Experimental study of a turbulent buoyant helium plume

Abstract

An experimental study has been performed on the dynamics of a large turbulent buoyanthelium plume. Two-dimensional velocity fields were measured using particle image velocimetry (PIV) while helium mass fraction was determined by planar laser-induced fluorescence (PLIF). PIV and PLIF were performed simultaneously in order to obtain velocity and mass fraction data over a plane that encompassed the plume core, the near-field mixing zones and the surrounding air. The Rayleigh–Taylor instability at the base of the plume leads to the vortex that grows to dominate the flow. This process repeats in a cyclical manner. The temporally and spatially resolved data show a strong negative correlation between density and vertical velocity, as well as a strong 90° phase lag between peaks in the vertical and horizontal velocities throughout the flow field owing to large coherent structures associated with puffing of the turbulent plume. The joint velocity an mass fraction data are used to calculate Favre-averaged statistics in addition to Reynolds-(time) averaged statistics. Unexpectedly, the difference between both the Favre-averaged and Reynolds-averaged velocities and second-order turbulent statistics is less than the uncertainty in the data throughout the flow field. A simple analysis was performed to determine the expected differences between Favre and Reynolds statistics for flows with periodic fluctuations in which the density and velocity fields are perfectly correlated, but have the phase relations as suggested by the data. The analytical results agreewith the data, showing that the Favre and Reynolds statistics will be the same to lead order. The combination of observation and simple analysis suggests that for buoyancy-dominated flows in which it can be expected that density and velocity are strongly correlated,phase relations will result in only second-order differences between Favre- and Reynolds-averaged data in spite of strong fluctuations in both density and velocity.

Application of FDS and FireFOAM in Large Eddy Simulations of a Turbulent Buoyant Helium Plume

“…In this study comparisons are made with experiments performed in a large building designed for indoor fire experiments: the Fire Laboratory for Accreditation of Models and Experiments (FLAME) at Sandia National Laboratories in Albuquerque, New Mexico, and reported by O'Hern et al [9]. The FLAME chamber is nominally a 6.1 m cubical enclosure with a 2.4 m in diameter chimney located on top of the chamber.…”

“…The values of concentration contain uncertainties in the order of 18%, plus fixed uncertainties of 5%, while the concentration fluctuation in the order of 21%. The above uncertainties also include run-to-run variability [9].…”

“…The simulations have been configured to replicate the experimental axisymmetric helium -air plume study of O'Hern et al [9]. The experimental set-up for all the simulations is simplified into an 3 …”

“…Yet, as explained later, this is not straightforward either. In this study, large eddy simulations (LES) performed with FireFOAM and the fire dynamics simulator (FDS 5) will be compared with the well-documented experiment performed by O`Hern et al [9]. FireFOAM is based on the OpenFOAM [10] platform, which is a set of object-oriented open source CFD toolboxes written in C++.…”

Validation experiments to determine radiation partitioning of heat flux to an object in a fully turbulent fire.

“…Flow field velocities will be meas-ured by cross-correlation of particle locations between subsequent frames in an image pair, so 15 velocity maps per second will be produced. PIV has previously been applied for measurements in methanol pool fires, methane and hydrogen gas fires, and nonreacting helium plumes in previous SNL programs (e.g., Tieszen et al, 2002Tieszen et al, , 2004O'Hern et al, 2005). …”