Experimental study of the structure of a thermal plume inside a rectangular tunnel

Abstract

The objective of this work is to experimentally simulate a plume developing inside a horizontal tunnel. The experimental device used in this simulation is essentially constituted of a hot disk, a rectangular tunnel and a ventilation system. The hot disk is heated by Joule effect to a constant and uniform temperature, and placed inside the tunnel. The hot source generates a thermal plume. We first studied the evolution of the thermal plume without ventilation system. The study of the average and fluctuating thermal and dynamic fields shows three zones during the vertical evolution of the free plume. A first zone close to the source, serving to the plume supply in fresh air, is characterized by the apparition of three escapes of the thermal plume. Followed by a second zone where the main escape undergoes a contraction. Finally, a third zone where the thermal plume accumulates and undergoes a flow upstream named backlayering and a flow downstream that borders the ceiling to leave by the free part of the tunnel. Keywords Fire plume, Fire tunnel, Thermal plume, turbulent natural convection. I. INTRODUCTION The thermal plume model is an experimental methodology usable easily to simulate the fire plume (1, 2). The thermal model contributes to better understand many practical fire problems such as problems associated with fire tunnel and flow encountered in fires of structural elements of buildings. The interaction of these material surroundings with the fire plumes reveals very complex physical mechanisms. From a fundamental viewpoint, the study of the interaction of the thermal plume with the vertical walls that surrounds it began with Agator's work (3) on the influence of a wall placed in the vicinity of the plume source. He noted that the plume is attracted toward the wall. A. O. M. Mahmoud and al. (4, 5, 6) are the first who were studied the evolution of a thermal plume in semi-confined geometry. They studied the evolution of a thermal plume produced by a flat disc heated at 300° C and placed at the entrance of an open-ended vertical cylinder. They noted that the plume interacts narrowly with the thermosiphon flow which develops along the internal wall of the cylinder. Contrary to previous works (7, 8, 9, 10), they noticed the appearance of a supplementary zone in addition to the two classic zones which characterize the vertical evolution of the free plume. Just above of the source, the instability zone is characterized by the formation of rotating rolls and by the existence of three extrema of temperature and velocity profiles. Higher, a second zone of turbulence pre- established followed by a last zone where the turbulence is fully established. J. Zinoubi et al. (11, 12) continued this experimental work by studying the form factors effect of the plume evolution inside a vertical cylinder. Using the visualization and analysis of the thermal and dynamic profiles of the flow, they showed the existence of three zones described previously. By studying the influence of the cylinder height, J. Zinoubi et al. (13) noted a blocking of the ascending flow in the third zone due to the lateral expansion of the plume. They also showed that a choice of the cylinder height not exceeding the second zone of the flow let us avoid this blocking. In order to determine the geometry effect, N. Taoufik et al. (14) studied the evolution of a thermal plume generated by a flat disc inside an open-ended rectangular canal. They noted the existence of the three zones observed in the cylindrical geometry. Also, they noticed the contraction of the rotating rolls size located in the first zone of the flow. Recently, A. O. Mahmoud et al. (1) studied the effects of source air entrainment on the flow structure induced by two heat sources, one placed at ground level, the other at a height above the ground. The experimental results permitted to specify that the additional vertical contribution of the air entrainment especially entails a substantial change of the flow structure of the plume, an important elongation of the height of the plume spread, a considerable increase of the flow rate of the plume and an important elevation of the thermal flux absorbed by the air. It is clear that these works were essentially interested to the determination of the effects of the emplacement, the heat release of the fire and the tunnel geometry on the critical ventilation velocity. The physical structure of the plume inside tunnel has not been studied. A fire plume inside tunnel has very complex