Fluid Dynamic Aspects Of Room Fires

Several fluid dynamic processes which play important roles in the development of accidental fires in structures are discussed. They include a review of information concerning the characteristic flow regimes of fire plumes and the properties of the flow in these regimes, and a brief review of flow through openings and in ceiling jets. Factors which lead to the development of thermal stratification in ceiling layers are also discussed

Characteristics Of Large Diffusion Flames Burning In A Vitiated Atmosphere

Experiments concerning properties of large diffusion flames burning steadily in a vitiated atmosphere under conditions similar to those which may arise in a room fire are described. The effects of vitiation on the products of combustion and flame lengths, and the extinction limits are described for natural gas and ethylene diffusion flames stabilized on 8.9-, 19- and 50-cm pool-type burners. As vitiation was increased and the flame extinction limit was approached, the flame length increased slightly. Close to the limit, radiation from soot in the flame became imperceptible, leaving only a weakly luminous blue flame. Even with significant reductions in both the flame height and luminosity near the limit conditions, the hydrocarbon fuels were completely oxidized in the flame to water and carbon dioxide and no measurable concentrations of products of incomplete combustion were produced. A. comparison of limiting oxygen concentrations and limiting flame temperatures for these experiments with the results of other investigations shows reasonably good agreement despite widely varying experimental techniques. These results are contrasted with those obtained in the unsteady situation which arises when a large buoyant diffusion flame burns in an enclosed space such that the upper part of the flame is in a strongly vitiated layer composed of a mixture of air and products of combustion, and the lower part in fresh air

An analytical and computational investigation of shock-induced vortical flows

Interaction of a shock wave with a jet of light gas surrounded by an ambient heavy gas generates vorticity around the perimeter of the jet. This rolls the jet into a pair of counterrotating, finite-core size vortices. The canonical problem is the two-dimensional, unsteady interaction in a finite channel. The dynamics of the vortex pair are controlled by the incident shock strength, the light/heavy gas density ratio, and the channel spacing. Analytical expressions are derived which describe the strength and motion of the vortex pair as a function of these parameters. Numerical simulations shQw good agreement with these models. Various perturbations on the single jet flow are investigated with the goal of destabilizing the vortex pair and further enhancing the mixing. Single jet shape perturbations are relatively ineffective. However, an array of jets can dramatically increase the mixing. Another effective method is to form a reflected shock. Finally, an analogy to the corresponding three-dimensional, steady flows is demonstrated both qualitatively and quantitatively. This allows an understanding of the dynamics and mixing of the two imensional, unsteady flows to be directly applied to three-dimensional, steady flows typical of SCRAMJET designs

The effect of wall cooling on a compressible turbulent boundary layer

Experimental results are presented for two turbulent boundary-layer experiments conducted at a free-stream Mach number of 4 with wall cooling. The first experiment examines a constant-temperature cold-wall boundary layer subjected to adverse and favourable pressure gradients. It is shown that the boundary-layer data display good agreement with Coles’ general composite boundary-layer profile using Van Driest's transformation. Further, the pressure-gradient parameter β_K found in previous studies to correlate adiabatic high-speed data with low-speed data also correlates the present cooled-wall high-speed data. The second experiment treats the response of a constant-pressure high-speed boundary layer to a near step change in wall temperature. It is found that the growth rate of the thermal boundary layer within the existing turbulent boundary layer varies considerably depending upon the direction of the wall temperature change. For the case of an initially cooled boundary layer flowing onto a wall near the recovery temperature, it is found that δ_T ~ x whereas the case of an adiabatic boundary layer flowing onto a cooled wall gives δ_T ~ x^½. The apparent origin of the thermal boundary layer also changes considerably, which is accounted for by the variation in sublayer thicknesses and growth rates within the sublayer

An experiment on the adiabatic compressible turbulent boundary layer in adverse and favourable pressure gradients

A wind-tunnel model was developed to study the two-dimensional turbulent boundary layer in adverse and favourable pressure gradients with out the effects of streamwise surface curvature. Experiments were performed at Mach 4 with an adiabatic wall, and mean flow measurements within the boundary layer were obtained. The data, when viewed in the velocity transformation suggested by Van Driest, show good general agreement with the composite boundary-layer profile developed for the low-speed turbulent boundary layer. Moreover, the pressure gradient parameter suggested by Alber & Coats was found to correlate the data with low-speed results

Measurements of interactions between acoustic fields and nonuniform mean flow

Two problems crucial to the stability of longitudinal acoustic waves in solid rocket motors are examined experimentally. The first is the dissipation of energy associated with an average flow inward at the lateral boundary. Measurements reported here, though subject to considerable experimental error, show that the actual losses are much larger than predicted by the approximate one dimensional analysis. The second problem is the attenuation of waves accompanying reflection by the nonuniform flow in a choked exhaust nozzle. Empahsis in this work has been on technique, to provide data relatively easily and inexpensively. It appears that good results can be obtained in a routine manner using small supersonic wind tunnel operated as an open cycle. At least for Mach numbers up to 0.04 at the nozzle entrance, difficulties with signal/noise are satisfactorily overcome with a tracking filter

Experimental Study of Environment and Heat Transfer in a Room Fire

This final report is written in three sections. The first two draw attention to work reported in detail in the Third Quarterly Progress Report and the third Section describes ongoing work which has not been previously discussed in a Progress Report

Spin-Hall and Anisotropic Magnetoresistance in Ferrimagnetic Co-Gd / Pt layers

We present the Co-Gd composition dependence of the spin-Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) for ferrimagnetic Co100-xGdx / Pt bilayers. With Gd concentration x, its magnetic moment increasingly competes with the Co moment in the net magnetization. We find a nearly compensated ferrimagnetic state at x = 24. The AMR changes sign from positive to negative with increasing x, vanishing near the magnetization compensation. On the other hand, the SMR does not vary significantly even where the AMR vanishes. These experimental results indicate that very different scattering mechanisms are responsible for AMR and SMR. We discuss a possible origin for the alloy composition dependence.Comment: 31 Pages, 9 figure

Role of Coulomb correlation on magnetic and transport properties of doped manganites: La0.5Sr0.5MnO3 and LaSr2Mn2O7

Results of LSDA and LSDA+U calculations of the electronic structure and magnetic configurations of the 50% hole-doped pseudocubic perovskite La0.5Sr0.5MnO3 and double layered LaSr2Mn2O7 are presented. We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons has a very different influence on the (i) band formations, (ii) magnetic ground states, (iii) interlayer exchange interactions, and (iv) anisotropy of the electrical transport in these two manganites. A possible reason why the LSDA failures in predicting observed magnetic and transport properties of the double layered compound - in contrast to the doped perovskite manganite - is considered on the basis of a p-d hybridization analysis.Comment: 11 pages, 3 figure