Byoungchul Kwon

Bio: Byoungchul Kwon is an academic researcher from Case Western Reserve University. The author has contributed to research in topics: Materials science & Flame spread. The author has an hindex of 2, co-authored 2 publications receiving 29 citations.

Concurrent flame spread over discrete thin fuels

Abstract: An unsteady two-dimensional numerical model was used to simulate concurrent flame spread over paper-like thin solid fuels of discrete configurations in microgravity (0 g with 20 cm/s) and in normal gravity (1 g). An array of ten 1 cm-long fuel segments was uniformly distributed in the flow direction (0 g) or in the vertical direction (1 g). A hot spot ignition source was applied at the upstream leading edge of the first fuel segment. The separation distance between the fuel segments was a parameter in this study, ranging from 0 (corresponding to a continuous fuel) to 3 cm. Using this setup, the burning characteristics, spread rate of the flame base, and the solid burning rate were examined. The flame base spread rates in both 1 g and 0 g cases increase with the separation distance. This is due to the flame jumping across the gaps. For the solid burning rate, the dependency on the separation distance is different in 1 g and 0 g cases. At a flow velocity of 20 cm/s in 0 g, the flame reaches a limiting length and the flame length is approximately the same for all fuel configurations. As the separation distance increases, the heating length (the fuel area exposed to the flame) decreases, resulting in a decreasing total heat input and a decreasing solid burning rate. In 1 g, the flame is long and extends to the last fuel segment before the first fuel segment burns out. This suggests that the heating length is approximately the same in all simulated cases (∼total fuel length). However, the flame standoff distance decreases when the separation distance increases. This results in an increasing total heat input and an increasing solid burning rate. Terrestrial experiments were conducted to validate the 1 g model. The experimental results agreed reasonably with the model predictions of burning characteristics, burn durations, and flame spread rates.

Effect of firebrand size and geometry on heating from a smoldering pile under wind

Abstract: Smoldering firebrands can be lofted over long distances, easily igniting spot fires. This poses a threat to structures in the wildland-urban interface (WUI), where wildland fires spread into and within communities. This study investigates the influence of firebrand size and makeup on heating from a pile to a recipient surface in a small-scale wind tunnel. Two sizes of fluted birch wooden pins and wooden discs, one size of cylindrical birch dowels, two lengths of eucalyptus sticks, and pine bark flakes were used to simulate firebrands; all with 4 g of smoldering brands deposited. The porosity of the different firebrand piles, the heat flux to an inert test surface, spatial measurements of surface temperature, and videos of the experiments were recorded and analyzed. It was found that the realistic fuels, i.e. pine bark and eucalyptus sticks, could achieve higher peak heat fluxes than artificial birch fuels at higher wind speeds, which exhibited little change with geometry. This change was not due to material makeup alone, but also the way in which the bulk density and height of the pile changed for different smoldering fuels.

Effects of spacing on flaming and smoldering firebrands in wildland–urban interface fires

Abstract: Firebrand (ember) attack has been shown to be one of the key mechanisms of wildfire spread into wildland–urban interface communities. After the firebrands land on a substrate material, the ignition propensity of the material depends on not only the attributes (e.g. shape, size, and numbers) but also the distribution of the firebrands. To help characterize this process, this study aims to investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments are conducted using nine wooden cubes, 19 mm on each side. These samples are arranged in a 3 × 3 square pattern on suspension wires and are ignited by hot coils from the bottom surface. The gap spacing (s) between the samples varies in each test (ranging from 0 to 30 mm). After ignition, the samples are left to burn to completion. The burning process is recorded using video cameras. Sample mass loss and temperatures are monitored during the flaming and smoldering processes. The results show that the flame height and the sample mass loss rate have non-monotonic dependencies on the gap spacing. When the gap spacing reduces, the flame height and the mass loss rate first increase due to enhanced heat input from the adjacent flames to each sample. When s ≤ 10 mm, flames from individual samples are observed to merge into a single large fire. As s further decreases, the air entrainment at the flame bottom decreases and the flame lift-off distance at the flame center increases, resulting in an increased flame height, decreased flame heat feedback to the solid samples, and a decreased mass loss rate. The decreased mass loss rate eventually leads to a decrease in the flame height as well. The gaseous flame height is correlated to the solid burning rate. The correlation generally follows previous empirical equations for continuous fire sources. For the smoldering combustion, compared to a single burning sample, the smoldering temperature and duration significantly increase due to the thermal interactions between adjacent burning samples. To help interpret the results of the burning experiments, thermogravimetric analysis is also performed in air and nitrogen, resulting in heating rates ranging from 10 to 100 K/min.

