I have been going through articles on Turbulent Combustion modelling and for simplification purposes, many authors have assumed Lewis number equal to 1. I know that it is the ratio of rate of thermal diffusivity to the rate of mass diffusivity. However, I am unable to understand the consequences of both the rates being equal.
Are you talking consequences from a numerical point of view?
When using Fick's law to evaluate the species diffusion velocities, we typically assume a mean species molecular diffusion coefficient D_s (mass diffusivity of species s relative to the mixture).
The Lewis number does come into play in it's evaluation: D_s = \nu / (Pr Le_s). If you assume it equal to 1 for every species, this implies that you get the same D_s.
From a modeling point of view, assuming Le=1 means that you do not have to worry about the relative thickness between the thermal and mass diffusion boundary layers, as they behave in a similar manner. This, in many situations, simplify the modeling treatment (This is quite analogous to how to treat the relative thickness between the momentum and thermal boundary layers for high/ow Pr-numbers).
This is a text book like question. There is so much written in text books.
In the case of laminar flow, molecular diffusion dominates and the use of the appropriate Lewis number (defines say as Le = D rho cp /k) which in reality is between two species (D should be D12) has been interpreted to mean diffusion of fuel in an averaged mixture of the rest of the gases. For methane it is close to 1. For many other gases except propane or hydrogen its deviation from 1 is small. For propane or gases with higher molecular weight it is less than 1 (with the above definition of Le) and for hydrogen it is higher. There are significant effects on the prediction of combustion behavior with Le. There is enormous literature on this.
However, for turbulent flow, the mixing dynamics is dependent on "eddy diffusivity" or better ideas and molecular diffusion has much less influence. It is for this reason that turbulent Prandtl number, Schmidt number (and so Lewis number) are taken as 1. It is only in the case of hydrogen, significant differences between prediction and experiments are observed. General statements on this have not much value. The physical problem where these approximations are used must be examined with care if predictions of significance are sought.
This values tell you if the mechanism of mass transfer and heat transfer are of similiar magnitude or not. So you may use thermal convection coefficients for determining mass transfer if you know Lewis number.
This values is quite often seen in drying literature as a correlation parameter for drying rate.
Nothing else to say.
From modelling pespective I see that if you solve a set of differential equations on the mesh, you usually may encounter problems due to that functions you operate are non-linear or of different form that the others. This gives an opportunity to ambiguate the notation to matrix algebra, instead of using numerical solvers everytime to evalutate anything because eveything works on same constitutive equation, you may just save on number of corellations what reduces the total error and stability of your solution.