Characteristics of hydrogen–hydrocarbon composite fuel turbulent jet flames

The stability properties of round variable-density low-Mach-number jets are studied by means of direct numerical simulation (DNS) and linear stability analysis. Fully three-dimensional DNS of variable-density jets, with and without gravity, demonstrate that the presence of buoyancy causes a more abrupt transition to turbulence. This effect helps to explain differences between normal gravity and microgravity jet diffusion flames observed in the laboratory. The complete spectrum of spatial eigenmodes of the linearized low-Mach-number equations is calculated using a global matrix method. Also, an analytic form for the continuous portion of this spectrum is derived, and used to verify the numerical method. The absolute instability of variable-density jets is confirmed using Brigg’s method, and a comprehensive parametric study of the strength and frequency of this instability is performed. Effects of Reynolds number, the density ratio of ambientto-jet fluid (S1), shear-layer thickness and Froude number are considered. Finally, a region of local absolute instability is shown to exist in the near field of the jet by applying linear stability analysis to mean profiles measured from DNS.

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Self-sustained oscillations in variable-density round jets

This paper discusses results obtained in a study of the transition to turbulence of gas jet diffusion flames in microgravity. The experiments were conducted at the 2.2-second Drop Tower of the NASA Lewis Research Center. Propane flames burning in quiescent air at atmospheric pressure were investigated. Significant differences have been observed between the transition processes of gas jet diffusion flames in microgravity and normal gravity These include the appearance of large scale structures in microgravity which are manifested at the flame base and are convected downstream. These structures appear intermittently at first, and, then, as Reynolds number is increased, a continuous train of structures is observed along the flame front. The experimental observations are explained in terms of flow transition and instability of the shear layer associated with the injection of the jet flow into quiescent surroundings

The transition to turbulence of microgravity gas jet diffusion flames

Recently, three major combustion experiments were carried out in the microgravity environment of the Space Shuttle on the Microgravity Science Laboratory-1 mission. These three efforts entailed a study of laminar soot processes in gaseous diffusion e ames, a study of the structure of e ame balls at low Lewis numbers, and a study of isolated droplet combustion. Descriptions of these three experiments and their preliminary results appear in the following three papers. The purpose of thispaperis to “ setthetable” forthese threepapers through a discussion of theimportanceof microgravity to the study offundamental combustion phenomena, a presentation of an overview of the current NASA microgravity combustion program, and a brief description (with references to morecomplete papers) of previous space-based microgravity combustion experiments carried out under the aegis of NASA.

Overview of the NASA Microgravity Combustion Program

Steady-state global chemistry calculations for 20 different flames were carried out using an axisymmetric Computational Fluid Dynamics (CFD) code. Computational results for 16 flames were compared with flame images obtained at the NASA Glenn Research Center. The experimental flame data for these 16 flames were taken from Sunderland et al. [4] which included normal and inverse diffusion flames of ethane with varying oxidiser compositions (21, 30, 50, 100% O2 mole fraction in N2) stabilised on a 5.5 mm diameter burner. The test conditions of this reference resulted in highly convective inverse diffusion flames (Froude numbers of the order of 10) and buoyant normal diffusion flames (Froude numbers ∼0.1). Additionally, six flames were simulated to study the effect of oxygen enhancement on normal diffusion flames. The enhancement in oxygen resulted in increased flame temperatures and the presence of gravity led to increased gas velocities. The effect of gravity-variation and oxygen enhancement on flame shape a...

Simulations of normal and inverse laminar diffusion flames under oxygen enhancement and gravity variation

Gas jet diffusion flames have been a subject of research for many years. However, a better understanding of the physical and chemical phenomena occurring in these flames is still needed, and, while the effects of gravity on the burning process have been observed, the basic mechanisms responsible for these changes have yet to be determined. The fundamental mechanisms that control the combustion process are in general coupled and quite complicated. These include mixing, radiation, kinetics, soot formation and disposition, inertia, diffusion, and viscous effects. In order to understand the mechanisms controlling a fire, laboratory-scale laminar and turbulent gas-jet diffusion flames have been extensively studied, which have provided important information in relation to the physico-chemical processes occurring in flames. However, turbulent flames are not fully understood and their understanding requires more fundamental studies of laminar diffusion flames in which the interplay of transport phenomena and chemical kinetics is more tractable. But even this basic, relatively simple flame is not completely characterized in relation to soot formation, radiation, diffusion, and kinetics. Therefore, gaining an understanding of laminar flames is essential to the understanding of turbulent flames, and particularly fires, in which the same basic phenomena occur. In order to improve and verify the theoretical models essential to the interpretation of data, the complexity and degree of coupling of the controlling mechanisms must be reduced. If gravity is isolated, the complication of buoyancy-induced convection would be removed from the problem. In addition, buoyant convection in normal gravity masks the effects of other controlling parameters on the flame. Therefore, the combination of normal-gravity and microgravity data would provide the information, both theoretical and experimental, to improve our understanding of diffusion flames in general, and the effects of gravity on the burning process in particular.

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Effects of Buoyancy on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames

This paper presents the ground-based results obtained to date in preparation of a proposed space experiment to study the role of large-scale structures in microgravity transitional and turbulent gas-jet diffusion flames by investigating the dynamics of vortex/flame interactions and their influence on flame characteristics. The overall objective is to gain an understanding of the fundamental characteristics of transitional and turbulent gas-jet diffusion flames. Understanding of the role of large-scale structures on the characteristics of microgravity transitional and turbulent flames will ultimately lead to improved understanding of normal-gravity turbulent combustion.

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Characteristics of transitional and turbulent jet diffusion flames in microgravity

An ongoing study of the transition to turbulence of gas jet diffusion flames conducted in a reduced microgravity environment at the NASA Lewis 2.2 second drop tower is described. The experiments were based on propane injected into quiescent air. Several differences with the corresponding normal gravity process were observed. In particular, large scale structures appeared intermittently in microgravity which are manifested at the flame base and convected down stream. As the Reynolds number increased, a continuous train of structures was observed along the flame front. These observations are explained by instability of the shear layer associated with injection of flow into quiescent surroundings.

Liming Zhou

Papers

Effects of Buoyancy on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames

Characteristics of transitional and turbulent jet diffusion flames in microgravity

Effects of Gravity on the Transition to Turbulence of Gas Jet Diffusion Flames