Convective heat transfer is the most important factor in activating fire detection and protection equipments, such as sprinkler systems. When a fire occurs inside a room, the convective heat transfer under the ceiling is closely related to the motion of the smoke and the circulation of the room air heated by the fire plume. Thus, it is essential to understand smoke layer formation and the vortical motion under a ceiling to design effective fire detection and protection equipment. In the early stages of a fire, the smoke that is generated from a fire rises and forms a layer under the ceiling; it then spreads radially along the ceiling surface, entraining ambient air. The formation of this smoke layer includes the initial impinging process of the fire plume on the ceiling surface and is related to the periodic pulsating phenomenon of the fire. Therefore, it is necessary to investigate the characteristics of smoke layer formation and the effects of pulsation frequency on the flow structures of the fire plume to obtain physical insights into the convective flow under a ceiling.
In this study, first to capture the features of the ceiling jets of real fires, the smoke front propagation distances and speeds of real gasoline pool fires inside a corridor-shaped room were measured by the thermocouples and the laser visualization technique. In addition, the smoke layer under the ceiling plate formed by a periodically pulsating pool fire was also visualized by the Schlieren.
Second, to understand the unsteady evolution process and the behavior of the ceiling jet formed by a pulsating jet, the vortex behavior and thickness of the nozzle fluid layer of pulsating jets under a ceiling were studied using the Mie scattering visualization technique. The thicknesses of the nozzle fluid layers of jets pulsating at different frequencies were compared with those of continuously issuing jets to investigate the effects of the vortex formation behavior on pulsating jets under a ceiling. To identify similarities between the features of pulsating cold jets and periodically pulsating pool fires, the frequencies of the pulsating jets were determined using the results obtained from previous pool fire studies. The formation and propagation characteristics of vortices in pulsating jets were investigated, along with the effects of the pulsation frequency on the thickness of the nozzle fluid layer.
An intuitive criterion for a smoke front tracking inside a corridor-shaped room based on temperature change was developed. For synchronized pictures of the smoke front along with temperature signals, the intuitive criterion was shown to be acceptable. The smoke visualization by the laser sheet proves to have several advantages over the convectional techniques such as the use of the smoke generators. The speed of the smoke front is measured by thermocouples and by the laser-sheet visualization. These are compared to show good agreement in the fully developed ceiling jet region.
In addition, it was found that the size of the vortex generated decreased as the pulsation frequency of the jet increased. The thickness of the nozzle fluid layer of pulsating jets was larger than that of continuously issuing jet, although the primary vortex was smaller. The thickness of the nozzle fluid layer increased with decreasing pulsation frequency in jets with the same Reynolds number. The results of this study imply that the thickness of the smoke layer formed from afire plume under a ceiling is strongly related to the pulsation frequency of the plume. Therefore, it is important to understand the vortex formation features and periodicity of fire plumes to determine the thickness of the smoke layer.
충돌제트 중 round jet나 pool fire와 같은 유동형태에서는 평판과 충돌하기 전의 유동에 규칙적인 거대구조들을 포함하고 있다. 이들의 주기적인 거대 구조들은 충돌 후에 형성되는 유동장의 유동구조 및 경계층 형성 등에 직접적인 영향을 줄 것으로 생각되지만 이에 대한 연구는 거의 전무한 실정이다. 화재 크기가 큰 경우에서는 화재 plume 자체가 난류가 되어 본 연구에서 관찰하고자하는 화원에서 생성된 주기적인 운동을 효과적으로 관찰하는 것이 불가능하게 된다. 또한 층류 영역의 화원에서 실험할 경우에는 화원크기가 고정되기 때문에 화원크기에 해당하는 고유한 변동주기만을 관찰해야 하는 실험상 제약을 가지게 되어 본 연구에서는 가진 주파수의 변동이 가능한 유동장을 실험조건으로 선정하였다.
본 연구는 화재에서 발생하는 주기적인 plume이 천장에 부딪혀 전개되는 ceiling jet의 거동을 이해하고자하는 목적을 가지고 있다. 이를 위해 먼저 실물 pool fire의 연기선단 가시화를 실시하여 천장 아래에 형성되는 연기선단의 거동을 파악하였으며, 이렇게 파악된 ceiling jet의 거동을 체계적으로 분석하기 위하여 등온 유동 조건에서 우선적으로 주기적인 변동이 없는 경우의 ceiling jet 거동을 파악하고 이를 주기적인 변동이 있는 경우의 결과와 비교하였다. 또한 각 조건에서의 DNS 수치해석을 수행하여 실험결과와 비교하였다.
