Combustion Science and Technology

Stufflebeam, John H.; Eckbreth, Alan C.

doi: 10.1080/00102208908947147pmid: N/A

The firstCARS measurements of temperature and species concentrations in the exhaust of solid propellants burning at elevated pressure (≤ 35 atm) are reported. Multiple species data are acquired at high temporal (10 ns) and spatial resolution from both a homogeneous, double-base and a composite nitramine propellant. CARS spectra have been obtained from three spectral regions that encompass the signatures of the major combustion products, CO2, CO-N2,and H2O. The CARS data are analyzed by comparison with computer synthesized spectra generated at various temperatures and species concentrations. Results in the postflame zone from nitramine combustion at 23 atm indicate temperatures as high as 2600 K with species concentrations of N2 ∼ 23%, CO ∼ 36%, and H2 ∼ 23%. These results compare favorably with predictions of temperature and concentration from a chemical equilibrium code that simulates the combustion parameters. Utilization of dual broadband approaches will allow the simultaneous acquisition of data from the three spectral regions with each laser pulse.

Winter, Michael; Long, Marshall B.

doi: 10.1080/00102208908947148pmid: N/A

The fuel-air mixture concentration has been measured as a function of time in the central plane of a turbulent premixed jet flame. A stoichiometric hydrogen-air mixture was uniformly seeded with submicron-sized aerosols. The aerosols were consumed at the reaction front, and their presence was used to map the concentration of the fuel-air mixture. A sheet of laser light was formed with an Ar- laser and the elastically-scattered light from the flame was recorded in two dimensions at 48 kHz using a framing camera to record 20 frames. The interface between the unburned fuel-air mixture and reaction products was clearly visible in the scattered intensity distribution. From the sequence of images, the motion of the flame front and the convective motion of the flow structures are evident. Information on the local burning velocity was obtained by observing the change in the size of structures in a convective frame of reference.

Coverdill, R. E.; Kirwan, J. E.; Brandon, l. E.; Peters, J. E.

doi: 10.1080/00102208908947149pmid: N/A

Abstract-The effects of throttle body injection on fuel spray atomization are examined in terms of their impact on fuel/air distribution in multi-cylinder engines. From exhaust product measurements of individual cylinders in a four cylinder engine, cylinder-to-cylinder uniformity of fuel/air ratio is shown to vary substantially with engine operating conditions. For example, cylinder-to-cylinder variations in fuel/air ratio ranged from 3% at light loads to nearly 15% at wide open throttle. In contrast, substantially different fuel injectors had little impact on the fuel/air distribution. These trends correlated well with previous measurements of throttle body injection systems where throttle position and air flowrate were shown to dominate the spray quality (in terms of drop size and spatial uniformity) and injector effects were of secondary importance. The improved uniformity of fuel/air distribution for light load conditions can be directly attributed to increased air velocity past the nearly closed throttle which produces an air blast atomization effect resulting in mean drop sizes as small as 131lm. These small drops can more easily track the air flow in the intake manifold which provides a more uniform cylinder-to-cylinder fuel/air distribution.

Kumar, Sunil; Tien, C. L.

doi: 10.1080/00102208908947150pmid: N/A

This study examines the extinction and scattering characteristics of randomly oriented agglomerates consisting of closely-packed small identical spherical particles such as soot in flames. An equivalent sphere is introduced as one that exhibits similar scattering or extinction cross-section as the agglomerate and has the same refractive index as the primary particles. A simple analytical reasoning is presented that establishes the ratio of the diameter of this sphere to the diameter of the primary particles to be proportional to the cube root of the number of particles in the agglomerate. Simple expressions for the proportionality constants for various morphologies are developed. Previous studies in literature have used extensive numerical computations to empirically correlate the above proportionality without considering the proportionality constants. Scattering patterns for various morphologies are also predicted which have significance for optical diagnostic techniques.

Birch, A. D.; Brown, D. R.; Fairweather, M.; Hargrave, G. K.

doi: 10.1080/00102208908947151pmid: N/A

Abstract-An understanding of the physical processes involved in the atmospheric venting and flaring of flammable gases is necessary to assessments of the consequences associated with these operations. In order to further such understanding, this paper presents the results of a wind tunnel study of a turbulent natural gas jet in a cross-flow. The results obtained relate the mean concentration field ofa single, non-reacting jet, through the ignition characteristics of that jet to the temperature and radiation fields associated with the cornbusting now. A more detailed study of the ignition characteristics of a number of jets has also been performed. The experimental data obtained provides insight into the connection between those phenomena of interest to consequence assessments, and will be of value to the validation of mathematical models of venting and flaring.

