Recently, Sabelnikov et al. (2019) developed a phenomenological theory of propagation of an infinitely thin reaction sheet, which is adjacent to a mixing layer, in a constant-density turbulent flow in the case of a low Damk\uf6hler number. In the cited paper, the theory is also supported by Direct Numerical Simulation data and relevance of such a physical scenario to highly turbulent premixed combustion is argued. The present work aims at complementing the theory with a new mathematical framework that allows for appearance of thick mixing zones adjacent to an infinitely thin reaction sheet. For this purpose, the instantaneous reaction-progress-variable c(x,t) is considered to consist of two qualitatively different zones, that is, (i) mixture of products and reactants, c(x,t)<1, where molecular transport plays an important role, and (ii) equilibrium products, c(x,t)=1. The two\ua0zones are separated by an infinitely thin reaction sheet, where c(x,t)=1 and |nabla c| is fixed in order for the molecular flux into the sheet to yield a constant local consumption velocity equal to the speed of the unperturbed laminar reaction wave. Exact local instantaneous field equations valid in the entire spaceare derived for the conditioned (to the former, mixing, zone) reaction progress variable,\ua0its second moment, and instantaneous characteristic functions. Averaging of these equations yields exact, unclosed transport equations for the conditioned reaction-progress-variable moments and\ua0Probability Density Function (PDF), as well as a boundary condition for the PDF at the reaction sheet.\ua0The closure problem for the derived equations is beyond the scope of the paper
