Diffusion flames exist in most practical combustors, and an accurate understanding of their structure is crucial to efficiency improvement and pollution suppression. A coflow laminar diffusion flame, which has well-defined boundary conditions, is the simplest configuration in which interactions between flow field and reactions can be readily modified and studied. Knowledge obtained from coflow laminar diffusion flames is not only of fundamental importance, but it also can facilitate the study of turbulent diffusion flames in practical industrial combustors.
In order to facilitate the computational investigation of laminar flames, a novel vorticityvelocity formulation of the Navier-Stokes equations — the Mass-Conserving, Smooth (MC-Smooth) vorticity-velocity formulation — is developed in this work. The governing equations of the MC-Smooth formulation include a new second-order Poisson-like elliptic velocity equation, along with the vorticity transport equation, the energy conservation equation, and Nspec species mass balance equations. The MC-Smooth formulation is compared to two pre-existing vorticity-velocity formulations by applying each formulation to confined and unconfined axisymmetric laminar diffusion flame problems For both applications, very good to excellent agreement for the simulation results of the three formulations is obtained. The MC-Smooth formulation requires the least CPU time and can overcome the limitations of the other two pre-existing vorticity-velocity formulations by ensuring mass conservation and solution smoothness over a broader range of flow conditions. In addition to these benefits, other important features of the MC-Smooth formulation include: (1) it does not require the use of a staggered grid, and (2) it does not require excessive grid refinement to ensure mass conservation.
The MC-Smooth formulation is then applied to two groups of coflow laminar diffusion flames of great fundamental and practical significance. In the first application, the influences of fuel dilution, inlet velocity, and gravity on the shape and structure of methane-air coflow laminar diffusion flames are investigated. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) on board the International Space Station is assessed numerically under microgravity and normal gravity conditions with CH4 mole fraction ranging from 0.4 to 1.0, inlet fuel velocity ranging from 23 to 90 cm/s, and inlet coflow velocity ranging from 16 to 65 cm/s. Very good agreement between computation and measurement is obtained, and the major conclusions are as follows.
1. Buoyant and nonbuoyant luminous flame lengths are proportional to the mass flow rate of the fuel mixture. Computed and measured nonbuoyant flames are noticeably longer than their 1 g counterparts. The effect of fuel dilution on flame shape is negligible when the flame shape is normalized by the methane flow rate. 2. Increasing coflow velocity reduces the size of the luminous flame shape, and the size of the luminous flame shape will decrease faster when gravity is eliminated or inlet fuel velocity is larger. 3. Soot volume fraction is very sensitive to variations in gravitational acceleration, fuel stream dilution, and inlet fuel velocity. Eliminating buoyancy, increasing fuel concentration, or increasing inlet fuel velocity will broaden and lengthen the sooting region of the flame, increasing the peak soot volume fraction, and shift its location to the vicinity of the flame wing region.
In the second application, the influence of fuel dilution and pressure on the structure and geometry of methane-air coflow laminar diffusion flames is examined A series of methane-fueled, nitrogen-diluted flames is investigated both computationally and experimentally, with CH4 mole fraction ranging from 0.50 to 0.65 and pressure ranging from 1.0 to 2.7 atm. For a broad spectrum of flames at atmospheric and elevated pressures, very good agreement between computation and measurement is obtained, and the major conclusions are as follows. (1) Maximum temperature increases monotonically with increasing CH4 concentration or pressure, while the peak temperature along the centerline changes in a non-monotonic way with respect to pressure. (2) Flame radius at a given height decreases with pressure approximately as rf ∝ P-1/2, and modified flame length is roughly independent of pressure. (3) Inclusion of a soot model significantly reduces the peak temperature along the centerline, and its effects on the maximum temperature and flame geometry are minor.
|Advisor:||Smooke, Mitchell D.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 77/06(E), Dissertation Abstracts International|
|Keywords:||Gravity, Laminar Diffusion Flame, Methane, Pressure, Soot, Vorticity-Velocity Formulation|
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