Elsevier

Combustion and Flame

Volume 116, Issue 3, February 1999, Pages 348-359
Combustion and Flame

Original Articles
Experimental and numerical study of flame ball IR and UV emissions

https://doi.org/10.1016/S0010-2180(98)00103-5Get rights and content

Abstract

Near-infrared (IR) and ultraviolet (UV) emission profiles of flame balls at microgravity conditions in H2-O2-diluent mixtures were measured in the JAMIC 10 s drop-tower and compared to numerical simulations and supplemental KC135 aircraft μg experiments. Measured flame ball radii based on images obtained in the JAMIC, KC135, and recent space experiments (IR only) were quite consistent, indicating that radius is a rather robust property of flame balls. The predicted IR radii were always smaller than UV radii, whereas the experiments always showed the opposite behavior. Agreement between measured and predicted flame ball properties was closer for UV radii than IR radii in H2-air mixtures but closer for IR radii in H2-O2-CO2 mixtures. The large experimental IR radii in H2-air tests is particularly difficult to interpret even when uncertainties in chemical and radiation models are considered. Experimental radii would be consistent with a chemiluminescence reaction of the form HO2 + HO2 → H2O2 + O2 producing an excited state of H2O2, since HO2 is consumed at large radii through this reaction and its exothermicity is sufficient to create excited states that could emit at the observed wavelengths, however, no appropriate transition of H2O2∗ could be identified.© 1998 by The Combustion Institute

Introduction

Microg ravity (μg) experiments in drop towers [1], aircraft [2], and orbiting spacecraft [3] have shown that stable, stationary spherical premixed flames (“flame balls”) can exist near flammability limits in mixtures with low Lewis number (Le), defined as the ratio of the thermal diffusivity of the bulk mixture to mass diffusivity of the stoichiometrically limiting reactant. Flame balls are supported by diffusion of reactants to the ball surface and heat and product diffusion away from the ball. Convection plays no role in these steady, stable flame structures; the mass-averaged fluid velocity is zero everywhere at steady state. While adiabatic flame balls are always predicted to be unstable 4, 5, as are flame balls in mixtures with Le close to or greater than unity [6], flame balls at low Le with significant volumetric heat loss (e.g., due to thermal radiation) are predicted to be stable 7, 8. Consequently, flame balls in low-Le near-limit mixtures represent probably the simplest possible flame and thus are attractive for comparison to theoretical and computational models of premixed combustion, particularly at conditions near flammability or extinction limits.

Despite this simplicity, to date the agreement between model predictions and experimental observations, particularly with regard to the flame ball radius, has not been satisfactory 2, 9, 10. One substantial problem is the uncertainty in the appropriate chemical model to employ for the very lean H2-O2-diluent mixtures in which most flame ball studies have been conducted; different published chemical mechanisms predict widely varying flame ball properties, even among models that predict the burning velocities of propagating planar H2-air flames quite well [10]. The flame ball calculations compare favorably with independent calculations [11] when the same chemical and radiation sub-models are employed; thus numerical accuracy issues are not considered a significant factor in these discrepancies.

The comparisons of model and experiment to date have only been made for flame ball radii based on images of the near-infrared and visible emissions of H2O because these emissions are readily detected by commercially available intensified video cameras. At the time of the early experiments using video cameras with sensitivity in the near-IR and visible region [2], unsuccessful attempts were made to image emissions from OH chemiluminescence using UV-sensitive intensified video cameras. Recent improvements in intensified video camera technology have now made OH imaging feasible, even in drop-tower and aircraft μg experiments. UV emissions from OH are more indicative of the location of heat release because they occur only where O, H, and OH radicals are present, whereas the near-IR/visible emissions are more indicative of the locations where H2O and other stable radiating species are present at high temperature. The former may provide a more meaningful test of H2-O2 chemical kinetic models. Consequently, the purpose of this study is a comparison of predicted and measured UV emissions from excited-state OH molecules, and a comparison of the UV emissions to near-IR/visible emissions from H2O (and to a lesser extent CO2 and SF6 in mixtures containing these molecules).

The comparisons are conducted as follows. Numerical predictions of UV and near-IR/visible emissions from flame balls were obtained from computations employing detailed chemical, transport, thermal radiation, and UV emission sub-models. Images of UV emissions from flame balls were obtained in the Japan Microgravity Facility (JAMIC) in Kamisunagawa, Hokkaido, Japan. This facility provides 10 seconds of fairly high-quality μg (< 10-5 go, where go denotes earth gravity). Supplemental tests were conducted in NASA’s KC135 low-gravity aircraft which enable more tests to be conducted with slightly longer durations (typically 15 s) at the expense of much poorer quality of μg (typically 0.02 go). These results are also compared to calculated near-IR/visible emissions and corresponding experimental data obtained in JAMIC and KC135 tests. Preliminary data on near-IR/visible radii obtained from space experiments conducted using the Combustion Module-1 (CM-1) facility on the STS-83 and STS-94 Space Shuttle missions [3] are also presented.

Section snippets

Numerical study

As in our previous numerical studies of flame balls 9, 10, a one-dimensional, time-dependent flame code with detailed chemical and transport sub-models 12, 13, was employed. The usual nonsteady equations for energy and species conservation were solved in spherical geometry at constant pressure. The compositions studied were H2-air, H2-O2-CO2, and H2-O2-SF6 mixtures. For the latter two mixture families, a fixed H2:O2 ratio of 0.5, corresponding to equivalence ratio of 0.25, was employed, as

Experimental apparatus and procedures

A total of 30 drop tests were conducted in the JAMIC facility. The experimental apparatus consisted of a combustion chamber, spark generator, and video imaging system. The entire experimental apparatus described below was mounted in a 0.92 m x 0.87 m x 0.43 m frame that was installed in the JAMIC drop capsule. The combustion chamber was a cylindrical vessel with inside diameter 200 mm and length 250 mm. Quartz windows on the side and on top of the vessel enabled observation of the flames by the

Results

Figures 4a and b show comparisons of predicted and measured flame ball near-IR/visible and UV intensity profiles, respectively, for a 3.44% H2-air mixture. The predicted near-IR/visible radius is smaller than the predicted UV radius, whereas the measured near-IR/visible radius is larger than the measured UV radius. This behavior was observed for all mixtures tested in all three facilities employed. For the H2-air mixtures, the predicted UV radius is larger than the measured value, whereas the

Discussion

The results show significant differences between model and experiments, even for H2-air mixtures where reabsorption effects are negligible [10], especially for near-IR/visible radii. Particularly surprising is that the relative sizes of UV and near-IR/visible radii are different in model predictions and experimental observations. The data obtained in all three experimental facilities are quite consistent, indicating that variations in acceleration level and experiment duration cannot account

Summary and conclusions

Near-IR/visible and UV emission profiles of flame balls in H2-O2-diluent mixtures were obtained in μg experiments employing drop tower, aircraft and space-based facilities. Data for both types of emissions obtained in all three facilities were quite consistent, indicating that radius is a rather robust property of flame balls. In marked contrast to experiments, the predicted near-IR/visible flame radii were always smaller than UV radii. The magnitude of discrepancy between measured and

Acknowledgements

The USC portion of this work was supported by NASA under grants NAG3-1816 and NAG3-2124. We are grateful to Dr. S. Vosen for helpful discussions concerning OH∗ emissions.

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