Computational/experimental studies of isolated, single component droplet combustion

Isolated droplet combustion processes have been the subject of extensive experimental and theoretical investigations for nearly 40 years. The gross features of droplet burning are qualitatively embodied by simple theories and are relatively well understood. However, there remain significant aspects of droplet burning, particularly its dynamics, for which additional basic knowledge is needed for thorough interpretations and quantitative explanations of transient phenomena. Spherically-symmetric droplet combustion, which can only be approximated under conditions of both low Reynolds and Grashof numbers, represents the simplest geometrical configuration in which to study the coupled chemical/transport processes inherent within non-premixed flames. The research summarized here, concerns recent results on isolated, single component, droplet combustion under microgravity conditions, a program pursued jointly with F.A. Williams of the University of California, San Diego. The overall program involves developing and applying experimental methods to study the burning of isolated, single component droplets, in various atmospheres, primarily at atmospheric pressure and below, in both drop towers and aboard space-based platforms such as the Space Shuttle or Space Station. Both computational methods and asymptotic methods, the latter pursued mainly at UCSD, are used in developing the experimental test matrix, in analyzing results, and for extending theoretical understanding. Methanol, and the normal alkanes, n-heptane, and n-decane, have been selected as test fuels to study time-dependent droplet burning phenomena. The following sections summarizes the Princeton efforts on this program, describe work in progress, and briefly delineate future research directions

Assessment of kinetic modeling for lean H 2 /CH 4 /O 2 /diluent flames at high pressures

Abstract Experimental measurements of burning rates, analysis of key reactions and kinetic pathways, and modeling studies were performed for H 2 /CH 4 /O 2 /diluent flames spanning a wide range of fuel-lean conditions: equivalence ratios from 0.30 to 1.0, flame temperatures from 1400 to 1800 K, pressures from 1 to 25 atm, CH 4 fuel fractions from 0 to 0.1. The experimental data show negative pressure dependence of burning rate at high-pressure, low-flame-temperature conditions for all equivalence ratios and with CH 4 addition. Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves -up to a factor of four at high pressures. Similar to our previous work that demonstrated that none of the recent kinetic models reproduced the measured pressure dependence of the mass burning rate for all diluent concentrations and medium to high equivalence ratios, here it is demonstrated that none reproduce the measured pressure dependence for very low equivalence ratios. The effect of pressure on the kinetics of lean flames is largely driven by competition of both H + O 2 (+M) = HO 2 (+M) and HO 2 + O/OH/HO 2 with the main branching reactions, in contrast to rich mixtures that are largely driven by competition of both H + O 2 (+M) = HO 2 (+M) and HO 2 + H with the main branching reactions. Methane addition is shown to influence the pressure dependence mainly through reactions of CH 3 with H and HO 2 . Given the nature of the modeling problem for high-pressure flames, it appears that a rigorous solution to improving predictive capabilities will require both empirical adjustments of multiple rate constant parameters as well as improved characterization of the functional temperature and pressure dependence of certain highly sensitive reactions. Furthermore, many of the reactions responsible for uncertainties in the pressure dependence of H 2 /O 2 flames at high pressures are shown to contribute significantly to uncertainties in the pressure dependence of flames of hydrocarbon fuels

Droplet Combustion Experiment (DCE)

The first space-based experiments were performed on the combustion of free, individual liquid fuel droplets in oxidizing atmospheres. The fuel was heptane, with initial droplet diameters ranging about from 1 mm to 4 mm. The atmospheres were mixtures of helium and oxygen, at pressures of 1.00, 0.50 and 0.25 bar, with oxygen mole fractions between 20% and 40%, as well as normal Spacelab cabin air. The temperatures of the atmospheres and of the initial liquid fuel were nominally 300 K. A total of 44 droplets were burned successfully on the two flights, 8 on the shortened STS-83 mission and 36 on STS-94. The results spanned the full range of heptane droplet combustion behavior, from radiative flame extinction at larger droplet diameters in the more dilute atmospheres to diffusive extinction in the less dilute atmospheres, with the droplet disappearing prior to flame extinction at the highest oxygen concentrations. Quasisteady histories of droplet diameters were observed along with unsteady histories of flame diameters. New and detailed information was obtained on burning rates, flame characteristics and soot behavior. The results have motivated new computational and theoretical investigations of droplet combustion, improving knowledge of the chemical kinetics, fluid mechanics and heat and mass transfer processes involved in burning liquid fuels

