2,111 research outputs found
Recent studies on flame stabilization of premixed turbulent gases
FLAME stabilization is of importance in the practical design
of ramjets and afterburners. It has been studied
extensively in recent years, particularly with reference to
bluff-body flame-holders. In the present survey we describe
the investigations relating to flame holding by bluff bodies as well as new techniques (e.g.,. flame holding by the use of reverse jets) which may prove to be of practical importance in new engine configurations. In Section II we consider the flow field downstream of a bluff-body flame-holder which includes the recirculation zone behind the body and a region of flame spreading farther downstream.
Explicit reference is made to crucial experiments which illustrate the nature and magnitude of the velocity field, the physical extent, the temperature, and the gas composition of the recirculation zone. Experimental studies and theoretical predictions of the angle of flame spreading, as well as some observations on unstable flow and the onset of blowoff, will be reviewed.
The variation of blowoff velocity with flame-holder design,
pressure, and mixture composition is considered briefly in
Section III both for single and for adjacent bluff bodies. Also included is a summary of results for blowoff velocities obtained with a reverse-jet flame-holder and with wall recesses. Theoretical studies on the mechanism of flame stabilization form the subject of Section IV. We shall indicate the points on which various proposed models agree and disagree with experiment and attempt to formulate a composite description which is consistent with most of the currently available experimental data both for bluff-body and for reverse-jet flameholders
The Theory of Steady, One-dimensional, Laminar Flame Propagation for One-step Chemical Reactions
The Theory of Steady, One-dimensional, Laminar Flame Propagation for One-step Chemical Reactions. The present status of the theory of one-dimensional, steady, laminar flame propagation for one-step chemical reactions is reviewed with particular emphasis on methods of solution and on the physical processes that dominate observable results
Filtered screens and augmented Teichm\"uller space
We study a new bordification of the decorated Teichm\"uller space for a
multiply punctured surface F by a space of filtered screens on the surface that
arises from a natural elaboration of earlier work of McShane-Penner. We
identify necessary and sufficient conditions for paths in this space of
filtered screens to yield short curves having vanishing length in the
underlying surface F. As a result, an appropriate quotient of this space of
filtered screens on F yields a decorated augmented Teichm\"uller space which is
shown to admit a CW decomposition that naturally projects to the augmented
Teichm\"uller space by forgetting decorations and whose strata are indexed by a
new object termed partially oriented stratum graphs.Comment: Final version to appear in Geometriae Dedicat
Interference effects during burning in air for stationary n-heptane, ethyl alcohol, and methyl alcohol droplets
Experiments have been conducted for the determination
of the evaporation constant and flame shapes of two and
of five closely spaced droplets burning in air. Droplets of
approximately the same and of different diameters were
used at various distances between the droplet centers.
The apparent flame shape, which was observed only for n-heptane droplets, changes very little during burning.
The square of the droplet diameter decreases linearly with
time for fixed spacing between droplet centers, at least
within the experimental limits of accuracy. In general,
the average evaporation constant for two droplets, K',
must be assumed either to vary continuously during burning
or else to be a function of average initial drop diameter,
D^0. The change of K' with time corresponds to the second
derivative in plots of the square of the diameter vs. time.
These second derivatives are not defined in our work because
of unavoidable scatter of the experimental data. Attempts at understanding the observed results by considering
published theories for single droplets, as well as groupings obtained from dimensional analysis, have been
unsuccessful. It appears that the diffusion model for
the heterogeneous burning of single fuel droplets will require serious revision and extension before the burning of
droplets arrays and sprays can be understood quantitatively.
Furthermore, the effective value of K' for a spray
probably depends not only on the fuel-oxidizer system but
also on the injection pattern. For this reason additional
studies had best be carried out under conditions corresponding to those existing in service models
Approximate theoretical performance evaluation for a diverging rocket
A simplified combustion model, which is motivated by available performance studies on the diverging rocket reactor, has been used as basis for an engine performance
evaluation. Comparison with conventional rocket configurations shows that an upper performance limit for the diverging reactor is comparable with performance
estimates for engines using an adiabatic work cycle. Development of the diverging reactor for engine applications may, however, offer some advantages for very hot, high-energy, propellant systems
Mechanistic investigations of nanometer-scale lithography at liquid-covered graphite surfaces
Pulse-induced nanometer-scale lithography has been performed on graphite surfaces that were in contact with pure water or other organic liquids. Very reproducible control over the pit diameter was observed in aqueous solutions, and a well-defined voltage threshold (4.0±0.2 V) was also apparent. Near the threshold voltage, 7 Å diameter×2 Å high protrusions were formed, while larger initial pulse voltages resulted in pits of diameter>~20 Å
Atomic resolution imaging of electrode surfaces in solutions containing reversible redox species
Procedures are described for insulating metal scanning tunneling microscope (STM) tips with either glass or polymer coatings. In solutions containing 0.10 M of a reversible redox couple, Fe(CN) - 3/-46 , the faradaic limiting current to polymer coated tips was 200–500 pA and that for glass coated tips was <10 pA. For polymer insulated tips, steady-state currents of 10–100 pA were observed at tip-sample displacements less than 0.3 µm. The suppression of faradaic current achieved by these coating procedures enabled the collection of the first atomic resolution STM images of highly ordered pyrolytic graphite electrodes in contact with redox-active electrolytes. Preliminary data for the in situ electrochemical characterization of these tips are also discussed
Diet Of Peary Caribou, Banks Island, N.W.T.
The results of analyses of rumen contents from 101 Peary caribou (Rangifer tarandus pearyi J. A. Allen 1902) collected on Banks Island are presented. Peary caribou on Banks Island were found to be versatile, broad spectrum grazers specializing on upland monocots, to ingest few lichens, and to exhibit significant seasonal and/or regional differences in diet
Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94948/1/grl13844.pd
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