Turns, S. R.; Lovett, J. A.

doi: 10.1080/00102208908947152pmid: N/A

Measurements of global emission indices for NO and NO,were made in unconfined,vertical,propane jet diffusion flames. Initial fuel jet velocities were varied to provide a range of jet Reynolds numbers from 10000 to 60000. The measurements showed that scaling of NO, emission indices with velocity changes as the flame character changes from transitional-turbulent (Re ≈ 10000) to fully turbulent (Re > 20000). At the lower Re condition. the observed scaling approaches that reported in the literature for similar conditions. Arguments are presented to show that radiation losses from the flames may account for the observed scaling. Significant amounts ofN02 were found in the flame plumes. The variation of NO2/NOx, ratios with jet velocity and with radial position was consistent with the hypothesis that NO2is formed in the cooler post-flame regions. in agreement with the findings of others for CO and CO/H2 fueled diffusion flames.

Kumar, R. K.; Dewit, W. A.; Greig, D. R.

doi: 10.1080/00102208908947153pmid: N/A

Combustion experiments were carried out in a 2.3-m diameter spherical vessel with venting to the atmosphere over a hydrogen concentration range of6 to 20% by volume. Three vent sizes (15 em, 25 cm and 45 em) and three igniter locations (near-vent, centre and far-vent) were employed. The vents were initially covered with cardboard or aluminum foil that ruptured between I and 10 kPa. It was found that venting was effective in reducing the peak pressure at low hydrogen concentrations and for large vent areas for which the combustion times were much longer than the characteristic vent times. In all cases, central ignition led to the highest peak pressures and shortest combustion times, and near-vent ignition resulted in the lowest peak pressures and longest combustion times. Instabilities and turbulence caused by the vent opening resulted in enhanced burning rates. A simple one-dimensional model produced results in reasonable agreement with the experimental pressure transients when two burning rates (a laminar burning rate prior to the onset of instability and an enhanced burning rate after the onset of instability) were used.

Purl, Ishwar K.; Libby, Paul A.

doi: 10.1080/00102208908947154pmid: N/A

Single droplets of n-heptane fuel are observed in two countcrflowing laminar streams with well-defined velocity and state fields. The simpler of the two involves identical isothermal streams of nitrogen and nonevaporating droplets. The second consists of a non-premixed flame leading to droplet evaporation on both sides of the reaction zone and to combustion when oxygen and suitably high temperatures exist in the ambient surrounding the droplet. In both flows the droplet trajectory and in the flame the radius history are observed by photographic means. The measurements are compared with engineering models for the drag and heat and mass transfer of droplets.Significant discrepancies between the data and predictions in both flows arc found. Particularly interesting is the magnitude of the lift that the droplets experience and the extent of the modification of droplet drag by acceleration.

Rehm, Ronald G.; Baum, Howard R.; Lozier, Daniel W.; Aronson, Jonathan

doi: 10.1080/00102208908947155pmid: N/A

Abstract A two-dimensional model of a constant-density diffusion-controlled reaction between unmixed species initially occupying adjacent half-spaces is formulated and analyzed. An axisymmetric viscous vortex field satisfying the Navies-Stokes equations winds up thc interface between the species as they diffuse together and react. A flame-sheet approximation of the rapid reaction is made using a mixture fraction dependent variable. The problem was originally proposed by F. Marble, who performed a local analysis and determined the total consumption rate along thc flame sheet. The present paper describes a global similarity solution to the problem which is Fourier analyzed in a Lagrangian coordinate system. The Fourier amplitudes are determined both by an asymptotic analysis, valid for large Schmidt numbers,and by numerical solution of the two-point boundary-value ordinary differential equations. The solution is evaluated in both Lagrangian and Eulerian coordinate systems. Comparisons are made between the asymptotic and the numerical solutions for a variety of values of the governing parameters, the Reynolds and Schmidt numbers.

Lockwood, F. C.; Mahmud, T.

doi: 10.1080/00102208908947156pmid: N/A

AbstraCt-A general mathematical model of pulverised coal combustion has been applied to predict burner flame stability data obtained in the Imperial College furnace. The data are for the most part well predicted with the proviso that the model is generally somewhat optimistic about the achievable stability performance. Flame stability is dependent on the precise nature of the near burner flow field. This flow region is also critical to pollutant emissions. Detailed data for it are extremely scarce. The ability to predict more easily obtainable flame stability data provides a ready measure of the quality of a mathematical model.