Andrew Lock

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.

Application of spatial frequency response as a criterion for evaluating thermal imaging camera performance

Police, firefighters, and emergency medical personnel are examples of first responders that are utilizing thermal imaging cameras in a very practical way every day. However, few performance metrics have been developed to assist first responders in evaluating the performance of thermal imaging technology. This paper describes one possible metric for evaluating spatial resolution using an application of Spatial Frequency Response (SFR) calculations for thermal imaging. According to ISO 12233, the SFR is defined as the integrated area below the Modulation Transfer Function (MTF) curve derived from the discrete Fourier transform of a camera image representing a knife-edge target. This concept is modified slightly for use as a quantitative analysis of the cameras performance by integrating the area between the MTF curve and the cameras characteristic nonuniformity, or noise floor, determined at room temperature. The resulting value, which is termed the Effective SFR, can then be compared with a spatial resolution value obtained from human perception testing of task specific situations to determine the acceptability of the performance of thermal imaging cameras. The testing procedures described herein are being developed as part of a suite of tests for possible inclusion into a performance standard on thermal imaging cameras for first responders.

Suite of proposed imaging performance metrics and test methods for fire service thermal imaging cameras

The use of thermal imaging cameras (TIC) by the fire service is increasing as fire fighters become more aware of the value of these tools. The National Fire Protection Association (NFPA) is currently developing a consensus standard for design and performance requirements for TIC as used by the fire service. This standard will include performance requirements for TIC design robustness and image quality. The National Institute of Standards and Technology facilitates this process by providing recommendations for science-based performance metrics and test methods to the NFPA technical committee charged with the development of this standard. A suite of imaging performance metrics and test methods based on the harsh operating environment and limitations of use particular to the fire service has been proposed for inclusion in the standard. The performance metrics include large area contrast, effective temperature range, spatial resolution, nonuniformity, and thermal sensitivity. Test methods to measure TIC performance for these metrics are in various stages of development. An additional procedure, image recognition, has also been developed to facilitate the evaluation of TIC design robustness. The pass/fail criteria for each of these imaging performance metrics are derived from perception tests in which image contrast, brightness, noise, and spatial resolution are degraded to the point that users can no longer consistently perform tasks involving TIC due to poor image quality.

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.

Dilution and Suppression of Partially Premixed Flames in Normal and Microgravity for Different Fuels

The suppression of fires and flames is an important area of interest for both terrestrial and space based applications. In this investigation we elucidate the relative efficacy of fuel and air stream inert diluents for suppressing laminar partially premixed flames. A comparison of the effects of fuel and air stream dilution are also made with other fuels. Both counterflow and coflow flames are investigated, with both normal and zerogravity conditions considered for coflow flames. Simulations are conducted for both the counterflow and coflow flames, while experimental observations are made on the coflowing flames. With fuel or air stream dilution, coflow flames are observed to move downstream from the burner after overcoming initial heat transfer coupling. Further increases in diluent result in increases in the flame liftoff height until blow off occurs. The flame liftoff height and the critical volume fraction of extinguishing agent at blow out vary with both equivalence ratio and with the stream in which diluents are introduced. Nonpremixed methane-air flames are more difficult to extinguish than partially premixed flames with fuel stream dilution; whereas, partially premixed methane-air flames are more resistant to extinction than nonpremixed flames with air stream dilution. This difference in efficacy of the fuel and air stream dilution is attributed to the action of the diluent. In leaner partially premixed flames with fuel stream dilution and richer partially premixed flames with air stream dilution the effect of the diluent is to replace the deficient reactant in the system, thus starving the flame. In leaner partially premixed flames with air stream dilution and richer partially premixed flames with fuel stream dilution the effect of the diluent is purely thermal in that it absorbs heat from the flame, until combustion may no longer be sustained. The dilution effect is more effective than the thermal effect. When gravity is eliminated from the 2-D flame the liftoff height decreases and the critical volume fraction of diluent for blow off is also decreased.© 2006 ASME

Lifted Dilute Partially Premixed Flames in Microgravity with Applications to Fire Safety

Partial premixing of fuel and oxidizer often occurs prior to the ignition of incipient fires. These fuel/air mixtures can also become additionally mixed with fully or partially oxidized products as well as with pyrolyzed species and then burn in the form of partially premixed flames. Often the flames are lifted since the pyrolysis products can issue in the form of jets from a substrate, or because of dilution with products. Therefore, we investigate the influence of dilution on the liftoff and blowout characteristics of axisymmetric partially premixed flames in this context. Dilution is accomplished by mixing the fuel lean and fuel rich streams with carbon dioxide (due to its use in fire suppression). The addition of this chemically inert diluent into a flame decreases its temperature and thus reduces the effective flame speed of a lifted flamefront. Increasing dilution leads to large liftoff heights and eventually to blowoff, which is important to characterize in the context of fire safety. We find that when carbon dioxide is injected directly into the fuel rich central jet of a partially premixed flame, it is more effective at suppressing the flame than when it is introduced into the air coflow.

Lifted partially premixed flames in microgravity

An experimental-computational investigation on the liftoff characteristics of partially premixed flames (PPFs) under 1- and µ-g conditions is presented. Lifted methane-air PPFs are established in coflowing jets in both 1- and µ-g conditions. A coflow configuration with equal jet and coflow velocities is employed in order to minimize the effects of the jet shear layer on the flame structure and liftoff behavior. The effects of gravity, jet velocity and equivalence ratio on the flame liftoff height and topology are characterized. Both the simulations and measurements indicate that under identical conditions, a lifted µ-g PPF is stabilized closer to the burner compared to the 1-g flame. In addition, the 1-g lifted flame base exhibits a triple flame structure, while the corresponding µ-g flame base indicates a nub or edge flame structure. The 1-g lifted flames also exhibit well-organized oscillations due to a jet shear layer instability caused by the buoyant acceraltion. The measurements and simulations further indicate that the liftoff height decreases as the equivalence ratio is increased, but increases with the jet velocity. Overall there is good agreement between measurements and simulation with respect to the topologies of 1- and µ-g lifted flames.