Erik L. Johnsson

Greatly enhanced soot scattering in flickering CH4/Air diffusion flames

Abstract Planar images of laser-induced flurescence from OH · radicals and elastic scattering from soot particles are presented in time-varying, laminar CH4/air diffusion flames burning in a co-flowing, axisymmetric configuration at atmospheric pressure. Acoustic forcing is used to phase lock the periodic flame flicker to the pulsed laser system operating at 10.13 Hz. For conditions where the tip of the flame is clipped, the intensity of the light scattered by the soot particles increases dramatically (by more than a factor of 7 for the maximum signals at a point) compared to a steady-state, laminar flame with the same mean fuel flow velocity. Comparison of the scattering signals integrated along the flame radius is carried out in the steady-state and time-varying flames as a function of height above the burner. Time-varying flames exhibit a larger range of combustion conditions than observed in corresponding steady-state flames, including different residence times, temperature histories, local stoichiometries, and strain and scalar dissipation rates. Thus, their investigation promises to yield new insights into a wide variety of chemistry-flowfield interactions which are prominent in turbulent combustion.

Design and testing of a new smoke concentration meter

The design of a new smoke concentration meter based on light-extinction measurements with a He-Ne laser is described. The measurement allows the determination of the mass-generation rate of smoke and smoke yield during a 5re test with little more time or labour than is required for performing heat-release-rate and mass-loss-rate measurements. The new smoke concentration meter was motivated by the 5nding from several studies of a nearly universal value of the speci5c extinction coe7cient of post-6ame smoke produced by over ventilated 5res. Key design features include the use of a stabilized laser, purge 6ow to eliminate smoke deposition on the optics, U channel construction to minimize the e4ect of heating on the optical alignment and beam correction optics. The facility was fabricated almost entirely from commercially available components to allow this design to be easily reproduced by5re research and testing laboratories. The smoke concentration meter was able to measure a smoke yield as small as 0.005 for a propane 5re to as large as 0.10 for a toluene pool 5re. A detailed uncertainty assessment was made. The result for a 50 cm diameter heptane pool 5re agrees well with previous smoke yield measurements made for the same fuel and pool diameter based on 5lter collection and weighing. Copyright ( 2000 John Wiley & Sons Ltd.

Carbon monoxide formation in fires by high-temperature anaerobic wood pyrolysis

Building fire fatalities often occur at locations remote from the room where the fire is actually burning. The majority of these fire deaths are the result of smoke inhalation, primarily due to exposure to carbon monoxide (CO). Although causing nearly 2500 deaths per year in the United States, the mechanisms for the formation of CO in building or enclosure fires remain poorly characterized. In order to test the hypothesis that high concentrations of CO can be generated by pyrolysis of wood in a high-temperature, vitiated environment, a series of natural gas fires, ranging from 40 to 600 kW in heat release rate, were burned inside a reduced-scale enclosure (RSE). The ceiling and upper walls of the RSE were lined with 6.4-mm-thick plywood. During each burn, the concentrations of CO, CO2, and O2 were monitored at two locations within the upper layer. Oxygen calorimetry was used to monitor the total heat release rate for each fire. Vertical temperature profiles for two positions within the enclosure were also recorded. Much higher levels of CO were generated with the wood-lined upper layer than with comparable fires fueled only by natural gas. Volume concentrations as high as 14% were observed. The fires with wood in the upper layer had higher heat release rates and depressed upper-layer temperatures. The major conclusions of this work based on the experimental findings are (1) the pyrolysis of wood in a highly vitiated, high-temperature environment can lead to the generation of very high concentrations of CO in enclosure fires; (2) the overall wood pyrolysis is endothermic for the experimental conditions studied; and (3) the maximum mass loss rate of wood under the experimental conditions is on the order of 10 gs−1 m−2 with the majority of released carbon being converted to a roughly 1:1 mixture of CO and CO2.

Studies on Fire Characteristics in Over- and Underventilated Full-scale Compartments

An experimental study was conducted to investigate the thermal, chemical, and flow environments of heptane fires in an ISO 9705 room. Fuel flow rates and vent size were manipulated to create overventilated fire (OVF) and underventilated fire (UVF) conditions. Numerical simulations were also performed, for the same conditions, with the Fire Dynamics Simulator (FDS) developed at the National Institute of Standards and Technology. Both OVF and UVF conditions were characterized with temperature distributions, and combustion product formation measured locally in the upper layer, as well as combustion efficiency and global equivalence ratio. It was shown that the numerical results agree quantitatively with measurements in both OVF and UVF. The internal flow pattern rotated in the opposite direction for the UVF relative to the OVF so that a portion of products recirculated to the inside of compartment. This flow pattern may affect changes in the complex processes of CO and soot formation inside the compartment due to an increase in the residence time of high-temperature products. The 3D flow structures including O2 and CO distribution were visualized inside the underventilated compartment fire using FDS. It was observed that the two gas sample locations in the upper layer of the room were insufficient to completely characterize the internal structure of the compartment fire.

