Jiann C. Yang

Ignition of mulch and grasses by firebrands in wildland–urban interface fires*

Firebrands or embers are produced as trees and structures burn in wildland–urban interface (WUI) fires. It is believed that firebrand showers created in WUI fires may ignite vegetation and mulch located near homes and structures. This, in turn, may lead to ignition of homes and structures due to burning vegetation and mulch. Understanding the ignition events that are due to firebrands is important to mitigate fire spread in communities. To assess the ignition propensity of such materials, simulated firebrands of uniform geometry, but in two different sizes, were allowed to impinge on fuel beds of shredded hardwood mulch, pine straw mulch, and cut grass. The moisture content of these materials was varied. Firebrands were suspended and ignited within the test cell of the Fire Emulator/Detector Evaluator (FE/DE) apparatus. The FE/DE was used to investigate the influence of an air flow on the ignition propensity of a fuel bed. Ignition regime maps were generated for each material tested as a function of impacting firebrand size, number of deposited firebrands, air flow, and material moisture content.

The Effect Of Sample Size On The Heat Release Rate Of Charring Materials

The burning of a horizontal wood slab situated atop an insulating substrate was modeled using three coupled submodels for the gas-phase, wood, and substrate processes. A global analytical model was used to determine the radiative and convective heat feedback from the gas-phase combustion to the wood surface. The char-fonning wood model was a one-dimensional numerical computation of the density change as a function of position and time. The backside boundary condition of the wood was treated as conductive heat loss into a substrate material modeled by the heat conduction equation. The condensed-phase model results were tested by exposing Douglas Fir samples to an external flux in a nitrogen environment (no combustion). Heat release rate calculations are compared to experimental results for Douglas Fir samples of 0.1 m and 0.6 m diameter. Both theory and experiments show that, for the conditions studied, the heat release rate is nearly independent of the specimen diameter except for the initial peak and the affect of this peak on the first portion of the quasi-steady settling period. Model predictions also indicate that the second peak, which follows the settling period, is very sensitive to the thickness of the insulating substrate.

On the collision dynamics of a water droplet containing an additive on a heated solid surface

An experimental study is presented for water–droplet impingement containing an additive upon a heated stainless–steel surface. A solution of 30% (mass fraction) sodium acetate trihydrate, CH3COONa3H2O, was used for the experiments. The impaction process was recorded using a high–speed digital camera at 1000 frames per second. The initial droplet diameter was fixed at 2.7 ± 0.1 mm and all experiments were performed in atmospheric air. Three different impact Weber numbers were considered, namely 15, 80 and 181. The droplet–evaporation lifetime was measured as a function of temperature for 30% (mass fraction) sodium acetate trihydrate. Collision dynamics were investigated for each impact Weber number, with the temperature of the stainless–steel surface varied from film evaporation to film boiling. The temporal variation of the liquid–film diameter was measured as a function of temperature for each impact Weber number. Experiments were also performed using water for direct comparison of the collision dynamics with the additive–containing droplets. The collision dynamics were observed to be different for water droplets containing an additive at low impact Weber number, but became increasingly similar as the impact Weber number was increased.

Measurement of Smoke Particle Size under Low-Gravity Conditions

Smoke detection experiments were conducted in the Microgravity Science Glovebox (MSG) on the International Space Station (ISS) during Expedition 15 in an experiment entitled Smoke Aerosol Measurement Experiment (SAME). The preliminary results from these experiments are presented. In order to simulate detection of a prefire overheated-material event, samples of five different materials were heated to temperatures below the ignition point. The smoke generation conditions were controlled to provide repeatable sample surface temperatures and air flow conditions. The smoke properties were measured using particulate aerosol diagnostics that measure different moments of the size distribution. These statistics were combined to determine the count mean diameter which can be used to describe the overall smoke distribution.

Dispersed Liquid Agent Fire Suppression Screen Apparatus

Abstract : The design, construction, demonstration, and operation of a bench-scale device capable of screening the fire suppression efficiency of liquid agents are described in detail in this report. The apparatus is based on a well-characterized flame, a means to facilitate the introduction of liquid agents, and a way to generate liquid droplets. A Tsuji-type burner, a porous cylinder used in a counterflow diffusion configuration, is used. Both wake and enveloped flames can be maintained over a wide range of fuel and oxidizer flows. The flame is easily observed, and critical stages such as the blow-off limit (abrupt transition from an enveloped flame to a wake flame) can be ascertained with ease and high reproducibility. A small-scale vertical wind tunnel, which allows for the delivery of a uniform flow of oxidizer to the burner at a low turbulence intensity and also assists in the delivery of liquid agent droplets to the flame, is used for the flow facility. Two techniques of generating droplets have been examined: (1) a piezoelectric droplet generator and (2) a small glass nebulizer. The piezoelectric droplet generator was found incapable of handling fluids with high loading of dissolved solid due to frequent clogging of the orifice opening. The nebulizer is used in the current liquid screen apparatus.