In Situ Gas Analysis and Fire Characterization of Lithium-Ion Cells During Thermal Runaway Using an Environmental Chamber.

Abstract: An experimental apparatus and a standard operating procedure (SOP) are developed to collect time-resolved data on the gas compositions and fire characteristics during and post-thermal runaway of lithium-ion battery (LIB) cells. A 18650 cylindrical cell is conditioned to a desired state-of-charge (SOC; 30%, 50%, 75%, and 100%) before each experiment. The conditioned cell is forced into a thermal runaway by an electrical heating tape at a constant heating rate (10 °C/min) in an environmental chamber (volume: ~600 L). The chamber is connected to a Fourier transform infrared (FTIR) gas analyzer for real-time concentration measurements. Two camcorders are used to record major events, such as cell venting, thermal runaway, and the subsequent burning process. The conditions of the cell, such as surface temperature, mass loss, and voltage, are also recorded. With the data obtained, cell pseudo-properties, venting gas compositions, and venting mass rate can be deduced as functions of cell temperature and cell SOC. While the test procedure is developed for a single cylindrical cell, it can be readily extended to test different cell formats and study fire propagation between multiple cells. The collected experimental data can also be used for the development of numerical models for LIB fires.

Urban-Wildland Fires: On the Ignition of Fuel Beds by Firebrands. (POSTER ABSTRACTS) | NIST

Abstract: Urban-wildland fires have plagued the United States for centuries. Recent urban-wildland fires include the 2002 Hayman Fire, the 2000 Los Alamos Fire, and the 1991 Oakland Hills Fire [1]. Fires in the urban wildland interface can have a devastating effect on human life, property loss, and local economies. Embers or firebrands are produced as trees and other objects burn in urban-wildland fires. These firebrands are entrained in the atmosphere and may be carried by winds over long distances. Hot firebrands ultimately come to rest and may ignite fuels far removed from the fire, resulting in fire spread. This process is commonly referred to as spotting. Understanding how these hot firebrands can ignite surrounding fuels is an important consideration in mitigating fire spread in communities. A major advance in urban-wildland fire research would be the development of a model to predict the ignitability of materials due to firebrand impact [2]. The lack of a detailed theory on the ability of firebrands to ignite remote objects limits the utility of detailed computational fluid dynamic models (CFD) that could be used to predict fire spread by firebrands [2]. Detailed experimental ignition studies of fuel beds typically found in the urban-wildland interface due to firebrand impact are required to validate such models. Consequently, an experimental apparatus has been built to investigate the ignition of fuel beds as a result of impact with burning firebrands. The apparatus allowed for the ignition and deposition of firebrands onto the target fuel bed. The moisture content of the fuel beds used was varied and the fuels considered were pine needle beds and shredded paper beds. Pine needle beds were intended to simulate gutters filled with pine needles. Shredded paper beds were used as a surrogate for typical cellulosic fuels that would be found in attic spaces. Firebrands were simulated by machining wood (pinus ponderosa) into small disks of uniform geometry. The firebrand ignition apparatus was installed into the Fire Emulator/Detector Evaluator (FE/DE) to investigate the influence of an air flow on the ignition propensity of fuel beds. Results of this study are presented.

Understanding the effects of inclination angle and fuel bed width on concurrent flame spread over discrete fuel arrays

Abstract: Concurrent flame spread over discrete fuel arrays is a dangerous flame configuration where flame spread has the same direction with induced airflow. However, inclined upward flame spread over discrete fuels is rarely concerned, especially for the varied fuel bed width. In this study, birch rods were mounted in the regular array spaced 9 mm apart. 36 groups of arrays by varying six inclination angles (θ, 15°–90° with an interval of 15°) and six kinds of column numbers (n, 1–11 with an interval of 2) were designed to study the combined effect of inclination angle and array fuel bed width on flame spread over discrete fuel arrays. Average flame spread rate was found to be independent on the fuel bed width for n > 1 , and mass loss rate per total mass is restricted by air entrainment when θ ⩾ 75 ° or θ = 60 ° and n > 5 . As the inclination angle increases, the heat transfer mechanism changed from radiant dominance to convective dominance. Moreover, in the cases of radiant dominance, average flame spread rate was proportional to the squared incident heat flux. By simplifying burning zone, a prediction model of mass loss rate was developed. Based on the aspect ratio, burning zone was further divided into line ( λ ⩾ 3 ) and rectangular ( λ 3 ) fire sources, and then dimensionless correlations between the flame length and the heat release rate were proposed both for λ ⩾ 3 and λ 3 . In addition, during the upward flame spread process, flame length was found to be controlled by the competition between the accelerated upward buoyancy flow and the restriction of lateral air entrainment. This study provides a comprehensive insight for flame spread behavior of discrete fuels.

