John B. Haggard

Sooting and disruption in spherically symmetrical combustion of decane droplets in air

Abstract Results of experiments are reported on the burning of individual decane droplets, initially between 1.0 and 1.2 mm in dia, in air at room temperature and atmospheric pressure. Use was made of the 2.2 s drop tower at the NASA Lewis Research Center and a newly designed droplet-combustion apparatus that promotes nearly spherically symmetrical combustion. Unanticipated disruptions were encountered and related to sooting behavior.

Droplet combustion experiments in spacelab

Individual droplets with diameters ranging from about 2 mm to 5 mm were burned under microgravity conditions in air at 1 bar with an ambient temperature of 300 K. Each droplet was tethered by a silicon carbide fiber of 80 μm or 150 μm diameter to keep it in view of video recording, and, in some tests, a forced air flow was applied in a direction parallel to the fiber axis. Methanol, two methanol-water mixtures, two methanol-dodecanol mixtures, and two heptane-hexadecane mixtures were the fuels. Droplet diameters were measured as functions of time, and they are compared here with existing theoretical predictions. The prediction that methanol droplets extinguish at diameters that increase with increasing initial droplet diameter is verified by these experiments. In addition, the quasi-steady burning-rate constant of the heptane-hexadecane mixtures appears to decrease with increasing droplet diameter; obscuration consistent with very heavy sooting, but without the formation of soot shells, is observed for the largest of these droplets. Forced convective flow around methanol droplets was found to increase the burning rate and to produce a ratio of downstream to upstream flame radius that remained constant as the droplet size decreased, a trend in agreement with earlier results obtained at higher convective velocities for smaller droplets having larger flame standoff ratios. Implications of the experimental results regarding droplet-combustion theory are discussed.

Observations on a Slow Burning Regime for Hydrocarbon Droplets - N-Heptane/Air Results

Experiments on n-heptane/air droplet combustion under reduced gravity have served for more than thirty years as a benchmark for much of the existing theoretical efforts on the modeling of sphero-symmetric droplet burning. New experiments conducted in the NASA-Lewis Research Center 2.2 second droptower (at less than 10−5 g) which emphasize the production of sphero-symmetry and low relative droplet/gas convection produce burning rates in air (for ∼1 mm droplets) as much as 40% lower than the classical result (0.78 mm2/s). The burning rate is proportional to the measured droplet/gas relative velocity, and the observed functional dependence is much larger than predicted by published convective correlations. At low relative velocities, a substantial amount of soot is formed and accumulated as a shell within the spherical diffusion flame surrounding the droplet. New results clearly indicate that the droplet/laboratory velocity does not correspond to the relative droplet/gas velocity. Thus, the convective effects on droplet combustion is not properly characterized by droplet motion alone. Differences in the burning rates are speculated to result from the effects of the accumulated soot as well as the asymmetry (caused by convection) in the temperature and species distributions surrounding the droplet. These changes affect diffusive transport characteristics near the droplet. Previous experiments did not observe these effects because droplet production, deployment and asymmetric single-spark ignition processes were performed in normal gravity which can induce droplet/gas relative motion and convective effects much larger than those estimated by motion of the droplet itself.

Microgravity combustion of isolated n-decane and n-heptane droplets

This paper presents recent experimental results on n-heptane droplet combustion from a 5.0 second Zero-Gravity Facility. For these experiments, droplet sizes from 1 mm to 1.75 mm were studied, oxygen mole fractions in nitrogen ranged from 12 to 21 percent, and the pressure was varied from 0.25 to 1 atm. Disruptive burning mechanisms were observed in some of the experiments conducted in air environments. However, this behavior was inhibited by reducing the oxygen concentration and/or the system pressure. The above results suggest that combinations of lower oxygen indices and reduced ambient pressures are important to reducing the effects of sooting on droplet vaporization-rates. 24 refs.

Computational/experimental basis for conducting alkane droplet combustion experiments on space-based-platforms

Recent advances in the understanding of droplet combustion clearly illustrate the serious experimental constraints imposed by the diagnostic capabilities and the short observation times available in current drop tower facilities. In this paper, the need for conducting spherically symmetric droplet combustion experiments on space-platforms is discussed and further analyzed utilizing a recently developed time dependent computational droplet combustion model that permits the incorporation of time and temperature dependent transport characteristics and complex combustion kinetics. A method first demonstrated for methanol droplet combustion, including full detailed elementary combustion kinetics, is applied with semi-empirical kinetics to estimate the combustion properties of n-heptane droplets for various pressures, oxygen indices, and diluents. Based upon the calculations, particularly for droplet extinction phenomena, results suggest two different regimes of behavior. At low oxygen indices, droplet burning extinction becomes a very strong function of oxygen index (strongly affected by kinetics), while at higher oxygen indices, it becomes a more weak function of oxygen index and in fact is more difficult to determine. The oxygen index separating the ranges over which these different characteristics are noted is dependent on the diluent chosen, being higher for helium, and lower for nitrogen.

N-Decane-Air Droplet Combustion Experiments in the NASA-Lewis 5 Second Zero-Gravity Facility

The burning of single fuel (n-decane) droplets in a microgravity environment (below 0.00001 of the earths gravity, achieved in the NASA-Lewis 5-Second Zero-Gravity Facility) was studied, as part of the development of the Droplet Combustion Experiment for eventual operation aboard either the Shuttle middeck or Spacelab. Special attention is given to the combustion equipment used and its operations and performance. Temporal analysis of the local burning rates in these tests showed increasing rates of change in the local burning as droplet combustion progressed. Result point to the need of studying large droplets, with long droplet combustion lifetimes as well as low gas/droplet motion to understand reasons for this unsteadiness.

N-Decane Droplet Combustion in the NASA-Lewis 5 Second Zero-Gravity Facility - Results in Test Gas Environments Other than Air

The burning rate of single droplets of n-decane in a microgravity environment of the NASA-Lewis 5 Second Zero-Gravity Facility was investigated as a function of time, together with the flame diameter/droplet diameter ratio, for a wide range of test environments other than normal air conditions, using an engineering model of the flight experiment. Oxygen mole fractions were varied from 18 to 50 percent, the total test chamber pressure was varied from 0.5 to 2 atmospheres, and the initial droplet diameter was varied from 0.98 to 2.41 mm. Measurements showed that the average burning rates for n-decane droplets exhibited the same qualitative trends as are found in two current models. Temporal analysis of the local burning rates showed variable rates of change in local burning as the droplet combustion progressed. The causes and implications of these findings are discussed.