Three-dimensional simulations of premixed hydrogen/air flames in microtubes

Abstract

The dynamics of fuel-lean (equivalence ratio φ = 0.5) premixed hydrogen/air atmospheric pressure flames are investigated in open cylindrical tubes with diameters of d = 1.0 and 1.5 mm using three-dimensional numerical simulations with detailed chemistry and transport. In both cases, the inflow velocity is varied over the range where the flames can be stabilized inside the computational domain. Three axisymmetric combustion modes are observed in the narrow tube: steady mild combustion, oscillatory ignition/extinction and steady flames as the inflow velocity is varied in the range 0.5 6 U IN 6 500 cm s −1 . In the wider tube, richer flame dynamics are observed in the form of steady mild combustion, oscillatory ignition/extinction, steady closed and open axisymmetric flames, steady non-axisymmetric flames and azimuthally spinning flames (0.5 6 U IN 6 600 cm s −1 ). Coexistence of the spinning and the axisymmetric modes is obtained over relatively wide ranges of U IN . Axisymmetric simulations are also performed in order to better understand the nature of the observed transitions in the wider tube. Fourier analysis during the transitions from the steady axisymmetric to the three-dimensional spinning mode and to the steady non-axisymmetric modes reveals that the m = 1 azimuthal mode plays a dominant role in the transitions.

The differential diffusion effect of the intermediate species on the stability of premixed flames propagating in microchannels

“…Different burning modes such as mild combustion, ignition-extinction, steady symmetric and non-symmetric flames, and oscillating or pulsating flames, including cellular and chaotic behaviour, have been identified in the numerical simulation of premixed hydrogen-air flames in micro and meso channels with a specified wall temperature and using detailed chemistry and transport [17,18]. A complex combustion mode, called a spinning flame, where the flame presents a non-symmetric shape and rotates in the azimuthal direction, has also been reported recently in three-dimensional studies of hydrogen-air flames in tubes [19]. In these simulations it was observed that the lighter species or radicals (H, O, OH) were the first species to move away from the tube axis when increasing the inlet velocity in an axisymmetric flame solution, anticipating the role of radicals in flame stability, as discussed below in Section 6.…”

“…For example, for Le F < 1 we choose a lean hydrogen-air mixture at equivalence ratio φ = 0.5, such as done in [17][18][19]. For Le F = 1 a lean methaneair mixture at φ = 0.6 is selected.…”

“…Therefore, before extending the one-step model results to a detailed chemistry, as done in [17][18][19] for premixed hydrogen-air flames, or to a global reaction, as in [28][29][30] for methane-air flames, the present work addresses the influence of the preferential diffusion of the intermediate species on the flame dynamics by using a simple two-step chainbranching model. This chain-branching kinetics is usually preferred as a better approach to real hydrocarbons and hydrogen flame description than that of the simple one-step model [31][32][33].…”

Numerical Modelling of Anisotropic Heat Recirculation in Microcombustors

“…To simulate a three dimensional microtube with premixed hydrogen-air flames, Pizza et al [42] prescribed a hyperbolic tangent temperature function along the axisymmetric wall similar to that shown in Figure 3.4 below. Here the initial (lower) temperature was set to that of the incoming reactants whilst the final (upper) temperature was set based on typical material limits of microcombustor materials (steel) at 960 K. Pizza et al noted that heat losses at the inlet would cause an initial temperature ramp not modelled by the hyperbolic function.…”

Low-dimensional modeling of flame dynamics in heated microchannels

“…Within the context of fundamental works on combustion at small scales, the studies of reactive flows in a straight heated channel with an inner diameter smaller than the ordinary quenching diameter at ambient conditions were shown to provide meaningful contributions (Maruta et al, 2005;Richecoeur and Kyritsis, 2005;Fan et al, 2009;Pizza et al, 2010). Provided that a temperature gradient at the channel's wall can be controlled, such a configuration lately gave significant insights into both the ignition and combustion characteristics of alternative fuels (Yamamoto et al, 2011) and the physico-chemical processes that govern the repetitive ignition/extinction regime (Nakamura et al, 2012).…”