Vedha Nayagam

Numerical modeling of isolated n-alkane droplet flames: initial comparisons with ground and space-based microgravity experiments

Transient, spherically symmetric, combustion of single and multi-component liquid n-alkane droplets is numerically simulated with a model that includes gas phase detailed, multi-component molecular transport and complex chemical kinetics. A compact semi-detailed kinetic mechanism for n-heptane and n-hexadecane oxidation consisting of 51 species (including He, Ar, and N2) and 282 reactions is used to describe the gas phase. Non-luminous, gas phase radiative heat transfer and conservation of energy and species within the liquid droplet interior are also considered. Computed quasi-steady flame structure for pure n-heptane droplets is compared with that produced using the kinetic mechanism of Warnatz (frequently used in the past for modeling both premixed and diffusion flame properties). Transient calculations are also compared with the numerical results of King, which consider infinite rate chemical kinetics, but temperature dependent molecular diffusion. Modeling results are in reasonable agreement with small-diameter, drop tower experiments, though slow convective effects and droplet sooting effects exist in the experimental data. Comparisons with isolated large-diameter free droplet data (1 atm, He/O2 mixtures and air) from recent space experiments are reasonable for droplet gasification rate, flame position, and flame extinction. Very small extinction diameters are predicted for small initial diameter droplets (<1 mm). As droplet size is increased, or oxygen index is decreased, the model predicts decreasing gasification rates and for an appropriate range of parameters, radiative flame extinction. Bi-component droplet combustion of n-heptane and n-hexadecane is also considered. Modeling results qualitatively reproduce experimentally observed, multi-stage burning, resulting from the volatility differential and diffusional resistance of the liquid components. Internal liquid convection effects are examined by parametrically varying an effective liquid mass diffusivity. Flame extinction can occur either in the initial or the secondary droplet heating period, with subsequent, continuing vaporization of the more volatile component from the residual heat within the liquid phase.

Microgravity n-Heptane Droplet Combustion in Oxygen-Helium Mixtures at Atmospheric Pressure

Results are presented from experiments on the combustion of freely floated n-heptane droplets in helium-oxygen environments conducted in Spacelab onboard the Space Shuttle Columbia during the first launch (STS-83) of the Microgravity Science Laboratory mission in April 1997. During this shortened flight, a total of eight droplets were burned successfully in nominally 300 K oxygen-helium atmospheres having oxygen mole fractions of 25, 30, and 35% at a total pressure of 1 atm. Initial droplet sizes ranged from about 2 to 4 mm. The results demonstrated both radiative and diffusive flame extinction during burning, whereas droplet surface regression followed the d-square law. The full range of possible droplet-burning behaviors was thus observed. The results provide information for testing future theoretical and computational predictions of burning rates, soot and flame characteristics, and extinction conditions.

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.

Hydroxyl radical chemiluminescence imaging and the structure of microgravity droplet flames

A procedure is outlined that uses hydroxyl (OH) radical chemiluminescence measurements along with detailed numerical modeling to determine flame position and gain further insight into the structure of microgravity droplet flames. To validate this procedure, microgravity n -heptane and methanol droplet combustion experiments were conducted using the 2.2-second drop tower and the ero Gravity Facility at NASA Lewis Research Center. The spontaneous emission from electronically excited hydroxyl radicals (OH o ) within the envelope diffusion flame was measured with a UV-sensitive video camera. The OH o emission data was deconvoluted using an inverse Abel transform to determine the time evolution of the location of peak intensity within the flame. Chemical reactions describing the production, emission, and quenching of OH o were incorporated into a transient, spherically symmetric droplet combustion model. The modeling and experimental results indicate differences in the route of OH o production between n -heptane and methanol flames. For n -heptane, the production of OH o emission-intensity profiles which agree well with experiment (both in terms of shape and location of maximum intensity) and are shifted from the position of maximum ground-state OH concentration. For methanol flames, which produce very little CH, the OH o appears to be the result of thermal excitation within the flame rather than from a specific chemiluminescent reaction. In both cases, the location of maximum OH o emission intensity is very near the location of maximum flame temperature, suggesting that OH o imaging is a good approach for measurement of the flame position.

Microgravity combustion of methanol and methanol/water droplets: Drop tower experiments and model predictions

To experimentally validate single and bicomponent droplet combustion models, microgravity methanol and methanol/water droplet combustion experiments were conducted in the 2.2-s drop tower facility at NASA Lewis Research Center. The experiments were then simulated using a transient, bicomponent droplet combustion model developed earlier. Tests were performed in oxidizing environments of 18%–35% O2/N2 with initial liquid water contents of 0–20%. Instantaneous droplet diameter measurements were made using back-lit, high-speed photography. The instantaneous flame position was determined by monitoring the chemiluminescence from electronically excited hydroxyl radicals (OH*). Analysis of the flame and droplet diameter data yielded burning rates and flame standoff ratios for a wide array of methanol and methanol/water droplet combustion conditions. For initially pure methanol droplets, the available burn time (≈1.5 s) was not, in general, sufficient to observe extinction or significant nonlinearity in the regression of diameter squared with respect to time. For each oxygen content, the numerical model predicted the burning rate to within 10% and the flame position to within one normalized diameter without any independent parameter adjustment. In the case of 10% and 20% initial water content, substantial nonlinearity in diameter squared was observed. The numerical model, which accurately accounts for changes in liquid transport properties caused by variable liquid water content, did not predict the nonlinearity to the extent that it was observed. A possible explanation is that with the addition of initial water content to the droplet, the internal mass and thermal transport is enhanced as a result of increased internal mixing. The sources of this internal mixing are likely multifold but are not due to relative gas/liquid convection effects at the droplet surface.

