Heat transfer across an air curtain for heat confinement in road tunnels: Influence of the heat source

Experiments were carried out to study the influence of the heat source using double-flow double-jet curtains (DSTJ) to confine heat in fire safety problems. For this purpose, a small-scale road tunnel facility in 1:34 was built. The heat source used is an electric resistance. Simultaneous two-dimensional temperature and velocity measurements were made across the DS-TJ air curtain and in its immediate vicinity using a fine thermocouple and a 2D laser Doppler anemometer (LDA), respectively. Four cases were studied, by modifying the air curtain velocities at the nozzle exits and the heat source power. It is noted that hot-cold jets and heat source play a role in the curtain heat confinement by modifying the development of the two jets. Both the entrainment of the upward flow rising from the heat source as well as the Kelvin-Helmholtz instabilities contribute to this. On the other hand, the nozzle exit velocities determine the behavior of the Reynolds stress profiles as well as the production of the velocities fluctuations across the curtain, showing a non-correlating effect on the hot side, so that the profiles take values around zero, suggesting a heat source influence. In contrast in the cold side negative values indicate energy-draining behavior. The intensity of thermal turbulence reveals that the hot jet has higher values than the cold jet, constituting a thermal barrier to turbulent heat flow, and all activity is relegated to the zone of interaction between the two jets. In fact, the production derived from temperature fluctuations occurs mainly in the interaction of hot-cold jets, and negative values are due to large turbulent eddies, such as Kelvin-Helmholtz instabilities and Go center dot rtler-type structures. The Richardson number shows that buoyancy forces prevail on both sides of the curtain (Ri > 0.1), especially on the hot side, due to the heat source. Lower Ri values within the curtain indicate the presence of a thermal barrier, attenuating heat transfer through the curtain and favoring thermal sealing.