Matthew F. Bundy

Suppression limits of low strain rate non-premixed methane flames

The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s 1 . The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N2 ,C O 2, and CF3Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N 2 concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 0.01 for N2, 0.773 0.009 for CO2, and 0.437 0.005 for CF3Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 0.004 for N2, 0.187 0.002 for CO2, and 0.043 0.001 for CF3Br.

Characterization of Candle Flames

Common household open flame and radiant ignition sources are the actual or suspected cause for many fires. The purpose of this research is to identify the burning behavior and properties of common candles in order to provide additional tools for use by fire investigators. The properties of paraffin wax are obtained from the literature and from experiments. The candles are burned under controlled laboratory conditions to measure the mass burning rate, candle regression rate, flame height, and heat flux. Using the properties of paraffin wax and characteristics of the candles, numerous simulations are performed with the NIST Fire Dynamics Simulator (FDS) to model the burning rate and heat flux profile of the candle flame. The modeling results are then compared with the flame height and heat flux data obtained experimentally. The model facilitates an enhanced understanding of the structure of candle flames.

The two-dimensional structure of low strain rate counterflow nonpremixed-methane flames in normal and microgravity

The structure and extinction of low strain rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinction mode and flame structure. A specific maximum heat release rate was introduced to quantify the local flame strength and to elucidate the extinction mechanism. The contribution by each term in the energy equation to the heat release rate was evaluated to investigate the multi-dimensional structure and radiative extinction of the 1-g flames. Two combustion regimes depending on the extinction mode were identified. Lateral heat loss effects and multi-dimensional flame and flow structure were also found. At low strain rates in 1-g flames (‘regime A’), the flame is extinguished from the weak outer edge of the flame, which is attributed to a multi-dimensional flame structure and flow field. At high strain rates, (‘regime B’), the flame extinction initiates near the flame centreline owing to an increased diluent concentration in the reaction zone, similar to the extinction mode of 1D flames. These two extinction modes can be clearly explained by consideration of the specific maximum heat release rate.

Creeping flame spread along fuel cylinders in forced and natural flows and microgravity

in which the terms containing the constant C account for the enhanced gas-to-surface heat transfer because of the cylindrical curvature, and those containing Lsy, the heated layer depth in the solid, account for a reduction in the solid volume preheated in the cylindrical compared to the flat geometry. The expression is tuned by comparison with complete numerical solutions to the flame spread problem from which the flame energy EFL is determined from the flat surface geometry and the constant C chosen from heat transfer correlations. Results compare favorably with numerical solutions for cylindrical spread in forced and natural flows and microgravity and with experiments on downward flame spread on cylindrical rods in normal gravity and microgravity.

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.

An uncertainty analysis of mean flow velocity measurements used to quantify emissions from stationary sources

Point velocity measurements conducted by traversing a Pitot tube across the cross section of a flow conduit continue to be the standard practice for evaluating the accuracy of continuous flow-monitoring devices. Such velocity traverses were conducted in the exhaust duct of a reduced-scale analog of a stationary source, and mean flow velocity was computed using several common integration techniques. Sources of random and systematic measurement uncertainty were identified and applied in the uncertainty analysis. When applicable, the minimum requirements of the standard test methods were used to estimate measurement uncertainty due to random sources. Estimates of the systematic measurement uncertainty due to discretized measurements of the asymmetric flow field were determined by simulating point velocity traverse measurements in a flow distribution generated using computational fluid dynamics. For the evaluated flow system, estimates of relative expanded uncertainty for the mean flow velocity ranged from ±1.4% to ±9.3% and depended on the number of measurement locations and the method of integration. Implications: Accurate flow measurements in smokestacks are critical for quantifying the levels of greenhouse gas emissions from fossil-fuel-burning power plants, the largest emitters of carbon dioxide. A systematic uncertainty analysis is necessary to evaluate the accuracy of these measurements. This study demonstrates such an analysis and its application to identify specific measurement components and procedures needing focused attention to improve the accuracy of mean flow velocity measurements in smokestacks.

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.

Experimental Analysis of Steel Beams Subjected to Fire Enhanced by Brillouin Scattering-Based Fiber Optic Sensor Data

This paper presents high temperature measurements using a Brillouin scattering-based fiber optic sensor and the application of the measured temperatures and building code recommended material parameters into enhanced thermomechanical analysis of simply supported steel beams subjected to combined thermal and mechanical loading. The distributed temperature sensor captures detailed, nonuniform temperature distributions that are compared locally with thermocouple measurements with less than 4.7% average difference at 95% confidence level. The simulated strains and deflections are validated using measurements from a second distributed fiber optic (strain) sensor and two linear potentiometers, respectively. The results demonstrate that the temperature-dependent material properties specified in the four investigated building codes lead to strain predictions with less than 13% average error at 95% confidence level and that the Europe building code provided the best predictions. However, the implicit consideration of creep in Europe is insufficient when the beam temperature exceeds 800°C.

Temperature measurement and damage detection in concrete beams exposed to fire using PPP-BOTDA based fiber optic sensors

In this study, distributed fiber optic sensors based on pulse pre-pump Brillouin optical time domain analysis (PPP-BODTA) are characterized and deployed to measure spatially-distributed temperatures in reinforced concrete specimens exposed to fire. Four beams were tested to failure in a natural gas fueled compartment fire, each instrumented with one fused silica, single-mode optical fiber as a distributed sensor and four thermocouples. Prior to concrete cracking, the distributed temperature was validated at locations of the thermocouples by a relative difference of less than 9 %. The cracks in concrete can be identified as sharp peaks in the temperature distribution since the cracks are locally filled with hot air. Concrete cracking did not affect the sensitivity of the distributed sensor but concrete spalling broke the optical fiber loop required for PPP-BOTDA measurements.

Evaluating measurements of carbon dioxide emissions using a precision source—A natural gas burner

A natural gas burner has been used as a precise and accurate source for generating large quantities of carbon dioxide (CO2) to evaluate emissions measurements at near-industrial scale. Two methods for determining carbon dioxide emissions from stationary sources are considered here: predicting emissions based on fuel consumption measurements—predicted emissions measurements, and direct measurement of emissions quantities in the flue gas—direct emissions measurements. Uncertainty for the predicted emissions measurement was estimated at less than 1%. Uncertainty estimates for the direct emissions measurement of carbon dioxide were on the order of ±4%. The relative difference between the direct emissions measurements and the predicted emissions measurements was within the range of the measurement uncertainty, therefore demonstrating good agreement. The study demonstrates how independent methods are used to validate source emissions measurements, while also demonstrating how a fire research facility can be used as a precision test-bed to evaluate and improve carbon dioxide emissions measurements from stationary sources. Implications: Fossil-fuel-consuming stationary sources such as electric power plants and industrial facilities account for more than half of the CO2 emissions in the United States. Therefore, accurate emissions measurements from these sources are critical for evaluating efforts to reduce greenhouse gas emissions. This study demonstrates how a surrogate for a stationary source, a fire research facility, can be used to evaluate the accuracy of measurements of CO2 emissions.