A computational and experimental study of coflow laminar methane/air diffusion flames: Effects of fuel dilution, inlet velocity, and gravity

Abstract

The influences of fuel dilution, inlet velocity, and gravity on the shape and structure of laminar coflow CH 4 -air diffusion flames were investigated computationally and experimentally. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) on board the International Space Station was assessed numerically under microgravity (μg) and normal gravity (1g) conditions with CH 4 mole fraction ranging from 0.4 to 1.0 and average inlet velocity ranging from 23 to 90 cm/s. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modeled by sectional aerosol equations. The governing equations and boundary conditions were discretized on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, flame shape and soot temperature were determined by flame emission images recorded by a digital color camera. Very good agreement between computation and measurement was obtained, and the conclusions were 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 1g counterparts; the effect of fuel dilution on flame shape (i.e., flame length and flame radius) is negligible when the flame shape is normalized by the methane flow rate. (2) Buoyancy-induced reduction of the flame radius through radially inward convection near the flame front is demonstrated. (3) Buoyant and nonbuoyant flame structure is mainly controlled by the fuel mass flow rate, and the effects from fuel dilution and inlet velocity are secondary.

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.…”

A review of motivations, methods, and achievements on microgravity combustion research

“…They found that, for the same fuel and oxidizer, the length of flame-sheet increases with the increase in fuel mass flow velocity, and regardless of the burner diameter. Cao et al [17] computationally and experimentally investigated the influence of fuel dilution, inlet velocity, and gravity on flame characteristics. Their works showed that the mass flow velocity of fuel dominates both the buoyant and nonbuoyant flame structures, followed by the fuel dilution and inlet velocity.…”

A numerical investigation of CO2 dilution on the thermochemical characteristics of a swirl stabilized diffusion flame

“…They argued that as the concentration of diluent increases, the flame shape becomes more cylindrical. Cao et al [13] demonstrated that flame length is proportional to the mass flow rate of reactants in co-flow diffusion flames.…”