unsymdimethyl allene, dimethyl ketene, 2‐methoxy propene, 1‐ethoxy propene, 2‐bromo propene, and N, N‐dimethyl isobutenyl amine. Chemiluminescence activation energies, representing the cycloaddition process, and absolute quantum yields for singlet excited product, ranging from 10‐‐4 to 2.5 × 10‐‐2. are reported for 10 alkenes. Several of the reactions, 1,1‐difluoroethylene, ketene, ethylene and allene give formaldehyde 1nπ* product with excess vibrational‐rotational energy and a higher quantum yield than reactions not displaying this phenomenon. This is an indication of at least partially statistical partitioning of the energy in excess of that needed to electronically excite the formaldehyde. The experiments with ketene and ketene‐d2 provide the first evidence for the existence of unsubstituted 1,2‐dioxetanone. The results from several of the experiments, particularly those with 2‐methoxy propene and I‐ethoxy propene are consistent with the mechanism of Goddard, which predicts regioselective and stereoselective attack of O2(1Δ) upon alkoxy substituted alkenes having allylic hydrogen.

Manipulation of Cool and Blue Flames in the Winged Vertical Tube Reactor

Abstract A technique of “flame programming” using a refined vertical tube reactor (VTR) has been developed whereby widely separated preignition zones (cool and blue flames, and intermediate dark zones) can be studied and compared. In addition, the reactor is fitted with removable side arms for doping and/or spectroscopic studies. The use of the VTR gives nearly adiabatic, planar reaction zones for the preignition events. This new VTR responds very rapidly to changes in experimental parameters and achieves equilibrium within a few minutes. The flames are stable to within ±0.5 mm over an eight hour period with two stage ignition separations of over 40 cm. Reproducible and point source sampling of the flame zones can be made very precisely with reference to the location of the flames. The “free floating” flame stages show a minimum interaction with the walls and are extremely sensitive to additives.

Extinction of non-premixed methane-and propane-air counterflow flames inhibited with CF4, CF3H and CF3Br

The extinction condition was studied for non-premixed counterflow flames of methane and propane impinging against an air stream with added amounts of CF4, CF3H, or CF3Br. Strain rates near extinction using laser Doppler velocimetry were determined for counterflow methane-air flames, with CF4 and CF3H added to the air stream. The value obtained for the case of methane impinging against an air stream with no agent added, 400±25 s−1, agrees with the experimental value reported in the literature. For both methane-and propane-air flames, increasing amounts of CF4, CF3H, or CF3Br in the air stream decrease the extinction oxidizer stream mass flow rate (or strain rate) monotonically until, at a given amount of agent, the extinction strain rate rapidly drops. If the molar percentage of agent in the air stream exceeds 2.5% for CF3Br and 11% for CF4 or CF3H, non-premixed counterflow methane-air flames cannot be maintained. Counterflow propane-air flames cannot be maintained if agent concentrations in the air stream are greater than 2.7% for CF3Br, 11% for CF3H, and >14% for CF4. For a given amount of agent added to the air stream that does not exceed the limiting agent concentration, the extinction strain rate is comparable for CF3H or CF4 addition in methane-air flames. CF3H is more effective than CF4 in propane-air flames, similar to the reported extinction behavior in heptane-air cup burner studies. CF3Br is significantly more effective than CF4 or CF3H in extinguishing methane-or propane-air flames. The net chemical effect of CF3H in the inhibition of non-premixed counterflow flames is more pronounced for propane-air flames than for methane-air flames. Explanations for this behavior are discussed briefly.

Observation of O2(b 1Σg+ → X 3Σg−) chemiluminescence from the self-reaction of isopropylperoxy radicals

Abstract We report the first direct observation of singlet oxygen product resulting from a gas phase self-reaction of alkylperoxy radicals. The evidence is a rotationally resolved O2(b 1Σg+ → X 3Σg−) chemiluminescence spectrum observed in a 12 l integrating sphere reactor containing less than 1014 isopropylperoxy radicals per cubic centimeter diluted in helium and O2 at a total pressure of 80 Pa (0.6 Torr). According to a mechanism proposed by Russell in 1957, the self-reactions of non-tertiary alkylperoxy radicals proceed by formation of a weakly bound dialkyl tetraoxide intermediate which can decompose to yield a carbonyl, an alcohol and O2. There are five energetically accessible product state potential energy surfaces. Spin conservation forbids the ground state products, but allows the formation of O2 in the 1Σg+ or 1Δg states or of T1 carbonyl. We claim these results to be strong evidence for the occurrence of Russells mechanism in the gas phase.