Buoyant flows in vertical channels relating to smoke movement in high-rise building fires

Citation

Benedict, Noel Lakshman (1999) Buoyant flows in vertical channels relating to smoke movement in high-rise building fires. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ynbt-qn25. https://resolver.caltech.edu/CaltechETD:etd-02072008-074758

Abstract

This experimental study is motivated by the widespread loss of life and property due to accidental fires in high rise buildings, and investigates the progress of hot, toxic, products during such fires. The Stack Effect and Turbulent Mixing, two of the primary factors responsible for smoke movement within tall buildings are the focus of this study. The results of this investigation could be used for the development of fire modeling codes that simulate high-rise building fires.

The experiments involve a 2.6 m tall square shaft with various cross sections and openings. The shaft was situated above a large temperature controlled hot air reservoir, and the two chambers were initially separated by a partition. At the start of the experiment the partition was removed in a rapid horizontal motion and the hot and cold gases were allowed to mix. For the shafts with openings, the gases were withdrawn from the apertures at different rates with the Reynolds number varying between 600 and 7200. The temperature of the gas and wall, and the heat transfer to the wall were measured as functions of time at various locations in the shaft. Additionally, hot wire anemometry techniques were used to obtain velocity data in the channel. Some experiments involved monitoring a tracer gas in the vertical channel. Simple one-dimensional analytical modeling was performed to validate the experimental results.

The experiments indicated an initial transient period followed by a "pseudo steady state." At each elevation measured the cross-section averaged gas temperature, reached and fluctuated about a steady state value soon after the initial front of hot gas arrived at that location. For the closed channel experiments, the front arrival time was a function of the initial density ratio, the shaft width, and the gravitational constant.

The tracer gas trials suggested that the molecular diffusion was insignificant in comparison to the turbulent mixing. For the closed channels the observed velocity fluctuations were the same order of magnitude as the mean velocities. The time averaged heat transfer coefficient was weakly dependent on the initial reservoir temperature.

The vented shaft experiments indicated that the front propagation times are significantly affected by openings in the shaft and that the effect is more pronounced the higher the vents are located. The venting caused a significant rise in the steady state temperatures, and a reduction in both the temperature and velocity fluctuations. The local Nusselt number was independent of the Reynolds number and a function only of the Rayleigh number, indicating that the heat transfer was dominated by free convection effects.

The predictions made by the one dimensional analytical model agreed reasonably well with the experiments, particularly in the case of the closed channel.

Thesis Files