Performance of metal and polymeric O-ring seals during beyond-design-basis thermal conditions

This paper summarizes the small scale thermal exposure test results of the performance of metallic and polymeric O-ring seals typically used in radioactive material transportation packages. Five different O-ring materials were evaluated: Inconel/silver, ethylene-propylene diene monomer (EPDM), polytetrafluoroethylene (PTFE), silicone, butyl, and Viton. The overall objective of this study is to provide test data and insights to the performance of these Oring seals when exposed to beyond-design-basis temperature conditions due to a severe fire. Tests were conducted using a small-scale stainless steel pressure vessel pressurized with helium to 2 bar or 5 bar at room temperature. The vessel was then heated in an electric furnace to temperatures up to 900 °C for a pre-determined period (typically 8 h to 9 h). The pressure drop technique was used to determine if leakage occurred during thermal exposure. Out of a total of 46 tests performed, leakage (loss of vessel pressure) was detected in 13 tests.

Enhanced Effectiveness of Liquid Thermal Fire Extinguishing Agents

This paper describes the findings of a study designed to identify and characterize super-effective thermal fire-fighting agents as possible halon replacements. Two existing thermodynamic databases have been searched in order to identify chemical compounds, which are predicted to extract large amounts of heat from a combustion zone. Additional substances that are not well represented in these databases were included. Compounds having high (1) heats of vaporization, (2) liquid-phase heat capacities, and (3) total heat absorption due to phase changes (if applicable), heating of a liquid (if applicable), and heating of the gas phase to combustion temperatures were identified. Two compounds, methoxy-nonaflurobutane (HFE7100) and lactic acid, were identified as being especially interesting. Both agents and water were tested in the Dispersed Liquid Agent Fire Suppression Screen (DLAFSS) apparatus. Lactic acid was tested in mixtures with water at various concentrations. The acid/water mixtures were found to be less effective than water alone, indicating that the lactic acid was reacting and releasing heat that more than compensated for the heat extraction. HFE7100 and water were also tested in a screening apparatus known as the Transient Application Recirculating Pool Fire (TARPF) that incorporates a flame stabilized behind a bluff body in a turbulent oxidizer flow. The agent failed to extinguish a propane flame when it was released at nominal mass fraction in air twice as high as the extinguishing mass fraction measured in the DLAFSS. This observation is attributed to ineffective mixing and entrainment of liquid droplets into the flame. A simple thermodynamic analysis under-predicts the performance of the liquid thermal agents in the DLAFSS. Additional studies of the effectiveness of fire extinguishing agents released as liquids should be performed with a focus on confirming and understanding their enhanced performance relative to that expected based on simple heat extraction.

Fire Suppression And Re-ignition Prevention Ina Full-scale Engine Nacelle Simulator

An analysis is presented on the full-scale fire suppression experiments conducted on the F-22 engine nacelle simulator at Wright Patterson Air Force Base. Experiments investigated the relative effectiveness of halogenated agents and solid propellant gas generators (SPGG) in suppressing a series of spray fires with and without a fuel re-ignition source. Several agents were tested including CF3Br (halon 1301), C2HF5 (HFC-125), and two basic types of SPGG, including one that produced inert gases in conjunction with a fine solid particulate composed of K2CO3 and one that produced inert gases only. The measured agent effectiveness was compared to the predicted effectiveness based on results from cup burner suppression experiments. Estimates of the suppression effectiveness of the SPGG were based on the effectiveness of the components of its effluent. The mass fraction of SPGG effluent required to extinguish heptane cup burner flames was estimated as 0.15 and 0.29 as compared to previously measured values of 0.14 and 0.28 for CF3Br and C2HF5, respectively. The predicted suppression requirements (relative to CF3Br) agreed with the full-scale measurements within 35% for the halogenated compounds, whereas the SPGG performed as much as a factor of 3.3 better than predicted. This difference suggests that a large fraction of the SPGG performance may have been related to its fast deployment, which enhances flame straining and thereby reduces agent mass suppression requirements. The SPGG effluent that contained a significant percentage of K2CO3 particulate was particularly effective for re-ignition protection, a scenario that dominates agent mass requirements for the compressed halogenated liquids. In this case, the SPGG required 27 times less mass than CF3Br. INTRODUCTION Halon 1301, or trifluorobromomethane (CF3Br), has been used as a fire extinguishing agent in many applications because of its many positive attributes. Due to its high ozone depletion potential, however, its production has been terminated. In the search for a suitable replacement, novel types of agents are under development. One class of such devices is the solid propellant gas generator (SPGG), a device in which solid-phase combustion yields products which can be used for fire suppression [1]. The key parameters that affect flame stability, control flame extinction and the prevention of re-ignition are agent effectiveness and flow field dynamics. Whereas there is a general understanding of the suppression mechanisms of gaseous agents, little has been published on SPGG effectiveness. Recently, the relative suppression effectiveness of a SPGG has been Copyright

Halon 1301 Surrogates for Engine Nacelle Fire Suppression System Certification (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.

Experimental Studies on Discharge of Alternative Agent/Nitrogen Mixtures in a Simulated Dry Bay (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.