Diffusion flame

About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.

Soot formation in diffusion flames: Flow rate, fuel species and temperature effects

Abstract: A detailed study of soot particle formation in diffusion flames has been made for a series of laminar diffusion flames using a laser scattering/extinction technique. The effects of flow rate, fuel species and temperature on the evolution of the soot particle field have been investigated. Temperature effects have been examined through nitrogen dilution of the fuel. Fluorescence measurements from gas phase species in the initial particle formation region have been used to examine the relationship between these potential soot precursors and the observed particle formation. Flow rate, fuel species and temperature variations have been observed to significantly effect the annular region of the flame, where particles first appear. Increasing the fuel flow rate results in a larger soot volume fraction while the maximum particle size observed in the flame remains nearly constant. Thus, particle number concentration appears to be the quantity most sensitive to the flow rate variation. Temperature measurements made in the flames indicate that fuel flow rate increases result in higher temperatures in the particle formation region, leading to increased soot formation. At later stages, the flames with larger soot concentrations exhibit lower temperatures which facilitates soot particle survival in the oxidation region of the flame. Measurements made in flames with different fuels but similar adiabatic flame temperatures, indicate that the temperature exerts the strongest influence on soot formation. However, distinct effects due to fuel structure still need to be considered.

Effects of the In-Cylinder Environment on Diffusion Flame Lift-Off in a DI Diesel Engine

Abstract: The diffusion flame lift-off length of isolated, free diesel jets in quiescent atmospheres is known to have a strong influence on soot formation by affecting fuel/air mixing prior to combustion. In realistic engine environments, the proximity and temperature of in-cylinder surfaces, in-cylinder gas flows (swirl), and interactions between adjacent jets may affect the behavior of the flame lift-off, and thereby affect soot formation. To better understand the influence of these factors on the lift-off length and on soot formation, optical imaging diagnostics were employed to measure the flame lift-off length in an optically-accessible heavy-duty Direct Injection (DI) diesel engine. A two-camera OH chemiluminescence diagnostic was developed and employed to measure the flame lift-off length for a range of injector nozzle geometries and engine operating conditions. A two-camera OH Planar Laser-Induced Fluorescence (OH-PLIF) diagnostic for side-on illumination of the jet,was also developed and utilized to aid in the interpretation of the OH chemiluminescence data. The diesel flame lift-off lengths in the optical engine were shorter than those observed previously for single, isolated jets in a quiescent atmosphere. Also, high cycle-to-cycle variation was observed in the lift-off length data from the engine, yielding considerable uncertainty in most of the reported trends. However, on average, the lift-off length was 7% longer on the windward side of the jet relative to the weak in-cylinder swirl flow. The proximity of the firedeck to the jets (14° down-angle) did not cause preferential vertical asymmetry in the flame shape, on average. A reduction in the interjet spacing angle from 90° (i.e., 4-holes) to 45° (8-holes) was correlated with a 35% reduction in the lift-off length, along with decreased sensitivity to ambient gas temperature and density. The mechanism by which the proximity of adjacent jets affects the lift-off length was not firmly identified, but fluid mechanical/thermal coupling between jets and/or internal injector fuel flow characteristics may be important. In realistic Dl diesel engine environments, high cycle-to-cycle variability, decreased temperature dependence, and shortening of the lift-off length may hinder soot reduction methods that rely on manipulation of the mean flame lift-off length.

The Effect of Oxygen Concentration on Sooting Diffusion Flames

Abstract: In a concentric diffusion flame arrangement, sooting heights were determined as a function of the oxygen concentration in the oxidizing stream. Changing the oxygen concentration has two different and competing effects on the tendency of a fuel jet to soot. Increasing the oxygen concentration increases the stoichiometric flame temperature which in turn increases the fuel pyrolysis and soot formation rates. However, increasing the oxygen concentration also increases the particle burn-up rate in the vicinity of the flame. The initial effect dominates at high oxygen concentrations and the latter at low concentrations. In most practical combustor systems, the temperature would appear to be the controlling parameter.