Structure of self-preserving plane buoyant turbulent free line plumes and adiabatic wall plumes.

Abstract

An experimental investigation of the self-preserving properties of buoyant turbulent plumes is described. The research is motivated by the need to resolve effects of buoyancy/turbulence interactions and to provide data required to benchmark models of buoyant turbulent flows for fire environments. The flows considered included free line plumes and adiabatic wall plumes in an attempt to learn more about buoyant turbulent flows typical of the environment of unwanted fires. Measurements included laser-induced iodine fluorescence (LIF) to find mixture fraction statistics and laser-Doppler velocimetry (LDV) to find velocity statistics. Present measurements emphasized self-preserving conditions far from the source where effects of source disturbances and momentum have been lost. The plumes were simulated using helium/air sources in a still and unstratified environment and rising along a smooth plane and vertical wall. Present measurements of plane buoyant turbulent plumes extended farther from the source (up to 155 source widths) and had more accurate specifications of plume buoyancy fluxes than past measurements. Self-preserving behavior of free line plumes was observed 76--155 source widths above the source, yielding smaller normalized plume widths and different scaled mean and fluctuating mixture fractions near the plane of symmetry than previously thought. Measurements of probability density functions, temporal power spectra and temporal integral scales of mixture fluctuations are also reported. Self-preserving behavior of adiabatic wall plumes was observed 92--155 source widths above the source, yielding smaller normalized plume widths and near wall mean mixture fractions than earlier measurements. Present measurements of velocity properties yielded smaller normalized plume widths and larger near wall mean velocities than observations within the flow development region nearer to the source. Unlike observations of concentration fluctuations, which are unusually large due to effects of streamwise buoyant instabilities, velocity fluctuation intensities were comparable to values observed in nonbuoyant turbulent wall jets. The entrainment properties of the present flows approximated self-preserving behavior in spite of the continued development of the wall boundary layer. Measurements of probability density functions, temporal and spatial integral scales of mixture fraction and velocity fluctuations are also reported. Self-preserving adiabatic wall plumes mix slower than comparable free line plumes because the wall prevents mixing on one side and inhibits large-scale turbulent motion. This reduced rate of mixing for turbulent wall flows is a concern in fires because it extends the length of the flame-containing region and reduces effects of dilution on reducing temperature levels and toxic gas concentrations in overfire plumes.