This work is concerned with the numerical investigation of the influence of the ratio of thermal conductivity to diffusivity of a combustible gas mixture on the dynamics of adiabatic flame propagation in narrow closed planar channels under zero gravity. Reynolds numbers are less than 100. A low Mach number limit of the Navier-Stokes system for the A --> B reaction was used. The appearance of a flame front bifurcation, resulting in flame inversion, was obtained numerically for low Lewis numbers. The combined effect of a specific hydrodynamic flow field in a closed channel and of the domination of diffusivity over thermal conductivity, similar to the mechanism of thermal-diffusive instability, was found to be the main driving force of the bifurcation. It was concluded that, both numerically and experimentally, the inverted flame front has the same general shape. However, this shape is unlikely to be attributed to the same physical mechanism of its formation over the entire range of regimes. Fragmentation of inverted flames into two segments is clearly observed for low Lewis numbers.