  The problem of premixed flame propagation in wide horizontal tubes is
revisited. Employing the on-shell description of flames with arbitrary gas
expansion, a nonlinear second-order differential equation for the front
position of steady flame is derived. Solutions to this equation, obtained
numerically, reveal two distinct physical regimes of laminar flame propagation
controlled by the strong baroclinic effect. They differ by the front shape and
flame speed, the ratio of the total consumption rates in the two regimes being
1.4 to 1.8, depending on the value of the gas expansion coefficient. Comparison
with the existing experimental data on methane-air flames is made, and
explanation of the main trends in the observed flame behavior is given. It is
shown, in particular, that the faster (slower) regime of combustion is realized
in mixtures close to (far from) the stoichiometric composition, with pronounced
changeover in between.