Effects of Fuel Location and Distribution on Full-Scale Underventilated Compartment Fires

An experimental study was conducted to investigate the effects of fuel location and distribution on full-scale underventilated compartment fires in an ISO 9705 room. Heptane fuel was burned in three different fuel distributions: single centered burner (SCB), single rear burner (SRB), and two distributed burner (TDB). It was experimentally observed that variations in fuel placement did not significantly affect the global steady state underventilated fire characteristics such as fuel mass loss rate, heat release rate, combustion efficiency, global equivalence ratio, and global CO emission outside the compartment for these simple distributions. Supplemental numerical simulations reveal that the local characteristics of thermal and chemical environments depend on the fuel placement between the front and rear region inside the compartment. At the front region, the local fire characteristics were nearly the same regardless of fuel placement. Changes in fuel location and distribution resulted in changes in temperature, total heat flux, CO2, and CO volume fraction at the rear region. Burner placement led to changes in the mixture fraction, flow dynamics, and variations in CO production in the back of the compartment.

Measurements in Standard Room Scale Fires

In this paper the results of a continuing effort to develop a comprehensive compartment fire database for validation of numerical fire modeling is presented. Natural gas fires were conducted inside a full-scale ISO 9705 room and are compared with previous results obtained in a 2/5 scale, reduced-scale enclosure. In these experiments, fires with heat release rates as large as 2.7 MW were used in the full-scale room. Gas species and temperature measurements were made inside the room at several locations in the upper layer and the doorway. Oxygen, CO/CO2, and total hydrocarbon gas analyzers were used in addition to gas chromatography to make gas species measurements. Temperature measurements were made in the upper layer of the room using aspirated thermocouples. Fires as large as 2.7 MW were observed not to produce underventilated compartment fire conditions in the full-scale enclosure despite the large heat release rate and temperatures observed in excess of 1200 °C. A comparison of the gas species in the upper layer of the reduced-scale and full-scale results showed similarities in terms of the gas species volume fractions when plotted as a function of mixture fraction, but the temperature results showed that the full-scale enclosure was reaching higher temperatures than the reduced-scale enclosure.

Fire-Induced Mass Flow Into a Reduced-Scale Enclosure (NISTIR 5499)

This paper presents the first measurements of the burning rate of premixed flames inhibited by three fluorinated hydrocarbons who’s chemistry is similar to agents which may he used as replacements for CF3Br. Measurements were made of the reduction in the burning rate of premixed methane-air flames stabilized on a Mache-Hebra nozzle burner. The burning rate was determined with the total area method from Schlieren images of the flame. The inhibitors were tested over a range of concentrations and fuel-air equivalence ratios. The measured burning rate reductions are compared with those predicted by numerical solution of the species and energy conservation equations employing a detailed chemical kinetic mechanism recently developed at the National Institute of Standards and Technology (NIST). This paper presents initial efforts at testing and validation of the mechanism using burning rate data. The mode of inhibition of these chemicals is inferred through interpretation of the numerical results.

Carbon Monoxide Production in Compartment Fires: Full-Scale Enclosure Burns (NISTIR 5499)

This paper presents the first measurements of the burning rate of premixed flames inhibited by three fluorinated hydrocarbons who’s chemistry is similar to agents which may he used as replacements for CF3Br. Measurements were made of the reduction in the burning rate of premixed methane-air flames stabilized on a Mache-Hebra nozzle burner. The burning rate was determined with the total area method from Schlieren images of the flame. The inhibitors were tested over a range of concentrations and fuel-air equivalence ratios. The measured burning rate reductions are compared with those predicted by numerical solution of the species and energy conservation equations employing a detailed chemical kinetic mechanism recently developed at the National Institute of Standards and Technology (NIST). This paper presents initial efforts at testing and validation of the mechanism using burning rate data. The mode of inhibition of these chemicals is inferred through interpretation of the numerical results.