Benjamin D. Shaw

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.

Development and field test of a laser-based nonintrusive detection system for identification of vehicles on the highway

A real-time laser-based nonintrusive detection system has been developed for the measurement of true travel time of vehicles on the highway. The detection system uses a laser line that is projected onto the ground as a probe. The reflected light is collected and focused into a photodiode array by an optical system. Vehicle presence is detected based on the absence of reflected laser light. By placing two identical laser/sensor pairs at a known distance apart, the speeds of both the front and rear of a vehicle are measured based on the times when each sensor is triggered. The length of each vehicle is determined by using these speed measurements and the residence time of the vehicle under each sensor. Using real-time software, the speed, acceleration, and length of a detected vehicle can be calculated and displayed simultaneously. A new prototype system has been tested on the highway with different types of vehicles and scenarios, and the results are presented here. The tests have also been carried out for different weather conditions and road materials. The results indicate that the laser system operates well under real highway conditions.

Studies of han-based monopropellant droplet combustion

Abstract Combustion of hydroxylammonium nitrate (HAN)-based monopropellant droplets and droplet components was considered. Individual droplets of 13 M HAN, XM46, HAN-glycine-water, TEAN-water, and HAN-methanol-water mixtures were suspended from a quartz fiber and heated with a wire-loop igniter at pressures ranging from 1 to 10 atm in nitrogen or air environments. High-speed video imaging of droplet shape changes with color CCD camera and UV ICCD camera imaging of flames were performed in addition to gas-phase thermocouple temperature measurements. The 13 M HAN droplets did not display any flames in nitrogen or air environments. However, XM46 and HAN-glycine droplets displayed colorful luminous flames in air but not in nitrogen environments. XM46 and HAN-glycine droplets of similar diameters had similar lifetimes, although their flame characteristics and flame lifetimes were quite different. The XM46 flame had a distinct rapidly expanding flame traveling at about 1 m/sec, and HAN-glycine flames lasted almost two times longer than the XM46 flames. Also, HAN-glycine droplets left a large residue behind and did not burn as completely as XM46. HAN-methanol droplets could be ignited in air depending on the initial methanol concentration in the liquid phase. TEAN-water droplets (in air) displayed an initial period of water evaporation followed by pyrolysis, ignition, and combustion of the remaining material.

Marangoni and Stability Studies on Fiber-Supported Methanol Droplets Evaporating in Reduced Gravity

Support fibers are commonly employed in microgravity droplet evaporation and combustion studies. and it is of interest to develop models of fiber-supported droplets. In this paper, a numerical study of the Marangoni forces on a methanol droplet supported by a fiber has been carried out for an evaporating methanol droplet in both dry air and humid air environments, with the focus being the development of instabilities that form early in the lifetime of a droplet. The thermal Marangoni effect has a stabilizing effect, and it is always driving the droplet surface toward an isothermal condition. The solutal Marangoni effect (from water absorption) is much larger in magnitude, and it tends to concentrate water on the surface. This tendency to concentrate water leads to surface waves and instabilities, which are relieved by diffusion of the water into the droplet interior. For large droplets (initial radii of 0.5 mm) the surface forces have generated liquid Reynolds numbers greater than one hundred, Smaller droplets (initial radii of 0.005 mm) give qualitatively similar results, however the Reynolds numbers are reduced by size and diffusion damping influences. The numerical simulations are sensitive to droplet and supporting fiber size, and it appears that experimental verification and comprehensive numerical studies should be fully three-dimensional. It also appears that the solutal Marangoni flows are unstable to very small wavelengths, as has been suggested by previous linearized stability analyses.

Influence of the gas phase inert on reduced-gravity combustion of decane/hexadecane droplets

Reduced gravity experiments were performed to investigate droplet combustion behaviors in environments with He, N2, or Xe as diluents. These diluents have significant differences in molecular weight, allowing investigation of Soret transport (species diffusion from temperature gradients) and differences in binary diffusion coefficients. The droplets, which were initially about 1 mm in diameter, were composed of decane/hexadecane mixtures with initial hexadecane mass fractions of 0, 0.05 and 0.20. Individual fiber-supported droplets were burned at 0.1 MPa with an ambient oxygen mole fraction of 0.21. Use of He produced the largest burning rates, while burning rates for N2and Xe were similar to each other. Flame standoff ratios were unsteady for all inert species investigated. Flame unsteadiness was largest when the inert was He, with N2and Xe producing successively more quasisteady flames, respectively. Using He as an inert did not produce any visible soot, while N2and Xe produced successively brighter radiant soot emissions, respectively. This behavior is interpreted in terms of variations in O2 Lewis numbers between the flame and the environment. The data suggest that Soret transport effects and differences in binary diffusion coefficients were important when He or Xe were used as inerts, but not when N2was used as an inert.