Multi-User Droplet Combustion Apparatus - Flame Extinguishment Experiment

Multi-User Droplet Combustion Apparatus Flame Extinguishment Experiment (MDCA-FLEX) will assess the effectiveness of fire suppressants in microgravity and quantify the effect of different possible crew exploration atmospheres on fire suppression. The goal of this research is to provide definition and direction for large scale fire suppression tests and selection of the fire suppressant for next generation crew exploration vehicles

Fiber Supported Droplet Combustion-2 (FSDC-2)

Experimental results for the burning characteristics of fiber supported, liquid droplets in ambient Shuttle cabin air (21% oxygen, 1 bar pressure) were obtained from the Glove Box Facility aboard the STS-94/MSL-1 mission using the Fiber Supported Droplet Combustion - 2 (FSDC-2) apparatus. The combustion of individual droplets of methanol/water mixtures, ethanol, ethanol/water azeotrope, n-heptane, n-decane, and n-heptane/n-hexadecane mixtures were studied in quiescent air. The effects of low velocity, laminar gas phase forced convection on the combustion of individual droplets of n-heptane and n-decane were investigated and interactions of two droplet-arrays of n-heptane and n-decane droplets were also studied with and without gas phase convective flow. Initial diameters ranging from about 2mm to over 6mm were burned on 80-100 micron silicon fibers. In addition to phenomenological observations, quantitative data were obtained in the form of backlit images of the burning droplets, overall flame images, and radiometric combustion emission measurements as a function of the burning time in each experiment. In all, 124 of the 129 attempted experiments (or about twice the number of experiments originally planned for the STS-94/MSL-1 mission) were conducted successfully. The experimental results contribute new observations on the combustion properties of pure alkanes, binary alkane mixtures, and simple alcohols for droplet sizes not studied previously, including measurements on individual droplets and two-droplet arrays, inclusive of the effects of forced gas phase convection. New phenomena characterized experimentally for the first time include radiative extinction of droplet burning for alkanes and the "twin effect" which occurs as a result of interactions during the combustion of two-droplet arrays. Numerical modeling of isolated droplet combustion phenomenon has been conducted for methanol/water mixtures, n-heptane, and n-heptane/n-hexadecane mixtures, and results compare quantitatively with those found experimentally for methanol/water mixtures. Initial computational results qualitatively predict experimental results obtained for isolated n-heptane and n-heptane/n-hexadecane droplet combustion, although the effects of sooting are not yet included in the modeling work. Numerical modeling of ethanol and ethanol/water droplet burning is under development. Considerable data remain to be fully analyzed and will provide a large database for comparisons with further numerical and analytical modeling and development of future free droplet experiments aboard space platforms

Detailed Results from the Flame Extinguishment Experiment (FLEX) March 2009 to December 2011

The Flame Extinguishment Experiment (FLEX) program is a continuing set of experiments on droplet combustion, performed employing the Multi-User Droplet Combustion Apparatus (MDCA), inside the chamber of the Combustion Integrated Rack (CIR), which is located in the Destiny module of the International Space Station (ISS). This report describes the experimental hardware, the diagnostic equipment, the experimental procedures, and the methods of data analysis for FLEX. It also presents the results of the first 284 tests performed. The intent is not to interpret the experimental results but rather to make them available to the entire scientific community for possible future interpretations

Comprehensive Mechanisms for Combustion Chemistry: An Experimental and Numerical Study with Emphasis on Applied Sensitivity Analysis

