Chang Bo Oh

Effects of Initial Condition and Opening Geometry of a Compartment on the Gravity Current in the Backdraft

Computational study of a gravity current prior to the backdraft was conducted using fire dynamic simulator (FDS). Various initial conditions of mixture compositions and compartment temperature as well as four opening geometries (Horizontal, Door, Vertical, and Full opening) were considered to figure out their effects on the gravity current. The density difference ratio () between inside and outside of compartment, the gravity current time () and velocity (), and non-dimensional velocity () were introduced to quantify the flow characteristics of the gravity current. Overall fluid structure of the gravity current at the fixed opening geometry showed similar development process for different conditions. However, for entering air to reach the opposed wall to the opening geometry increased with . Door, Vertical, and Horizontal openings where openings are attached on the ground showed similar development process of the gravity current except for Horizontal opening, which located on the middle of the opening wall. The magnitude of at fixed was, from largest to smallest, Full > Vertical > Door > Horizontal, but it depended on both the size and location of the opening. On the other hand, was found to be independent to , and only depended on the geometry of the opening.

Radiation Effects on the Ignition and Flame Extinction of High-temperature Fuel

The radiation effects on the auto-ignition and extinction characteristics of a non-premixed fuel-air counterflow field were numerically investigated. A detailed reaction mechanism of GRI-v3.0 was used for the calculation of chemical reactions and the optically-thin radiation model was adopted in the simulations. The flame-controlling continuation method was also used in the simulation to predict the auto-ignition point and extinction limits precisely. As a result, it was found that the maximum H radical concentration, (YH)max, rather than the maximum temperature was suitable to understand the ignition and extinction behaviors. S-, Cand O-curves, which were well known from the previous theory, were identified by investigating the (YH)max. The radiative heat loss fraction (fr) and spatially-integrated heat release rate (IHRR) were introduced to grasp each extinction mechanism. It was also found that the fr was the highest at the radiative extinction limit. At the flame stretch extinction limit, the flame was extinguished due to the conductive heat loss which attributed to the high strain rate although the heat release rate was the highest. The radiation affected on the radiative extinction limit and auto-ignition point considerably, however the effect on the flame stretch extinction limit was negligible. A stable flame regime defined by the region between each extinction limit became wide with increasing the fuel temperature.

A Numerical Study of the Backdraft Behavior with the Variation of the Ignition Location and Time

The behavior of backdraft in the compartment with different ignition locations and times was numerically investigated. The Fire Dynamics Simulator (FDS) v5.5.3 with a model-free simulation option was used in the numerical simulation of backdraft. The ignition source was located near the inside wall, at the compartment center and near the window opening, respectively. The ignition was started at the instance when the fresh air reached the ignition location or when a sufficient time passed compare to the instance of the arriving of the fresh air to the ignition location. As a result, for the ignition source was located near the inside wall, a strong fire ball was observed at once and the result was similar to the previous experimental result. For the ignition source was located at the center of the compartment, a strong fire ball was occurred and two strong fire balls were observed consecutively for the ignition time was delayed. For the ignition source was located near the window opening and longer time was given for the ignition compare the duration of the fresh air arriving to the ignition location, the rapid temperature variation was not observed because there was no flame. However, for the ignition was started at the instance when the fresh air reached the ignition location, the ignition could be initiated and a intensive fire ball was observed. The pressure measured at the upper inside part of the window opening provided a similar trend with the previous experimental result of compartment backdraft.

A Study of Numerical Reproducibility for the Backdraft Phenomena in a Compartment using the FDS

A numerical reproducibility of the backdraft phenomena in a compartment was investigated. The prediction performance of two combustion models, the mixture fraction and finite chemistry models, were tested for the backdraft phenomena using the FDS code developed by the NIST. The mixture fraction model could not predict the flame propagation in a fuel-air mixture as well as the backdraft phenomena. However, the finite chemistry model predicted the flame propagation in the mixture inside a tube reasonably. In addition, the finite chemistry model predicted well the backdraft phenomena in a compartment qualitatively. The flame propagation inside the compartment, fuel and oxygen distribution and explosive fire ball behavior were well simulated with the finite chemistry model. It showed that the FDS adopted with the finite chemistry model can be an effective simulation tool for the investigation of backdraft in a compartment.