An upper bound for the bulk burning rate for systems
  We consider a system of reaction-diffusion equations with passive advection term and Lewis number not equal to one. Such systems are used to describe chemical reactions in a flow in a situation where temperature and material diffusivities are not equal. It is expected that the fluid advection will distort the reaction front, increasing the area of reaction and thus speeding up the reaction process. While a variety of estimates on the influence of the flow on reaction are available for a single reaction-diffusion equation (corresponding to the case of Lewis number equal to one), the case of the system is largely open. We prove a general upper bound on the reaction rate in such systems in terms of the reaction rate for a single reaction-diffusion equation, showing that the long time average of reaction rate with $\Le \ne 1$ does not exceed the $\Le=1$ case. Thus the upper estimates derived for $\Le=1$ apply to the systems. Both front-like and compact initial data (hot blob) are considered. 
Alexander Kiselev and Leonid Ryzhik