Combined CARS/LDA instrument for simultaneous temperature and velocity measurements

The performance of a combined CARS/LDA instrument capable of measuring temperature and two velocity components with a time coincidence of about 4 s is evaluated in a turbulent premixed propane-air Bunsen-burner flame. Measurements near the base of the flame exhibit negative axial correlations, indicative of normal gradient transport; those near the flame tip show strong positive axial correlations, indicative of transport counter to the temperature gradient. The radial correlations are positive both in the reaction zone and in the plume. An analysis of temperature data from measurements made (1) independent of and (2) coincidental with LDA measurements indicates that the CARS/LDA instrument provides a density-weighted velocity, temperature, and velocity temperature correlation due to the density variations in the flame.     

A study of a bluff-body combustor using laser sheet lighting

Laser sheet lighting is used to study reacting flows with and without heat release in an axisymmetric, unducted and vertically mounted bluff-body combustor. The fuel, which is seeded with titanium tetrachloride vapor, is ejected from a jet located in the center of the bluff-body. The TiCl₄ in the dry fuel reacts spontaneously with the water in the annulus air to form titanium dioxide particles. High speed movies and visual observations of vertically and horizontally located sheets of laser light provided remarkably detailed visualization (via Mie scattering) of the vortex dynamics in the near-wake region of the bluff-body.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 and printed in AMD, Vol. 66     

Soot studies of laminar diffusion flames with recirculation zones

This paper describes the unusual sooting structure of three flames established by the laminar recirculation zones of a centerbody burner. The vertically mounted burner consists of an annular air jet and a central fuel jet separated by a bluff-body. The three ethylene fueled flames are identified as: fully sooting, donut-shape, and ring-shape sooting flames. Different shapes of the soot structures are obtained by varying the N₂ dilution in the fuel and air jets while maintaining a constant air and fuel velocity of 1.2 m/s. All three flames have the unusual characteristic that the soot, entrained into the recirculation zone, follows discrete spiral trajectories that terminate at the center of the vortex. The questions are what cause: (1) the unusual sooting structures and (2) the spiral trajectories of the soot? Flame photographs, laser sheet visualizations, and calculations with a 2D CFD-based code (UNICORN) are used to answer these questions. The different sooting structures are related to the spiral transport of the soot, the spatial location of the stoichiometric flame surface with respect to the vortex center, and the burnout of the soot particles. Computations indicate that the spiral trajectories of the soot particles are due to thermophoresis.     

Efficiency and scaling of an ultrashort-pulse high-repetition-rate laser-driven X-ray source

Technical issues and performance of a high-repetition-rate ultrafast-laser-based X-ray source have been studied experimentally in the context of developing a dedicated laboratory-based tool for combustion diagnostics. X-ray emission from numerous elemental materials have been investigated to compare with analytical based expectations for yield and efficiency, as well as to evaluate advantages of some materials for operational issues such as debris production and degree of efficiency enhancement utilizing various illumination configurations. A weak inverse scaling of conversion efficiency with atomic number was observed. Broadband energy conversion efficiency of approximately 10^?5 and yield greater than 10¹⁰ photons/s have been measured with numerous target elements. Application of a pre-pulse significantly enhances conversion efficiency, and the enhancement factor depends on material. Thus, previous optimizations must be performed in the atomic number variation as well. Additionally, the efficiency enhancement associated with p-polarization incidence (relative to s-polarization) is observed to depend on base material reflectivity.     

A numerical/experimental study of the dynamic structure of a buoyant jet diffusion flame

An overview of a joint numerical/experimental investigation of the dynamic structure of a low-speed buoyant jet diffusion flame is presented. The dynamic interactions between the flame surface and the surrounding fluid mechanical structures are studied by means of a direct numerical simulation closely coordinated with experiments. The numerical simulation employs the full compressible axisymmetric Navier-Stokes equations coupled with a flame sheet model. Counterrotating vortex structures both internal and external to the flame surface are seen to move upward along with flame sheet bulges. These buoyancy-driven dynamic features compare well with those observed experimentally by means of phase-locked flow visualizations over entire flame-flickering cycles. The flicker frequencies measured both computationally and experimentally also compare well. Other aspects of this investigation which are discussed include sudden jumps in flicker frequency with increasing coflow velocity and the utilization of background pressure changes to simulate gravitational force variations experimentally.     

Role of flow visualization in the development of UNICORN

This paper describes how visualizations have been used in the development and evaluation of a reacting-flow-simulation model known as UNICORN (UNsteady Ignition and COmbustion with ReactioNs). UNICORN, which solves full Navier-Stokes equations, has evolved over a 6-year period and is perhaps one of the most thoroughly evaluated codes of its kind. It evolved hand-in-hand with experiments that have been conducted to test its ability to predict ignition, extinction, and the dynamic characteristics of diffusion and premixed flames of hydrogen, methane, and propane fuels and that are stabilized in different ways. This paper also describes how UNICORN has been used, in conjunction with experiments, to provide new insights into combusting flows. Also, predictions of unobserved phenomena that were later confirmed by experiments are described. This paper demonstrates that the judicious use of a well-validated simulation in conjunction with laser based diagnostics is an effective way of understanding complex combusting flows.