This project was an integrated experimental/numerical effort to study pyrolysis and oxidation reactions and mechanisms for small-molecule hydrocarbon structures under conditions representative of combustion environments. The experimental aspects of the work were conducted in large-diameter flow reactors, at 0.3 to 18 atm pressure, 500 to 1100 K temperature, and 10<SUP>-2</SUP> to 2 seconds reaction time. Experiments were also conducted to determine reference laminar flame speeds using a premixed laminar stagnation flame experiment and particle image velocimetry, as well as pressurized bomb experiments. Flow reactor data for oxidation experiments include: (1)adiabatic/isothermal species time-histories of a reaction under fixed initial pressure, temperature, and composition; to determine the species present after a fixed reaction time, initial pressure; (2)species distributions with varying initial reaction temperature; (3)perturbations of a well-defined reaction systems (e.g. CO/H<SUB>2</SUB>/O<SUB>2</SUB> or H<SUB>2</SUB>/O<SUB>2</SUB>)by the addition of small amounts of an additive species. Radical scavenging techniques are applied to determine unimolecular decomposition rates from pyrolysis experiments. Laminar flame speed measurements are determined as a function of equivalence ratio, dilution, and unburned gas temperature at 1 atm pressure. Hierarchical, comprehensive mechanistic construction methods were applied to develop detailed kinetic mechanisms which describe the measurements and literature kinetic data. Modeling using well-defined and validated mechanisms for the CO/H<SUB>2</SUB>/Oxidant systems and perturbations of oxidation experiments by small amounts of additives were also used to derive absolute reaction rates and to investigate the compatibility of published elementary kinetic and thermochemical information. Numerical tools were developed and applied to assess the importance of individual elementary reactions to the predictive performance of the developed mechanisms and to assess the uncertainties in elementary rate constant evaluations

Uncertainty Analysis in the Use of Chemical Thermometry: A Case Study with Cyclohexene

A general method to evaluate the absolute uncertainties in temperatures derived using chemical thermometry is developed and applied to the retro Diels–Alder reaction of cyclohexene. Experiments from previous studies of this reaction are reanalyzed to establish the minimum absolute uncertainty limit. Chemical thermometry results are compared with thermocouple measurements in experiments performed in a flow reactor at 6.1 atm pressure and at temperatures from 957 to 978 K . Using conservative uncertainty estimates, our analysis yields absolute (1σ) uncertainties of temperature through chemical thermometry using this reaction greater than ±20 at 1000 K. Neither more refined experimental techniques nor computational theory is likely to refine rate correlation parameters sufficiently to reach the absolute temperature uncertainties often reported in the literature for chemical thermometry using the retro Diels–Alder reaction of cyclohexene. Published chemical thermometry uncertainty estimates typically have not quantitatively considered the absolute uncertainties of the original data from which the reference rate correlations were based

Dehydration Rate Measurements for tertiary-Butanol in a Variable Pressure Flow Reactor

Fundamentally, the dehydration reaction of tertiary-butanol is frequently used as an internal standard for relative rate studies of other decomposition reactions. We report here a study using radical trappers to isolate this path in tertiary-butanol pyrolysis experiments conducted in the Princeton variable pressure flow reactor between 658 and 980 K. A novel technique that determines the rate constant value by applying a global least-squares fit incorporating all experimental species (tertiary-butanol, isobutene, and water) evolution data is developed and applied to yield six rate constant values at two reaction pressures (6.1 and 18 atm) and at temperatures between 949 and 980 K. Data from previously reported studies are reanalyzed to evaluate their “absolute” uncertainties, and new Arrhenius parameters are derived based upon the present and previous measurements. The recommended rate constant (uncertainties) for the dehydration reaction is k = 2.88(0.91) × 107T2.21(0.10) s–1 exp(−62.4(0.9) kcal mol–1/RT). The new correlation is in excellent agreement with other independent experimental and theoretical studies appearing in the literature