Diffusion flame-holes

Recent models of straight diffusion flame edges are extended to consider the effect of a curved edge forming the perimeter of an axisymmetric ‘hole’, where a burning flame surrounds a quenched inner region. For ‘free’ flame-holes (without a heat sink near the axis), at small Damkohler number (Da), the holes grow if the initial radius is large but shrink if it is small. For large Da, the holes shrink for any initial radius. Thus, free flame-holes are not stable for any Da, which is consistent with experimental observations. When the flame-hole is ‘anchored’ by a heat sink near to the axis, stationary holes of finite radius can exist for sufficiently high Da, but the solutions revert to ‘free’ hole behaviour for radii sufficiently larger than the heat sink radius. Based on these results, it is suggested that quasi-stationary flame-holes are not likely to be a common feature of turbulent diffusion flames, except possibly when large lateral gradients of Da are present due to intense vortices passing through t...

Lewis-number effects on edge-flame propagation

Activation-energy asymptotics is employed to explore effects of the Lewis number, the ratio of thermal to fuel diffusivity, in a one-dimensional model of steady motion of edges of reaction sheets. The propagation velocity of the edge is obtained as a function of the relevant Damkohler number, the ratio of the diffusion time to the chemical time. The results show how Lewis numbers different from unity can increase or decrease propagation velocities. Increasing the Lewis number increases the propagation velocity at large Damkohler numbers and decreases it at small Damkohler numbers. Advancing-edge and retreating-edge solutions are shown to exist simultaneously, at the same Damkohler number, if the Lewis number is sufficiently large. This multiplicity of solutions has a bearing on potential edge-flame configurations in non-uniform flows.

Freezing dynamics of molten solder droplets impacting onto flat substrates in reduced gravity

Abstract The axisymmetric impingement of solidifying molten solder droplets onto smooth metallic substrates in a reduced gravity environment is investigated numerically to provide basic information on the heat and fluid flow phenomena and determine the governing parameters of the process. The numerical predictions are also tested against experimental data. Millimeter-sized droplet impact events in reduced gravity are employed for scale up modeling of the impingement of picoliter size droplets of molten eutectic 63%Sn–37%Pb solder used in electronic chip packaging. The present article reports on both numerical (the main focus of the paper) as well as experimental work (for the purpose of verification). To this end, the employed numerical model considers the axisymmetric impact and subsequent solidification of an initially spherical, molten solder droplet on a flat, smooth, metallic substrate. The laminar Navier–Stokes equations, combined with the energy transport equations are solved simultaneously in the liquid region (melt) using a Lagrangian approach. In the solid (substrate and solidified droplet material) the heat conduction equation is solved. A time and space averaged (but phase dependent) model of the thermal contact resistance between the impacting droplet and the substrate is also incorporated in the formulation. The numerical model is solved using a Galerkin finite element method, where a deforming, adaptive triangular-element mesh is employed to accurately simulate the large-domain deformations caused by the spreading and recoiling of the impinging droplet fluid. The experimental work has been conducted in reduced gravity in the range 2×10 −4 to 5 ×10 −4 g with technically relevant impact velocities of ∼0.2 m/s, in order to provide validation of the numerical predictions. These impact conditions correspond to Re=O(100), We=O(1), and Fr=O(10,000), Ca=O(0.001). Presentation of the numerical results in terms of the Froude and the Ohnesorge numbers aids their interpretation. Among the results that stand out is the formation of a large number of frozen ripples on the droplet surface as a result of the simultaneous manifestation of rapid fluid oscillations and solidification. Furthermore, a non-intuitive behavior of the solidification times is reported. Specifically, the dependence of the solidification time on the Froude number is not monotonic, but features a minimum for each distinct value of Ohnesorge number considered in this study. Despite the complexity of the phenomena, the numerical model captures well the main features of the experimental results. In addition, the model offers key insights on the influence of the Ohnesorge and Froude numbers on the dynamics of the solidification process.

Partial-Burning Regime for Quasi-Steady Droplet Combustion Supported by Cool Flames

A simplified model for droplet combustion in the partial-burning regime is applied to the cool-flame regime observed in droplet-burning experiments performed in the International Space Station with normal-alkanes fuels resulting in expressions for the quasi-steady droplet burning rate and for the flame standoff ratio. The simplified predictions are found to produce reasonable agreement with the experimentally measured values of burning-rate constants but not with their apparent dependencies on pressure or on the initial droplet diameter. Good agreement is found, however, with newly measured and numerically calculated flame standoff ratios in this droplet combustion supported by cool flames.

Lunar Resource Utilization: Development of a Reactor for Volatile Extraction from Regolith

The extraction and processing of planetary resources into useful products, known as In- Situ Resource Utilization (ISRU), will have a profound impact on the future of planetary exploration. One such effort is the RESOLVE (Regolith and Environment Science, Oxygen and Lunar Volatiles Extraction) Project, which aims to extract and quantify these resources. As part of the first Engineering Breadboard Unit, the Regolith Volatiles Characterization (RVC) reactor was designed and built at the NASA Glenn Research Center. By heating and agitating the lunar regolith, loosely bound volatiles, such as hydrogen and water, are released and stored in the reactor for later analysis and collection. Intended for operation on a robotic rover, the reactor features a lightweight, compact design, easy loading and unloading of the regolith, and uniform heating of the regolith by means of vibrofluidization. The reactor performance was demonstrated using regolith simulant, JSC1, with favorable results.