We study analytically and numerically a model describing front
propagation of a KPP reaction in a fluid flow. The model consists
of coupled one-dimensional reaction-diffusion equations with
different drift coefficients. The main rigorous results give lower
bounds for the speed of propagation that are linear in the drift
coefficient, which agrees very well with the numerical
observations. In addition, we find the optimal constant in a
functional inequality of independent interest used in the proof.