The control of turbulent mixing on the reactivity of species is studied by analyzing the contribution of the chemical terms to the governing equations for reactants. For reacting scalars with a short chemical lifetime or with a lifetime at the same order of magnitude as the turnover time of the convective boundary layer (CBL), one should expect that the chemical terms have a large contribution and, as a result, fluxes and (co-)variances deviate from the inert profiles. In this paper, we attempt to give a comprehensive view of the magnitude of these deviations and of their control parameters.

The effect of the chemical terms on transport and mixing of reacting scalars is evaluated through exact decomposition of second-order moment budgets using large-eddy simulation (LES). Compared to temperature, moisture and inert scalars for which extensive studies have shown the contribution of the different dynamical terms to the budget of fluxes and (co-)variances, only few studies were devoted to reacting scalar second-order moment budgets. In particular, a complete analysis of the chemical contribution to these moments is still missing. As representative cases, we have simulated turbulent reacting flows in the CBL with a second-order irreversible reaction and a chemical mechanism in equilibrium.

From the budget analysis, we found that the chemical terms become more relevant when the chemical timescale is similar to the turbulent timescale. In order to determine the importance of the chemical term, we have compared this term to the dynamical terms of the budget equations. For the flux of reactants, the chemical term becomes the dominant sink in the bulk of the CBL. As a result, non linear flux profiles of reacting scalars are found. For the covariance, which accounts for the segregation of species in the CBL, the chemical term acts as a sink or source term depending on the reaction rate. In consequence, reacting scalar (co-)variance profiles deviate largely from the inert scalar one. When the chemistry is in equilibrium, the chemical term becomes negligible and therefore the flux and (co-)variance profiles are similar to the ones of inert scalars. Based on the previous results, we develop a parameterization to represent the segregation of reacting species in the CBL.