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Small separation between reactants, not exceeding $10^{-8}-10^{-7}cm$, is thenecessary condition for various chemical reactions. It is shown that randomadvection and stretching by turbulence leads to formation of scalar-enrichedsheets of {\it strongly fluctuating thickness} $\eta_{c}$. The molecular-levelmixing is achieved by diffusion across these sheets (interfaces) separating thereactants. Since diffusion time scale is $\tau_{d}\propto \eta_{c}^{2}$, theknowledge of probability density $Q(\eta_{c},Re)$ is crucial for evaluation ofchemical reaction rates. In this paper we derive the probability density$Q(\eta_{c},Re,Sc)$ and predict a transition in the reaction rate behavior from${\cal R}\propto \sqrt{Re}$ ($Re\leq 10^{4}$) to the high-Re asymptotics ${\calR}\propto Re^{0}$. The theory leads to an approximate universality oftransitional Reynolds number $Re_{tr}\approx 10^{4}$. It is also shown that ifchemical reaction involves short-lived reactants, very strong anomalousfluctuations of the length-scale $\eta_{c}$ may lead to non-negligibly smallreaction rates.

Dissipation Scale Fluctuations and Chemical Reaction Rates in Turbulent Flows