Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (17 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2010)
Experimental tests were conducted on a single cartoned, unexpanded Group A plastic commodity to evaluate an approach to commodity classification in complex fuel geometries. The approach is based on comparing the chemical energy released during the combustion process with the energy required to vaporize the fuel. The ratio of these two quantities is often called the mass transfer number, B-number or Spalding number. The Group A commodity tested was essentially a three-dimensional cardboard box subdivided by cardboard placeholders for individual unexpanded polystyrene cups. Experiments consisted of burning the front face of a single box with all remaining faces uniformly insulated. Measurements of gas temperatures, mass-loss rate and heat flux from the flame were used to calculate a modified B-number, which includes consideration of the role of radiant heat transfer. Results from experiments have produced time-averaged B- numbers for stages of burning. Comparison to a concurrent study using a cone calorimeter on smaller-scale material samples shows good agreement between these methods. Future use of the results to improve commodity classification for better assessment of fire danger is discussed
    Cover page: A fundamental approach for storage commodity classification
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2012)
    Experimental techniques have been used to investigate three upward flame spread phenomena of particular importance for fire safety applications. First, rates of upward flame spread during early-stage burning were observed during experiments on wide samples of corrugated cardboard. Results indicated a slower acceleration than was obtained in previous measurements and theories. It is hypothesized that the non-homogeneity of the cardboard helped to reduce the acceleration of the upward spread rates by physically disrupting flow in the boundary layer close to the vertical surface and thereby modifying heating rates of the solid fuel above the pyrolysis region. The results yield alternative scalings that may be better applicable to some situations encountered in practice in warehouse fires. Next, a thermally thick slab of polymethyl methacrylate was used to study the effects of the inclination angle of a fuel surface on upward flame spread. By performing experiments on 10 cm wide by 20 cm tall fuel samples it was found that the maximum flame- spread rate, occurring nearly in a vertical configuration, does not correspond to the maximum fuel mass-loss rate, which occurs closer to a horizontal configuration. A detailed study of both flame spread and steady burning at different angles of inclination revealed the influence of buoyancy-induced flows in modifying heat-flux profiles ahead of the flame front, which control flame spread, and in affecting the heat flux to the burning surface of the fuel, which controls fuel mass-loss rates. Finally, vertical arrays of horizontally protruding wood matchsticks were used to investigate the influence of the spacing of discrete fuel elements on rates of upward flame spread. Rates of upward flame spread were found to increase dramatically for spacings between 0 cm and 0.8 cm and experienced only a slight increase thereafter. Based on these observations, the influence of convective heating was hypothesized to dominate this spread mechanism, and predictions of ignition times were developed using convective heat-transfer correlations. Mass-loss rates followed a similar pattern and were predicted along with matchstick burnout times using a droplet burning theory extended for a cylindrical geometry
      Cover page: Studies on upward flame spread
      • Thesis
      • Peer Reviewed
      UC Berkeley Electronic Theses and Dissertations (2023)

      Wildfires have increased in frequency and severity over recent decades, resulting in serious impacts on both the environment and human lives. These fires are often fueled by dry conditions, high temperatures, and strong winds, making them difficult to contain. The consequences of these extreme wildfires are enormous, leading to the destruction of vast forested areas, the loss of biodiversity, and the release of harmful gases and particulate matter into the atmosphere. These emissions pose a serious health threat to nearby populations and firefighters who are tasked with controlling the fire.

      In this study, the concentration of gaseous and particulate matter emissions resulting from flaming and smoldering combustion of different wildland fuels found across the United States is characterized. Furthermore, the effect of fuel moisture content (FMC) on these emissions is explicitly studied, which has not been considered in the literature before. To achieve this, a custom linear tube-heater apparatus is constructed, enabling the steady production of emissions in different combustion modes across a wide range of FMC. The results obtained demonstrate that smoldering combustion exhibits increased emissions of CO, particulate matter, and unburned hydrocarbons when compared to flaming combustion. Moreover, it is observed that the concentrations of CO and particulate matter in the flaming mode are significantly correlated with FMC, while FMC has little influence on emissions during smoldering mode combustion when considering the dry mass of fuel burned. These variations are observed in certain vegetative fuel species but not others, highlighting the importance of fuel type. Additionally, Fourier-transform infrared (FTIR) spectroscopy is employed to provide detailed characterization of the unburnt hydrocarbons.

