Temperature and radiation correlations in microgravity turbulent diffusion flames

Abstract

This paper presents the results of an experimental and numerical study of the radiation and temperataure fields of turbulent jet diffusion flames in a reduced gravity environment. Tests were conducted for non-premixed, nitrogen-diluted propane flames burning in quiescent air in the NASA Glenn 5.18 Second Zero Gravity Facility. Flame radiation from horizontal, wedge-shaped slices of the flame were measured. The radiation data are compared with those obtained from corresponding normal gravity tests. With the aid of a k-c numerical model, the effects of flame radiation on the thermal field is demonstrated and qualitative agreement is obtained between the predictions and measurements. In addition, power spectra of the measured flame radiation are used to assess the turbulent structure of the flame. INTRODUCTION The first investigations on hydrocarbon turbulent diffusion flames in microgravity were initiated recently13. These investigations have been conducted for flames with moderate injection (cold jet) Reynolds numbers under 10000. Scaling relationships for measured flame heights were obtained3 and found to be similar to the relationships for corresponding flames in normal gravity, in particular, the scaling of flame height with fuel dilution and nozzle diameter. However, the heights of the microgravity flames are about twice the normal gravity flame heights. The difference in flame heights of the microgravity and normal gravity flames indicates that the Froude number of the investigated normal gravity flames is not high enough for these flames to be momentum-controlled and buoyancy plays an important role in their behavior. For example, buoyant acceleration counteracts the centerline velocity decay that would otherwise occur due to jet spreading and momentum diffusion. That such flow differences yield different flame characteristics may be explained by noting that the primary time scales in diffusion flames have components of flow time and chemical time. Flow time scales are related to flow processes uch as mixing of fuel and oxidizer prior to reaction whereas chemical time scales are related to the times for the various reactions occurring in the flame. In most cases, flow times are several orders of magnitude larger than chemical times, with the result that flow processes control characteristics of diffusion flames such as size and shape. Hence, factors such as buoyancy that modify the flow patterns in these flames are expected to have significant influence on flame behavior. Flame radiation is an important quantity that is impacted by buoyancy as has been shown in previous studies by the authors4 and also by Urban et a15. It was found that jet diffision flames burning under microgravity conditions have significantly higher radiative loss (about five times higher) compared to their normal gravity counterparts because of larger flame size in microgravity and larger convective heat loss fraction from the flame in normal gravity. These studies, however, were confined to laminar flames. l Principal Researcher. Associate Fellow, AIAA. + Senior Scientist. Senior Member AIAA. “Copyright