Velocity and temperature structure of medium-scale pool fires

Abstract

A series of measurements was conducted using a bidirectional probe to characterize the upward speed in the plumes of medium-scale pool fires burning a variety of gaseous and liquid fuels. Time-averaged local measurements of the upward velocity were conducted in the plumes of methanol, ethanol, and acetone pool fires burning in a 30 cm diameter liquid pool burner and methane and propane gaseous pool fires established on a 37 cm diameter gas burner. The gas-phase thermocouple temperature was measured separately using many kinds of fine bare-wire thermocouples with different data acquisition rate, which temperatures were corrected considering radiative loss and thermal inertia effects. The bidirectional probes were connected to multiple pressure transducers with various time responses, ranging from 20 ms to 160 ms. A 25 ?m wire diameter, bare-bead, Type R or S, thermocouple with an approximately spherical bead of diameter 125 ?m was positioned about 0.5 cm upstream of the probe. The time series of temperature was corrected for radiative loss only. The Bernoulli equation was solved using the time series of pressure, the corrected temperature time series, and the calculated K factor based on the Re (and its properties) calculated from the instantaneous velocity and temperature. The spatial separation between the probe and thermocouple was assumed to be of negligible importance. The experiments were typically repeated 2 to 3 times and the results were repeatable to within a few percent. The upward component of the velocity on the plume centerline in the methanol pool fire compared favorably to previous measurements and Baum's plume velocity correlation. The results showed that the upward velocity increased with distance above the plume for about 1 to 2 diameters above the fuel surface and then decreased with distance. The upward component of the velocity on the plume centerline in the methanol pool fire compared favorably to previous measurements and followed the general trends of Baum and McCaffrey's plume theory. A single location radiative heat flux measurement was used to determine the radiative fraction in 30 cm diameter methanol pool fires. The results were favorably compared to previous measurements using multi-location and single-location heat flux measurements.