Structure of laminar coflow spray flames at different pressures

Abstract

Experiments were conducted on laminar spray diffusion flames of ethanol/argon burning in oxygen at pressures of 1 and 3 atm. The flames were physically characterized by measuring droplet velocities and sizes by phase-Doppler anemometry, and gas temperature by thin-filament pyrometry and thermocouples. The flames exhibited a cold core, where little vaporization occurred, surrounded by a primary diffusion flame enveloping most of the droplet cloud. Their structure was strongly influenced by the Peclet number for heat transfer that, by regulating the heat penetrating into the core, and, consequently, the growth of the thermal boundary layer, affected the droplet vaporization history. At pressures above the atmospheric, because of density effects on thermal diffusivity, the boundary layer growth above the burner was reduced and so was the vaporization region. Complete evaporation of the droplets before they reached the primary diffusion flame was ensured if a suitably defined Damkö hler number of evaporation, Da v , was smaller than 1. Conversely, if Da v Ͼ 1, droplets penetrated the flame, ignited by a flame transfer process that was captured photographically, and burned isolated on the oxidizer side. These conditions of internal or partial group combustion prevailed in the lower part of the flame. Farther up in the flame, this combustion regime progressively shifted toward that of external group combustion, with fewer and fewer direct droplet/flame interactions.

Physical characterization of laminar spray flames in the pressure range 0.1–0.9 MPa

“…Further details on scaling and selfsimilarity, as well as their controlling parameters will be given in the discussion of the results. The present set of measurements broadens considerable the pressure range and the Richardson number range, as compared to our earlier work discussing only Flames 1 and 3 [7].…”

“…In that region, the temperature remained constant since the flow had not been influenced yet by the flame generated heat. The dimension of the cold region can be inferred from droplet size or radial velocity measurements [7,8].…”

“…This figure is adapted from Ref. [7] for comparisons purpose. The trajectory is tracked by the axial coordinate in the abscissa.…”

“…Particular attention is devoted to the vaporization history of droplets that do not fully vaporize before reaching the diffusion flame in a regime of internal group combustion. Results on two of the four flames studied were presented in [7].…”

Thin-filament pyrometry with a digital still camera

“…Another 8-bit CCD camera, of unspecified pixel dimensions, was used by Bundy et al [19]. As a part of a larger study, Russo and Gomez [20] used 320 × 240 infrared camera in the TFP measurements.…”