      Secondly, this study quantifies the limiting conditions for the smoldering-to-flaming (StF) transition, a commonly observed phenomenon during wildfire events, specifically focusing on cellulose powder, which constitutes approximately 50% of vegetative wildland fuel. It is discovered that the external heat flux required for the StF transition increases as the oxygen (O2) concentration decreases from 21% to 10% at fixed flow velocities. However, for a constant O2 concentration, an increase in flow velocity leads to a higher required heat flux due to the growing significance of convective heat losses. Under the experimental conditions, an oxidizer flow velocity of 5 cm/s is determined to be a limiting value for the StF transition. Finally, the efficacy of six different respiratory protection (RP) materials commonly worn during wildland fire events is tested against simulated wildland fire smoke. The findings indicate that cloth bandanas, currently the most commonly used RP material, offer minimal benefits in terms of reducing both particulate matter (PM) and gaseous emissions. On the other hand, all other filter materials demonstrate significant advantages. N95, P95, and P100 filters prove highly effective in removing PM and show some effectiveness in filtering certain gaseous species, particularly those with nuisance volatile organic compound (VOC) capabilities, although this effect may not be sustained over longer durations.

        Cover page: An experimental study of smoldering-to-flaming transition and emissions from wildland vegetative fuels
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2016)
        Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.
          Cover page: Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2016)
          Fire whirls are powerful, spinning disasters for people and surroundings when they occur in large urban and wildland fires. Whereas fire whirls have been studied for fire-safety applications, previous research has yet to harness their potential burning efficiency for enhanced combustion. This article presents laboratory studies of fire whirls initiated as pool fires, but where the fuel sits on a water surface, suggesting the idea of exploiting the high efficiency of fire whirls for oil-spill remediation. We show the transition from a pool fire, to a fire whirl, and then to a previously unobserved state, a blue whirl. A blue whirl is smaller, very stable, and burns completely blue as a hydrocarbon flame, indicating soot-free burning. The combination of fast mixing, intense swirl, and the water-surface boundary creates the conditions leading to nearly soot-free combustion. With the worldwide need to reduce emissions from both wanted and unwanted combustion, discovery of this state points to possible new pathways for reduced-emission combustion and fuel-spill cleanup. Because current methods to generate a stable vortex are difficult, we also propose that the blue whirl may serve as a research platform for fundamental studies of vortices and vortex breakdown in fluid mechanics.
            Cover page: From fire whirls to blue whirls and combustion with reduced pollution.
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2016)
            Fire whirls are powerful, spinning disasters for people and surroundings when they occur in large urban and wildland fires. Whereas fire whirls have been studied for fire-safety applications, previous research has yet to harness their potential burning efficiency for enhanced combustion. This article presents laboratory studies of fire whirls initiated as pool fires, but where the fuel sits on a water surface, suggesting the idea of exploiting the high efficiency of fire whirls for oil-spill remediation. We show the transition from a pool fire, to a fire whirl, and then to a previously unobserved state, a "blue whirl." A blue whirl is smaller, very stable, and burns completely blue as a hydrocarbon flame, indicating soot-free burning. The combination of fast mixing, intense swirl, and the water-surface boundary creates the conditions leading to nearly soot-free combustion. With the worldwide need to reduce emissions from both wanted and unwanted combustion, discovery of this state points to possible new pathways for reduced-emission combustion and fuel-spill cleanup. Because current methods to generate a stable vortex are difficult, we also propose that the blue whirl may serve as a research platform for fundamental studies of vortices and vortex breakdown in fluid mechanics.