Experimental study of upward flame spread over discrete thin fuels

Abstract: Experiments are performed to study upward flame propagation over discrete combustibles separated by air gaps. An array of ten 1 cm-long 5 cm-wide filter papers is uniformly distributed on a vertical sample holder subjected to double-sided burn. The distance between the samples was varied from 0 to 4 cm. After being ignited from the bottom end, the flame spread process is recorded by front and side video cameras. A precision balance with 0.01g resolution is used to monitor the mass loss and deduce the solid burning rate. The results show that both flame spread rate and solid burning rate have a non-monotonic relationship with the gap size. The presence of gaps decreases the fuel load (fuel mass per unit length), which results in an increasing apparent flame spread rate as the gap size increases. The gaps also allow the lateral entrained air to push the flame closer to the sample surface, enhancing the conductive heat input to the samples. This results in an increased solid burning rate and flame spread rate. However, when the gap size is large, the effective heating length of the sample and hence the total burning rate decrease as the gap size increases. Eventually, the flame fails to spread.

Identifying the criterion for discrete flame spread over single-row birch rods

Abstract: Although discrete flame spread is a common phenomenon in the practical fire scenarios, such as a group of burning trees under bushfire, research gaps still exist to address the flame spreading criteria and the related spread characteristics. Therefore, through this study, a group of birch rods with different lengths (denoted by l within 60–100 mm) and spacings (S, 1–9 mm) were analyzed experimentally and theoretically. It was known that the critical criterion of discrete flame spreading is determined by the l and S, where a criterion of l≥3d (sample thickness) could predict the critical spacing between the separated birch rods. Theoretical models were also developed to predict the global flame spread rate and mass loss rate under various S and l. The predictions based on the newly developed models agree reasonably well with those experimental data. Global flame spread rate, mass loss rate and flame height increase first and then decrease along with a bigger S, where the dimensionless flame height shows a piecewise exponential relationship with the dimensionless heat release rate. The research outcomes of this study provide a theoretical basis for the fire risk evaluation of those discrete fire spread.

Concurrent-flow flame spread over thin discrete fuels in microgravity

Abstract: Microgravity experiments are performed to study concurrent-flow flame spread over an array of thin cellulose-based fuel samples, using NASA Glenn Research Center's 5.18 s drop tower. Sample segments are distributed uniformly, separated by air gaps, on a sample holder. The exposed width of each sample segment is 5 cm. Two segment lengths, 0.5 cm and 1 cm, are tested. The gap sizes are varied in different tests, ranging from 0.5 to 5 cm. In all tests, a low-speed air flow (30 cm/s) is imposed and the upstream-most fuel segment is ignited by an electrical ignition wire. Upon ignition, the flame spread is recorded by two video cameras from the front and side-view angles. Spread rates, flame lengths, and burning durations are extracted using a custom video processing code. Similar to continuous fuels, flame spread over discrete fuels is a continual process of ignition. A burning discrete fuel segment, before it is consumed, needs to ignite the subsequent segment in order to have flame propagation across the gap. During this process, larger gaps between samples reduce the effective fuel load, increasing the apparent flame spread rate. However, larger gaps also reduce the heat transfer between adjacent samples, decreasing the sample burning rate. As a result, as the gap size increases, the flame spread rate increases but the burning rate decreases. At the same gap size, the flame spread rate is higher for the shorter tested sample segments. When considering sample configurations of the same fuel ratio (fuel length over the summation of the fuel and gap lengths), the spread rates are similar. This trend remains until a critical gap size is reached and flame fails to propagate across the entire array of samples. The critical gap sizes are similar for the two tested sample segment lengths and are suspected to be determined by the flame length.