Z.-G. Yuan

Shapes of buoyant and nonbuoyant laminar jet diffusion flames

Flame shapes were measured for buoyant and nonbuoyant laminar gas jet diffusion flames burning, methane, ethane, and propane in quiescent air. Test conditions involved burner diameters of 0.19–5.5 mm, ambient pressures of 0.25–2 atm, and fuel flowrates of 0.04–4.6 mg/s. Care was taken to minimize interference from soot emissions and from ignition disturbances. Microgravity conditions were obtained in the 2.2-s drop tower at the NASA Lewis Research Center. Normalized lengths of both buoyant and nonbuoyant flames were proportional to source Reynolds number, but the nonbuoyant flames were 40% longer on average. Normalized widths of the nonbuoyant flames were constant for Re ≥ 100, whereas buoyant flame widths scaled with source Froude number. Several nonbuoyant flame models are evaluated with the present shape data.

Shapes of nonbuoyant round luminous hydrocarbon/air laminar jet diffusion flames

Abstract The shapes (luminous flame boundaries) of round luminous nonbuoyant soot-containing hydrocarbon/air laminar jet diffusion flames at microgravity were found from color video images obtained on orbit in the Space Shuttle Columbia. Test conditions included ethylene- and propane-fueled flames burning in still air at an ambient temperature of 300 K, ambient pressures of 35–130 kPa, initial jet diameters of 1.6 and 2.7 mm, and jet exit Reynolds numbers of 45–170. Present test times were 100–200 s and yielded steady axisymmetric flames that were close to the laminar smoke point (including flames both emitting and not emitting soot) with luminous flame lengths of 15–63 mm. The present soot-containing flames had larger luminous flame lengths than earlier ground-based observations having similar burner configurations: 40% larger than the luminous flame lengths of soot-containing low gravity flames observed using an aircraft (KC-135) facility due to reduced effects of accelerative disturbances and unsteadiness; roughly twice as large as the luminous flame lengths of soot-containing normal gravity flames due to the absence of effects of buoyant mixing and roughly twice as large as the luminous flame lengths of soot-free low gravity flames observed using drop tower facilities due to the presence of soot luminosity and possible reduced effects of unsteadiness. Simplified expressions to estimate the luminous flame boundaries of round nonbuoyant laminar jet diffusion flames were obtained from the classical analysis of Spalding (1979); this approach provided successful correlations of flame shapes for both soot-free and soot-containing flames, except when the soot-containing flames were in the opened-tip configuration that is reached at fuel flow rates near and greater than the laminar smoke point fuel flow rate.

Analytical predictions of shapes of laminar diffusion flames in microgravity and earth gravity

Flame shape is an important observed characteristic of flames that can be used to scale flame properties such as heat release rates and radiation. Flame shape is affected by fuel type, oxygen levels in the oxidiser, inverse burning and gravity. The objective of this study is to understand the effect of high oxygen concentrations, inverse burning, and gravity on the predictions of flame shapes. Flame shapes are obtained from recent analytical models and compared with experimental data for a number of inverse and normal ethane flame configurations with varying oxygen concentrations in the oxidiser and under earth gravity and microgravity conditions. The Roper flame shape model was extended to predict the complete flame shapes of laminar gas jet normal and inverse diffusion flames on round burners. The Spalding model was extended to inverse diffusion flames. The results show that the extended Roper model results in reasonable predictions for all microgravity and earth gravity flames except for enhanced oxygen normal diffusion flames under earth gravity conditions. The results also show trends towards cooler flames in microgravity that are in line with past experimental observations. Some key characteristics of the predicted flame shapes and parameters needed to describe the flame shape using the extended Roper model are discussed.

Modeling of Heat and Mass Transfer in a TEC-Driven Lyophilizer

ABSTRACT Dewatering of wet waste during space exploration missions is important for crew safety as it stabilizes the waste. It may also be used to recover water and serve as a preconditioning step for waste compaction. A thermoelectric cooler (TEC)- driven lyophilizer is under development at NASA Ames Research Center for this purpose. It has three major components: (i) an evaporator section where water vapor sublimes from the frozen waste, (ii) a condenser section where this water vapor deposits as ice, and (iii) a TEC section which serves as a heat pump to transfer heat from the condenser to the evaporator. This paper analyses the heat and mass transfer processes in the lyophilizer in an effort to understand the ice formation behavior in the condenser. The analysis is supported by experimental observations of ice formation patterns in two different condenser units. INTRODUCTION This paper describes modeling efforts carried out to develop an energy efficient condenser for a lyophilization system for solid waste dewatering and recovery for space exploration missions. Wet wastes are projected to be a significant issue for these missions [1] and processing of these wastes for resource recovery can reduce overall system mass impacts. For short-term missions, solid waste stabilization and water recovery are desirable but conversion of organic waste to CO

INVESTIGATION OF NONBUOYANT LAMINAR JET DIFFUSION FLAMES: A PARADIGM FOR SOOT PROCESSES IN TURBULENT FLAMES

The structure and soot properties of steady nonbuoyant round laminar jet diffusion flames at microgravity were studied based on measurements obtained on orbit during three flights of the Space Shuttle Columbia (Flights STS-83, 94 and 107). The test conditions included ethylene- and propane-fueled flames burning in still air at ambient temperature of 300 K and ambient pressures of 35-130 kPa, for jet exit diameters of 0.40- 2.70 mm and jet exit Reynolds numbers of 46-1186, to yield steady nonbuoyant round laminar jet diffusion flames with most of the flames near the laminar smoke- point. The first phase of the study involved evaluation of the classical analysis of the structure of steady nonbuoyant round laminar jet diffusion flames due to Spalding (1979), after empirically extending it to account for the presence of luminosity due to the presence of soot within the flames. It was found that the extended Spalding (1979) analysis provided excellent predictions of the flame shape properties of the test flames when radiative heat losses were small so that quenching and flame-tip opening were avoided. This analysis also shows that flame properties are identical functions of time for nonbuoyant laminar *