              Cover page: From fire whirls to blue whirls and combustion with reduced pollution
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2020)
              Large outdoor fires are an increasing danger to the built environment. Wildfires that spread into communities, labeled as Wildland-Urban Interface (WUI) fires, are an example of large outdoor fires. Other examples of large outdoor fires are urban fires including those that may occur after earthquakes as well as in informal settlements. When vegetation and structures burn in large outdoor fires, pieces of burning material, known as firebrands, are generated, become lofted, and may be carried by the wind. This results in showers of wind-driven firebrands that may land ahead of the fire front, igniting vegetation and structures, and spreading the fire very fast. Post-fire disaster studies indicate that firebrand showers are a significant factor in the fire spread of multiple large outdoor fires. The present paper provides a comprehensive literature summary on the role of firebrand mechanisms on large outdoor fire spread. Experiments, models, and simulations related to firebrand generation, lofting, burning, transport, deposition, and ignition of materials are reviewed. Japan, a country that has been greatly influenced by ignition induced by firebrands that have resulted in severe large outdoor fires, is also highlighted here as most of this knowledge remains not available in the English language literature. The paper closes with a summary of the key research needs on this globally important problem.
                Cover page: Role of Firebrand Combustion in Large Outdoor Fire Spread.
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2021)
                High-frequency temperature measurements were carefully conducted for a 15 kW buoyant turbulent ethylene diffusion flame over a 15.2 cm diameter gas burner with air co-flow. A dual-thermocouple probe, consisting of two fine-wire thermocouples with 25 μm and 50 μm wire diameters, was used to determine a compensated turbulent gas temperature. A sensitivity analysis shows that temperatures resolved using this dual-thermocouple technique are less sensitive to changes in thermocouple bead size, therefore, uncertainty is greatly reduced even when soot deposition on the thermocouple bead occurs in sooty flames. Mean and root-mean square (rms) fluctuations of gas temperature were recorded in a two-dimensional plane across the flame centerline. The mean gas temperature monotonically decreases away from the flame centerline at most flame heights, except for 1 diameter above the burner, where a temperature dip is observed. The rms temperature peaks shift from the edge of the flame to the center as the height increases. This is due to the enhanced mixing between fuel and air, which is further shown using probability density functions of the local gas temperature. A systematic temperature dataset with high spatial resolution is established for sooty flames, which is valuable for future soot and radiation model validation.
                  Cover page: Temperature measurement of a turbulent buoyant ethylene diffusion flame using a dual-thermocouple technique
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2021)
                  Emission measurements are available in the literature for a wide variety of field burns and laboratory experiments, although previous studies do not always isolate the effect of individual features such as fuel moisture content (FMC). This study explores the effect of FMC on gaseous and particulate emissions from flaming and smouldering combustion of four different wildland fuels found across the United States. A custom linear tube-heater apparatus was built to steadily produce emissions in different combustion modes over a wide range of FMC. Results showed that when compared with flaming combustion, smouldering combustion showed increased emissions of CO, particulate matter and unburned hydrocarbons, corroborating trends in the literature. CO and particulate matter emissions in the flaming mode were also significantly correlated with FMC, which had little influence on emissions for smouldering mode combustion, when taking into account the dry mass of fuel burned. These variations occurred for some vegetative fuel species but not others, indicating that the type of fuel plays an important role. This may be due to the chemical makeup of moist and recently live fuels, which is discussed and compared with previous measurements in the literature.
                    Cover page: Effect of moisture content and fuel type on emissions from vegetation using a steady state combustion apparatus
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2020)
                    Large outdoor fires are an increasing danger to the built environment. Wildfires that spread into communities, labeled as Wildland-Urban Interface (WUI) fires, are an example of large outdoor fires. Other examples of large outdoor fires are urban fires including those that may occur after earthquakes as well as in informal settlements. When vegetation and structures burn in large outdoor fires, pieces of burning material, known as firebrands, are generated, become lofted, and may be carried by the wind. This results in showers of wind-driven firebrands that may land ahead of the fire front, igniting vegetation and structures, and spreading the fire very fast. Post-fire disaster studies indicate that firebrand showers are a significant factor in the fire spread of multiple large outdoor fires. The present paper provides a comprehensive literature summary on the role of firebrand mechanisms on large outdoor fire spread. Experiments, models, and simulations related to firebrand generation, lofting, burning, transport, deposition, and ignition of materials are reviewed. Japan, a country that has been greatly influenced by ignition induced by firebrands that have resulted in severe large outdoor fires, is also highlighted here as most of this knowledge remains not available in the English language literature. The paper closes with a summary of the key research needs on this globally important problem.
                      Cover page: Role of firebrand combustion in large outdoor fire spread
                      Top