An experimental and computational study of soot formation in a coflow jet flame under microgravity and normal gravity

Abstract

Upon the completion of the Structure and Liftoff in Combustion Experiment (SLICE) in March 2012, a comprehensive and unique set of microgravity coflow diffusion flame data was obtained. This data covers a range of conditions from weak flames near extinction to strong, highly sooting flames, and enabled the study of gravitational effects on phenomena such as liftoff, blowout and soot formation. The microgravity experiment was carried out in the Microgravity Science Glovebox (MSG) on board the International Space Station (ISS), while the normal gravity experiment was performed at Yale utilizing a copy of the flight hardware. Computational simulations of microgravity and normal gravity flames were also carried out to facilitate understanding of the experimental observations. This paper focuses on the different sooting behaviors of CH 4 coflow jet flames in microgravity and normal gravity. The unique set of data serves as an excellent test case for developing more accurate computational models. Experimentally, the flame shape and size, lift-off height, and soot temperature were determined from line-of-sight flame emission images taken with a color digital camera. Soot volume fraction was determined by performing an absolute light calibration using the incandescence from a flame-heated thermocouple. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the chemically reacting flow, and the soot evolution was modeled by the sectional aerosol equations. The governing equations and boundary conditions were discretized on an axisymmetric computational domain by finite differences, and the resulting system of fully coupled, highly nonlinear equations was solved by a damped, modified Newton's method. The microgravity sooting flames were found to have lower soot temperatures and higher volume fraction than their normal gravity counterparts. The soot distribution tends to shift from the centerline of the flame to the wings from normal gravity to microgravity.

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

“…The HACA growth on the wings was found sensitive to the soot surface reactivity parameter whereas soot on the centerline is shown to be insensitive to it due to reduced H radical concentration. Computed temperature (K) isopleths and measured soot temperature (K) from [18] under (left) normal gravity and (right) microgravity for a CH 4 flame with 3.23 mm ID nozzle and average fuel speed of 46 cm/s. Figure 2.…”

“…Comparison of soot volume fraction maps (ppm) with measured soot volume fractions (ppm) from [18] under (left) normal gravity and (right) microgravity for a CH 4 flame with 3.23 mm ID nozzle and average fuel speed of 46 cm/s. Total mass contribution by HACA surface growth, PAH addition (nucleation and condensation), and the amount of soot mass depleted by oxidation for a soot particle travelling (top) along the pathline of maximum soot on the wings, and (bottom) along the centerline for a CH 4 flame with 3.23 mm ID nozzle and average fuel speed of 46 cm/s under normal and microgravity.…”

MC-Smooth: A mass-conserving, smooth vorticity–velocity formulation for multi-dimensional flows

“…The flow's small Mach number implies that pressure can be approximated as being independent of location in the flame, and mixture density can be directly obtained from the ideal gas law. Although larger chemical mechanisms and sectional soot models have been employed in other MC-Smooth simulations [51,72,73], the chemical mechanism adopted in this work is a C1 mechanism, which involves 16 species and 46 reactions [74] and has been utilized in previous studies [28,75]. The result is a model that consists of a total of 20 strongly coupled, highly nonlinear partial differential equations.…”

“…Since the flame is surrounded by an air coflow and occupies less than 5% of the cross-sectional area of the burner, the square duct is approximated as a coaxial tube with an identical cross-sectional area (radius r O = 4.288 cm, see Figure 1), and the computational domain for this application extends radially from the centreline (r = 0) to r max = 4.288 cm and axially from the burner exit plane (z = 0) to z max = 12.2 cm. Further information on the burner construction and operation can be found in [72,73]. In this study, the fuel is methane, which is diluted with nitrogen to reduce soot formation.…”

“…These values are selected to match corresponding experiments [72,73]. Since temperature measurements near the fuel tube exit are unavailable, we will follow the strategy employed in [72,73] and set the inlet temperature T 0 to 298 K. The wall temperature T wall is also set to 298 K.…”