Droplet/flame interactions including surface tension influences

Numerical simulations of the vaporization and burning processes around a moving methanol droplet have been studied. The results have shown that the influences of the flame/droplet interaction and surface tension effects cause both the internal and external flow to dramatically change. The droplet was studied under practical spray thermodynamic conditions, and a small droplet of 20 μm in diameter was chosen to be typical of modern spray injectors. Under the conditions studied, the approach of the flame caused a partial reversal of the external flow, and a large increase in the heat transfer to the droplet. The increase of the temperature in the rear of the droplet caused a very large “thermal Marangoni” effect, which reverses the surface velocity. The thermal Marangoni effect interacted strongly with the liquid water that had accumulated inside the droplet during the vaporization of methanol. Before the arrival of the diffusion flame, the “water or concentration Marangoni” effect dominated the surface velocity of the droplet. The surface velocity is characterized by high-frequency oscillations caused by water circulating in the droplet. The results clearly show that both surface tension and diffusion flame interactions exert strong forces on liquid droplets.

Surface tension influences on methanol droplet vaproiation in the presence of water

A computational model has been developed to study the influences of both temperature-and mass-fraction-driven surface tension forces on the transient vaporization of a methanol droplet in a hot environment containing water vapor. The model is based on the Navier-Stokes equations and includes variable transport and thermodynamic properties as well as simultaneous condensitation and vaporization. Numerical simulations have been carried out for practical flow conditions and intermediate Reynolds numbers. The results obtained show that surface tension forces dominate over surface shear forces and that the mass fraction-driven surface forces of water vapor are important during a significant portion of the droplet lifetime. The primary physical reasons for the dominance of the water-driven capillary forces are the low values of liquid mass diffisivity and the large variation of surface tension in water/alcohol mixtures. It is the conclusion of the paper that surface tenstion forces caused by surface mass fraction gradients of water should be included in alcohol droplet flows.

Optical and electronic design for a field prototype of a laser-based vehicle delineation detection system

A field prototype of a laser-based non-intrusive vehicle detection system has been developed for the measurement of delineations of moving vehicles on the highway. This prototype is based on our previous research on the principle of the measurement. The detection system uses two laser lines that are projected onto the ground as probes. The reflected light is collected and focused onto a photodiode array by an optical system. Vehicle presence is detected based on the absence of reflected laser light. By placing two identical laser/sensor pairs at a known distance apart, the speed of both the front and rear of a vehicle can be calculated based on the times when each sensor is triggered. The detector data are acquired and processed by a real-time system to obtain speed, acceleration, and length of a detected vehicle. The travel time of a vehicle can be acquired by detecting a vehicle at the beginning of a link and re-identifying the same vehicle at the end of the link. Several tests have been done with the field prototype system on the highway. The testing results show that the system can obtain the accuracy of measurement necessary to distinguish between moving vehicles on the highway. This article describes the design and implementation of each functional component of an advanced version of the field prototype system.

Studies on Combustion of Single and Double Streams of Methanol and Methanol/Dodecanol Droplets

Results are presented on combustion characteristics of single and double droplet streams injected into hot gas environments. The droplets were composed of methanol/dodecanol mixtures or methanol. A flat-flame burner generated hot gases that were directed down an electrically heated flow tube. Droplets initially about 193 µm in diameter were injected through the center of the flat-flame burner and along the tube, where they ignited. Staged burning was observed for methanol/dodecanol droplets but not for methanol droplets. Burning rates decreased as the number of droplet streams was increased from one to two, and burning rates increased as the initial mass fraction of dodecanol was increased. The experiments indicated that droplet stream interactions influenced sooting and droplet disruption. The data also indicated that liquid species mixing rates were significantly larger in these droplets than in methanol/dodecanol droplets burned in previous reduced-gravity experiments--these differences are attributed to the droplet generation methods.

Droplet Combustion in a Simulated Reduced-Gravity Environment

Abstract Experimental results are presented on combustion of freely-falling droplets in an apparatus that promotes spherical symmetry. Design of the apparatus is based upon the principle that gravitation-ally-induced pressure gradients in gases may be significantly reduced by accelerating the gases with properly-contoured tubes. Droplets travelling with the gases experience substantially reduced forced-convective and buoyant (gravitational) flow effects, strongly promoting spherically symmetrical combustion. It was found that burning rates for single-component alkane fuel droplets were significantly lower in our contoured tube than in a straight tube where similar initial conditions were applied. The results suggest that use of a contoured drop-tube significantly diminished the degree of natural and forced convection experienced by the falling droplets, allowing droplet burning rate measurements to be made under very weak buoyant and forced-convective flow conditions. This is desirable so that fundamental...